The correct handling and processing of acacia seed prior to storage is essential. Seed coming from the field is rarely fit for immediate storage; drying, extraction from the pods and further cleaning are commonly required. It is important to ensure that these operations do not harm the seed, that the seedlots are not mixed accidentally and that the identification of each seedlot is maintained. As each stage requires careful supervision, it is preferable that the facilities for final drying, extraction, cleaning and storage be centrally located and that responsible staff supervise each operation.
Processing techniques vary according to species and objectives. With small-scale research collections, for example, it is usually impractical to employ sophisticated machinery developed for processing bulk collections. The normal sequence of operations in extraction and cleaning is -
When harvested material arrives at the depot there should be an immediate inspection to determine its condition. Moist and fermenting pods must be spread out to dry and does require treatment with fungicide to avoid further deterioration. The removal of branches, twigs and other large impurities will facilitate drying and further cleaning (Fig. 17).
|Fig. 17||Preliminary cleaning by hand and a coarse sieve prior to mechanical threshing.|
The most straightforward way to dry the collected material is to expose it to sun and wind by spreading it in a thin layer on sheets on the ground, or above the sheets on screens or platforms which improve air circulation (Fig. 18). This method is ideally suited to the hot, rainfree environment of the dry-zone. If there are heavy dews the material may have to be covered or bagged at night. Someone must be on hand at all times to protect and to regularly turn the crop to permit the lower layers to dry. Where the climate is less reliable facilities such as roofed structures, glasshouses, or even specially designed drying rooms may be required. In these situations good air circulation is essential. Problems can arise where ants and rodents remove or eat the seed (insecticidal sprays poison or baits can help), domestic or wild animals scatter the seed (dogs, fowls etc.), or interested by-standers inadvertently mix the seedlots.
|Fig. 18||Drying material on a temporary frame.|
The time required for natural drying depends on a number of factors including species, the degree of maturity of the fruit and weather conditions. When the pods and seed of acacias are collected in the near-dry, mature state (i.e. the pods are already cracking and releasing the dark-coated seed) there are few problems and the final drying is quick. ‘Green’ pods, however, are high in moisture content and are likely to ferment. In this case, the pods should be dried at moderate temperature with good ventilation.
The rapid, artificial drying of pods is especially useful during rainy or humid weather. A temperature treatment of 45°C for a maximum period of 48–72 hours is a common schedule (e.g. Bowen 1981). In species where a large proportion of seed remains either enclosed in the pod (e.g. A. nilotica, A. aneura) or firmly attached to it by the funicle (e.g. A. mangium) even after lengthy periods of natural drying, a gentle artificial heat treatment at 40–45°C for 24 h can promote a brittleness in the pod that assists in the extraction process (Bowen 1981).
There is a wide range of drying equipment available ranging from small ovens to large seed drying kilns (Boland et al. 1980). For small seedlots a forced draught oven can be used. Large kilns are expensive to establish and operate and are warranted only if very large amounts of seed are processed.
After drying the seed must be removed from the pod without affecting seed viability. The ease of extraction depends on the species, stage of pod maturity, and the available equipment. With some species, a mere shaking of the dried material will release all the seed from the pods. In other species the seeds remain locked in sections of the pod or remain attached to the pods so strongly that special efforts are needed to release them.
The seed of many acacias can be extracted from the dried pod by beating with a flail or slender pole (Fig. 19), crushing the pods between canvas sheets by trampling underfoot or, with small samples, simply by breaking up the pods by hand. More robust methods such as pounding the dried pods with a heavy wooden pestle or mallet may be necessary for very hard pods.
As manual threshing can be very time consuming and labour intensive, it is often advantageous to utilize either hand or power driven machinery to assist in this task. A wide range of equipment is available for threshing agricultural grain crops; some can be adapted for the extraction of acacia seed. The tolerance of acacia seed to threshing depends on species. Machinery must be carefully tested for harmful effects on each seedlot before the bulk of the seed is subjected to the treatment. Examples of machinery used to extract acacia seed in Australia are -
|Fig. 19||Flailing Acacia aneura collections with slender pole.|
(Photo: J. O'Connell, University of Utah, USA)
The construction and performance of the Hand Type, Resilient Tapered Thresher is discussed by Ridgway (1978). Now refined (Fig. 20) the machine is manufactured under licence by Alf. Hannaford & Co Ltd, Woodville, South Australia.
This machine has successfully threshed research samples of A. aneura and A. victoriae and, according to the manufacturers, is used extensively for processing grain crops in countries where labour is readily available. The machine is simple, with few parts, relatively cheap to purchase and operate, and is easily cleaned. It is, however, slow and labour intensive when handling large quantities of seed.
A sometimes effective method of extraction is to place the dried pods in a rotating drum or cement mixer with several large loose weights (e.g. stones, sand bags) which, when tumbled, break the pods and facilitate the release of seed. This technique is simple but slow and its effectiveness varies with species.
|Fig. 20||The Resilient Tapered Thresher - Hand Type Model manufactured by Alf. Hannaford & Co Ltd., Woodville, South Australia.|
A hopper-fed thresher designed by CSIRO has proved effective for a number of Australian acacias (Fig. 21).
|Fig. 21||CSIRO 15 cm Flailing Thresher -|
|(A) Feeding material into thresher|
|(B) View showing essential parts|
|(C) View showing essential parts|
|(D) Material that has been threshed and is ready for cleaning|
The machine's electric motor rotates a metal shaft bearing four replaceable flailing rubber strips inside a chamber. The pods are drawn downwards from the overhead hopper and forced across and through a selected interchangeable brass sieve at the internal base of the specially designed concave profiled chamber; this concludes the threshing operation and the harvest falls into a container, where the operator can examine the degree of success of the treatment. To achieve complete separation of seed from pod, it may be necessary to pass the material through again using a different sieve but this is very simple to do. The design of the thresher allows easy cleaning to all parts, thus preventing contamination between seed lots.
Peg drum threshers (Fig. 22), composters and hammer mills are examples of other equipment that have been used to advantage in the threshing of acacia pods. These designs tend to be more severe on the seed passing through them than the machines described earlier, and pilot trials are recommended to assess seed damage before general usage on a particular crop.
Seed coming directly from the field or through the extraction process usually requires a final, thorough cleaning before storage. The aim of cleaning is to separate full seeds from impurities (including empty seeds) and thus obtain homogeneous seedlots of high viability. Complete cleaning of a seedlot is not always possible or necessary. The level of cleanliness adopted is usually a compromise between time, effort and loss of viable seed. It may also depend on plant quarantine requirements.
Mechanical cleaning usually depends on sieving, winnowing, flotation, or a combination of these.
A sequential sieving through a gradation of mesh sizes can produce a high level of cleanliness with many acacia seedlots. This process has been mechanised by the vibrating screens of machines like the ‘Vac-A-Way Gravity Screen’ (Fig. 23). Problems arise when there are large proportions of impurities similar in size to the viable seed. In this case a combination of methods may be required to achieve acceptable results.
Winnowing is widely used for cleaning grain and other seeds in many parts of the world. Wind or an air stream is used to separate light material (chaff) from the heavier remainder (i.e. full seed). In its crudest form, the uncleaned seed may simply be thrown into the air on a windy day. Various fractions of the seedlot separate out, and the desired ones are retained. This principle has been refined in seed cleaning machines commonly referred to as blowers. Many seed cleaning machines use a combination of winnowing and screening. The screens are used to remove the largest and finest fractions and the intermediate fractions containing the seed are winnowed.
|Fig. 22||Peg Drum Thresher|
|(A) Open for cleaning-material for extraction in the background|
|(B) Feeding material into thresher.|
|Fig. 23||The Vac-A-Way Gravity Screen.|
For cleaning acacia seed, CSIRO has successfully made use of a blower of Austrian manufacture called a ‘Kurt Pelz Saatmeister Mark 2’ (Fig. 24). The machine comprises three compartments as shown in the photograph. An automatically agitated hopper feeds material to be cleaned vertically down into the first compartment. Heavy material and seed normally fall straight down into the bucket. Controllable suction vents are located in the second and third compartments, which extract the light material as the uncleaned seed is fed into the first compartment. By adjusting the vents, it is possible to control the fraction of light material which is separated out.
The South Dakota Blower (Fig. 25) is useful for cleaning small lots of acacia seed. The machine provides an updraught within a transparent tube. The updraught carries off the lighter particles which are trapped by the baffles near the top of the tube. By careful adjustment of the updraught, only the desired heaviest fraction is retained.
|Fig. 24||Cleaning threshed acacia material by the Kurt Pelz Saatmeister Mark 2.|
|Fig. 25||The South Dakota Blower for cleaning seed.|
Flotation in water is an effective final treatment to remove small impurities. The method relies on differences in density with the sound seed sinking to the bottom of the water bath and the lighter material, including empty seeds, floating on the surface. After removal of the impurities, the water is drained off and the seed thoroughly dried before storage.
The removal of the funicle before storage and distribution may be desirable for plant quarantine purposes and because of possible adverse effects on germination (Bowen 1981). Normal mechanical extraction and cleaning methods will usually take care of this, but in some cases special efforts may be required. The tumbling of these seeds between two sheets of grooved rubber matting followed by sieving is usually sufficient to remove the funicle.