1.3.3 Tamr

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In the previous chapters the use of khalaal and rutab was discussed which, for local climatological or other reasons, are harvested at an earlier stage than tamr. It was shown that, apart from a limited percentage being consumed "fresh", the maturation of these dates could be hastened by artificial means (curing, heat, chemicals), to give them longer storage life and more marketing possibilities. At the rural level sun-curing and compacting of the dates have proved efficient methods. Boiling of certain varieties combined with sun drying established itself in several areas as a major rural processing technique to turn a perishable commodity into a durable one. Refrigerated storage, freezing and the use of anti-microbial agents have, though on a limited scale, also contributed to prolong storage life and thus marketing opportunities.

This Chapter deals with fruit harvested at the tamr stage, i.e. when the date has dried down on the palm to below 24%, and its flesh has firmed through moisture loss but is pliable. The fruit at this stage is non-perishable that is, micro-organisms will not be able to grow on it, but it does not exempt it from insect infestation, moisture uptake and its consequences, and compositional changes (darkening and changes in flavour) during subsequent storage, unless special precautions are taken. Appreciation of these constraints and taking into account consumer preference and market demands will facilitate understanding how the markets for tamr have developed. A useful subdivision of these outlets is as follows:

i. home consumption, local markets
ii: wider regional distribution, also overseas, through merchants, primitive packaging in bulk,
iii. collecting/bulk packing centres,
iv. small-, medium-, and large-scale packing plants for bulk shipments and retail packs.

Pertinent figures on the size of each of these marketing channels are not available but it is most likely that the statement in Dowson's book of 1962, (139): "Most of the dates of the world are never submitted to any processing operation except pressing" still holds true. In other words, home consumption, local markets and the somewhat wider regional distribution of loose dates or those pressed in baskets, second hand tins and bags occupies still the better portion of the annual crop. But it is also true that organized commercial date packing has increased substantially over the last 30 years. There is hardly any date producing country in the "old" world that does not have its one or more date packing plants which operate mainly for local but also for foreign markets.

The development in this direction has not been easy, however. The external markets being dominated in either size (and price) or quality, by a few major date producing countries, and local consumer appreciation of quality and convenience of food products not having been developed (or trained?) to such an extent to be prepared to pay for the higher price of a packed product, have jeopardized the economic operation of date packing plants considerably, especially in the smaller date producing countries. To be added here, that for going into date packing, for exports in particular, one needs to be assured of a raw material, constant in quality and desired quantity. With the large variety of dates, the many smallholders and the sometimes difficult conditions of climate and logistics this has been a severe bottleneck in the development of date commercialization. Even when world demand was there and foreign importers were looking for supplies, when the major traditional suppliers had dropped out of the market for reasons of war or politics, the less developed date producers could not fill the gap qualitatively and quantitatively. Going from diffused traditional marketing channels where much local produce is traded directly between grower and consumer, the establishment of date packing plants is a quantum jump, which can only be successful when raw material supply and markets are reasonably assured, apart from the time it takes to adapt a local subsistence economy into an externally oriented market economy.

Dates intended for domestic use, for bulk export or for retail packs may vary widely in the treatment they undergo before reaching the consumer. Below, in summary form, are listed the major operations that are in use in post-harvest technology, from harvest to delivery to the consumer:

a. Transport of Dates

From the moment the dates are harvested (Fig. 38) they need a container for being transported to the drying yard, or a central collecting point in the garden, the home or the local market, or eventually to a date packing plant. A multitude of baskets, mats and crates, made from palm materials is in use, their size and shape adapted to the type of date and transport means. They may range from the small shallow baskets made of palm leaflets carried on the head by the women, to the rigid baskets made of the midrib of the palm leaves and carried on the back by men, to the twin baskets and folding mats, slung over the donkey's back (Fig. 39). In the more advanced marketing systems where dates are purchased on a contract basis by a central authority or date packing plant the use of garden boxes, usually supplied by the purchaser of the dates, is widely used. Though with some variation in size, basically the box is of sturdy wood construction without lid, provided with handholes, and holding 10-15 kgs of dates. Use of standardized boxes gives the purchaser the chance to streamline transport to and handling of the incoming dates at the plant, but it requires extra investment, administration, transport, cleaning and storage costs of empty containers. A more recent development has been the use of plastic containers which apart from being a more hygienic and cleanable material, does not however reduce the other cost factors of empty boxes. A container that does reduce transport and storage costs is the so-called nesting box, which by a clever arrangement of the design allows the boxes to be stacked, and by turning them alternatively by 180 C around their vertical axis, to be nested. In this way a saving of 2/3 of the space required for storage and transport of the empties is obtained (Fig. 40).

Selective picking and use of cloth containersField sorting on trays and wooden boxes for further storage and treatment in packing plant 

Figure 38: Harvesting Tamr (California)
(a) Selective picking and use of cloth containers
(b): Field sorting on trays and wooden boxes for further storage and treatment in packing plant

Figure: 39: Different ways of  transporting dates in and from the field  Figure: 39: Different ways of  transporting dates in and from the fieldFigure: 39: Different ways of  transporting dates in and from the field

 Figure: 39: Different ways of transporting dates in and from the field

Stacked  emply containers 

 Comparative volume of empty, normal and nested containers

Figure 40: Plastic Containers (Saudi Arabia) (a) Stacked emply containers, (b): Comparative volume of empty, normal and nested containers

In the United States the increasing trend of mechanized harvesting has made room for the use of bigger containers, the so-called lug bins. They measure about 120 cm x 120 cm but are not deeper than maximum 45 cm to avoid crushing of the fruit. They are pallatable for transport to the plant. The usual garden boxes are, however, also still in use.

b. Storage

A major first concern after harvest is to prevent or control insect infestation and, traditionally, the date farmer has made use of locally available materials to provide protection against insects. Soft tamr is normally pressed into containers such as goatskins, small to large size jars made out of clay, baskets made of palm leaflets or other similar materials, old kerosene tins and oil drums (Fig. 41). The more tightly the dates are pressed the better they will be protected from insects, although exposed surface areas are liable to be attacked. For prolonged storage the exposed areas are therefore covered by cloth, clay or a layer of oil. One step further and more particular for larger scale containers or heaps, cloth or mats treated with malathion were found to be an effective protection (219). Jars are also used to store and pack dates in syrup. A typical example of larger scale storage in the field is the mud bin (mudibsa) of the Shatt-el-Arab region. It is made of thick mud walls and measures about 3 by 2 m and 2 m high. The bottom is corrugated which allows syrup oozing from the (soft) dates by their own weight, to be collected from a common spout at the bottom of the bin. A variation of this system is found in Saudi Arabia where sun cured dates are pressed in jute bags and stacked on ridges at the bottom of the store. Syrup will exude through the bags and eventually collect in channels between the ridges as an incidental by-product (Fig. 42). The bags which are covered with syrup and very sticky, are traded this way; a not attractive bulk pack, but it has its effect on preventing insect damage. Pressing soft dates into an impenetrable mass is therefore an effective way to reduce insect infestation.

 Figure 41: Pressing Dates in Baskets
Figure 41: Pressing Dates in Baskets

Figure 42: Local Date Store for Bagged Dates with Ridges on the Floor to Accommodate Syrup Collection 
Figure 42: Local Date Store for Bagged Dates with Ridges
on the Floor to Accommodate Syrup Collection

At the same time pressing excludes air, which is responsible for some of the compositional changes taking place in dates during storage. It also prevents the quick exchange of moisture with the surrounding air, retarding either drying out or getting wet according to the humidity of the air.

Storage of semi-dry and dry dates, unless very dry like desert dates and boiled dates, which insects do not find very attractive, is a difficult proposition unless preventive measures in the field are taken by covering heaps with cloth to reduce initial egg laying by storage pests. When left uncared for in storage it was found in experiments in Saudi Arabia that infestation by the fig moth started one month after harvesting (20 October) and increased progressively to 100% after 7 months, involving 3 generations of the insect (346).

Dry dates are usually packed in bags, boxes or are transported loose on trucks to the market or packing plant. The prevailing climate during the date harvesting season, i.e. hot and quite often humid, makes the conditions ideal for infestation and unless this is accepted, one has to resort to field fumigation and move the crop out of the growing areas as quickly as possible.

As an example of how persistent the threat of infestation in dates can be, it is cited that one of the greatest worries and problem areas in the past has been to keep insect infestation within the permitted levels imposed by the Food and Drug Administration of the US, for the annual bulk shipment to the US (up to 15,000 tons) of dates from the Shatt-el-Arab region, in spite of the most strict precautionary measures from harvest to delivery.

The major techniques to prevent and contain insect infestation are four: (i) fumigation (ii) heat treatment (iii) cold storage and (iv) irradiation, of which fumigation is by far the most applied technology. Heat treatment and cold storage are rather "benefits in disguise" when these are applied to dates for other reasons, and not solely for the purpose of containing insect infestation.

Irradiation may be classified as an effective but a not yet for other reasons materialized technology.

(i) Fumigation: Fumigation consists of exposing dates under an airtight cover or in a container or store room to a noxious gas at the appropriate temperature and time with the aim of killing insect life in all its stages of development: egg, larva, pupa and adult. Amongst the various fumigants (solid, liquid and gaseous) that have been used over the last 70 years or so, the main fumigant now in use is methyl bromide (CH3Br). It is a gas (boiling point -10 C), highly noxious to insects but also to man, heavier than air, non-inflammable and almost non-corrosive.

With regard to fumigation by gases two types can be distinguished: at atmospheric pressure and under vacuum. The penetration of the gas in the vacuum method is more intense, the time of treatment is reduced and more dictated by the time of evacuating the air than dispersion of the gas, but the investment costs are much higher. Capacities range from small batch cabinets to large drive-in chambers of several tons capacity. Fumigation at atmospheric pressure can be subdivided in fumigation in temporary enclosures, like under tarpaulin or plastic and in permanent storeroom space equipped for fumigation with airtight doors, and circulation and exhaust fans. The practical, average dose of effective fumigation of dates has been l lb of gas/1000 ft3 of storerooms for 12 hrs (which corresponds to 15 grams/m) at 15 C. If the temperature is lower, the amount of gas has to be increased (25% for every 3 C drop) but can be reduced when time of treatment is increased.

A second fumigant which has gained popularity over the last thirty years or so is hydrogen phosphide more commonly known under the trade name Phostoxyn. Tablets of a standard format, consisting of aluminium phosphide, ammonium carbamate and paraffin, upon contact with the air and depending on temperature and humidity, release hydrogen phosphide, the active component. The ammonium carbamate is decomposed into ammonia and carbon dioxide, jointly acting as a warning and fire suppressing agent. The residue of the tablet is a powder which can be removed after treatment. One standard tablet of 3 g (there are also available pellets of 0.6 g) will release 1 g of hydrogen phosphide. Compared to the use of methyl bromide, which is fast in its action, the use of hydrogen phosphide is slow and it will take at least 48 hours before the gas has fully developed. On the other hand, the application is very simple and also suitable for small containers like airtight bags and small storerooms without the need for special equipment, and where the time factor is not important. Effective dosage for the fumigation of dates is 50 to 60 tablets per 1000 ft3 of storage space, which corresponds to 1.5 to 2 g gas/m3.

Apart from effectiveness in killing all stages of insect life (which is also correlated to the type of insects involved), a common point for consideration is the residual amount of gas remaining in the date after treatment. Both methyl bromide and hydrogen phosphide when properly applied and aerated after treatment stay within the limits by law for these residues, though for the latter fumigant the time involved for dissipation could extend to 9 days (429 and 384). In summary therefore methyl bromide and hydrogen phosphide provide two effective fumigants with slightly different fields of application: methyl bromide for large-scale quick turnover treatments and hydrogen phosphide for simpler applications on small lots in storage conditions where the time factor is not that important.

(ii) Heat treatment: as intimated before, heat treatment would not normally be applied in isolation for the single purpose of killing insects though the idea has been tinkered with in order to replace pesticides and fumigants in consequence of tightening residue regulations and increasing insect resistance towards these insecticides. It may therefore be useful to review what kind of time/temperature relations are required to cause a 100% insect kill in all life stages, illustrated by the following examples, summarized in Tables 9, 10 and 11 (53, 55).

Table 9
Lethal times in minutes for 100% mortality of different stages of Carophilus Hemipterus, L (Dried Fruit Beetle), exposed to 40 - 60 C (70% Rel. Humidity)

Temperature C

Stage 40 45 50 55 60
Egg 1080 240 25 10 5
Larva 5760 240 35 17 10
Pupa 4320 210 30 20 15
Adult 9060 480 25 20 10

Table 10

Lethal time in hours for 100% mortality of different stages of the Fig Moth (Ephestia Cautela (Walker))
exposed to temperatures of 40 -60 C, and 20% and 70% rel. humidity (R.H.)

  20% R.H.

70% R.H.

Stage 40 50 55 60 C 40 50 55 60 C
Egg 15.0 1.50 0.50 0.33 15.0 3.00 0.50 0.33
Larva 18.0 1.75 1.17 0.50 18.0 1.50 1.11 0.58
Pupa 10.0 4.00 0.75 0.50 10.0 3.00 0.75 0.50
Adult 12.0 1.25 0.66 0.41 12.0 1.25 0.50 0.33

In practical applications these treatments will be somewhat longer because of the time required to reach the target temperature in the dates. Above results were obtained by directly placing insects in the environment indicated. It is further noted from Table 10 that relative humidity does not seem to have a great influence on the time, though in practice higher humidity would have a shortening effect on the heating-up period because of better heat conductivity.

A variation of singular heat treatment is to combine it with vacuum which would have importance for preserving vacuum packed dates mainly in small retail packs.

The following table gives the results of tests along this line, including comparative figures where heat treatment was given without vacuum (54):

Table 11
Exposure time in minutes required for 100% mortality of different stages of Ephestia Cautella under the effect of temperature alone (T) and temperature-cum-vacuum treatment (T+V) (vacuum 25-30 mm Hg abs.)

  45°C 50°C
Stage T T+V T T+V
Egg 900 190 180 40
Larva 1080 40 90 30
Pupa 600 40 180 20
Adult 720 15 75 20

Though not all figures under T correspond with Table 10, these results show enough of the effect of added vacuum treatment, i.e. sharply decreasing the lethal time, though one may still remain amazed that an insect can resist 20 minutes at 50 C in a vacuum of 30 mm abs.

Other empirical references on heat treatment with the aim of destroying insect life recommend 2.5 hrs at 54°C, 30 minutes at 65oC, or 20 minutes at 71°C.

The lethal temperature for microorganisms is generally higher than for insects especially when it concerns spores of the spore forming bacteria. Complete sterilization which would require temperatures over 100°C is therefore impractical in view of the damage caused to the date. However at the level of 60-65oC a partial pasteurization will take place. Moreover at the usual moisture levels of the date i.e. below 24%, microorganisms have no chance to redevelop.

More or less the same applies to enzymes, which, each within his own range have a minimum temperature below which action is stopped, an optimal activity range, usually between 35-45oC and an upper range usually starting at 50oC at which gradually the enzyme activity decreases. Most enzymes are inactivated at 80-90°C.

The above figures tell us that heat treatment of dates, provided it is applied at a maximum of 60-65oC may have the combined beneficial effect of destroying insect life, reducing the microbial count and decreasing enzyme activity, all factors that work in favour of creating a product with a prolonged storage life.

(iii) Refrigeration: Cool storage is applied to prolong storage life of dates by stopping or retarding biochemical and biological processes. Thus, enzymatic reactions slow down as well as the activity of microbial and insect life. The extent to which this reduction of activities goes is governed by three parameters: the inherent charateristics of the date, the microbe or insect, to the moisture content of the raw material, and to the temperature. An example of the latter two is shown in Figure 43 which shows the relation of temperature and moisture content for good quality storage of Deglet Noor (478). For instance, dates of 20% moisture can be kept well for a year at 40 F (4.5 C), whilst dates of 30% moisture would not keep longer than 4 months at the same temperature. Maintaining the correct relative humidity in cold storage at the prevailing temperature is important in order to prevent either drying out or moisture uptake of the date (unless these are packed in airtight containers).

Figure 43: Time/Temperature/Moisture Relationships for Storage of Dates 
Figure 43: Time/Temperature/Moisture Relationships for Storage of Dates

With respect to keeping insect infestation down, as a general rule it can be stated that below 4 C no insect activity takes place, but at these levels the insects will not necessarily be destroyed.

(iv) Irradiation

A substantial amount of research has been carried out on irradiation by gamma rays to control infestation of storage insects in dates. This research has apparently been motivated by the desire to create an effective new way of controlling insects which would overcome the, reportedly, increasing problems of pesticide residues on treated fruit and increasing insect resistance to fumigants. However, introduction of radiation treatment of foodstuffs, though already accepted and practised in a number of cases, requires a long procedure of testing, especially with respect to possible after effects. Dates are no exception and available literature on the subject over the last 20 years reflects this cautious approach. Without pretending to be able to conclusively treat the subject in this chapter, a number of results are reported which centre around answering three main questions:

- at what dosage is irradiation effective against all stages of insect life, i.e. eggs, larvae, pupa, and adult,

- to what extent, if any, is the composition and quality of the fruit compromised,

- could the consumer of irradiated fruit in any way be affected.

With regard to dosage, the following examples are given:

- tests on Ephestia Cautella and Oryzaephilus Surinamensis in dates over a range of 30-100 Krad irradiation treatment showed 100% mortality at 50 Krad for the most resistant development stages of the insects,

- 25 Krad was effective in preventing the development of Oryzaephilus Surinamensis, Ephestia Cautella and Batrachedra Amydraula in dates at all stages of their development (508),

- In Ephestia Cautella and Oryzaephilus Surinamensis 100% mortality was obtained with irradiation at 20 Krad at the most damaging developmental stages of the species, except for late-instar larvae, which show a high resistance to gamma radiation (11).

The latter conclusion has given rise to proposals for combined treatments such as fumigation/irradiation (11), heat/irradiation (15, 12), cool storage/irradiation (584), all aimed at keeping radiation to the minimum and exploiting the complementary effectiveness of the control methods.

Above dosage figures may serve to indicate the order of magnitude where irradiation of dates appears effective in the control of insect infestation. As a comparison it can be stated that 100 Krad was already approved for dried fruits in the sixties in the USSR and that 100 Krad is permissible for disinfesting grain and cereal products (131) in the USA.

Concerning the compositional and quality changes the date may undergo from irradiation, the following references are of interest:

- dates treated with 50, 100 and 150 Krad gamma irradiation and stored for 3 weeks were vacuum distilled at low temperature. Both acid and non-acid fractions of the distillate were found to retain initial aroma. Also, chemically, no major changes in the components could be detected (236)

- no qualitative or quantitative change in sugars in dates irradiated with 150 Krad and stored for 3 weeks could be detected (237)

- several Iraqi varieties irradiated with gamma rays, ranging from 30 to 500 Krad, were stored at 25-35 C in wooden boxes and plastic bags. Quantit-ative and qualitative analyses at regular intervals did not show significant changes in either sugar or protein content of the control and treated fruit (52)

- chromotographic and spectophotometric analyses of extracts of irradiated (up to 270 Krad) and non-irradiated dates revealed no quantitative or qualitative changes in glucose, fructose, sucrose, proteins, amino acids, carotenoids and pectin (49)

- no significant changes in the nutritional value of irradiated dates (25 Krad) (carbohydrates, proteins and amino acids) could be detected after 3, 6, 9 and 12 months storage respectively at 20-35 C and 85-95% RH. In the same dates evaluated by a test panel of 10 judges in a triangular test (2 irradiated dates, 1 control) no sensory differences could be detected between treated and untreated fruit (508)

The above results on compositional and organoleptic changes for irradiated dates should be comforting and in favour of the introduction of irradiation.

Answering the last question, concerned with the possible effects on the consumer from eating irradiated fruits, is more difficult, because direct target tests are obviously impossible and the results have to be extrapolated from simulations on insects and animals (cells). But in a field where consumer consciousness and public opinion is highly senstive, a very prudent approach has to be taken. Some of the types of tests and results of same to prove the "wholesomeness" of irradiated dates are given below:

- 6 lots of dates treated with high dosage of irradiation (0, 625, 1250, 2500 and 5000 Krad) were each, after treatment, seeded with 200 eggs of Ephestia moths and incubated at 35 C and 50-60% RH. Dosages of 2500 and 5000 Krad caused a significant increase in softness of the fruit and lowered the rate of Ephestia development. At treatments of 625 and 1250 Krad (10 to 20 times the normally needed dosage) there was no significant difference with the control, neither was there an increase in malformed moths (191)

- possible toxilogical effects from interaction between gamma radiation (100 Krad) and phosphine residues were studied by feeding treated dates to Ephestia Cautella. Results showed a comparable development, fecundicity and fertility of the insects fed on, respectively, the control, fumigated, irradiated and fumigated/irradiated dates (194)

- irradiated dates (50 and 100 Krad) were fed to Fig Moth (Ephestia Cautella). Statistical analyses of the results showed no significant difference in: average number of larvae and pupae, adult survival, mating frequency, eggs laid per female, and egg hatch in the control and treated dates (192 I)

- long-term feeding of irradiated dates (100 and 200 Krad) on 5 generations of Ephestia Cautella did not show signficant differences in: development from egg to adult, female fecundicity, mating frequency, and egg hatch for, respectively, control and irradiated dates, leading to the conclusion that the irradiated diet had no genetic effect (192 III)

- cultured Chinese hamster ovary cells and Salmonella Typhimurium were subjected in vitro to extracts of irradiated dates following special testing techniques for mammalian cells and micro-organisms. No genetic damage was observed (442). The same result was obtained by in vivo genetic tests using hamsters, rats and mice fed on irradiated dates (454).

In spite of these apparent positive results of irradiation technology for dates, the last word has not been spoken and the underlying reasons for not having found practical applications are perhaps two-fold: the slumbering consumer resistance and suspicion towards irradiation of foods and the cost. With regard to the latter, it has been calculated that grain irradiation can be only competitive with fumigation at an annual throughput of 200,000 tons (131). There is probably only one date producing country at the moment that would have the capability to put such an amount of dates through one facility.

c. Sorting and cleaning

In any organized date handling and packing undertaking, be it on a small or large scale, sorting and cleaning must form part of the operation if a clean, homogeneous product is to be offered to the market. However, the extent to which these operations have to be applied is closely related to the way the dates are harvested and handled in the field and during transport.

As mentioned before, dates do not ripen simultaneously, neither on one particular bunch, nor on different bunches of the same palm. The time lapse between the first date to reach maturity and the last one on one palm differs with the variety and may last from 3 to 4 weeks for early maturing varieties and 2 to 3 months for the later ones (393). Assuming that the early stage of full maturity of tamr is the preferred quality for marketing, a choice has to be made between carrying out frequent selective pickings, harvesting only those fruits that have reached the desired maturity and thus, the highest economic value, or, opting for less pickings and dividing the crop in different quality classes either to be marketed at their own merit, or, to be technologically adjusted into a higher quality grade (Fig. 38). Considering some of the disadvantages connected with selective picking, such as labour cost and availability, increasing risk of weather and insect damage with the advancing season, and pressure of marketing demand, make it clear that selective harvesting at optimum maturity is not practical and feasible.

Actually in those countries where either labour costs have increased or skilled labour in the date garden has become increasingly scarce for this type of work, which is also not without danger, the tendency has been to reduce the number of pickings. For example, in California, Deglet Noor which used to be picked as many as seven times a season, is now reduced to one or two (478), with a trend towards mechanical harvesting of principally dry dates. Experimentally it was observed that all dates can be processed into a high quality product when 75-80% of the fruit on the bunch is fully mature (288).

Dates are therefore delivered to the packer/processor as a mixture of qualities which he, in the first instance, will sort and clean into the grades required by the market. Sorting in many instances will result in not more than three of four qualities, i.e. culls, 1st, 2nd and 3rd choice. Culls will go for animal feed, 3rd choice may be used for date products, whilst first and second quality will be retained for packaging. However, for the Deglet Noor of North Africa and California the classification has become much more specific as shown in Table 12 (363).

Table 12
Classification and treatement of Deglet Noor for Export (North Africa)

Classification and treatement of Deglet Noor for Export  (North Africa)

Besides the resulting types of products, this diagram also shows how and with what means the packer/processor will technologically adjust the quality of the fruit, i.e. dehydration, artificial maturation and hydration.

Sorting is principally done by eye, whether on fixed tables or along moving belts (Fig. 44), quite often in two stages. Parameters for sorting may be colour, texture, size, moisture content and blemishes. Average capacity of one grader (South Algeria) is about 200 kgs per 8 hour shift, which, however, is increased to 400 kgs when use is made of moving belts (363), which should not move faster than about 9 m/minute.

 Figure 44: Date Grading on Moving Belts (Iraq, Oman)
Figure 44: Date Grading on Moving Belts (Iraq, Oman)
Figure 44: Date Grading on Moving Belts (Iraq, Oman)

Dates, because of their shape, size and sometimes stickiness, do not lend themselves well to mechanized sorting. Moreover, in contrast to the eye of an experienced grader, a machine can focus only on one aspect of quality. To cover the combined aspects which determine the ultimate quality grade, the date would have to undergo several mechanized operations. Some experimental success was obtained with a diverging roll sizer, and a system based on resilience of the date (228).

Separation on colour basis is not sufficiently selective to justify sophisticated colour sorting equipment and separation based on difference in specific gravity could only have some importance for sorting of immature dates (228). An experimental separation system based on applying different degrees of vacuum could remove up to 98% of high moisture fruit (109).

It would appear from the above that date sorting and grading will have to rely for the greater part on the human eye and on the efforts to make the crop more uniform in order to reduce the degree of necessary sorting.

Cleaning dates runs from simple hand-operated water spraying by hose of the fruit in baskets or wiremeshed trays, to the more mechanized dry or wet cleaning systems for larger operations.

Dry cleaning of dates is done by moving dates over damp towelling, on mechanical shakers or inside inclining, slowly rotating cylinders, also lined with towelling, or passing the dates over rotating soft brushes. The operations are preferably preceded by a pre-cleaning with an air blast (suction) over a coarse screen to remove a major part of coarse debris and dust. These methods are particularly suited for the more delicate fruits, but the danger of microbial build-up necessitates frequent changing and cleaning of towelling and brushes. Capacities of this type of operation are rather limited. For the larger scale date packing operations mechanical washers based on spraying the dates from water jets have now been mainly adopted.

A complete washing unit consists of an inclined feeding belt made of coarse screen which takes the dates to the enclosed washing tunnel where they are subjected to strong water sprays, in the first instance by a spray of recirculating water containing a detergent, and at the end of the tunnel by a fresh water rinse (Fig. 45). During this process the dates are turned around by the water sprays for a complete wash from all sides. Moving on, the dates will pass under a strong air blast which removes the adhering water from the date surface. Coming out of the air blast the dates are carried on over a moving belt which allows the possibility of an after grading before being collected on trays (Fig. 46) or in boxes awaiting further treatment.

 Figure 45: Mechanical Date Washer
Figure 45: Mechanical Date Washer

Figure 46: Washed Dates Loaded on Trays (Libya)

Figure 46: Washed Dates Loaded on Trays (Libya)

In Bahrain a prototype date washer/stripper/grader was developed for cleaning dates on the bunch, at the khalaal stage (Fig. 47). The unit consists of an endless overhead chain with hooks, which moves intermittently (at variable, selected intervals) over a rectangular course. Date bunches coming from the field are hooked on and are inspected for possible removal of culls or damaged fruit. These culls are dropped on the moving belt underneath and collected in boxes. Bunches now enter the enclosed washing compartment which consists of two or three positions where strong water jets are removing dust from the fruit. The last washing position uses fresh water after which a strong air blast removes adhering water from the bunch. Leaving the washing compartment, the bunches continue their course and workers alongside the moving belt will strip the fruit from the bunch which drops on the belt, where, if needed, they are subjected to further grading, before being collected in boxes. The empty bunches are removed from the hooks and replaced by full bunches to keep up the continuous process. The collected khalaal, in the case of Bahrain, were put in deep freeze for subsequent packaging.

 Figure 47: Prototype Washer/Stripper/Grader for Dates on the Bunch (Bahrain)

Figure 47: Prototype Washer/Stripper/Grader for Dates on the Bunch (Bahrain)
Figure 47: Prototype Washer/Stripper/Grader for Dates on the Bunch (Bahrain)
Figure 47: Prototype Washer/Stripper/Grader for Dates on the Bunch (Bahrain) Figure 47: Prototype Washer/Stripper/Grader for Dates on the Bunch (Bahrain)

Figure 47: Prototype Washer/Stripper/Grader for Dates on the Bunch (Bahrain)


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