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It mentioned on p. 78, almost invariably it is the fruits, not the seeds, of forest trees which are harvested. In some species it is also the fruits which we sown in the nursery and they are then often referred to loosely as “seeds”, e.g. teak. In the majority of species, however, fruits are collected but seeds are sown, therefore at some stage the seeds must be extracted from their covering of fruits. As stated in Chapter 5, extraction is sometimes done close to the site of collection, but is frequently carried out at a central processing and storage depot. The purpose of extraction and associated processes is the maximum production of clean seed having high viability (Stein et al. 1974). The processes involved include one or more of the following: maceration and depulping, drying, separation, tumbling and threshing, dewinging and cleaning.

Operations Prior to Extraction

Temporary storage at the processing depot

No matter how much trouble is taken, conditions for fruits and seeds are seldom ideal during transit from the forest to the seed processing depot. This is particularly the case if the journey is prolonged over several days. It is important therefore that fruits should be unloaded as soon as they arrive at the depot, inspected and placed in storage conditions which provide protection from rain, exclusion of rodents and birds and continuous free air circulation round the fruits (Aldhous 1972). Without such care fruits and seeds may suffer considerable deterioration between arrival at the depot and the extraction of seed. If moulds are allowed to develop on cones or fruits, they not only degrade the batch on which they occur but can form a source for infection of other batches by fungal spores.

Because of the seasonal nature of most fruit and seed harvesting, large quantities of fruits are often collected within a short period. Seed extracting machinery has a limited capacity, so some of the fruits cannot be processed immediately on arrival at the depot. Temporary storage is thus unavoidable. In some species it is also highly desirable, because it allows time for maturation and drying of seeds prior to further processing. When deliberately applied for this purpose, this type of storage/air drying is known as precuring and is described on pp. 89–91.

Storage of cones or fruits for an extended period before extraction is necessary when an exceptionally heavy seed crop occurs, perhaps once in a decade or two, and very large quantities are collected in a single season. Research in British Columbia on 5 species of conifer showed that, provided good cone handling techniques were used, cones could be stored, either in covered outdoor cone sheds or in refrigerated coolers at 2°C, for six months from October to March. In most cases germination was as good or better after six months than it was in the freshly collected seed. Controlled temperature storage offered no advantage over outdoor storage (Leadem 1980).

In most cases, pre-cleaning of fruits (see next section) is done soon after their arrival at the depot but, if the volume of fruits arriving at the same time is very high, pre-cleaning of part of the consignment may have to be deferred; in this case part of the storage period will precede the pre-cleaning operation and part will occur between pre-cleaning and extraction.

Fruits should be stored in a dry, cool, well-ventilated place to prevent moulding or heating. Depending on species, condition of fruits and processing techniques, sacks of fruits or cones may be emptied, loosely refilled and placed on storage racks; or the fruits may be spread in trays, on a storage floor or, under cover, on the ground (Stein et al. 1974). Floors of brick or wood are suitable, but fruits should not be placed directly on concrete floors because of moisture problems (Morandini 1962, Turnbull 1975 c). In Honduras slatted wooden storage bins supported at intervals on concrete blocks have proved very satisfactory for temporary storage of cones of Pinus caribaea and P. oocarpa. Gaps between the slats allow good ventilation and cones can be stored to a depth of 30–40 cm without the need to move them (Robbins 1983 a, b). In filling and stacking trays of cones, allowance must be made for a two- or three-fold expansion of cones as the scales open (Stein et al. 1974). Fruits may also be spread on tarpaulins.


Cones and fruits need to be cleaned of twigs, bark, foliage and other impurities before they go for extraction, cleaning, storage or sowing. In large extraction plants cleaning is accomplished with oscillating screens or vibrators. Precleaning by flotation is an alternative method. In small operations the major debris may be removed by hand (Turnbull 1975 c). The impurities take up needless space. In addition leaf and twig fragments may carry fungal spores of e.g. needle cast disease of which the seeds themselves are free. These spores are a potential threat not so much to the seeds as to the newly germinated seedlings and to nursery stock and plantations in the vicinity of the nursery. Impurities are more easily removed before than after extraction.

Special measures are sometimes taken to remove sticky resin exudations from cones. Stein et al. (1974) reported that one company dried cones of Pseudotsuga sufficiently to harden the resin, then wetted and tumbled the closed cones to remove both dirt and resin.

For some species, pre-cleaning, together with drying in some cases, is the only operation needed before storage or sowing. They are stored or sown as fruits. Pre-cleaning may include the removal of appendages to the fruit such as the involucre of Quercus, Fagus or Tectona. Winged fruits are often sown complete with wings e.g. Ulmus, Fraxinus, Acer, Triplochiton, Pterocarpus, several genera of dipterocarps.


Pre-curing is the deliberate storage and slow air drying of fruits and contained seeds in order to render them more suitable for subsequent operations of kiln drying, extraction and longterm seed storage. The processes assisted by precuring are seed maturation and fruit drying.

Fruits do not all ripen at the same time, even for the same species and in the same forest (Morandini 1962). Thus, even when a collection is perfectly timed at the peak maturity of the crop, there will always be a proportion of sound seeds which are not fully mature. The minimum period for maturation in certain species is two weeks, but many demand more than 6 – 8 weeks (Morandini 1962).

Abies procera is one species which benefits from post-harvest storage of cones. It was found that during six weeks' storage, the seeds in the cones increased their dry weight by 10% as a result of accumulating organic materials from the cones; during this period carbohydrate and starch content in the seeds decreased but crude fat content steadily increased (Rediske and Nicholson 1965).

In a few species e.g. Fraxinus excelsior, Magnolia spp., Gingko biloba, the entire crop of seeds is dispersed with underdeveloped embryos which must complete their growth and development before the seeds can germinate (Gordon and Rowe 1982). In most such cases of morphological dormancy, simple precuring is insufficient to induce germination, but warm, moist pretreatment is needed; this is often followed by a cold moist pretreatment to remove the physiological dormancy which also occurs in the same species. These pretreatments are described on p. 184.

The deliberate collection of immature fruits was mentioned on p. 37. It has shown promise on a research scale in several species of temperate conifers and two hardwood genera, Liquidambar and Liriodendron (Bonner 1970, 1972). A cool moist environment is best for artificial ripening and can be maintained by mixing the cones or fruits with peat moss or other inert moisture-retaining material in polyethylene bags. Satisfactory temperatures were 5°C for Liquidambar (Bonner 1970) and 17°C for Pseudotsuga (Silen 1958). For Pinus sylvestris collection was done when specific gravity of the cones was 1.1 and the cones were stored for at least one month (Remröd and Alfjorden 1973).

In New Zealand the correct sowing season for Pinus radiata is in October but ripe cones are not available for collection until November – December. The seeds must therefore be stored for nearly a year. Research has shown that immature green cones can be collected in June or July and artificially ripened by storing loosely in paper bags at a room temperature of 20°–24° C for 10 weeks (Wilcox and Firth 1980). After kiln-drying and extraction, the germination of the seeds and the growth of the resulting seedlings were no different from those of seeds from mature cones collected in January. Collection of immature cones can thus reduce the interval between pollination and sowing from three to two seasons, an important consideration when raising genetically improved stock from controlled pollinations.

For most species of the humid tropics immature fruits can be encouraged to ripen by storage at room temperature in a sheltered, well-ventilated place. Temperatures below 20°C and above 35°C are likely to be detrimental (Ng 1983). Ventilation can be provided by keeping the fruits loosely packed in open bags or boxes so that normal respiration can take place. Rapid or excessive drying should be avoided. The aim should be to keep the fruits alive and healthy as long as possible so as to gain time for their seeds to mature. The fruits should be inspected daily and those that are ready should be removed for processing. Two categories of fruits require special attention, pulpy fruits and capsules. Pulpy fruits (drupes and berries) are ripe as soon as the pulp turns soft. After this point, the pulp begins to decay and ferment and this leads to seed deterioration. Hence pulpy fruits that have become soft should have their seeds extracted as soon as possible. Capsules are ready when they split open of their own accord. Seeds forcibly extracted from unopened capsules are likely to be immature and non-viable.

Precuring promotes a gradual decrease in moisture content of fruits (and seeds) which will shorten the period of kiln-drying necessary to make the fruits open. This saves time, power and money. It also prevents “case-hardening” of fruits, which can occur when fruits with a high moisture content are subjected to rapid drying and which makes subsequent extraction of the seeds very difficult (Morandini 1962, Turnbull 1975 c). Seed yield of Pinus elliottii, P. taeda and P. palustris was increased by 5 weeks of precuring before kiln drying (McLemore 1975). The effect was most marked in cones collected early in the season, as shown in the following table:

Number of seeds extracted per cone - Pinus elliottii

Date of collectionPrecured for 1 weekPrecured for 5 weeks
19 August 060
16 September2782

The germination of the extracted seed was also improved somewhat in P. elliottii, but not consistently in P. taeda or P. palustris. In Honduras the general rule for P. caribaea is to precure until all tissues have changed colour from green to brown.

Conditions for precuring are similar to those described above for temporary storage. Promotion of free air circulation is of paramount importance, therefore fruits should be spread thinly (one or a few fruits deep) and should be regularly turned and stirred by raking. Raised trays with fine wire-mesh bottoms are the ideal containers and, for best aeration and uniform curing and opening, a layer one cone deep is recommended (Stein et al. 1974). The fine wire-mesh will retain any seeds which may be released.

In large processing depots it may be advantageous to raise the temperature gradually during the precuring operation. Morandini (1962) records that the final stage of precuring, before cones are passed into a kiln, can be carried out effectively by placing the cones near the top of the kiln where warm air from the exhaust passes through them.

Methods of Extraction

The methods of extracting seeds from fruits are determined mainly by the characteristics of the fruits. Fleshy fruits are treated by a depulping process which usually involves a combination of soaking in water with pressure or gentle abrasion. Cones and other woody or leathery fruits are first dried until cone scales open or seeds become detached from the placenta of the fruit, and then treated manually or mechanically by tumbling or threshing in order to separate the dry seeds from the dry fruits.

As mentioned on p. 87, some indehiscent fruits, mainly nuts, achenes and winged samaras, do not require extraction but are stored or sown as fruits. Some species with seeds covered by a thin fleshy covering may be dried and sown with the dried skin intact (Stein et al. 1974). Drying under cover with frequent turning of the fruits is appropriate. Examples are Vitex parviflora in the Philippines (Seeber and Agpaoa 1976), Crataegus in temperate regions (Goor and Barney 1976) and Podocarpus spp. and Maesopsis eminii in Africa. However, in some of these species germination may be improved by removal of the pulp (e.g. Vitex parviflora, see p. 94).

Bonner (1978) has classified hardwood seeds into three classes according to their requirements for pre-storage handling and storage. The classes are: (1) seeds that must be dried for extraction and for storage; (2) seeds that must be kept moist at all times, both during cleaning and during storage (i.e. recalcitrant species); and (3) seeds that must be kept moist for extraction, then dried for storage. His table of some important hardwood genera classified in this way is reproduced on the following page.


Depulping of fleshy fruits should be done soon after collection to avoid fermentation and heating. Small lots of seed are usually macerated by hand. After soaking, the flesh is hand squeezed or mashed by a wooden block, rolling pin or fruitpress. Alternatively flesh may be macerated by rubbing it against or through a screen (Stein et al. 1974). The pulp and skins can usually be separated from the seed by washing through appropriate sieves or by differential flotation in a deep bowl through which a slow stream of water is flowing (Aldhous 1972). The seed sinks while the pulp rises to the surface.

Table 6.1 Some important hardwood genera classed according to the pre-storage handling and storage requirements of their seeds.

dry for extraction and storagealways keep moistmoist for extraction and dry for storage
AcaciaAcer (some species)Gmelina
Acer (some species)AesculusMalus
Casuarina Sorbus
Cedrela Ziziphus

In the Philippines the fleshy fruits of Aleurites spp., Canarium ovatum, Syzygium cumini and other species are placed in barrels or cans with water. After a day or two the pulp becomes soft. The fruits are then mashed carefully with a tamper without crushing the seeds. When plenty of water is added, the pulp will float while the seeds sink to the bottom (Seeber and Agpaoa 1976). This method is also suitable for the fruits of Gmelina arborea, Azadirachta indica, Ocotea usambarensis and Cinnamomum camphora and for the syncarps or multiple fruits of Chlorophora and Morus. Because of the minute size of the seeds of Anthocephalus chinensis (2.6 million per kilo), a special technique is needed to extract them from the fleshy fruit. The outer portion of the fruit, which contains the seeds, is rubbed lightly against a ½ inch (12.5 mm) wire mesh. The mixture of pulp and seeds which passes through is placed on a screen box with a 1/16; inch (1.5 mm) mesh. The operators pour water over the mixture while rubbing it carefully by hand, so that the seeds together with some fine pulp pass the screen and drop into a water-filled container below. The seeds sink while the fine pulp remains afloat. If the floating pulp still contains seed, it is returned to the screen box and the operation is repeated (Seeber and Agpaoa 1976). In the Jari project in Brazil fruits of Gmelina arborea are depulped by macerating them against a wire mesh or mechanically by the use of a modified coffee depulper (Woessner and McNabb 1979). Thorough cleaning of the stones is important. It has been found that thoroughly cleaned stones gave 10 % better germination than depulped but uncleaned stones. Fresh green fruits sown whole without depulping gave only 10 % germination while whole fruits sown after a period of drying failed to germinate. The stones may be cleaned in water or a combined cleaning and drying operation may be carried out in a rotating baffled steel cylinder which reduces the moisture content to 8 – 10 % after 20 hours at 45° C.

Removal of the pulp from drupes of Vitex parviflora in the Philippines has improved germination, both in full-size green fruits and in the riper purple fruits (Umali - Garcia 1980). The best improvement was from 26 % whole green fruits to 65 % depulped green fruits and the least from 38 % whole purple fruits to 52 % depulped purple fruits.

Seeds may be washed free of flesh hydraulically. Fruit is placed in a mesh bag or wire basket and subjected to a stream of water from a high-pressure nozzle until all of the flesh and most of the skins are washed away (Stein et al. 1974).

6.1 Precuring sheds (background) with open-air racks in foreground in Zimbabwe. (Forestry Commission Zimbabwe)6.2 Top view of the Dybvig separator. Flesh abraded by a flanged spinner plate is washed away and cleaned seeds are retained. Clearance around the adjustable plate is set smaller than the seed to be cleaned. (USDA Forest Service)
6.3 Solar drying of pine cones under clear polythene roofing in Zimbabwe (A) Wire netting base which will be lined with hessian (B) Final stages showing fully opened Pinus taeda cones ready for tumbling. (Forestry Commission Zimbabwe)



6.4 Solar cone drying of Pinus kesiya and Pinus merkusii in rotatable drums in Thailand. (Pine Improvement Centre, Thailand)

After separation orthodox seeds should be carefully air dried under cover, with frequent turning. Thereafter they can go forward for shipment to the nurseries or for treatment to adjust moisture content to the correct value for the species, before storage.

Where large quantities of fruits have to be depulped, various designs of machine are available. They include feed grinders, concrete mixers, hammer mills and macerators. Most machines only free seeds from the flesh; a part or all of the residue must be removed in later cleaning. The Dybvig separator, however, pulps the flesh and fully cleans the seeds in one operation (Stein et al. 1974). Small quantities of small-seeded fleshy fruits can be processed speedily by use of an electric mixer.

Drying of Fruits Without Artificial Heat

Drying, using either natural or artificial heat sources, must be used in the extraction of seeds of many important tree species and is almost always used for the cones of pines and other conifers and the capsules of eucalypts. Drying should imitate the natural drying process so that the fruits are subjected to progressive drying which causes a continuous release of moisture. Air coming in contact with the fruits must always be drier than the fruits and this can be obtained by continuous circulation of air (Turnbull 1975 c).

Drying under cover

This is the slowest and least severe method of drying fruits for seed extraction. The technique is the same as that described for precuring, but is employed as the sole method of drying rather than in combination with a later application of sun-heat or kiln-heat. Fruits must be in well ventilated rooms, spread thinly, stirred regularly if on a solid surface or, preferably, placed on trays with a wire mesh bottom to allow all-round air circulation.

Air-drying under cover is effective for cones of Abies and Cedrus, which readily disintegrate under this treatment and which can easily be damaged by heating in the sun or in kilns. It is also used for separating fruits of some hardwoods, such as Quercus and Fagus, from their involucral coverings (Morandini 1962). At the same time it is suitable for a modest degree of drying of these species and of others like them which need to be stored at a relatively high moisture content in order to retain viability. Dipterocarpus, Hopea and Triplochiton are tropical genera for which the method is suitable. It can be used for drying the thin fleshy fruits covering of Vitex, Maesopsis and other species which are then stored or sown as dried fruits.

The drying process is slow under this method and the period of time needed is dependent on the natural air humidity and temperature. But it is the safest method for any “delicate” species which will not stand heating or very rapid drying.

Sun drying

This method is well suited to drying of cones and fruits of species which will withstand the rather high temperatures involved. It is commonly used in the dry season in tropical, sub-tropical or warm temperate climates where it can be 100 % effective in causing fruits to open and can make kilns unnecessary. In moist cool temperate climates it is much less reliable and may need to be supplemented, if not replaced, by kiln drying.

Spreading the fruits in layers on screens, platforms, canvas or other sheeting in the sun is one of the simplest methods of air drying and requires little investment in equipment (Turnbull 1975 c). In the Mediterranean, pine species such as P. pinea and P. halepensis are handled by this method (Morandini 1962), as are P. kesiya and P. merkusii in Thailand and the Philippines (Bryndum 1975, Seeber and Agpaoa 1976). Fruits can be laid on wire screens with meshes of suitable size to let seeds drop through onto canvas or polyethylene sheets (Morandini 1962, Turnbull 1975 c). The main requirements are:

  1. Frequent stirring and turning to promote uniform drying and opening of the cones and release of the seed.

  2. Arrangements for immediate covering of fruits in the event of rain, either by moving them indoors or by erecting a temporary shelter over them.

  3. Care to avoid overheating of the fruits while they still have a high moisture content. This may involve preliminary precuring under cover or the avoidance, in the initial stages, of devices such as a base of corrugated iron sheeting or a covering of glass or polythene, which are designed to trap heat and raise temperature. The importance of this varies greatly according to the strength of sunlight locally and to the heat tolerance of individual species.

  4. Frequent removal of any seeds which have separated from the fruits, prevent their being exposed too long to intense direct sunlight.

  5. Protection against birds, rodents and insects which may pose a more serious threat in open air drying than in drying inside a building. Ants will carry off a large proportion of eucalypt seed unless they are rigorously excluded from the extraction area (Turnbull 1975 c), while rodents and birds have a taste for the seed of pines.

Pines. Cones of Mediterranean pines exposed to sun drying take 3 to 10 days to open, depending on the drying conditions (Goor and Barney 1976). In Thailand Pinus kesiya takes 5 – 7 days and P. merkusii 2 – 3 days (Bryndum 1975). At Chiang Mai in Thailand, where the mean maximum temperature of the hottest month is 36.5°C, Bryndum (1975) found that putting the cones on a flat tray and then covering them with a clear polythene sheet, to produce a greenhouse effect with higher temperatures, gave nearly twice the seed yield in P. kesiya as the same treatment without the polythene sheeting; respective yields after 7 days were 15.6 g and 8.2 g of seed extracted per kg of cones. He also found that stirring the cones frequently (8 times a day) gave a significantly higher seed yield than a single stirring at the end of the drying period. The trays used have 10 cm wooden sides with a bottom of 12.5 mm wire mesh. The seeds fall through the mesh into a sheet-metal funnel and from there they drop into a bag attached to the outlet. The trays rest on a wooden scaffold at a convenient height. A total tray area of 125 m2 is needed to extract 6 hectolitres of cones per day, and it was calculated that cost of materials and labour to construct it was one eighth of the cost of an imported electrical extractor of the same capacity. In addition to the tray method, locally made drums have also been used with success. The material used is transparent roofing sheeting and there is an internal rotatable metal grid to hold the cones, which is rotated manually six times a day to ensure thorough mixing of the cones. Extraction period is similar to that in trays. Another method of multiplying sun heat which has been used effectively for P. kesiya in the Philippines is to place the cones on corrugated iron sheeting (Cooling 1967).

In Honduras Pinus caribaea cones are sun-dried either in trays or on tarpaulins. The trays are spread out individually in the sun but can be stacked and put under cover at night or in case of rain, or else the stacks may be covered with tarpaulins at those times. In the tarpaulin method heavyweight waterproof canvas is used, commonly of 5 × 7 or 5 × 10 m, but smaller sizes can be used if it is necessary to keep small seed lots separate (e.g. provenance collections). Cones are spread out one cone deep, leaving a perimeter strip of about 30 cm uncovered to avoid losses of cones by spilling off the tarpaulin. During the day's exposure to sun the cones are moved every 2 – 3 hours by raking and lightly knocking the cones with the back of the rake. About 1 ½ hours before sunset or whenever it is about to rain, the two long sides of the tarpaulin are, in turn, pulled upwards and inwards abruptly until at least one third of the tarpaulin area on each side is free of cones. The cones finish up piled in a strip down the middle third of the tarpaulin. The pile is then covered by first lifting the short sides of the tarpaulin upwards and inwards by about 1 m, then the first long side (away from the prevailing wind), which is folded over the cones so that it just reaches the far side of the cone pile and finally the other long side which is pulled up and over the pile, on top of the first cover, ensuring that the edge reaches the ground. It may be secured by weights or by tucking it under the pile. Extracted seed is best removed at the start of each day by uncovering the cones, tapping the cone pile with the back of a rake to dislodge the seeds, gently raking the cones towards the perimeter and then brushing the seeds together in the central strip and placing them in a suitable container. The tarpaulin should be laid on a well-drained site (Robbins 1983 a).

In Zimbabwe sun-drying of cones of Pinus patula, P. elliottii and P. taeda is done on a large scale in open-sided sheds covered with clear plastic roofs supported on wire netting (Seward 1980). Each shed is 15.2 m long by 12.2 m wide and contains 8 wire netting troughs on a pole framework. Fresh cones are spread not more than two layers thick on hessian in the troughs. Those collected at the beginning of the season have a first spell of drying in the sheds followed by an intermediate period in half-filled sacks in racks in precuring sheds, followed by a final period of drying and cone opening in the troughs. The drier cones collected towards the end of the season need no precuring and open after a single short period in the drying troughs. The whole system of drying sheds has a capacity of 2000 bushels (720 hectolitres) of cones.

Eucalypts. Sun-drying is also effective for the capsules of Eucalyptus spp. wherever local climatic conditions are favourable. The following account is based on that of Turnbull (1975 f).

Small seed collections usually involve drying individual capsules or clippings of capsule-bearing branches. These can be spread out in a thin layer on canvas, calico or plastic sheeting, in a dry and well-ventilated position either in the sun or in shade. A small seed extractor can be made by placing the capsules on wire mesh a few centimetres from the bottom of a box and placing a sheet of clear plastic or glass over the top (Boden 1972). The capsules should be shaken daily and the seeds removed, so that they are not exposed for longer than necessary to high temperature.

Bulk seed collections are often spread on tarpaulins on the ground, placed in special concrete enclosures, or raised up above the ground in frames on wires. Spreading the capsule bearing branches on the ground requires little equipment but is expensive because of the frequent turning required to permit the lower layers to dry out.

A more practical method of drying large quantities of capsules is to spread them over a frame covered with wire netting. The wire mesh permits a greater air circulation around the capsules than can be obtained if they are placed on the ground. A canvas sheet underneath the frame catches the seed as it is shed. Such structures may be large or small, permanent or temporary. Small temporary frames are suitable for use by the migratory seed collector and the large, permanent structures are placed in central seed extraction areas.

Another method sometimes used in drying is to hang capsule bearing branches over a single taut strand of wire. It has the advantages of providing very good air circulation between the leaves and the fruits and there is no risk of the material becoming compacted and heating up. The seed is collected on sheets placed under the branches. This method can be used for extraction in the forest, but can be equally well set up in a well-ventilated shed, the latter being used when there is a high risk of rain falling during the drying period.

A flexible system which is used in Brazil consists of placing the capsules in special trucks provided with shelves. Each truck has a capacity of about 50 kg of capsules. During the day the trucks are left in the sun and at night or during rain they can be wheeled under cover. The fruits usually open in about three days (Cavalcanti and Gurgel 1973).

The rate of seed release during natural drying varies with the capsule characteristics of each species, capsule maturity, and most importantly, on the drying conditions. Very mature capsules of some species can release their seed within a few hours under optimum drying conditions but under average conditions most species will dry sufficiently in 3 – 4 days. Some eucalypts characteristically retain unopened capsules on the tree for several years, these capsules may become very woody and are often difficult to open.

Although drying in full sun leads to rapid opening, there is an inherent danger that, if the temperature becomes too high, any primary dormancy in the seed may be strengthened. The capsules should not be placed directly on metal which is exposed to the sun, since this may result in high temperatures which damage the seed.

During natural drying ants and birds can cause considerable seed losses. Ants will carry away viable seeds and leave the chaff behind. Spraying an aerosol insect repellant or spreading an insecticide dust around the seed bearing material will usually discourage pilfering. Seed-eating birds such as the domestic sparrow can also be responsible for seed losses during extraction.

Other species which may be sun-dried include dry zone legumes such as Acacia and Prosopis spp. and various species of Toona, Lagerstroemia, Leucaena, Casuarina, Albizzia falcataria and Pithecellobium dulce (Seeber and Agpaoa 1976). In India the “cones” of Casuarina equisetifolia are placed in trays in the sun and a thin cloth is secured to the top of the trays to prevent the “seeds” from being blown away by wind. The “cones” are treated with BHC 10 % powder or other insect repellant to prevent ants from removing the “seeds”. In three days the “seeds” separate from the “cones”, if left longer other “cone” fractions get mixed with the “seeds” (Kondas 1981). In general, hardwood fruits and seeds are more easily damaged by overheating than those of conifers, so care must be taken to avoid prolonged exposure of the seeds to intense direct sunlight. Intermittent shade may be necessary.

Drying of Fruits with Artifical Heat

Drying of fruits in heated kilns may be necessary for a variety of species where the climate is not suited to air drying i.e. in cool moist climates. It may also be necessary for a few refractory species which will not respond to sun-drying even in a dry climate. It is used most often for the cones of coniferous species, but also for some eucalypts in cool moist areas (Turnbull 1975 f, Boland et al. 1980). Although seed of most species of Casuarina may be readily extracted by sun-drying, some species have serotinous cones which require the higher temperatures of a kiln (or, in nature, bush fires) before they open (Turnbull and Martensz 1983). Kiln drying is usually preceded by a period of air-drying or precuring.

The main difficulty in drying cones by natural methods is the impossibility of controlling air humidity and temperature. An increase in the humidity of the air may cause a reclosing of the cones (Morandini 1962, Turnbull 1975 c). Artificial heating, in contrast, permits control of air moisture and temperature, a much shorter period of treatment and, with a continuous process, more effective work organization.

Artificial heating requires expensive equipment and installations, which are not used throughout the year. This makes their unit cost exceptionally high. A very careful appraisal must therefore be made of the capital cost, in relation to the amounts of seed to be processed annually, before establishing large, permanent kilns (Turnbull 1975 c). As much use as possible should be made of air drying. Sometimes a combination of the two methods is appropriate; a small kiln can be installed in order to supplement air drying of some species by a final short period of kiln drying, or to deal with species or batches of cones which prove resistant to air drying alone (Cooling 1971).

Cone drying by artificial heat should be carried out in such a way as to obtain drying in the shortest possible time without damaging the viability of the seed. For this purpose the following recommendations, based on those of Morandini (1962), are applicable:

  1. Cones should be properly precured before they are put in the kiln.

  2. Air temperature should be controlled and kept to the minimum level sufficient to dry cones.

  3. Cones should not be heated nor seeds kept in the kiln longer than necessary.

  4. Air in the kiln should be kept as dry as possible.

The principle of kiln drying is the provision of a regulated flow of warm dry air, so that all the cones dry uniformly and as quickly as possible without risk of overheating or “case-hardening” (Aldhous 1972). In most modern kilns the temperature of the air-stream is raised progressively as drying proceeds; there is forced-draught air circulation and seeds are speedily removed from the source of heat as soon as they are freed from the cones. Cone kilns vary in size from small cabinets to large buildings, but they have in common a source of heat, a means of controlling movement of heated air by forced-draught or convection and some tray, shelf or other system of exposing cones to the moving air (Stein et al. 1974). Kiln controls may be simple and hand-operated or complex and automated. Some kilns also have sensitive humidity controls.

In many kiln schedules the first two or three hours of drying is at a considerably lower temperature than the effective drying temperature. This prevents the combination of high temperature and high moisture which is the most common cause of reduced seed viability during drying. The safe temperature of all cones is about 30°C, rising to 60°C when the moisture content of cones is below 10 % (Aldhous 1972). Humidity control is available in some modern kilns but optimum schedules have yet to be determined for many species (Stein et al. 1974).

Types of Drying Kiln

Turnbull (1975 c) classified kilns under the following types (a) Stationary tray kilns (b) Vertical progressive kilns (c) Horizontal progressive kilns (d) Rotating drum kilns (e) Portable kilns. The following description of these kiln types is based on the accounts of Morandini (1962) and Turnbull (1975 c, 1975 f).

Stationary tray kilns

The simplest kilns are convection kilns consisting of anything from a stove-heated room to a more complex structure. Basically they consist of a heating unit above which is an extraction chamber. Heat enters the chamber at the base and passes up through tiers of trays containing the fruits or cones. Since the air becomes cooler and moister as it rises through the cones, the efficiency of extraction is not uniform in all parts of the kiln. Some trays must often be moved to different levels and re-treated. Such kilns are relatively cheap and they require no special technical skills to operate. They have the disadvantages that they are relatively inefficient, fire risk is often high, and temperature and humidity control are rarely adequate. They can be improved to some extent by the provision of forced ventilation.

In Zimbabwe most cones are air-dried, but this is supplemented by a simple kiln using tobacco-curing flues. Because there is no mains electricity in the area air circulation is by convection and the kiln is wood-fueled. Safe maximum temperatures are 48° C for Pinus elliottii, P. taeda and P. kesiya and 60° C for P. patula. The kiln holds 36 trays, equivalent to 9 bags (about 22 bushels or 8 hectolitres) of cones, which will open in one shift of 8 hours (Seward 1980).


6.5 Solar drying of pine cones in Thailand, showing polythene lids. (Pine Improvement Centre, Thailand)


6.6 Kiln seasoning. Stacked trays of Pinus radiata cones entering a kiln in New Zealand. (F.R.I. Rotorua, photography by H.G. Hemming)

6.7 Interior view of a kiln in Denmark, with trays (DANIDA Forest Seed Centre)6.8
6.8 Rotating kiln with discharge end open to show drum; control panel is on the left. (USDA Forest Service)

In Honduras cones of Pinus oocarpa and P. caribaea, after a period of precuring, are successfully dried in a forced draught hot air kiln modified from a solar kiln for drying timber (Robbins 1985). Ventilation is provided by two large diameter fans powered by a one horse-power electric motor, while heating is provided by flue radiator and a furnace using waste cones and firewood. Air can be recirculated if required by opening a valve which connects the air entry and exit passages. The kiln holds 32 hectolitres of closed cones, arranged in 8 stacks of 8 trays each, with 50 litres of cones per tray. The temperature regime is 4 hours at 40°C followed by 10–14 hours at 45°C for P. caribaea, while the thicker cones of P. oocarpa receive a constant 50°C. Cone opening is usually complete in 12–18 hours. Seed yield is 160–250 g per 100 litres of closed cones for P. oocarpa, 125–625 g per 100 litres for P. caribaea.

Electric ovens, preferably of forced draught type, are useful for drying small lots of eucalypt capsules. In Brazil electric ovens, consisting of a large chamber with a series of sliding shelves and a capacity of 80 kg of material, are used for drying eucalypt capsules (Cavalcanti and Gurgel 1973). Drying time is 24 – 36 hours at 45° C. A forced draught oven has been used sucessfully in the USA to dry pods of Prosopis (Brown and Belcher 1979). The pods are placed in the oven at 32° C for 18 hours. The dried pods are then placed in an electric scarifier for 10 – 15 seconds, after which the seeds are separated by removing light debris in an air-column blower and sieving out the pod fragments with a size 11 or 12 screen (mesh size 1.85 or 1.70 mm).

In Sabah pods of Acacia mangium are dried in a simple drying chamber which incorporates an electric heater and a domestic fan (Bowen and Eusebio 1981b). Pods are held in stacks of trays 0.7 × 0.7 m square, each with a wire mesh base. Air entry temperature is 45°C. Pods are separated by colour and treated as follows: (1) Black pods - no precuring, dried for 24 hours in drying chamber (2) Brown pods - precure in shade for 48–72 hours, then dry in chamber for 24–48 hours (3) Green but full-sized pods - precure in shade for 72–120 hours, dry in chamber for 48 hours. The same method is used for Albizzia falcataria.

In place of trays, steel-mesh cylindrical drums are sometimes used to hold the seed-bearing branch clippings of eucalypts (Boland et al. 1980). Kilns of this type are in use at more than one site in Tasmania (Anon. 1972, Anon. 1974). One kiln is approximately 9 m × 4 m and has an extraction capacity of about 4500 kg of seed per annum. It has insulated walls with a transparent roof, formed of two layers of acrylic sheeting with an air space between, which permits solar heating in summer. In winter a false reflective ceiling is put in to reduce loss of radiant heat. The kiln is maintained at a temperature of 40° C. After 36 hours the capsules have opened and the cylinders are rotated and shaken to dislodge the seeds.

If no special equipment is available and only a few fruits or cones are to be dried, they may be placed on a warm radiator (Aldhous 1972). They should never be left to open where it is too hot to rest the hand comfortably.

Vertical progressive kilns

The cones are placed in the kiln on a set of trays, one above the other, in a tower-like structure; trays descend during cone treatment. From the bottom of the kiln the hot dry air hits the lowest tray, containing fairly dry cones; passing through the cones, the air loses some heat and takes some moisture from the cones. In the second tray from the bottom, cones have somewhat higher moisture content and the air is less hot and less dry. In the topmost tray, the cones, still having the original moisture content, are engulfed by warm, humid air.

At the right moment, a tray is taken away at the bottom, and the whole tray set descends one step, while a new tray with fresh cones is put on the top of the pile.

Hot air can circulate simply by convection, but forced ventilation makes the operation quicker and more regular. The distance between two successive trays should be sufficient to contain open cones which have a volume 2 to 3 times that of closed cones. The time required to complete drying varies according to the initial moisture content of the cones, the volume of circulating air, and the type of air circulation (convection or forced ventilation).

In some kilns, cones are forced to fall down from one tray to the next below by opening the tray bottom, or by inclining the tray. In other types, trays are replaced by slow moving bands; cones move on a band till they fall to the one below.

In the last two types, cones are shaken as they fall from tray to tray, so that most of the seeds come out: special devices under the bottom tray collect the seed, while cones pass to further handling.

Horizontal progressive kilns

The operation of this type of kiln can be illustrated by the Italian designed horizontal kiln described by Gradi (1973). A continuous conveyor belt of perforated steel plate on which the cones are spread passes through an insulated galvanized tunnel. By means of radiators and ventilators installed in chambers on one side of the horizontal belt, an air movement in the form of a spiral is obtained from the entrance to the exit of the drying tunnel. In the first few metres of travel the cones undergo a pre-drying stage at a temperature about 10°C lower than the final temperature. In successive sectors of the drying tunnel higher temperatures are used, usually in the range of 45°–50°C. The spiral movement of the air creates a turbulence which greatly accelerates the drying process.

The kiln described by Gradi (1973) is designed to process large quantities of cones. More versatile equipment has been produced to process and maintain the identity of small seed lots as well as large ones (Isaacs 1972). This comprises a long horizontal kiln having small bins with removable trays rather than a conveyor belt. The kiln is provided with portable heaters which are rotated each day when unloading the kiln to ensure that the hottest air is on the driest cones. The static pressure and air flow is controlled through each bin. This equipment has a particular advantage in being very easy to clean. An alternative simple method of preserving the identity of small seed lots is to use small nylon bags. They allow free air circulation and the same bag can be used for the cones from the time of their insertion in the forest throughout transport and extraction.

The advantages of the horizontal kiln over the vertical kiln are (1) the expensive process of elevating cones to the entrance of the conventional vertical drier is avoided; (2) any possibility of damage to the cones in falling from one level to the next is avoided, because they remain stationary on the conveyor during all phases of the drying process; (3) the high productive capacity; (4) the ease of installation, inspection and maintenance. The relatively high cost of the equipment requires a high through-put of cones for optimum economy.

Rotating drum kilns

This type of kiln can be found in many modern seed extraction plants. The basic operation is as follows:

Cones are put in a cylinder made of perforated steel plate, rotating on a central axis. The cylinder is contained in a box, where forced air circulation is provided by an electric fan. During the extraction the cylinder rotates continuously, shaking the cones. The air temperature increases gradually, from room temperature to the maximum fixed level. The strong ventilation, the progressive heating and the continuous shaking make the cones dry and open in a short time. Seeds released from cones pass through the holes of the plate, and are blown at once out of the kiln. Temperature and ventilation control is generally fully automatic.

Seed is taken out of the hot air as soon as it comes out of the cone, in order to avoid seed damage. The capacity of the cylinder is reduced so that extraction can be done separately for cones from different provenances, even if the quantity is small. Kilns of this type may have electrical heating or a separate heating unit which can use different fuels.

The cylinder, the electric motor, the ventilator and the equipment for air and temperature control are contained in a steel box, above which is a pre-drying box with two floors. From this box, cones fall into the cylinder, where they are exposed to gradually increasing temperatures (40–45–50–60°C) for three to four hours. At the end of the extraction, reversing makes the cylinder open, the open cones fall down and other cones come into the kiln.

Modern rotating drum kilns are generally of compact metal construction and of moderate size so that they can be placed in small, inexpensive buildings. Their output is relatively low and they are generally planned to work in a set of two or three depending on the requirements of the extractory. Their advantages include the relatively short drying time and the shaking of cones during drying which extracts the seed and makes a separate tumbling operation unnecessary.

This type of kiln is widely used for modern seed extraction of conifers and is being used for the extraction of eucalypt seed in Australia.


6.9 Portable cone kiln, Beech Creek Seed Orchard, Murphy, North Carolina, USA. (USDA Forest Service)


6.10 Cone tumbler in Humlebaek, Denmark. (DANIDA Forest Seed Centre)


6.116.11 Double-storied tumbler room in Zimbabwe showing (A) inclined ramp to upper floor (B) cowl at discharge end of tumbler. (Forestry Commission Zimbabwe)

Portable kilns

For small-scale extraction or to provide seeds for modest plantation programmes or for research, large commercial cone drying kilns are neither satisfactory nor economical. In research work it is often vital that seed be extracted from small lots of cones without the possibility of the seeds being mixed with other lots.

A kiln designed to dry small lots of pine cones is described by McConnell (1973). It features economy, safety, portability, versatility and a small load size. The kiln consists of a heating mechanism based on bottled gas, a fan, a control panel, and both a drawer type chamber and a cabinet chamber to hold the cones. The capacity is 50 bushels (18 hectolitres or 1.8 m3). The control panel is designed to sound an alarm or provide a visual warning of any malfunction in the equipment, and thermostats ensure that the temperature cannot, under any circumstances, exceed 74°C.

The portable kiln described above could find wide application for small operations through its economical operation and advanced safety features.

Safety Precautions

Any artificial heating involves a fire hazard and this is particularly true in handling cones, since dust, resin and dry cone scales are all highly inflammable (Morandini 1962, Stein et al. 1974). Stringent fire precautions including a ban on smoking should be enforced, fireproof, non-wooden construction materials should be used and arrangements made for frequent removal of inflammable dust and debris by vacuum equipment or other means.

Other precautions are necessary when drying certain dry fruits and seeds of certain species. Dust masks need to be worn when handling species such as Platanus spp. which release fine hairs which might be breathed into the lungs (Stein et al. 1974).


When fruits and cones open after drying, some seeds fall out easily as a result of manual stirring, rotation in rotating drum kilns or, in certain vertical progressive kilns, from the shaking of the cones as they fall from one tray to another. But many seeds are left inside, especially in those drying techniques where the cones remain static. They must be removed as soon as possible after drying is complete.

In some species a thorough manual shaking is sufficient to extract the remaining seed. The capsules of eucalypts need to be vigorously shaken, particularly if not fully mature, because abscission of the seeds from the placenta may be only partially complete (Turnbull 1975 f). Failure to shake slightly immature capsules properly can result in only the chaff being released. The fertile seeds are usually attached to the placenta near the bottom of the loculus so that, after dispersal of the chaff, immature capsules may, on superficial examination, appear to be empty.

Cones may be shaken in coarse sieves to release the seeds, but more vigorous treatments are needed for some species. Those most widely used are tumbling for conifers and threshing for hardwoods.


A tumbler is a rectangular or round container or drum mounted horizontally on its long axis (Stein et al. 1974). As it turns, cones tumble about; interior baffles often accentuate the jarring and tumbling action. Seeds fall from open cones through the high-strength wire mesh which forms the sides of the tumbler and into a hopper or trays or onto a moving belt.

The tumbler can be operated by hand or mechanically driven, depending on the scale of the operation. Some drums may be closed at both ends and emptied and refilled at the end of each operation cycle (Morandini 1962). In more modern designs a continuous operation can be achieved with an inclined cylinder open at both ends. The cones are fed in at one end and during rotation roll slowly to the other end where they are discharged. Variable speeds and tilt are set for each species. The speed determines the rolling and pitching effect on the cones, while the tilt determines the length of time the cones remain in the tumbler (Turnbull 1975 c). Small types of tumbler are easily transportable. Fisher and Widmoyer (1977) describe a small tumbler of ½ bushel (18 litre) cone capacity made from a modified domestic washing machine.

In Zimbabwe a hand-operated drum tumbler 2.43 m long is used, fed from a chute from the floor above. By building the tumbling room on a slope, the cones can be easily transported into the upper floor (Seward 1980). The drum holds one bag of cones and it takes one minute to tumble one charge and to recharge the drum. The seeds drop through the 18 mm mesh of the drum into a collecting tray below and the empty cones are discharged into a trolley.

It is important that tumbling should be carried out as soon as possible after drying, because open cones exposed to cold wet air can reclose in a short time (Morandini 1962). If tumbling cannot follow immediately after drying, the opened cones should be stored in warm, dry conditions in the interval between these operations. The time required for tumbling depends on the species and the condition of the particular lot of cones being handled. Seed in some species such as Larix decidua and Picea abies may be held tightly in the cone and long periods of tumbling are sometimes required to extract them (Aldhous 1972). Special machines, such as a large potato-peeling machine or a tumbler incorporating saw blades are effective on difficult species like these. Alternatively, the cones may be rewetted and then redried to promote fuller opening of the cone scales. Haverbeke (1976) found that, after the first tumbling in Pinus sylvestris, soaking the cones in trays of water at 30° C for about half an hour until the cones softened and started to close, followed by thorough air-drying until the cone scales opened again, gave good results. The yield of seeds from the second tumbling averaged 36 % of that from the first. There was a marked difference between provenances, from 18 % for a Scottish to 84 % for a Spanish origin. The additional yield from the second tumbling fully justifies the extra cost for rare and valuable seeds such as those obtained from controlled pollination.

Mechanical damage can easily be inflicted on seeds if excessive tumbling speeds are used or if the tumbler is filled with too many cones. Speed of rotation and time of treatment should be adapted to the cone and seed characteristics of the species being handled. It is better to leave some seed in the cones than to spend money on extracting seeds most of which will be severely damaged in the process (Morandini 1962).


Extraction of seed from dry fruits of many hardwood species is accomplished by threshing. Seed extraction in species such as Cercis, Catalpa, Robinia and Liriodendron is easily accomplished by spreading the fruits on a platform, sometimes on a straw mat or other suitable material, and beating them with a flail or slender pole. For large quantities mechanical threshers used in agriculture can be adjusted for tree fruits by altering the distance between the crushers. In chile pods of Prosopis tamarugo are ground in a stone mill set at 4 mm and the seeds are then separated by sieving and floating the milled product (Habit et al. 1981). A modified cereal huller has been used as a thresher for Prosopis pods in the USA. The machine, described in Ffolliot and Thames (1983), will thresh 1 bushel (36 litres) of pods in 1 ½ hours; approximately 160 hours would be required to thresh the same quantity by hand. In the Philippines fruits which do not readily release their seeds are put in a sack and beaten. Separation is done by use of screens. For every seed size one screen is used with a mesh larger than the seeds, to separate them from fruit fragments and other large impurities, and another with a smaller mesh which retains the seeds and allows the fine impurities to pass through (Seeber and Agpaoa 1976).

In Sabah seeds of Acacia mangium are separated from the pods, after drying, by rotating them for 10–15 minutes in a cement mixer together with blocks of hard timber 10 × 10 × 15 cm (Bowen and Eusebio 1981b). A similar use is made of a cement mixer for pods of Albizzia falcataria but, because separation of the seeds is easier in this species, it is not necessary to include the timber blocks (Bowen and Eusebio 1981a). Several types of mechanical thresher suitable for Acacia pods are described by Doran et al. (1983); they include a hand type model of resilient tapered thresher, a rotating drum, a flailing thresher and a peg drum thresher. Many acacias give off a very irritating dust during threshing and protective equipment should be worn by operators.

More robust methods, such as pounding the fruits with a wooden pestle or putting them through hammermills, must sometimes be applied. For some species special equipment has been developed such as the dehuller of Juglans (Churchwell 1964). Hammermills consist of a hooded inlet or hopper, a central chamber containing a series of hammers which rotate about a central shaft, and removable outlet screens of different mesh (Stein et al. 1974). The outlet screen must have holes large enough to let seeds pass through without damage. Fruits are fed into the mill continuously during separation. Care must be taken to operate hammermills at comparatively low speeds, 250 – 800 revolutions per minute, to avoid injury to the seeds. The Dybvig separator works well on dry as well as on fleshy fruits.

Other Methods of Extraction

Seed extraction of some species is difficult, even after standard treatments of drying and tumbling or threshing. In the Philippines indehiscent hard pods of leguminous species such as Delonix regia, Pithecellobium saman, Cassia fistula, C. javanica and Parkia javanica have to be opened with a machete or knife and the seeds picked out individually (Seeber and Agpaoa 1976). The pods of P. saman are sweet and relished by termites; if they are piled in a dark place, after some time only the clean seeds are left behind.

6.12 The Resilient Tapered Thresher, hand model made by Alf. Hannaford & Co. Ltd., Woodville, S. Australia, and used for dry zone acacias. (FAO/Division of Forest Research, CSIRO Canberra)6.13
6.13 CSIRO 15-cm Flailing Thresher, (A) Feeding material into thresher (B) General view showing material threshed and ready for cleaning (C) View of essential parts. (FAO/Division of Forest Research, CSIRO Canberra)

6.14 Cement mixer used for dewinging. (DANIDA Forest Seed Centre)
6.16 Liriodendron tulipifera before and after dewinging. Upgrading is easier after dewining. (USDA Forest Service)
6.15 Missoula dewinger for small seedlots. (USDA Forest Service)

Goor and Barney (1976) recommend that Cedrus seed be stored in the cones, since extracted seeds quickly lose their viability. When removed from storage, the cones must be soaked in water to facilitate extraction. After treatment the cones can be easily broken apart by hand and the seeds extracted for immediate sowing.

Serotinous cones of species such as Pinus brutia, P. halepensis, P. contorta and P. radiata may need special treatment to induce opening. Dipping them in boiling water for 10 to 120 seconds (up to 10 minutes for some especially refractory seed lots), followed by very high temperatures in the drying kiln (75° – 80° C), has been effective in some cases. High temperature is needed to melt the resin which forms a strong bonding adhesive between overlapping cone scales (Stein et al. 1974, Krugman and Jenkinson 1974).

Immature green cones may also need special treatment. It was found that green cones of Pinus merkusii from Zambales (Philippines), after soaking for 48 hours, followed by 80 hours' drying at a starting temperature of 30° C and a final temperature of 50° C, released only 7 % of the contained seeds; if the cycle of alternate soaking and drying was repeated 5 or 6 times, 79 % of seeds were released (Gordon et al. 1972). The total period of 4 – 5 weeks would be uneconomic in operational forestry. The recommended alternative is to collect only ripe brown cones which released 91 % of their seeds in a single cycle.

Operations after Extraction

When seeds have been extracted from their fruits, several operations are needed before they are fit to go into storage. Sound seeds must be separated from empty and non-viable seeds and from inert fragments of fruits; winged seeds of some but not all species need to be dewinged; if seeds are to be stored, their moisture content must be tested and, if necessary, raised or lowered to the percentage most suitable for storage. If uniformity in growth of nursery stock is considered desirable, seeds may also be graded by size.

Inert material takes up space both in storage and transport and may cause uneven stocking in the nursery seed beds. It also carries a greater risk of introducing pests or diseases than do the seeds themselves; for example spores of needle cast are carried on needle fragments rather than seeds. Cleaning to a high standard of purity is easy in some species, but more difficult in others. Cleaning of seeds to a purity higher than a given percentage is undesirable in some species; beyond that, an increasing amount of good seed is separated out with the impurities (Goor and Barney 1976). Besides, the added effort of extra cleaning is time-consuming and expensive. Morandini (1962) recommended that seeds of Larix should not be cleaned to a purity higher than 65 %, since it has been shown that further cleaning causes a drastic loss of good seed. This is because the great thickness of the seedcoat in comparison with total seed size makes empty seeds nearly as heavy as full seeds. In many species of eucalypt, especially in the subgenera Monocalyptus and Idiogenes, it is difficult to separate the fertile seeds from the chaff which is released from the capsules at the same time. Viable seeds are frequently very similar in size, shape and colour to the chaff particles and the proportion by weight of chaff to viable seeds is usually in the range of from 5 : 1 to 30 : 1 (Grose and Zimmer 1958, Boland et al. 1980). Commercial seed lots of eucalypts are therefore cleaned of leaves, twigs and other large fragments but the remaining “seed” consists of a mixture of seed and chaff; this practice is accepted by both buyer and seller. Provided that seed lots are accompanied by test results giving the number of viable seeds per unit weight of seed plus chaff, the seed user will not be too concerned that they contain a certain amount of impurities. Cleaning seeds for operational use should therefore be applied with discretion. It may be necessary when special techniques are used e.g. if the seeds are to be pelletted or if precision sowing is to be employed. For research purposes also, when basic knowledge on germination or other seed characteristics is sought, repeated cleaning to a very high standard is necessary.


Winged seeds or winged fruits are a feature of many forest trees and almost all conifer seeds have a wing which may vary from long and hard to very short and soft (Morandini 1962, Turnbull 1975 c). In order to make seed processing and nursery sowing easier, the wing is usually removed whenever it is larger than the seed (or fruit).

Wings are small or impractical to remove from seeds in several coniferous genera e.g. Thuja, Chamaecyparis, Cupressus; in a few genera they cannot be removed without impairing seed viability e.g. Libocedrus (Stein et al. 1974). Many winged hardwood fruits e.g. Casuarina, Betula, Ulmus are stored and sown intact, but the larger wings of e.g. Swietenia fruits may be broken off (Robbins 1982b).

For small quantities of seed dewinging may be done by hand, either by rubbing seeds between the hands or against a screen or roughened surface, or by handrubbing in a cloth bag or by rolling them between two cloth sheets or in a cloth bag between a rubber surface below and a roller above (Stein et al. 1974, Turnbull 1975c). For large quantities mechanical dewinging is in common use.

Dewinging machines range from those which are hand operated to large semiautomatic equipment which gives a continuous output. Corn mixers and cement mixers are frequently used. Mechanical dewinging, if carelessly done, may cause damage to seeds by crushing, cracking or abrasion (Kamra 1967, Wang 1973).

Most mechanical dewingers are rotating devices in which the seed is pressed by brushes or pads against the walls of a cylinder, or rotating knobs or pads force the seeds to pass through narrow apertures while retaining the wings. If the clearance between the knobs or brushes is too small the seed may be damaged (Morandini 1962). A dewinger specially designed for small seed lots of 5 kg or less and for easy cleaning between seed lots has been described by Lowman and Casavan (1978). It consists essentially of a rubber-lined cylinder with a rotating central shaft to which are attached pure gum rubber flaps. There is a variable inclination setting so that seeds and wings will pass out of the dewinger by gravity.

Nartov et al. (1979) describe a combined dewinging and seed cleaning machine in use in the USSR. It is transportable and weighs 50 – 70 kg. A seed hopper and feed auger pass seeds into the dewinging unit which has brush-type beater vanes. A fan then removes wings and lighter material, while the heavier material falls onto a series of inclined screens with various mesh sizes. The clean seeds fall into different containers, graded according to seed size.

Mechanical injury can be avoided in some cases by moist dewinging. Wang (1973) describes a safe method of dewinging coniferous seeds in Canada. The seeds are moistened with water and left to soak for 20–30 minutes before they are stirred with a soft brush or sponge in a rotating cement mixer to remove the wings. A similar principle is used by Isaacs (1972) in dewinging pine seeds. A large tank with slow-moving pipes gently agitates the seeds, which have been moistened at a rate of about 2 litres of water to 45 kg of seed. The wing absorbs moisture and is shed from the seed. In Honduras moist dewinging of Pinus caribaea and P. oocarpa is done through the use of a small cement mixer or rotating drum. The capacity of the drum should be at least double the amount of seed to be dewinged and the speed of rotation about 1 revolution every 2–3 seconds (Robbins 1983a,b). The winged seeds are tumbled dry in the drum for 15 minutes; water is then sprinkled slowly over the seeds as evenly as possible while the drum continues to rotate, at a rate of about 1 litre to every 50 litres of seed. Tumbling is continued for 45 minutes after adding the water, and then the mixture is emptied out on to a gauze-bottomed tray, after which the seeds are separated from the wings. Moist dewinging is also in common use in Sweden. In moist dewinging the seeds absorb water and have subsequently to be dried to an acceptable moisture content.

Seed Cleaning Methods

The main characteristics by which sound seeds may be distinguished from inert matter including sterile and empty seeds are size and shape, specific gravity, colour and surface texture. The ease with which sound seeds can be differentiated depends on (1) the degree of difference which exists between the seeds and the matter to be separated from them and (2) the degree of uniformity among the seeds themselves (Turnbull 1975 c). Colour, size and shape are useful criteria for visual separation, while most seed cleaning machines make use of seed size and specific gravity. Screening and sieving methods separate by seed or particle thickness or diameter; the indented centrifugal cylinder by particle length; liquid flotation and blowing, fanning and winnowing methods by specific gravity; while frictional cleaning methods rely on differences in surface texture. Modern cleaning machines often combine more than one method, so that the cleaning process is both effective and quick. However, the species and the amount of seed to be handled will determine whether cleaning is best carried out by hand, by improvised equipment or by specialist machinery. The following account of cleaning and grading methods is based on Turnbull (1975 c).

Screening or sieving

In most cases a number of sieves with different sized perforations are used and the cleaning is a process of gradually sifting out smaller and smaller particles. It is not only the size of the perforations which determines the quality and quantity of the seed cleaned; other important factors include the precision of the perforations, the angle at which the sieves operate, the amplitude and speed of movement of the sieves, and the correct cleaning and maintenance of the equipment.

Sieves or screens may be made of flat perforated plate or wire mesh, and sometimes they may be three dimensional such as funnelshaped sieves. For small samples hand held sieves are adequate, but in larger scale cleaning a series of shaking screens is commonly used.

In Brazil sieving with a mesh size of 32 per inch (approx. 12.5 per cm) was effective in separating seeds from chaff in Eucalyptus grandis. 84 % of good seeds were retained and 89 % of chaff was eliminated (Cavalcanti and Gurgel 1973).

Sorting according to length

Cleaning with sieves relies on the separation of seeds, with diameter the critical factor. Fractionating according to length cannot be done with sieves, but this is possible with an indented cylinder. In addition to its use for separating good seed from the impurities, the equipment is used in agriculture for separating seed mixtures and can also be used for grading seeds.

The equipment consists of a slightly inclined horizontal rotating cylinder and a movable separating trough. The inside surface has small closely spaced hemispherical indentations. Small material is pressed into the indents by centrifugal force and can be removed. The larger material flows in the centre of the cylinder and is discharged by gravity. Depending on the type of impurities, the seed may be separated via the indentations or by passing down the cylinder.


Cleaning by blowing is a very important and widely used method. It is based on the principle that any object can float in an airstream of sufficient velocity.

For separation in an airstream there are three possibilities: falling, floating or rising. The behaviour of the seed and other matter will depend on their weight, their resistance to the flow of air (volume and shape), and the velocity with which the air moves.

The operation of blowing is often referred to as winnowing or fanning. In its simplest form the uncleaned seed is thrown into the air on a windy day. The components separate out and the desired ones are retained. Indoors the airstream from a fan can be utilized.

Hand winnowing has been used successfully in Thailand to separate full seeds from empty seeds of Pinus kesiya (Bryndum 1975). The initial separation was followed by a second winnowing of the discarded fraction. Cutting tests showed that the original full seed percentage of 82 % was raised to 98 % in the improved fraction, while the discarded fraction (approximately 10 % of the total volume) contained only 18 % full seed or about 2 % of the total full seed in the seed lot. One operator took 8 minutes to winnow 1 kg. Loss of the very smallest and lightest full seed in the separation process is usually not serious since these seeds are likely to germinate slowly and to be low in vigour.

Simple winnowing machines can be easily constructed and Yim (1973) illustrates a machine made entirely of wooden components which is used in Korea. Laboratory blowers can be classified as either the pneumatic type - a fan is situated near the air intake and pushes air through the system, or the aspirator type - a fan is situated near the air outlet and pulls air through the system, creating a partial vacuum. Small laboratory cleaners such as the ‘Brabant’ cleaner, and the ‘Kamas’ laboratory aspirator are available.

Another seed blower in common use is the South Dakota Blower. The principle of blowing is that a sample of seed, when suspended in an upward stream of air of a certain velocity, will split into a light fraction and a heavy fraction, the light fraction being carried upward and the heavy fraction falling down. The two fractions are caught apart from each other. The heterogeneity of the heavy fraction can be further reduced by subjecting it to a second blowing at a higher wind velocity. In that way a light fraction, a medium fraction and a heavy fraction are obtained. The blowing apparatus in the South Dakota Blower consists essentially of a centrifugal blower, the outlet of which is connected to the bottom end of a vertical tube of a few centimeters' internal diameter and about 50 cm in length. The sample is held on a fine wire gauze at the bottom of the tube. A built-in valve allows the wind velocity to be set at a rate that has been found optimal for the species. The lighter particles are blown up and trapped by baffles near the top of the tube, while the heavier are retained at the bottom.


6.17 Electrically operated laboratory seed blowers: (A) South Dakota blower (Division of Forest Research, CSIRO Canberra, photography by Allan G. Edward). (B) Barnes Tree Seed Separator (International Reforestation Suppliers)



6.18 Locally made seed cleaner used in Zimbabwe (A) Internal view of conical shield showing diffusing baffles (B) In operation, showing the compartmented receptacle. (Forestry Commission Zimbabwe)

A more complex blower developed in Canada is described by Edwards (1979). It consists of four plexiglas tubes of varying diameters which cause differential air velocity and, by choosing the appropriate combination of tubes, it is possible to use the equipment either to separate seed from chaff or full seeds from empty seeds. It works well with large seeds e.g. Abies amabilis but is not suitable for very small, light seeds such as Betula or Chamaecyparis.

In Zimbabwe a home-made cleaner was constructed by fitting a tapered aluminium sleeve with internal baffles onto a constant speed domestic fan (Seward 1980). The narrow end of the sleeve projects over a compartmented receptacle; full seeds fall into the nearest compartment, while the lighter impurities and empty seeds are blown into the further compartments. A similar locally made seed winnowing chamber, which uses a stream of air from an electric fan, has been successfully used in the Solomon Islands to separate debris and empty seeds from dry seed batches of Swietenia macrophylla and Campnosperma brevipetiolata (Chaplin 1984).

Many seed cleaning machines use a combination of winnowing and screening. A coarse upper screen removes larger material, a lower fine screen stops the seeds and lets through fine matter, and then the seed fraction passes through a transverse or nearly vertical airstream from a fan to remove chaff and empty seeds. The air-screen cleaner is the basic equipment of seed cleaning plants. The size of air-screen cleaners varies from the small two-screen model to a modern precision model, which uses several top and bottom screens and in one operation as many as three air separations.

Liquid flotation

Cleaning by flotation relies on the principle that the density of the seed of a given species is specific both for filled and unfilled seed.

There are two basic methods used:

  1. density method in which liquids with a density or specific gravity between that of the full and empty seed are used. The specific gravity of the liquids used is usually below 1.0 and such that the full seed sinks and the empty seed and light debris float.

  2. absorption method in which water is used and, although both full and empty seeds float initially, after some time the full seeds absorb water, become heavier and sink. The time of soaking can vary from a few minutes to several hours. This method is useful where there is a very small difference between the specific gravities of the full and empty seeds. The seed must be re-dried after being separated.

The flotation methods can separate out insect attacked, mechanically damaged, and immature seeds from filled mature seeds. The density method can only be applied if a liquid of suitable density is available which is not harmful to the seed. The application and the problems encountered are discussed by Simak (1973); both this and a method for separating viable from non-viable full seeds which was developed by the same author, involving an element of pregermination, are described on pp. 187–189.

Friction cleaning

Most debris can be removed from the seed by air-screen combinations, but leaf fragments, resin particles and other objects of similar size and density to the seed are difficult to remove.

Friction cleaning relies on the principle that any object falling or sliding over a surface encounters a certain friction. The movement of the particle is proportional to its weight and to a coefficient of friction which depends on the nature of the particle's surface and the surface on which it moves. Separation of seed from debris is made on an inclined cloth or rubber belt on the basis that the angle necessary for the run-off of the seed differs from the angle necessary for run-off of the debris. A continuous upward moving belt removes seeds downwards by gravity and the lighter debris upwards by friction.

A friction machine suitable for cleaning small samples of tree seeds is described by Hergert et al. (1971).

Specific gravity separation

This method makes use of a combination of weight and surface characteristics of the particles to be separated. It is a method which is finding increasing use in separating and grading tree seeds.

The specific gravity (SG) separator employs a flotation principle. A mixture of seeds is fed onto the lower end of a sloping perforated table. Air, forced up through the porous deck surface and the bed of seeds by a fan, stratifies the seeds in layers according to density, with the lightest seeds and particles of inert matter at the top and the heaviest at the bottom. An oscillating movement of the table causes the seeds to move at different rates across the deck; the lightest seeds float down under gravity and are discharged at the lower end, whilst the heaviest ones are kicked up the slope by contact with the oscillating deck and are discharged at the upper end.

Deck coverings of linen, plastic, and wire mesh have been used to distribute the air uniformly beneath the seeds and provide the correct push to the heavier seed in contact with the deck. A closely woven covering such as linen gives the best results for small seeds. On modern gravity separators, it is possible to control the feed speed, the tilt of the table in two directions, the speed of oscillation and the force of air differentially at various points along the deck. The combination of these different controls makes it possible to adapt the machine to handle a wide variety of species and seed lots (Thomas 1978).

The specific gravity separator will separate particles of the same density but of different size, and particles of the same size but of different densities. It will not separate efficiently particles which differ both in density and size, i.e. separate a larger but less dense particle from a smaller but denser particle. It has been found practical for cleaning the chaff from some eucalypt seeds and for grading pine seeds (Guldager, 1973). Purity % of uncleaned seed of E. grandis after extraction is about 10 %. This was raised to 95 % purity with 95 % germination by SG separator treatment.

Other cleaning methods

A number of other methods of seed cleaning have been used experimentally, but are not yet in widespread operational use. They include electronic and electrostatic separators, magnetic separators, electronic colour separators and shaking tables, which separate seeds by the angle at which they rebound when thrown against fixed walls. They are described by Klein et al. (1961) and Oomen (1969).

Seeds of Ochroma can be successfully cleared of their floss by placing the uncleaned mass on a wire sieve of 0.3 mm mesh and setting fire to the floss Goor and Barney 1976). The fire flashes across and the seeds drop through the mesh. Good results are obtained when the seeds are allowed to drop through the mesh into a pan of water. The inflammable oil in the floss burns with intense heat and experience in Honduras has shown that the floss must be spread out thinly to avoid damaging the seed (Robbins 1982b). This method has also been tried to clear Populus seeds of floss but may damage more than 50 % of the seeds in this genus.

Seeds of Prosopis are often embedded in a gummy matrix within the pods. One way of obtaining clean seeds is (1) To remove one side of the pod mechanically with a knife (2) To soak the pod contents in a 0.1 normal solution of hydrochloric acid for 24 hours (3) To wash in water for one hour, then dry in the direct sunlight (4) To beat or pound the dried mass to separate the clean seeds from the gummy coating. This method has been successful in India, yielding clean seeds with a germination rate of 65 % in 12 days (Vasavada and Lakhani 1973).


Within a single species there is a variation of seed dimensions due to environmental influences during the development of the seed and to normal genetic variability. The performance of the seed immediately after germination is related to seed size and, in order to produce a crop of seedlings which will emerge and grow evenly in the nursery, size grading of seeds can be a useful practice. Size grading can also assist mechanical sowing of seeds. The practice should be used with caution in the case of seeds collected from seed orchards having a restricted number of clones. Since part of the variation in seed size and shape is genetic, grading of seed orchard seed could lead to excessive genetic differentiation and loss of genetic diversity within each of the fractions obtained by grading (Simak 1982).

The methods of grading seed vary little from those used in the cleaning process. Sieving and screening, cylinders, air blowing, flotation and specific gravity separation can all be used effectively for size grading tree seeds.

Although grading itself is a relatively simple operation, seeds of certain species have to be properly processed before they can be graded, for example the spongy exocarp of teak fruits and the calyx tubes of dipterocarps must be removed in order to get the full benefits of grading. The cleaned stones of Gmelina arborea have been graded by using square-mesh screens of 7,9 and 11 mm mesh (Woessner and McNabb 1979); germination varied from 84 % for the smallest to 111 % for the largest size class (there are usually 1 to 3 seeds per stone).


6.19 Air/Screen seed cleaner used in Humlebaek, Denmark. (DANIDA Forest Seed Centre)

6.206.20 Damas gravity seed separator. (Damas Maskinfabrik, Denmark)


6.21 Equilibrium moisture content (fresh-weight basis) of wheat seed, showing separate curves for desorption and absorption. (source Harrington 1970)


6.22 Moisture content percentages of fresh seed of Pinus palustris (1938 crop, Mississippi) in equilibrium with air at various temperatures and relative humidities. - Derived from Wakeley 1954, with conversion of seed MC % to wet weight basis.

Control of Moisture Content

After seeds have been cleaned and graded, they are ready for sowing in the nursery. If, however, they are to be put into storage, it is necessary to check their moisture content (MC) and, if necessary, adjust it to the optimum level for storage of the species in question. Adequate facilities for testing MC should be available in the seed processing depot. Methods for testing MC are described on pp. 227–229.

For orthodox*) seeds, which comprise most coniferous and many hardwood seeds, adjustment of MC if needed means further drying. This is described in the following section. Much less commonly and only in the case of recalcitrant*) seeds which must be stored at a high MC it may be necessary to moisten the seeds in order to raise MC to the optimum for storage. For example promising results have been achieved for Acer pseudoplatanus by soaking the seeds in water for two or three days and immediately afterwards freezing and storing them at about -7°C in plastic sacks (Barner 1975b). Other genera, such as Quercus and Castanea, which have been lightly dried under cover in order to loosen their husks or involucral attachments, may benefit from soaking to restore MC to the optimum level (e.g. 40 – 45 % for Quercus robur, Holmes and Buszewicz 1956, Suszka and Tylkowski 1980), before being placed in moist, cool storage.

*) See glossary for definition of these terms.

Relationship of Seed Moisture Content to Atmospheric Humidity

Seeds, like cones and fruits, are hygroscopic materials and, when detached from the parent tree, they lose or gain moisture to or from the surrounding atmosphere until their moisture content (MC) reaches a point of equilibrium with the humidity and temperature of the surrounding air. This is known as the equilibrium moisture content (EMC). Once it has been reached, it will be maintained as long as the humidity and temperature of the air remain constant; if they change, the seeds will again lose or gain moisture until a new EMC is reached. Wood is another good example of a hygroscopic material and behaves in a similar way.

“Wet” seed surrounded by “dry” air will lose moisture and therefore weight, while “dry” seed surrounded by “moist” air will gain them. In order to devise the most suitable methods of drying and storing seed, it is necessary to be able to quantify the moistness of both air and seed.

Air humidity. Moisture in the atmosphere is in the form of water vapour, but air can hold only a limited quantity of water vapour. If this is exceeded, the air is said to be saturated and the excess moisture condenses as dew. The exact weight of water vapour (WV) which the air can hold at saturation depends on the temperature, as shown in the following table:

Temp. °C-10 0      10    20   30   40   50   60  
Weight of WV at saturation
(g WV per kg of dry air)
1.6 3.8 7.6 15   27  49  87  152 
Density of dry air (kg per m3) at a pressure of 760 mm1.341.
Weight of WV at saturation
(g WV per m3 of dry air)
2.1  4.9  9.5  18   31  55   95   161

Most of the time the content of water vapour in the air is less than that at saturation. Relative humidity (RH) is defined as the ratio (usually expressed as a percentage) of the quantity of water vapour actually present in the atmosphere to the quantity which would saturate it at the same temperature; alternatively, as the actual vapour pressure in the air as a percentage of the saturation vapour pressure at the same temperature. For the seedsman relative humidity is the most important measure of atmospheric humidity because the equilibrium moisture content of seeds is most closely correlated with it. For example the MC of seeds will be very nearly the same when in equilibrium with air at an RH of 50 %, whether the air temperature is at 10°C (absolute humidity or weight of water vapour present = 7.6 / 2 = 3.8 g/kg dry air) or at 50°C (absolute humidity or weight of water vapour present = 87 / 2 = 43.5 g/kg dry air). Though the absolute humidity of one is over ten times that of the other, the relative humidities are the same and it is relative humidity which has the greatest effect on the EMC of seeds. The importance of RH to the EMC of seeds and the dramatic effect of temperature changes on RH explain the importance of heat in the drying of many seeds. From the table above it can be seen that air with 3.8 g water vapour/kg at a temperature of 0°C would be saturated; at 100 % RH it would be useless as a medium for drying seeds. But if the same air were heated to 30°C and provided no additional moisture were introduced from outside the system, its RH would be reduced to 14 % and it would become a highly effective drying medium.

Moisture content of seeds. The amount of moisture in seeds is usually expressed as a percentage of their weight. Methods of measuring MC are described on pp. 227–229. Moisture content can be expressed in two ways (a) The weight of water expressed as a percentage of the initial “wet-weight” or “fresh-weight” of the seeds (= dry matter + water) or (b) The weight of water expressed as a percentage of the final oven-dry weight of the seeds (= dry matter only). One of the greatest difficulties in the understanding and application of published results on moisture content derives from the fact that, in the past, both the “wet-weight” and “dry-weight” methods have been used, often with no indication as to which of them were applied in a particular case.

According to the ISTA Regulations seed moisture content should always be expressed on a wet-weight basis. For orientation both formulae are given here with a conversion table.

Moisture content, % of dry matter (dry basis)

Moisture content, % of total weight (wet basis)

Because of the limited amount of water vapour which it takes to saturate air, a relatively small quantity of seed can hold as much moisture as a great deal of air. One litre of seed dried from 50 % to 9 % MC (wet weight) at 30°C would lose about 450 gms of moisture to the surrounding atmosphere, enough to change the RH of about 15 m3 (or 15,000 times its own volume) of air from 0 to 100 %. In the case of sun-drying, the atmosphere is so vast that it can absorb this moisture without difficulty, but in an enclosed building the ambient air can quickly become saturated. This explains why so much emphasis is put on adequate ventilation in kiln drying, in order to ensure that moist air is removed as it approaches saturation and is replaced by fresh, dry air.

The same feature is an advantage when storing dry seeds in sealed containers. Provided that the seeds are correctly dried and the containers properly sealed, a relatively small volume of seed will come into equilibrium with a much larger volume of enclosed moist air without increasing its own MC significantly. If one litre of seed of oven-dry specific gravity 0.5, dried to 9 % MC (wet weight), were to be enclosed in a 10 litre sealed container with 9 litres of moist air at 100 % RH at 20°C, the total moisture content of the air would be only: Even if the seeds were to absorb all this moisture, it would only raise their MC from 50 to 50.16 g or MC % from 9.09 to 9.12 %. The common prescription to fill sealed containers as full as possible with seed is a sound one, but it is based on the deleterious effects on many species of the oxygen in the enclosed atmosphere, not of the water vapour.

Other factors affecting EMC. Although relative humidity is the most important single factor affecting the equilibrium moisture content of seeds, it is not the only one.

(1) Temperature. As already explained, temperature has a large indirect effect on EMC because, if absolute humidity is kept constant, relative humidity is directly related to temperature. It has an additional effect because EMC varies slightly with temperature even when relative humidity remains constant. The effect varies with species but very few data on forest trees have been published (but see also p. 147). An example quoted for an agricultural crop, sorghum, by Justice and Bass (1979) shows that at 50 % RH the EMC varies from 12 % at 49°C to 14 % at -1°C. The difference is slightly greater in some other crops, but in all cases EMC decreases with increasing temperature and constant RH (even though absolute air humidity increases with temperature at the same RH).

(2) Absorption and desorption. For any species there is a difference of 1 – 2 % in the EMC according to whether a moist seed is losing moisture to a drier atmosphere (desorption) or a dry seed is gaining moisture from a moister atmosphere (absorption). The EMC is always higher on desorption and it is the desorption curve which is important in the common situation of drying orthodox seeds from a higher to a lower MC for storage.

(3) Variation in EMC according to species. The MC of seeds in equilibrium with a given RH and temperature varies with species. The EMC for each species must be determined by trial. An important component in interspecific variation is the percentage of oil content in the seeds. Seeds which store most of their food reserves as proteins or starch have a higher EMC at a given RH than seeds which store food as fats and oils, because the former are relatively hydrophilic, the latter hydrophobic. Among agricultural seeds wheat, with a low oil content of 2% and an EMC at 45% RH and 25°C of 10.4%, may be compared with Brassica oleracea, with a high oil content of 35 % and an EMC under the same conditions of 6.0 % (Harrington 1970).

Equilibrium Moisture Contents for 3 Orthodox Species


6.23 Equilibrium moisture contents for 3 orthodox species. (source F.T. Bonner)

Equilibrium Moisture Contents for 4 Recalcitrant Species


6.24 Equilibrium moisture contents for 4 recalcitrant species. (source F.T. Bonner)

Detailed information on the EMC of tree species is sparse, and lacking entirely for tropical species. Some selected examples are given in the following table and in the attached graphs provided by F.T. Bonner.

EMC % (wet weight basis)
SpeciesTemperatureRH 1020304040–5550607095
Fraxinus sp.1)not given4.
Picea abies 1)not given2.
Pinus taeda 2)4 – 5 °C----10---17

1) Source: Touzard (1961) cited in Roberts (1972). Absorption curves.
2) Source: Bonner (1981)

The graphs show EMC for four recalcitrant (Quercus) species and three orthodox broadleaved species. It should be noted that the EMC among the Quercus spp. is correlated positively with carbohydrate content and negatively with fat content. Q. alba has the highest EMC, the highest carbohydrate content and the lowest fat content, followed by Q. muehlenbergii, Q. shumardii and Q. nigra. Similarly, Liquidambar has the lowest EMC, the lowest carbohydrate content and the highest fat content of the three orthodox species.

The same process of tissues reaching an MC in equilibrium with the ambient RH occurs during the drying of fruits for seed extraction as in the drying of the seeds themselves for storage. Usually the exact EMC is less critical in drying fruits, since shrinkage, splitting or scale-opening take place over a range of MC and the process is only carried on to the stage when the fruits release their contained seeds.

Drying of Orthodox Seeds

For medium- or long-term storage of many species, a moisture content of 4–8 % is recommended (see p. 143). This is considerably less than the MC of freshly collected seeds. Reduction of MC can be achieved in most species by placing the seeds in an ambient atmosphere of relative humidity (RH) 15–20% for a period sufficiently long to allow the seeds to reach an MC in equilibrium with the RH.

The effectiveness of air-drying depends on local climatic conditions. Reduction to an MC of 12–18 % is frequently possible, provided that attention is paid to adequate aeration of the seed. Reduction to less than 8 % is impossible in most temperate situations and in some areas of the wet tropics, because average RH remains too high. Thus RH in moist tropical West Africa is commonly over 80 % in the wet season and over 70 % in the “dry” season (Ogigirigi 1977). An MC of less than 8 % is unlikely to be achieved in these conditions. In areas where insolation is high, many species can be successfully dried to 6–8 % MC by exposing them to direct sunlight, because the seeds and surrounding microclimate heat up and RH is consequently reduced. Also, for a constant RH, the equilibrium MC of seeds decreases with increasing temperature. Care must be taken to ensure that the seeds are as dry as possible prior to exposure and that they are moved frequently. In Honduras this method works well with Pinus spp. but is not recommended for Cordia because seed of this genus dries too quickly and can reach 4 % MC, with damage to the tissues (Robbins 1982b).

As pointed out by Harrington (1970), artificial drying can be accomplished in two ways. One method is to raise the temperature of the air which, provided that no additional water vapour is introduced from outside the system, automatically reduces the RH. The other is to remove moisture from the air without changing the temperature, which also reduces RH. He quotes the example of air at 5°C and 90 % RH which is heated to 35°C. Its RH is thus reduced to 15 % and it can be blown through the seed by forced ventilation until the latter reaches equilibrium MC. On the other hand, air at 30°C and 90 % RH, typical of moist tropical areas in the rainy season, would still have an RH of 40 % even if heated to 45°C. High temperatures can be extremely injurious to seeds, especially those which have a high MC. Generally, drying temperatures should not exceed 40°C and recent trends have been towards lower temperatures and greater air-flows to ensure safety in drying (Holmes and Buszewicz 1958). Barner (1975b) and CATIE (1979) recommend a temperature not exceeding 30°C in the early stages. One possibility is to dry seed in two stages, the first to about 11 % MC using a temperature below 40°C, the second to about 5 % using a temperature of 60°C. Provided that the MC is reduced to about 11 % in the first stage, a second stage temperature of 60°C is considered safe for most agricultural species (IBPGR 1976). On the other hand, there is some evidence that a high drying temperature, which does not affect immediate germination, may still affect subsequent longevity (IBPGR 1981). For long-term storage for genetic resources purposes, a combination of low RH and low temperature (15 % RH and 15° C) is recommended.

Where climatic conditions make it impossible to achieve a low enough RH by heating the air, it is necessary to reduce it by removing water vapour without raising the temperature. This can be done by either: (a) refrigerating the air to below the dew point, condensing the water vapour on the cooling coils and then reheating the air to 35°C or (b) blowing air through a chemical desiccant, which removes the water vapour, and then through the seed (Harrington 1970). Various desiccants are available e.g. silica gel, CaO, H2So4, Lithium chloride or anhydrous CaCl2 but the most indestructible and easiest to reuse is silica gel (Magini 1962, Harrington 1970). A good example of one drying technique is that furnished by the seed bank of the Regional Genetic Resources Project at Turrialba, Costa Rica (Goldbach 1979). Because of the permanently high air humidity and high day temperatures there, hot air drying is not possible without damage to the seeds. Therefore a silica-gel type dryer, which maintains an RH of less than 15 % at 25°C, is used, situated outside the drying room and ducted into it.

Small lots of seed that have first been air-dried to an MC below 20 % may be placed in a sealed container with an equal quantity of silica gel that has been freshly dried at 175°C and cooled. The silica gel, seed and enclosed air will come to an equilibrium suitable for storage (Harrington 1970). See also the section on pp. 156–157 on “Use of dessicants in containers”.

The period of time needed for forced-draught air circulation to dry seeds to equilibrium moisture content is partly dependent on the accessibility of individual seeds to the air current. Seeds should be spread in thin layers on trays, with space for air circulation between the trays.

The MC of coniferous seeds, which have been extracted from the cones during kiln-drying, may already be close to that recommended for storage. But moisture may be reabsorbed during cleaning and dewinging, and in some species it may be added deliberately to reduce damage during these operations. Therefore MC must be tested and, if necessary, further reduced immediately before storage. At the same time, if cleaning and dewinging are carried out in warm, well-ventilated rooms, this will decrease the amount of drying needed subsequently (Morandini 1962).

Mixing before Storage

When a large seed lot is to be stored in several containers, it is desirable that seed homogeneity between containers be maintained as far as possible, so that each container is equally representative of the seed lot as a whole. Methods of mixing samples for testing are described in chapter 9 and those methods can be adapted to mixing the larger quantities of seed handled during seed processing, before placing them into storage containers.

If seeds have been size-graded, mixing is carried out on the ultimate seed lots separated by the grading operation. Thus, a given seed collection may be graded into a “large seed” fraction and a “small seed” fraction. Each fraction becomes a separate seed lot with its own seed lot number and separate mixing within each of these two seed lots may be done before storage to ensure homogeneity between containers.

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