GRAPE DRYING IN AUSTRALIA HOME

Grape Drying in Australia

This booklet has been prepared to provide basic information on the process of drying grapes in Australia. It should assist secondary and tertiary students, extension and advisory workers, the many people in the marketing and distribution chain, and be useful to growers and processors of dried grapes.The booklet was written by members of a committee comprising representatives from the Victorian, New South Wales, and South Australian Departments of Agriculture and the CSIRO Division of Horticultural Research, at the request of the Dried Fruits Processing Committee. Publication has been made possible with the funds provided by the Dried Fruits Research Committee.

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Metric (S.I.) equivalentsin	mm1/8	3.21/2	12.7      25.412 ( 1 ft) 304.336 (1 yd) 914.41 ha   = 2.47 ac1 cubic m = 35.3 cubic ft1 gal   = 4.5511 kg   = 2.205 Ib1 tonne = 1000 kg1 degC = 2.7degF 
Metric measurements in the text have generally been derived, with appropriate rounding off, from the British measurements used by growers in the past

A glossary of terms used in the industry is appended.

Varieties

In Australia, the main varieties of grapes (Vitis vinifera L.) grown for drying are, in order of importance, Sultana (Sultanina, Thompson Seedless, Kish-mish), Currant (Zante Currant, Carina), Muscat Gordo Blanco (Muscat of Alexandria), and Waltham Cross (Rosaki, Dattier, Regina, Malaga). The dried fruit of the last two is collectively called raisins in Australia. Occasionally, small quantities of other varieties such as Ohanez are dried.

For many years there was no systematic selection of planting material. However research within the superficially uniform population of Australian Sultana vines has led to the selection of clonal' Sultanas, which are more vigorous and higher yielding than the general Sultana population. The main clones now recommended are known as HS, and M12.

Work is continuing with clonal selection of Muscat Gordo Blanco. The clones recommended at present are S.A.138, G5, J2, C6 and C.F.O. 125.

Commercial plantings of the CSIRO bred Carina Currant are steadily increasing. It is much more resistant to rain damage than Zante Currant, but has problems of berry setting and evenness of berry size. These are areas of continuing research by the Victorian Department of Agriculture.

CSIRO has also released a potential replacement for sultana named Merbein Seedless for commercial testing. The Victorian Department of Agriculture has established a major trial, and small observational plantings have been made on district properties.

Production and vineyard area.

Statistics relating to the grape industry are given in Tables 1 and 2. Factors responsible for year-to-year yield differences include variations in bud fruitfulness poor growing conditions, disastrously poor drying conditions, and the diversion of varying amounts to wineries.

Climate

Grapes for drying purposes ideally can be grown and dried only in areas providing certain climatic requirements. These are: a high effective heat summation (the product of the mean daily temperature in degC above 10°C and the number of days) of over 2200 degree C days during the growing season September to March, a low spring frost incidence; a low spring rainfall; and a hot, rainless summer. Australian dried grape growing areas generally meet these conditions, apart from sporadic rains during summer and early autumn.

Weather records show that in most areas although the probability of rain does not increase as the drying season progresses, the frequency of good drying days (maximum temperature 27 degreesC or more) decreases. At the same time day length is decreasing, and the average relative humidity of the atmosphere is rising.

The possibility of rain during the Australian drying season led to two early developments in drying methods. First was the adoption and improvement of pre-drying treatments which originated in the Middle East . By the use of these treatments, drying times have been shortened, thus reducing the probability of rain damage and allowing more drying to be completed earlier in the season, while the average air temperature is higher. The second development was the evolution of the drying rack from individual wooden drying trays to its present structure.

A very recent development is the use of pre-drying treatments on fruit while it is still on the vine.

Harvesting systems

There are two basic harvesting systems used to produce dried vine fruit in Australia. They are the traditional Australian "rack drying system" and the developing "trellis drying" systems.

For rack drying the grapes are hand-picked into containers ( 'dip tins' or 'buckets') measuring about 36 x 25 x 18cm. The most commonly used buckets are made of sheet steel and have perforated bottoms and sides to allow free flow of oil emulsion through the fruit during dipping. Unperforated plastic buckets are used when the fruit is not to be dipped, and are becoming more popular.

One of the problems associated with hand picking is the contamination of the fruit by soil picked up in the containers. Fresh fruit which is damaged in harvesting or in subsequent handling is even more likely to retain soil particles, thus lowering the quality of the dried product.

Mechanical harvesting of fresh grapes for drying purposes, although still in the early experimental stages, may well be developed in the future.

The trellis drying system involves drying the fruit while it is still attached to the vine foliage, to the point where it can be mechanically harvested. "Finish drying" then proceeds in a similar manner to that used for rack dried fruit after it is shaken from the rack.

Drying racks

In Australia a drying rack is usually either 46 or 92 m long. It consists of 8 to 12 tiers, spaced about 23 cm apart vertically and made of galvanized wire netting 1.2 m wide with a 5 cm mesh and 1.4 mm in diameter (17 gauge). At 3 m intervals along the rack are pairs of light intermediate uprights or posts, 2.4-3.0 m high and about 1.5 m apart, carrying cross pieces that support the tiers of netting. The section of rack between two pairs of intermediate posts is called a 'bay'. If the rack is roofed, the tops of the intermediates carry the roof bearers. The wire netting is reinforced along both edges with fencing wire, and is prevented from sagging or narrowing when loaded by 1.3 m x 4.8 cm x 1.9 cm wooden 'spreaders'. The spreaders are slotted at each end to receive the edge of the netting and the reinforcing wire, and are placed across the underside of each tier at 1.5 m intervals. At each end of the rack the load is taken by two heavy end-posts, about 1.5 m apart, sloped and stayed against the strain, with about 1.5 m of their 3.7-4.3 m length below the ground. Each pair of posts carries 8 to 12 strong end cross-pieces, to which the wires and netting are fastened, with some provision at either or both ends for taking up slack.

On such a structure, even unroofed, most of the drying takes place in the shade (especially before the fruit starts to shrivel) and is effected largely by the natural movement of hot, dry air through the rack. Consequently, racks are preferably positioned on a rise, clear of any obstructions to free air flow, and well spaced apart (9-12 m). To compensate for slower air movement near ground level, the nominal 23 cm tier spacing can be varied from 36 cm between the bottom tiers to 18 cm at the top. In the later stages of drying on a rack direct solar radiation plays a larger part, particularly in the early morning and late afternoon, and more even drying is achieved where the racks run north and south.

The area surrounding the racks, called the drying green or drying ground, usually has some sort of cover, such as couch grass or rolled grass growth, to reduce wind-blown grit contaminating the fruit. The ground beneath the racks is kept clean by the use of sterilant herbicides or other methods.Before fruit is loaded onto a rack, a 'berry hessian' of jute or polypropylene mesh is placed on the bottom tier to catch loose berries. The edge of the berry hessian on each side is pinned to a wire outside and supported by the intermediate posts, about 15 cm above the bottom tier. Beginning from the next tier, the fruit is emptied directly onto the netting from the buckets, 10-14 buckets per bay per tier. Various devices ( 'tier-jacks' ), some commercial, some home-made, are used to hold the tiers apart while loading.The fruit is then spread out evenly, about one bunch thick. Extremely large, tight bunches are split from the tails. Foreign matter such as leaves is removed at this stage.

Sultanas after spreading on drying rack

Drying

Theory of Drying
The grape berry is naturally very resistant to water loss, as it has a heavy, hydrophobic (water-repellent) 'bloom' consisting of over-lapping wax platelets. The cuticle, in particular the outer waxy layer, is the structure which limits the drying rate of the grape berry.

During drying, water moves outward through the cells of the flesh and skin, then passes through the 'bloom' and into the air surrounding the berries. This air becomes very humid and its removal is essential for rapid drying. Ideal drying conditions are provided by a light wind on a hot dry day, when heat is supplied rapidly to the berries and moist air around them is continually removed.

Drying of Currants
Currants are placed directly on the rack. They require no pre-drying treatment because of their small berry size. Bunches of large, seeded berries ('bucks') are segregated because they dry more slowly and seeds are not acceptable to the trade.Better-quality Currants can be produced by shading the drying fruit from direct sunlight. Hessian side-curtains are hung on the rack, but these are removed if drying conditions deteriorate.

Drying of Sultanas on Racks
Early in the season, Sultanas may bedried by simply placing them out in the directsun, without any pre-drying treatment. In thisway 'naturals' similar to American Thompson Seedless raisins are produced. They aredark reddish brown, with a bluish tinge due tothe 'bloom' and have a distinctive flavour.

However, in Australia pre-drying treatments that hasten drying are used. Sultanas are dipped in or sprayed with an alkaline oil-in-water emulsion known to growers as 'cold dip'. The commercially produced 'grape dipping oil' used in the emulsion is a mixture of ethyl esters of fatty acids and free oleic acid.

This is emulsified in a solution ofpotassium carbonate in water.The standard cold dip solution is made upof 2.4 kg of carbonate of potash and 1.5 litresof dipping oil in l00 litres of water.

Formerly it was believed that the emulsion treatment removed the waxy bloom, butis has since been shown that almost no wax isremoved and that the effect is reversible bywashing. The mode of action seems to be aphysical or chemical modification of thestructure of the outer wax layer so that itspermeability to water is increased. Also,the skin of emulsion-treated berries appearsto be more transparent to infrared rays,which would allow a better radiant heatenergy uptake.

The major active components of the emulsion are the ethyl esters contained in the oil, but potassium carbonate is required to achieve proper emulsification and to maintain the alkalinity of the emulsion. The alkalinity contributes to the increase in drying rate and is necessary to prevent fermentation of the emulsion in a bulk dip under some conditions.

Drying times are reduced from about 4-5 weeks for untreated Sultanas, to 8-14 days for treated Sultanas dried on a rack under average conditions. Such fast drying leads to a rapid rise in sugar concentration, which inhibits the action of the enzyme (polyphenoloxidase) responsible for darkening in untreated fruit. The enzyme is localised in the skin of the berry where drying begins. The natural golden-yellow colour of fresh Australian Sultanas is thus preserved in treated grapes dried under good conditions. However, when rain or high humidity interrupts fast drying, conditions may become suitable for the darkening reaction to proceed, and darker fruit is produced.

The emulsion may be applied to the berry surface in various ways, but the aim is always the same— complete wetting of the fruit until the 'bloom' is no longer visible. Parts of the bunches not wetted will dry more slowly, resulting in 'blobs' of higher moisture content and darker colour in the final produce.

Bulk dipping is the total immersion in a large tank of oil emulsion of a frame holding 75-100 perforated 'dip tins' of fruit. The level of emulsion in the tank is maintained by daily additions from a smaller stock tank, which generally keeps the dip sufficiently alkaline. Alkalinity may be measured using a suitable pH indicator paper. For a freshly made mixture the pH is about 11. If the pH falls below 9.5, the dip is likely to ferment, so potassium hydroxide (caustic potash) is added to raise the pH to 10 or more. Usually this is necessary only when treating fruit that has been heavily damaged by rain.

Rack spraying

The most recent way of applying oil emulsion is the rack spraying method. Fresh Sultanas are spread on the rack where they are sprayed with standard strength emulsion using a specially designed multiple-nozzle forked wand. Various wand designs are available and usage varies with local preference. Spraying is best done at the end of each day, or when a rack is filled. The amount of spray applied is 450 litres per l000 buckets each of about 8 kg capacity.

The fruit may be left for several days before spraying but in the meantime will dry very slowly. Unsprayed fruit is also liable to some sunburn damage in very hot weather.

When rack spraying, it is particularly important that the fruit is spread evenly and all leaves are removed, so that total wetting of the berry surfaces is possible.

A successful modification developed by CSIRO and the Victorian Department of Agriculture is a two-spray program. The first spray consists of 2/3 strength standard emulsion, followed four days later by a 1/3 standard strength spray, both applied with a multi-nozzle forked wand. The first spray is applied at the rate of 4501itres and the second at the rate of 360-450 litres per l000 buckets.

The drying of bulk-dipped fruit or rack sprayed fruit sprayed with standard strength emulsion, can also be hastened by spraying about 4 days after the first treatment. The emulsion used is 1,/3 standard strength, and is applied by a two-jet wand at the rate of l00 litres per 1000 buckets of fruit. This equipment and rate of application are common to all respraying procedures.

Treatment after rain. If the emulsion applied by spray or dip is washed off by rain, the fruit must be resprayed as soon as the weather improves, using standard strength emulsion. Affected areas only are resprayed.

Drying on the Trellis

Trellis drying, sometimes called summer pruning, was developed by CSIRO in the late 1960's. About 1% of Sultana growers have made a total commitment to the system. These growers have adapted their cultural system and developed or purchased specialised machinery for the method.

Research has also been done with Currants, and some growers have used it with this variety. Currants are normally spur pruned, so they must be changed to cane pruning for trellis drying.

About 50% of Sultana growers have adopted the system for at least part of their property in years when rain damage occurs on the vine, when labour is scarce or as a salvage operation.

It is expected that the system will become more generally used as grower innovators gain experience, and with research workers overcome some of the practical problems.

The main barrier to adoption is the difficulty in adequately wetting all the fruit on the vine. Although CSIRO research indicates that after an early drop in yield of about 10%, there is no further decline in vine performance, the fear of a progressive yield decline is also a barrier to widespread adoption.

Trellis drying is facilitated by pre-season preparation in the vineyard. Modifications to trellis tend to separate foliage from fruit making cane cutting easier and assisting wetting with spray.

Trellis dried fruit is usually harvested with a mechanical grape harvester so that vine headlands and rows should be flat and even.

The harvesters used for wine grapes are satisfactory. Local innovators have developed smaller and cheaper tractor mounted harvesters specifically for the purpose .

The first trellis drying operation is cane cutting or summer pruning. The canes wrapped on the fruit wire in the previous year are cut with secateurs at the point they would normally be cut in winter. This is just beyond the replacement canes needed for wrapping down in the following season. When cane cutting, the leaf area must not be reduced by more than 50%.

After cane cutting, leaves immediately start to wilt and dessicate, exposing the grapes, and slowly starting the grape drying process. It is the loss of turgidity and increased air movement in and around the fruit coupled with the speed that it can be done, which makes trellis drying so appealing when rain occurs and conditions conducive to mould develop.

Normally the grapes are sprayed with drying emulsion within 2 days after cane cutting. Delay will give limp bunches which are more difficult to penetrate. The unsprayed bunches also become susceptible to sun burn.

Under some circumstances especially following rain damage, growers prefer to spray before cane cutting. This is satisfactory provided cane cutting is done within 4 days, and a follow up spray as below is applied.

For quick and even drying it is essential that all berries in each bunch are thoroughly wetted with the spray of dipping mixture. The recommended mixture is 2/3 standard strength ( 1.7 kg of carbonate of potash plus 1 litre of dipping oil in 100 litres).

When a 2 spray program is used, the second treatment must be within 5 days of the first and be of 1/3 standard strength (.8 kg of carbonate potash plus .5 litres of dipping oil in 100 litres of water).

There are usually a small number of bunches on the uncut replacement canes in the centre of the vine. In most cases these are picked by hand after spraying and dried on the rack as for conventionally harvested fruit. Sometimes these bunches are removed from the vine and laid on the trellis wire with the other fruit.

A variety of commercial and grower built machines are used for spraying trellis dried fruit. The most successful units have operated on relatively low pressures, up to 700 kPa and delivering rates of 1800 to 2250 litres of spray per .4 ha of vines. Success in trellis drying depends on complete and thorough wetting of the fruit.

Fruit will dry to the point of removal from the vine in 2-3 weeks. It is easiest to machine harvest when stems are brittle. Late in the season harvesting in the afternoon is preferable. Excessive leaves and stems incur a penalty on receival at packing houses, however most mechanical harvesters have effective winnowing fans for removing trash.Trellis dried fruit always needs finish drying. The methods used for rack dried sultanas are used, sometimes with modifications to fit with the overall trellis drying system.

Drying Gordos and Waltham Cross
Until around 1973, most Muscat Gordo Blanco and Waltham Cross had a special "hot dip" predrying treatment. This dip produced fine cracks in the grapes which increased the rate of moisture loss, considered important because of the larger berry and the later harvest date. While drying was hastened in good weather, the cracked skin led to moisture uptake and attracted vinegar flies in poor drying conditions.

Hot dip treatments have now given way to treatments based on the Sultana cold dip emulsion and applied either through the dipping process or as rack sprays. Up to l% potassium hydroxide is sometimes added to the initial treatments for these varieties.

While drying is slow, there is less risk from mould and the final product is less inclined to become sticky. Rack dehydration can be used to hasten drying and to darken fruit colour if desired.

Bulk dipping is as for Sultanas except that the fruit is held in the solution for 15 minutes instead of simply wetting thoroughly as is done with Sultanas. This treatment is followed after 4 days with a fine "touch-up" spray using a two jet wand, applying 100 litres of a 2/3 strength cold dip emulsion per 1000 buckets of fruit. A similar 1/3 strength touch-up spray is applied to any areas wetted by rain or dew but only when the weather is fine.

Rack spraying can also be used as a total system. The requirements are as for Sultanas except that 2 sprays of the standard strength mixture are applied with a fork shaped wand using 450 litres to every 1000 buckets of grapes, 4-5 days apart.

A third spray of the 2/3 strength mixture is used if necessary to maintain the drying rate.

As with the dipping treatment, a fine touch-up spray of 1/3 strength mixture with a 2 jet wand is applied when the weather is fine, to any areas wetted by rain or dew.

A small quantity of Muscat Gordo Blanco, Waltham Cross and some other varieties is dried in dehydrators without any predrying treatment. The bloom is retained and the bunches are kept intact as much as possible. These 'muscatels' or 'table raisins' are used as components of special dried fruit or glace fruit packs, and are also marketed through health-food stores.

Mould Control
In some seasons, under persistent wet and humid conditions, mould begins to spread through the fruit on the racks. The best method of arresting its spread is by using a rack dehydrator to dry the fruit to a safe stage.

This is replacing the rack sulphuring method whereby sulphur is burnt to produce sulphur dioxide. To rack sulphur, the berry hessian is removed and the rack is surrounded, on both sides and above the top tier, with a double thickness of hessian or with dehydrator curtains, the aim being to make it as air-tight as possible. Approximately 2 kg of good quality burning sulphur per bay is burnt in shallow tins. A wick of old hessian is used in lighting the sulphur, which will also burn more readily if 1% of potassium nitrate is mixed with it.

After rain it is also advisable to remove the berry hessian and its contents from the rack for separate drying. This action prevents the spread of mould and vinegar fly from wet, damaged fruit on the berry hessian.

The practice of "drying on the vine" or "trellis drying" is now commonly used when fruit is damaged in the field. This reduces the amount of damaged fruit reaching racks effectively reducing the amount of mould control work to be done on racks.

Removal of Dried Fruit from the Rack

When the drying fruit has reached a suitable moisture level, it is shaken from the rack using a mechanical rack shaker mounted on the three-point linkage of a tractor and driven by the power take-off. Readiness for shaking is indicated by the ease with which a small quantity of fruit compressed in the hand resumes its original friable condition, by the brittleness of the bunch and cap-stems, and by the absence of juice when a berry is broken and squeezed. Fruit shaken before this stage is liable to be badly damaged, and if finished off on the ground will pick up more grit.

The rack is prepared or 'dressed' for shaking by running out a 1.8 m x 46 m hessian each side of the rack to catch spillage.

Modern mechanical rack shakers are fitted with a catching tray. Alternatively the berry tier is dropped to ground level and three to five 3-m lengths of hessian per bay are laid crosswise on it to catch the fruit, as it is shaken down.

Currants are usually left on the rack until they reach 15% or less moisture, and are then boxed immediately after shaking. They may be finished in the sun if necessary, but excessive handling and exposure to direct sunlight lower quality by spoiling the appearance of the bloom.

Finish Drying

Sultanas and raisins are ready for "finish drying" at about 18% moisture content. There are three methods in use with the two artificial methods steadily replacing the sun finish drying method.

To "finish dry" in the sun the fruit is spread out on hessian or polypropylene mesh sheets 46 m long x 1.8 m wide, with sisalcraft or plastic undersheets to prevent moisture rising from the ground into the fruit. Usually one or two days of good weather are sufficient to dry the fruit down to, or below, the maximum moisture level acceptable to the packing houses. This is 15% for Currants, 13% for Sultanas, and 14% for raisins.

In the case of rain or heavy dews fruit on the ground is protected by folding over the sisalcraft or plastic sheet and weighting it down at intervals to prevent unfolding. Late in the season air humidity at night is usually high, therefore fruit on the ground is protected by folding over the sisalcraft as in the case of rain.

Early-harvested Sultanas, especially when dried on a roofed rack, may have a strong green tinge to their colour. The degree of greenness is related to the amount of chlorophyll remaining in the dried fruit. Bleaching of the green tinge can be achieved by watering the hot fruit on the hessians in the middle of the day, using a hose or watering can. About 1 litre per metre of hessian is used, the fruit rolled in and left for several hours to sweat then raked out again. With very green fruit this procedure may have to be repeated on successive days.

Bin Dryers
The Hudson bin dryer, or various modifications of it, is often used as a means of artificially finishing off fruit in lieu of sun drying on hessians. The bin dryer was specifically designed to do this job, and it is undesirable to use it for drying fruit of moisture content above 18%.

The dryer consists of two bins 6m x 2.4m x 0.46m high each with a horizontal wire mesh floor located 15 cm above ground level. Lengths of hessian are placed on the floor of the bins, which are filled with fruit to a depth of about 15 cm. On top of each bin is placed a semi-cylindrical wire frame which supports a plastic cover. The cover is made airtight by carefully securing it to the flanged top edges of the bin with pegs or clips. The two bins are positioned end to end and connected to a centrally placed heating unit equipped with a fuel burner and a fan. The dryer forces hot air through the fruit from top to bottom, and discharges the moisture-laden air through vents set in the sides of the bins near ground level.

Rack Dehydration
Over the last 10 years there has been increased interest in artificial drying of fruit on the rack, not just for salvage purposes but as a part of a system of drying management.

Dehydration of the fruit replaces the finishing off operation completely. Any green tinge in the fruit is removed, and the considerable sand contamination which can be associated with finishing on the ground is avoided. Labour costs are also reduced so much that dehydration is economically justified.

Rack dehydrator

The rack dehydrator in use today was developed by Mr W. F. Yann, an engineering consultant engaged by the A.D.F.A. The unit consists of a kerosene or liquefied petroleum gas burner and a fan capable of discharging 280 cubic metres per minute of hot air to 60 degreesC into a duct which conveys and distributes the heated air throughout the full length of a 46 m drying rack. The drying rack is made reasonably airtight by means of heavy-duty plastic curtains.

A rack dehydrator can be used to dry fruit from a quite high moisture content- as a salvage operation when the fruit is deteriorating, or as an expedient to make a rack available for refilling. Mainly, the rack dehydrator is used with fruit of lower moisture content for finishing on the rack. The fruit is dried right down to the required maximum moisture content, then shaken off the rack and delivered directly to the packing company.

Boxing

For delivery to packing sheds, fruit is'boxed', i e. placed in wooden containers 94cm x 64cm x 20cm which hold about 55-65kg.ln these 'sweat boxes' the moisture content of the dried berries equalises , and the fruit has some protection from the weather until delivery is made.

For boxing the fruit should be cold, as hot fruit in large stacks of boxes cools very slowly and may darken. Currants and Sultanas, if sufficiently dry, may be boxed immediately after shaking down. If the fruit has been finished on the ground, it is boxed from the hessians. Fruit from dehydrators is cooled before boxing.

Sweat boxes are no longer in widespread use for dried grapes and most fruit is sent to packing sheds in 1/2 ton wooden bins as shown below

Boxing machine being used on Sultanas

Bin of dried sultanas being emptied at packing shed

Quality grading

Most of the fruit is delivered to the private or cooperative packing companies over the two months from late February to late April. The sweat boxes are unloaded at the receiving door and roughly classed for stacking on pallets. A representative sample of each grade or type of fruit present in the load is taken for more thorough classing,and is kept for two weeks to allow the grower to appeal. If the fruit appears tobe too wet, the moisture content is determined using a moisture meter. Weight deductions and also dehydration charges are made according to anymoisture content in excess of the maximum levels set for each variety. Hand-picking charges may be levied on fruit damaged by mould or vinegar fly, or in which there is unacceptable foreign matter, and further weight deducted.

Grit in dried fruit is the biggest single factor detracting from the quality ofAustralian fruit. Tests have shown that soil particles smaller than about 50 micronsare not detected by chewing. Particles 50 microns in diameter can taste slightlygritty if present in quantities above about 100 mg per 100g of fruit Larger particles of about the size of fine sand (up to 250 microns) can be detected if there is as little as 20-30 mg per 100 g of fruit.

The fruit may be contaminated at a number of stages of processing. Freshfruit hanging on the vine can pick up some wind-blown grit but this is generally considered a relatively minor source of contamination. Serious contaminationcan occur if perforated dip tins are dragged over loose ground or if bucketscoated with sticky grape juice are thrown on the ground and not washed beforere-use. During dipping the soil adhering to the bottoms of the tins is distributedthroughout the fruit and tends to stick to it as it dries. Damaged berries holdmore grit than intact berries and this grit is difficult to remove.

Fruit on the rack may be contaminated with dust blown up from the dryinggreen. Maintaining a grass cover on this reduces contamination. During finishing off on hessians spread on the ground, dust can be blown onto the fruit. In windy conditions the fruit may need to be temporarily covered.

At the packing shed samples of fruit canbe tested for soil contamination using a SIROGritometer. Developed by CSIRO, thisinstrument measures the amount of soil orgrit in the fruit which would be apparent tothe consumer as a gritty taste. The method involves washing the fruit, collecting the soil ina fine sieve, separating the organic matterfrom the soil in saturated potassium carbonate, and measuring the column of soil collected in a narrow-bore plastic tube. Particlesless than 50 micron are discarded. Theamount of grit is reported in gritometer units—the number of millimetres of sand collectedin the plastic tube of the gritometer. Packingsheds impose a penalty on fruit containingmore than 100 units of grit.

Grade standards for Australian driedvine fruits are set each year by the Grade Fixing Committee of the Australian Dried FruitsAssociation following guidelines set out in theDried Fruit Regulations. Broadly, goodquality fruit should be firstly of a uniform colour, typical to the variety; the berries shouldbe uniformly well filled and of a pliable texture .

Official samples of the grades of both unprocessed and packed Sultanas, Currants,and raisins are issued to all packing companies as soon as suitable fruit becomesavailable. The grower's sample is graded byvisual comparison with the Official Samplesunder standardised lighting conditions. EachOfficial Sample represents the minimumstandard required for that grade. Gradesrange from one to seven 'crown'. The higherthe number of the crown, the better the quality of the fruit and the more uniform its colour.

Processing

Processing of dried grapes involves theseparation of the good fruit from stems, capstems, poor fruit, grit, and other foreign matter.

The main bunch stems are first separatedfrom the fruit when it passes through a coarseriddle. Fruit is then lifted by an elevator anddropped onto the upper, smaller end of a vertical, rapidly revolving (500-700 rpm) metalcone. The cone is surrounded at a distance of20-22 mm by a stationary open-ended cone-shaped 'cage' of heavy wire mesh. As the fruitfalls it is bounced between the cone and thecage and the cap-stems are thus beaten off, atthe cost of some damage to the skin of the berries .

From the stemmer the fruit is conveyedto various shaking riddles of different openings, which together with blowers separate out the cap-stems, undersized or empty berries, any remaining clumps of berries, andvery large or very small particles of foreignmatter. Some heavy particles are also removed, and iron and steel objects picked up bymagnets. The fruit next falls onto a broadslow-moving belt for hand-picking of anyundesirable fruit or foreign matter notremoved by the machinery. Hand-picking isfollowed by washing, often in the recently introduced riffle washers. The fruit passesthrough a large, slowly revolving mesh drumwhere it is heavily sprayed with water, thenflows in a stream of water over a series of riffles which separate the heavier foreignmaterial.

Riffle washer in action

At the end of the riffle is a de-watering screen. Further de-watering takesplace in a spinner and finally, 0.2-0.3% byweight of paraffin oil or a stabilised vegetableoil is sprayed onto the fruit as a dressing, toimpart an attractive gloss and to preventstickiness. Ethyl formate (Eranol) is addedat this stage for control of insects in the fruit.

At this stage the seedless varieties arepacked for sale to wholesalers or retailers inAustralia or abroad and the final pack is inspected by officers of the Department ofPrimary Industry for compliance with theDried Fruit Regulations. However, before raisins are packed further processing is required to remove theseeds. Most packing companies do not de-seed the relatively small amount of raisinsthey receive, but after initial processing sendthe fruit to a central processor, Co-operatedDried Fruit Sales. After de-seeding and packing, export raisins are also inspected beforeconsignment.

Pest control in stored dried vine fruit Insects can cause considerable damageto dried vine fruits if not controlled. The threespecies of insects mainly responsible are:Plodia interpunctella (Indian meal-moth)Ephestia fugulilla ( raisin-moth ) Silvanus surinamensis(saw-toothed beetle).

As mentioned above, Eranol (ethyl formate) is added to each carton of cleaned fruitfor insect control. Because this control is of atemporary nature, packed fruit is further protected from insect infestation by fumigationat regular intervals with methyl bromide gas.The methyl bromide is introduced under atarpaulin which covers the stack of packedfruit during fumigation. The usual dosage is2.4 kg of methyl bromide per 100 cubic metres2 lb per 1000 cubic feet) at 15°C or above. In addition, packing-house premises arekept free of insects by regular fogging withpyrethrum mist (pyrethrins and piperonylbutoxide) produced by special fogging-guns.

To prevent migration of larvae(caterpillars) from unprocessed fruit to car-tons of packed and cleaned fruit, lines ofgrease containing pyrethrins are put on thefloor around stacks of unprocessed fruit.

GLOSSARY

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