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SOLAR DRYING OF GRAPESW. Eissena; W. Mhlbauera; H. D. Kutzbachaa Institute of Agricultural Engineering Hohenheim University, Garbenstrasse

Online publication date: 27 September 2010

To cite this Article Eissen, W. , Mhlbauer, W. and Kutzbach, H. D.(1985) 'SOLAR DRYING OF GRAPES', DryingTechnology, 3: 1, 63 74

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DRYING TECHNOLOGY, 3 ( 1 ) , 63-74 (1985)

SOLAR DRYING OF GRAPES

W. Eissen, W. Muhlbauer and H.D. KutzbachInstitute of Agricultural EngineeringHohenheim University7000 Stuttgart 70, Garbenstrasse 9Federal. Republic of Germany

KEY WORDS AND PHRASESB at ch ; d r y i n g r a t e ; e c on om ic s; l o s s e s ; m e t e o r o l o g i c a l i n f l u e n c e s ;r a i s i n s ; s t or a ge ; t e m p er a tu r e; v e n t i l a t i o n ; w a te r p e r m e a b i l i t y o fs k i n ( i n c r e a s i n g ) .

ABSTRACT

Mass losses and low quality are the most serious dis-advantages of traditional grape drying methods. For theproduction of high quality raisins an increase in thedrying rate is required and the grapes should be pro-tected from rain, dust and insects during drying.Under the terms of a joint German-Greek research pro-gram low-cost solar grape dryers were developed in theInstitute of Agricultural Engineering of HohenheimUniversity and were tested in Greece in cooperationwith the Crete Agricultural Research Center.The required data basis for optimizing solar grapedryers was obtained by additional laboratory tests mea-suring the influence of various drying parameters ondrying rate and quality.Tests with the solar dryers have shown that it is pos-sible to reduce the drying time and improve the quali-ty significantly compared to the traditional dryingmethods. Also mass losses due to rain can be prevented.

CopyrightO 985 b y Marcel Dekker, n c . 0737-3937/85/03014063S3SO~O

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EISSEN, MUHLBAUER, AND KUTZBACH

The solar dryers can be constructed by the farmersthemselves or by craftsmen using locally availablematerials and simple tools.

INTRODUCTION

The world raisin market demands a hiqh quality product.The raisins must be absolutely free of foreign materi-als and insects and in addition must be of uniformcolour and size.In Mediterranean countries raisins are produced mainlyon small farms with an acreage of less than one hec-tare. About 8 tons of fresh grapes are harvested manu-ally with a moisture content of 78 70 80 percent w.b.To increase water permeability the grapes are dippedin an alkaline solution before drying. For storage con-ditions the qrapes must be dried to a moisture contentof 14 percent w.b., meaning that 3000 kg of water mustbe removed per ton raisins.For drying, the qrapes are traditionally spread on theground where they are exposed to sun and wind. Naturalsun drying results in low-quality raisins because theyare exposed to rain, dust and insects. Furthermore,direct exposure to solar radiation results in undesi-red colour changes. There is also the risk that largepercentage of the crop can be destroyed by heavy rain-falls.To increase the income of the raisin producers it isessential to develop drying systems with which hiqhquality raisins can be produced. The investment andoperating costs of mechanical dryers are too prohibi-tory, considering the socio-economic situation of thesmall farmers.

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SOLAR DRYING OF GRAPES

An analysis of the meteorological conditions in theMediterranean countries during the drying period andthe drying behaviour of the grapes showed that solardryers may be a possible alternative to the traditio-nal methods.

OBJECTIVESThe general purpose of the study described in this pa-per was the development of low-cost solar dryers forgrapes. The specific objectives were:

conducting laboratory drying tests to determine theinfluence of air temperature, air velocity and che-mical pre-treatment on drying rate and quality,development of various types of solar dryers of dif-ferent drying capacities using locally availablematerials,field tests of the newly developed solar dryers withrespect to drying time, product quality and relia-bility,economical evaluation of the solar drying systemsin comparison with the traditional methods.

LABORATORY TESTS-For optimal design of solar grape dryers it is neces-sary to investigate the effects of the drying-air tem-perature, air velocity and chemical pre-treatment onthe drying rate and quality. Laboratory drying-testswere carried out to determine the drying behaviour ofthin layers of freshly harvested grapes (Eissen, 1 9 8 4 ) .Figures 1 to 3 show the results of the thin layerdrying tests:As shown in Figure 1 drying-air temperature has a greateffect on the drying rate. For example an increase in

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6 6 EISSEN, MUHLBAUER, AND KUTZBACH

Figure 1 :Influence of the dryingair temperature on thedrying rate of sultanagrapes

83Bi m-6 LO;.Ie

00 20 30 LO SO

Dry~ng m. hours

Figure 2 and 3: Influence of the air velocity and thechemical pre-treatment on the dryingrate of sbltana grapes

temperature from 40 to 60C reduces drying time from90 to 20 hours. However the increase in temperature islimited to approximately 70C, since higher drying-airtemperatures result in juice leaving the berries andcaramelization of sugars. This phenomenon causes lowquality raisins.Drying rate is also affected by the air flow-rate(Figure 2 ) . An increase in air velocity from 0 . 1 to0:25 m/s shortens the drying time approximately 30 per-cent. Air velocity higher than 0.5 m/s only results ina slight increase in drying rate.

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SOLAR DRYING OF GRAPES 67

F u r th e rm o r e t h e c h e m i c a l p r e - t r e a t m e n t h a s a g r e a t i n -f l u e nc e on t h e d r y i n g r a t e ( F i g u re 3 ) . Water permea-b i l i t y o f t h e s k i n i s i n c r e a s e d by d i p p i n g t h e ? r a p e si n t o an a l k a l i n e o i l - in - w a t e r e m ul si on b e f o r e d r y i n g( G rn ca re v ic e t a l . 1968, P o n t i n g e t a l . 1970). Comparedt o u n t r e a t e d g r a p e s, d i p p in g i n t o 7% K2C03 + 0.4%o l i v e o i l s o l u t i o n d e c r e as e s d r y i n g t i m e by app rox i -m a t e l y 30 p e r c e n t . A n ot he r d i p p i n g m a t e r i a l b a s e d o ne t h y l o l e a t e r e d u c e s t h e d r y i n g t i m e ev e n more( F i g u r e 3) .The l a b o r a t o r y t e s t s h a ve shown t h a t f o r o p t i m a l d r y in gs o l a r d r y e r s s h o u l d b e d e s i g n ed i n a way t h a t d r y in g -a i r t e m p er a tu r e i s a b o u t 50 t o 60C a nd t h e a i r v e lo -c i t i e s 0.25 t o 0.5 m / s . Fur the rmore it i s p a r t i c u l a r l yi m p o r t an t t o c h e m i ca l l y i n c r e a s e g r a p e p e r m e a b i l i tyb e f o r e d r y i n g . On t h e s e p r e m i s e s it a p p ea r s p r a c t i c a lt o r e d uc e d r y i n g t i m e s i g n i f i c a n t ly i n o r d e r t o producer a i s i n s of h i g h er q u a l i t y .

FIELD TESTS-M a t e r i a l s a n d M et ho dsS e v e r a l t y p e s o f l ow - co st s o l a r g r a p e d r y e r s w er e de-s i g n e d a t H ohenheim U n i v e r s i t y a nd t e s t e d o n t h e C r e t eI s l a n d i n c o o p e ra t i o n w i th t h e C r e t e A g r i c u l t u r a l Re-s e a r c h C e n t e r d u r i n g 1980 an d 1982. F o r t h e c o n s t r u c -t i o n o f t h e s o l a r d r y e r s l o c a l l y a v a i l a b l e m a t e r i a l ss u c h a s p l a n k s a nd b o a r d s , w i r e m es h, t r a n s p a r e n t an db la ck p l a s t i c f o i l s were u s ed . The s o l a r d r y e r s w e r ea l s o d e s i g ne d i n s u ch a way t h a t t h ey c o u l d b e b u i l tw i th s i m pl e t o o l s .A l l t e s t s w e r e c a r r i e d o u t w i t h f r e s h l y h a r ve s te d s u l -t a n a g r ap e s . B e f or e d r y i n g c h a r a c t e r i s t i c s o f t h e

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68 EISSEN, MUHLBAUER, AND KUTZBACH

grapes such as moisture content, size, sugar contentand acidity were determined. During the tests, directand diffuse radiation, temperature and relative humi-dity of the ambient air, as well as wind speed and pre-cipitation, were measured. Solar collectors, used fordrying purposes, were tested according to the specifi-cations of the BSE-Standards, 1980, and temperaturerise, pressure drop and collector efficiency were deter-mined to evaluate the collectors.The solar dryers.were assessed by measuring the weightlosses of the grapes. Changes in moisture content du-ring drying were measured to evaluate the uniformityof drying. The drying process was surveyed by measuringtemperature and relative humidity of the drying air aswell as the temperature of the berries at different lo-cations. The quality of the raisins was evaluated interms of colour, microorganism count and uniformity ofdrying. To compare the solar dryers with natural sundrying, a control sample was always dried under thesame weather conditions. The meteorological and thermo-dynamic data were recorded continuously. Moisture con-tent of the grapes and quality characteristics weremeasured daily. All data were processed by a computer.

Solar Dryer with Natural VentilationIn Mediterranean countries grapes are generally driedin the vineyard where electricity is not available.Therefore a solar dryer, which does not require any ex-ternal power source, was developed.The solar dryer with natural ventilation consists of asolar collector and a drying chamber (Figure 4).Dryingair is heated in the solar collector consisting of a

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SOLAR DRYING OF GRAPES 69

transparent foil cover and a black absorber sheet. Thedrying chamber holds the trays for the grapes. A trans-parent sheet covers the dryinq chamber and protects thegrapes from rain and dust. Ventilation is provided bynatural convection inside the collector and the dryingchamber, and is enhanced by a sucking effect at theair outlet caused by wind. Similar systems have beendescribed in previous publications (Szulmayer, 1971,Bolin, 1980 and Puigalli, 1981).The maximum drying-air temperature reached was close to50C at an ambient temperature of about 30C. The loa-ding capacity of the drying chamber was 100 kg freshgrapes per square meter of the drying chamber and 25 kggrapes per square meter of collector area.With the dryer described above drying time could not bereduced significantly compared to natural sun dryinq.However, it offers a means of protecting the productcompletely from rain and dust and improves the qualityof the raisins. The disadvantage is the small capacityof the dryer. Therefore it seems to be more suitablefor drying agricultural products which are produced insmaller quantities.

Figure 4: Solar dryer with natural ventilation-

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EISSEN, MUHLBAUER, AN D KUTZBACH

S o l a r T u n n e l D r ye r-A l l d r y i n g s y s t e m s b a s ed on n a t u r a l c o n v e c t i o n , l a c kc o n t r o l o f m o i s t u r e r em ov al . T h e r e f o re s p o i l a g e c a n n otb e e x cl u d e d . To a s s u r e t h e r e q u i r e d m o i s t u r e r e m o va l ,f o rc e d a i r c i r c u l a t i o n , e . g . w i th a f a n , i s n e c e s s a r y .T he m o st s i m p l e me th od t o r e a l i z e t h e s e d em an ds i s t ou se t h e g r a p e s a s a n a b s o r b e r and t o f o r c e a i r b et we ent h e g r a p e s and a t r a n s p a r e n t c o v e r ( F i g u re 5 ) . T he s o l a rt u n n e l d r y e r t e s t e d w as 1 . 5 m i n w i d t h a nd 1 6 . 0 m i nl e n g t h , w as e q ui p p e d w i t h a 1 0 0 W a tt f a n a n d h a d a c a -p a c i t y o f 2 5 kg o f f r e s h g r a p e s p e r s q u a r e m e t e r .The ma in a d v a nt a g es o f t h e s o l a r t u n n e l d r y e r a r e a3 t o 4 d a y s h o r t e n i n g o f t h e d r y i n g t i m e , u n i f o r md r y i n g an d c o mp le te p r o t e c t i o n o f t h e f r u i t f ro m r a i n ,d u s t and i n s e c t s . The q u a l i t y o f t h e r a i s i n s i n termso f c o l o u r a nd f o r e i g n m a t e r i a l s was s i g n i f i c a n t l yh i g h e r c om pa re d t o n a t u r a l s un d r y i n g .

S o l a r M u l t ip l e - L a y e r B a t c h D r y e rF or d r y i n g q u a n t i t i e s o f m ore t h a n t e n t o n s p e r y e a r amore s o p h i s t i c a t e d b a t ch - ty p e s o l a r d r y e r was d e v e l o p d .

F i g u r e 5: S o l a r t u n n e l d r y e r

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SOLAR DRYING OF GRAPES

T h i s d r y e r i s s u i t a b l e f o r u s e on f a rm s w it h a n a c r e -a g e o f m ore t h a n o n e h e c t a r e . The d r y e r i s composed ofa f l a t - p l a t e s o l a r a i r c o l l e c t o r , a f a n and a m u lt ip le -l a y e r b a t c h d r y e r ( F i g u re 6 ) .The s us pe nd ed a i r c o l l e c t o r c o n s i s t s o f a t r a n s p a r e n t -f o i l c o v e r a nd a c o r r u g at e d - m e ta l a b s o r b e r p a in t e db l a ck .To p r e v e n t d u s t c o n t a m i n a ti o n o f t h e a b s o r b e r s u r f a c e ,t h e a i r i s s u ck e d u n d e r n e a th t h e a b s o r b e r i n s t e a d o ff o r c i n g t h e a i r be tw ee n t h e a b s o r b e r an d t h e t r a n s p a -r e n t c o v e r . The c e n t r i f u g a l f a n c a n be d r i v e n b y ane l e c t r i c m otor o r a d i e s e l e ng in e a t w i l l . The ba tch-t y p e d r y e r i s d e s ig n e d a s a c o n t a i n e r i n w hi ch f i v ew ir e- m es h t r a y s c a n b e i n s t a l l e d . The maximum d e p t h o fe a c h l a y e r s s h o u l d n o t e x c e e d 0. 2 m t o p r e v e n t s q u as -h i n g o f t h e g r a p e s u n d e r t h e i r own w e i g h t . T he di me n-s i o n s o f t h e c o l l e c t o r a nd t h e d r y e r a r e shown i nF i g u r e 6 . The c a p a c i t y o f t h e d r y e r i s 5 00 k g o f f r e s hg r a p e s p e r s q u a r e m e te r o f t h e d r y e r s u r f a c e o r 3 8 kgo f g r a p e s p e r s q u a r e m e te r o f c o l l e c t o r s u r f a c e . Thea i r - f l o w r a t e i s a p p r o x i m a t e l y 2 0 0 0 m 3 a i r p e r h o u r ,c o rr e sp o n di n g t o a n a i r v e l o c i t y o f 0 .3 5 m / s i n t h ed r y e r . The pow er r e q u i r e d f o r d r i v i n g t h e f a n i s0 .8 kW a nd a t o t a l p r e s s u r e d r o p o f 50 mm WC i n c o l l e c -t o r a nd d r y e r i s n e c e s s a r y .

8"-

A-P

F i g u r e 6 : S o l a r m u l t i p l e - l a y e r b a t c h d r y e r

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72 EISSEN, MUHLBAUER, AND KUTZBACH

Based on the drying behaviour of the qrapes in thefirst stage of drying the dryer was operated with anair-flow rate of 2000 m3/h. In the second stage theair-flow rate was reduced to 1400 m3 /h , correspondinqto an air velocity inside the dryer of 0.25 m/s. Withthe higher air-flow rate and a total radiation of900 W/ m2 , a temperature rise of 20 K was reached at acollector efficiency of 70%. The reduction in the air-flow rate causes a temperature rise close to 30 K ata collector efficiency of 50%.

The tests have shown that dryins can be completedwithin 5 to 6 days, which means a reduction of appro-ximately 50 percent compared to natural sun dryins.However, the main advantage of this system seems to bethe excellent quality of the raisins produced. Theprotection of the grapes against direct radiation cau-ses the desired golden colour. During drying, the pro-duct is completely protected against rain, dust andinsect contamination.

EVALUATION OF THE SOLAR DRYING SYSTEMSTable 1 summarizes the features of the various solardrying systems tested and those of the traditional sundrying methods. In terms of quality, protection fromrain, dust and insects, as well as in terms of produc-tion reliability, the solar tunnel dryer and the batchtype solar dryer offer extraordinary advantages overthe traditional drying methods. Moreover a significantshortening in drying time can be observed.For the application of the new drying systems they mustbe economically superior in addition to beinq techni-cally feasible. An economical analysis of the newly

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SOLAR DRYING OF GRAPESTable 1: Results of drying tests

developed solar-drying methods shows certain advan-tages compared to traditional methods (Eissen, 1984)The increased income of the farmers resulting from re-ductions in mass losses and from the hiqher prices forbetter quality raisins should more than compensate forthe higher investment for the solar dryers.

ACKNOWLEDGEMENTThe authors acknowledge the German Ministery of Re-search and Technology for funding the inves tigation~and the Crete Agricultural Research Center for thecooperation.

REFERENCESBolin, H.R., Huxsoll, C.C. and D.K. Salunkke 1980Fruit drying by solar energy. Confructa 25 ( 3 ) :147-160BSE-Richtlinien und Hinweise fiir die Bestimmung derGebrauchstauglichkeit von Solarkollektoren. Bundes-verband Solarenergie 1980. Wehlmann Press, Essen,Fed.Rep.Germany

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E i s s e n , W . 1984. Trocknung von Trauben m i t S o l a r e n e r g i e .R e s e a r c h R e p o r t T 84 -0 89 , F e d e r a l M i n i s t r y o f R es ea rc hand Tec hno logy , Bonn, Fed .Rep.GermanyG r n c a r e v i c , M . , F . R a d l e r and J . V . Poss i ngham 1968 .Th e d ip p i n g e f f e c t c a u s i n g i n c r e a s e d d r v i n g o fg r a p e s d em on st ra te d w i t h a n , a r t i f i c i a l c u t i c l e .J o u r n a l o f E no lo gy a n d V i t i c u l t u r e 1 9 ( 1 ) :2 7 -2 9P o n t i n g , J . D . a n d D .M . McBean 1970. Te mp era tur e andd i p pi n g t r e a t m e n t e f f e c t s o n d r y i ng t imes o f g r a p e s ,p r u n e s a n d o t h e r waxy f r u i t s . Foo d T e ch n o lo g y 2 4:1403-1406P u i g a l l i , J . R. 1 98 1. D i s p o s i t i f s d e Campagne po u r l eS Qc ha ge S o l a i r e d e P r o d u i t s A g r o- A l im e n t ai r e s, F r u i t s

e t G r j\ in s . P h . D . - T h e i s s , U n i v e r s i t y o f B o r d e au x ,F r a n c eS z u l m a y e r , W . 1971 . F r om sun - d r y i ng t o s o l a r d eh yd ra -t i o n . F oo d Te c h no l og y i n A u s t r a l i a 2 3: 44 0- 50 1