Grape Drying Principles in Relation to Trellis DryingP. R. Clingeleffer. C.S.I.R.O. Division of Horticultural Research, Merbein

Presented to the 2nd Dried Fruits Research Trust Seminar, December 1985

IntroductionAustralia's main dried fruit markets require high

quality, light coloured sultanas. These are pro-duced by dipping or spraying the fruit with analkaline oil-in-water solution to hasten the dryingand prevent darkening of the fruit. The main prin-ciples involved in this process, particularly forrack drying have been discussed by Grncarevicand Lewis (1 976). This paper briefly coversthese principles and draws together furtherresearch results relevant to the system of trellisdrying

A. BASIC PRINCIPLES

l Thermodynamic considerationsThermodynamic parameters which affect the

movement of water between a grape berry andthe surrounding air have been discussed bySzulmayer (1 973). In summary, water move-ment from the berry is dependent on its availabili-ty at the berry surface, the rate of transfer beinggoverned by differences in vapour pressure (VP)between the fruit and the surrounding air (i.e.vapour pressure differential,(>'VP)

For an emulsion-treated berry 3 stages of dry-ng are recognized. In stage 1 , water evaporatesfrom the berry surf ace at a constant rate as iffrom a body of water. The only decrease in dry-ing rate is due to berry shrinkage. In the secondstage, water movement within the berrybecomes the limiting factor and the drying ratedecreases as the berry approaches ''dryness'Under normal ambient conditions on either therack or trellis the P'VP is small at this stage ofdrying and ''finishing off ' is necessary to reducethe moisture level to the 1 3%, required for pack-ing. In the 3rd stage of drying ''bound water '' islost by volatilization of sugars, normally at a slow

Differences between rack, trellis and grounddrying and dehydration can be related to ther-modynamic parameters. On the rack, drying oc-curs under ambient conditions. The berrytemperatures are not elevated above airtemperature except briefly in the early morningand evening when fruit is exposed to direct solarradiation. Good drying conditions are achievedwith high air and fruit temperatures and lowhumidities which create a large >VP. Duringperiods of high humidity or rain uptake ofmoisture can occur, in particular when airtemperatures are quite high.

In the trellis drying system, more of the fruitmay be exposed to direct solar radiation for alonger period than on the rack. The direct inputof solar radiation raises berry temperaturesabove ambient. increases the P'VP and conse-quently the drying rate. Berries within the bunchmay also receive some solar radiation as it istransmitted through a number of berry layersIClingeleffer 1984). Thus more rapid dryingrates may be achieved with trellis drying,provided emulsion application has beensatisfactory.

When berries are dried on the ground in-terception of solar radiation is maximized and theresulting high berry temperatures and low airhumidity result in large >'VP values.

rate

Differences between rack. tro I s and ground drying can bo related to tnermodynamicparameters.

For example, during March berry outerlayerof waxplateletssothatitspermeabili-temperatures may exceed 60'C and relative ty to water is increased (Grncarevic and Lewishumidity values approach zero close to the 1976, Possingham 1972). As water is rapidlyground sheet. At night little movement of lost from a treated berry, the sugar concentra-moisture occurs, even under dewy conditions bon rises rapidly and inhibits the enzymaticbecause air and fruit temperatures are both low darkening associated with polyphenol oxidaseand consequently P'VP values are small which occurs in slower drying untreated fruitIClingeleffer 1 984). Should rain and high humidity interrupt drying,

darkening also occurs (Grncarevic and Lewis

To be completely effective the emulsion mustcover all berry surf aces. If not, the berries drymore slowly and become 'blobs' of highermoisture content. Clingeleffer et al. (1980)found that excessive 'blob ' numbers, not only in-creased the number of dark berries but causeddarkening of the surrounding light colour fruitdue to moisture equilibration in storage. Blobs,may also contribute to sugaring and excessivedamage during processing

Thus, it is obvious that in the system of trellisdrying, emulsion application is vitally important.Growers should aim to completely cover all ber.ries to ensure a high quality product, free of darkberries, blobs and 'bloomy ' fruit

2 1976)Solar radiation considerations

The rack, trellis and ground drying techniquesare effective drying systems which rely on solarradiation energy either directly or indirectlythrough its influence on air temperature andhumidity. Inputs of solar radiation decreaserapidly over the drying period due to the combin-ed effects of decreasing light intensity andshorter days. Clingeleffer (1 984), showed thatthe maximum value recorded for total daily radia-tion in 1978 dropped from 9.0 x IO ' (Whm:) inearly February to 3.5 x I O ' (Whm:) at the end ofMay. These differences cause slower drying asthe season progresses, because P'VP valuesare less (e.g. for ground drying, berrytemperatures on clear days in early March, Apriland May were 65, 55 and 45'C respectively,(Clingeleffer 1 984)3. Emulsion effects

B. FIELD EXPERIMENTSGrower experiences with trellis drying in the

early 1 970's indicated a need to improve the ap-plication of drying emulsion to maintain fruitquality and minimise costs. Although excellentfruit was produced with high volume recyclingspray units the volumes used. i.e. greater than7000 1/ha, were regarded by many as excessiveand uneconomic. Vine management and emul-sion application studies were made to in-vestigate these problems

Untreated grapes lose water very slowly asthey are covered by a hydrophobic bloom con-sisting of irregular intersecting and overlappingway platelets which act as a barrier to watermovement. The mode of action of the alkaline oil-in-water emulsion appears to be a physical orchemical modification of the structure of the

1. Preliminary emulsion applicationstudies

A wide range of spray nozzle types, flow ratesand droplet sizes were tested in preliminarystudies. It was shown to be impossible topenetrate inside large bunches by direct spray-ing. It was necessary to apply sufficient emul-sion to the top of the bunch so that it could flowthrough to all berries. This problem was furthercompounded with the standard trellis systemsby the clumping together of bunches wherecanes from adjacent vines met. Further ex-periments with spray equipment fitted with 'canelifters' to lift the foliage and open up the buncheswere unsuccessful. The use of foam as a carrierfor the drying emulsion was also unsuccessful.Best coverage was achieved by spraying atclose range while moving the fruit.

5. Emulsion quantity experimentsStarwheel sprayer experiments indicated that

at least 4000 1/ha of spray emulsion were need-ed to give an adequate drying rate, cover all ber-ries and ensure good quality fruit (Clingeleffer etal. 1 977). The vines used in these experimentshad a similar yield to average vines in the districtli.e. 5 tonne of dried fruit per hectare) sug-gesting that approximately 800 litres of emulsionis required to dry a tonne of trellis dried fruit. Thiswould be equivalent to 6000 1/ha for a higheryielding 7. 5 t/ha (3t/ha) crop. Standard rack dry-ing of 7.5 dry tonne would have used approxi-mately 940 1/ha (1 25L per dry tonne. May et al.1983) if "bulk dipped" or 1880 1/ha if racksprayed. Thus trellis drying appears to requireapproximately 3 times as much emulsion pertonne of fruit as rack spraying. This is not unex-pected if one takes into account the non-uniformity of fruit spread along the trellis (i.e.only one half of the wire is filled with fruit as thereis a space around the trunk and basal parts ofcanes) and loss of emulsion on leaves, trunksand canes.

periences relate to differences in spray equipment, vine vigour and production.

C. LABORATORY EXPERIMENTSSmall scale laboratory experiments (some un

published) provide some interesting resultsrelevant to the system of trellis drying

1. Emulsion quantity effectsMay et al. (1 983) treated sultana berries with

different quantities of emulsion by partial or totalimmersion, spotting from a micropipette andspraying. The results show that the amount ofemulsion to produce light coloured fruit is muchless than that adhering to a fully immersed berryI'dipped '). Thus it should be possible to avoidproblems associated with excessive amounts ofemulsion on dried berries (i.e. uptake ofmoisture in storage during periods of highrelative humidity which then leads to darkening,sugaring and increased stickiness of the fruit,May et al. (1 973) by trellis drying and reducingthe quantity of emulsion retained per berry

In these experiments 37 ul of emulsion (85%accumulating as a run-off droplet) adhered to adipped berry. This would correspond to 125 1per dry tonne of fruit which is similar to that usedwith commercial dipping. Approximately 9 ul ofemulsion (30 litres per dry tonne) placed byspotting or part-immersion gave similar fruitquality to complete immersion despite slower in-itial drying rates. Lower quantities lead to slowerdrying and darker fruit. When the emulsion wasapplied by spraying, the best treated, 1 .5 ul ofemulsion per berry (5.0 litre per dry tonne) wasalmost as good as that produced by immersion,indicating the potential to reduce emulsion quantines if spray technology and bunch penetrationcan be improved

2. Vine management to improveemulsion application

Clingeleffer (1 981 ) summarises results with anumber of trellis systems which are suited tomechanisation but also spread the fruit, reduceclumping of bunches, favour small bunchdevelopment and separate the fruiting andreplacement shoots. These systems improvethe efficiency of emulsion application andminimise wastage on leaves. The trellis systemsare arched cane pruning, the split system of vinetraining on a wide T-trellis, hanging canes andthe swing-arm trellis. The use of fruiting canes ofdifferent lengths on standard trellises can alsobe used to avoid clumping of fruit.

6. Spraying before or after canecutting

A divergence of opinion among growers as tospraying before or af ter cane cutting exists.Some favour spraying about 2 days af ter cuttingwhen the leaves have wilted while others spraybefore cutting to maintain an open bunch andberry turgidity. In the starwheel sprayer ex-periments no difference was found when thesemethods were compared (Clingeleffer et al.1 977). A follow-up second spray af ter somedrying had occurred was also found to be un-necessary. It is likely that the grower ex-

3. Spray equipmentHand held spray nozzles were first used for

trellis drying. These were very slow and labourntensive. The starwheel spray (Clingeleffer etal. 1 977) was designed to spray bunches fromall sides, including from inside the canopy, tomove the fruit to facilitate emulsion spread andminimise emulsion quantities, in particular thewastage on foliage. (it is interesting to note thatthe starwheel principal is now being used inEuropean equipment designed to apply her.bicides under vines). The high volume, recyclingspray equipment in use today is used to producef ast drying, high quality fruit. Growers now areless concerned with the high volumes of emuldion used(i.e. a minimum of 7000 1/ha) providedfruit quality is high. These sprayers do wasteemulsion on leaves. Costs are decreased byreducing component concentrations in the emulsoon

4. Emulsion concentrationexperiments

Starwheel sprayer experiments were con-ducted using normal (2% oil, 2.5% K:CO '),0.75, 0.5 and 0.25 emulsion strengths(Clingeleffer et al. 1 977). Low strength emul-sions (i.e. O.$ and 0.25 Of normal) producedslower drying and darker fruit as the emulsiontended to run off the berries and spread poorlyThe normal and 0.75 treatments were similar toeach other. Laboratory experiments, reportedlater in this paper indicate that the oil componentcan be reduced to a low level (i.e. to about onequarter of normal strength) but that thepotassium carbonate component needs to bemaintained . Trellis Dried Fruit on the Vine

2. Movement of emulsion on berrysurface

Grncarevic and Hawker (1 971 ) and May et al(1 983) showed that complete cover of the berrysurf ace with drying emulsion is essential to pro-duce a high quality product and a fast dryingrate. May et al. (1 983) observed that emulsionspread from treated areas to untreated parts ofmature berries. Complete cover was achievedwhen sufficient quantities of emulsion were ap-plied to one part of the berry by part-immersionor repeated spotting. The spread of emulsionwas linearly related to sugar content ('Brix).This effect helps to explain the excellent resultsthat can be achieved with trellis drying, in particular with the high volume recycling spray unitswhich apply sufficient emulsion to move throughthe bunches. It also provides a reason for poorresults with immature fruit and differencesbetween seasons. May et al. (1 983) suggestedthat growers test their emulsion formulations andfruit maturity effect by partial immersion (1%4

berry) and observing the emulsion spread overiy2 hr period.

4. Spray droplet sizeA comparison between small and large

droplets for emulsion application by sprayingli.e. 190 and 440 um mean diameter) showedthat small droplets were about 20% more effec-tive than large droplets, resulting in faster dryingand better fruit quality (May et al. 1 983). Underfield conditions, bunch penetration, wind driftand droplet evaporation would effect such acomparison and may give a different result.

to test the effect on droplet spread and subse-quent oven drying rate. Of the 3 berrytemperatures tried (1 5, 25 and 35'C), the dry-ing rate was slowest for the 1 5'C applicationtreatment, indicating that early morning sprayingshould be avoided. Of the 4 emulsiontemperatures (15, 25, 35, 45'C), the 15 and45 'C treatments had slower drying rates. Thus,there would appear to be no advantage inheating the emulsion to improve the spread ofemulsion

5. Reversibility of the emulsionaffect

Grncarevic and Lewis (1 976) reported thatthe emulsion effect is reversible and that theemulsion is easily washed off the grapes by rain,especially in the early stages of drying. More re-cent studies (Clingeleffer and Newman unpublished) indicate that reversibility of the emul-sion effect occurs right up to the final stages ofdrying, when water movement inside the berrybecomes the limiting factor rather than move-ment through the cuticle. Thus after heavy rain,growers must consider re-spraying or beprepared to accept slower drying and darkerfruit

8. Timing of emulsion applicationGrowers who may be unable to spray im-

mediately af ter cane cutting (e.g. due to rain)may wonder how long they can leave the fruituntreated and what effect this may have on quality. A laboratory experiment where untreated fruitwas dried at 40'C, then dipped at differentstages of weight loss, was established to investigate this problem. A response to emulsionapplication on drying rate was found up to thelatter stages of drying (i.e. up to 50% weightloss). However, lower fruit quality was observ-ed, due to fruit browning if the emulsion was applied beyond a weight loss of 20%. In reality,delays of more than one week of natural dryingshould be avoided

3. Maturity effectsAs reported above, the spread of emulsion

and its effects on drying rate are dependent onmaturity. However, the results reported by Mayet al.(1983) also indicate that even with dipping.consistent golden fruit of high quality could onlybe produced with mature fruit. i.e. 22' Brix. Im-mature fruit always had an ''olive green tinge ''. Iftrellis dried fruit is mechanically harvested andthen ''finished off '' in a dehydrator the greentinge, due to presence of cholorophyll, will re-main and lower fruit quality. It can be bleachedout by ''finishing off '' on groundsheetsIGrncarnevic and Lewis 1 976)

6. Emulsion formulationFactorial experiments in which the oil and

potassium carbonate components of the emul-sion were varied indicate that the cost of theemulsion can be decreased by reducing the oilconcentration. Satisfactory oven drying rate andfruit quality was achieved with oil concentrationsone quarter of normal (i.e. 0.5%) providedpotassium carbonate levels were maintained.

D. CONCLUSIONThe results presented in this paper indicate

that high quality, light coloured sultanas can beproduced using the trellis drying system. Toachieve this, complete emulsion coverage of allberries is essential. Factors which must be con-sidered to produce a consistent, quality productare discussed. The results also suggest thatcosts of emulsion can be lowered by alteringemulsion formulations and improving equipmentfor its application. While drying should be begunas early as possible, growers should ensure thatthe fruit is fully mature to minimise green tingeand poor spread of emulsion on berries

7. Temperature effects on emulsionspread

Fruit and emulsion temperatures were varied

References

Clingeleffer, P. R. 1 981 CSIRO sultana vine management research.Aust. Dried Fruit News, 8 (5), 4-9Clingeleffer, P. R. 1 984 Effects of time of season, fruit depth and coverng at night when ground drying on acceptable moisture content ofsultana raisins. J. Sci. Food Agric., 35, 1 73-81 .Clingeleffer, P. R., Trayford, R. S., May, P. and Brien, C. J. 1977. Useof starwheel sprayer for applying drying emulsion to sultana grapes to bedried on the trellis. Aust. J. Exp. Agric. Anim. Husb., 17. 871-80Clingeleffer, P. R., May, P. and Brien, C. J. 1 980. The effects of mixingturgid ('blob ') and dried sultanas during storage. Fd. Technol. Aust.. 32.332-5

Grncarevic, M and Hawker, J. S. 1 971 . Browning of sultana grape ber-ries during drying. J. Sci.Fd. Agric., 22. 270-2Grncarevic, M. and Lewis, W. J. 1976. Drying of grapes in Australia. Fd.Technol. Aust., 28. 66-67. 69-71 . 76May, P., Clingeleffer, P. R. and Simons, 1. D. 1 983. Effects of varyingthe method of applying alkaline emulsion on the drying on sultana grapesJ. Sci. Food Agric., 34, 121 5-28Possingham, J. V. 972. Surf ace wax structure in fresh and dried sultanagrapes. Ann. Bot., 36, 993-6Szulmayer, W. 1 973. Thermodynamics of sun drying. Proc. Int. CongrThe sun in the service of mankind Unesco House, Paris, 24, 1-10

CSIRO Division of Horticultural ResearchReprinted from Australian Dried Fruit News ns. 14(2)3 Dec 1986