Use of Solar Energy to Produce High Quality Dried Vine Fruit .

DRIED FRUITS RESEARCH COUNCI L -FINAL REPORT.

DAV 39F Use of Solar Energy to produce high qua lity dried vine fruit.

Organi sation: Department of Agriculture and Rural Affairs, Victoria .

Project Supervisors: M. Schache, Sunraysia Horticultural Centre, Irymple .

Time Span:

Objectives:

R. Fuller, Engineering Centre, Food Research Institute, Werribee .

1989 - 1990 .

1 . To evaluate a commercial-sized prototype "solar drying rack" .

2. To evaluate the performance of the "solar drying rack" in drying grape varieties which mature, or are dried in the latter part of the harvest period.

3. To further develop and refine the design and use of the "solar drying rack" to maximise its effi ciency .

4. To Disseminate information regarding the use of technology in the production of high quality sultanas.

solar dried

3

INTRODUCTION

Grapes have been dried in Sunraysia for over 90 years and very few changes have occurred in the drying practices during this time. Drying racks were developed in about 1910 and their design has changed little since then.

During the months of February and March periods of very good drying conditions usually occur in Sunraysia . However, rain occurs in these months in approximately two out of every three years and can adversely effect the quality of the dried fruit . Rain is even more likely to occur later in the harvest season (April) .

Rain may extend the drying time which may result in the fruit on the rack drying to a dull brown colour instead of the much sought after light golden. In more s e verely rain affected fruit, moulds may grow which can result in wastage penal ties being i mposed . During prolonged periods of rain the extended drying time , coupled with excessive mould growth , can result in the production of dried fruit which i s down graded to manufacturing grade (the lowest grade) or even to no commercial value in extreme cases .

Increasing the dry ing rate of the grapes has two advantages . Firstly, dried fruit quality is directly linked with drying time, with quality increasing with a faster drying rate. The second advantage is that the sooner the fruit is dried, boxed and transported to the packing shed, the less chan ce there is of rain damage.

With Australia's main advantage on the export market being high quality and light golden colour any system which can be u sed to produce high quality, light golden sultanas more cons istently can only be of benefit to the indus t ry .

On ce the frui t drying on the racks reaches a moisture conten t of approximately 25 - 30 % it becomes prone to reabsorption of moisture, particularly from the cool, night air. This results in the fruit remaining on the drying racks for a longer period of time .

The solar drying rack was developed to improve the drying rate of grapes in the latter stages of drying .

GENERAL INFORMATION

In the two years prior to this project, research had been carried out at SHC to determine the most cost effective solar drying system of the three developed wh ich actively utilise d s olar energy in drying grapes (see Department of

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Agricu l ture and Rural Affairs Research Report No. 86, June 1989 for further details)

A small research model of the solar drying rack had shown that the increased temperatures generated by the system enhanced drying during the day, while the presence of the curtains retarded the cool moist, air from penetrating the rack space at night or during rain periods. It was also found that the fruit could be dried down to a moisture content where it could be boxed directly from the rack thus eliminating the time consuming and contaminate-prone ground finishing process.

The most successful system tested in the 1987 and '88 seasons was one which utilised a simple glazed solar collector constructed from the existing rack roof, a fan, ducting and clear polyethylene curtains which enclosed the partially dried grapes.

This rack has been designated the 'experimental rack' Individual bays incorporating the solar system (a total of 6 in a 12 bay rack) were located in the experimental rack . Five control bays were also included . This rack was used to refine design modifications as well as evaluate the suitability and efficiency of drying late maturing grape varieties with the solar system.

The design concepts of this research model were adopted to construct a full scale commercial solar drying rack prototype. This rack was tested thoroughly to ensure that it performed as well as the research model had in the previous two seasons .

Although identical in concept to the experimental rack, some components of the full size prototype solar rack differ from those on the small experimental rack system.

These are: - unglazed collector replacing the glazed unit. - 46m continuous curtains replacing the 3m single bay curtains. - inflatable poly ducts below the grapes replacing the rigid PVC pipes.

This rack has been designated the 'demonstration rack'. A small, single bay conventional, roofed rack was built alongside the solar drying rack to provided a comparison in drying rates and fruit quality between the demonstration rack and a conventional one.

The performance of both racks was measured throughout both harvest seasons .

5

The fruit used to fill the racks was picked from relatively uniform areas of vines wherever possible. The fruit was picked and the rack filled in 1-1~ days and then sprayed with a % strength commercial drying oil / potash preparation using a 'Gaulke-type' wand. Approximately 4 days later a % strength oil mix was applied.

After very promising results from the commercial prototype two other solar drying racks were introduced into the district, one at the Sunraysia College of TAFE and the other in Robinvale. The results from the three solar drying racks has created much interest in the industry and one grower has already designed and constructed his own solar system based on the solar drying rack.

EXPERIMENTAL RACK

Prior to tl.'e rack being filled, thermocouples were installed in specified positions in the rack to record the wet and dry bulb temperatures in the bays. Data were automatically logged every hour.

The experimental unit used in this trial was an individual rack bay. Each of the 12 bays in this rack was sealed off from its neighbours with plastic partitions. This created a slightly artificial situation, particularly in the control bays, as it restricted air movement which may naturally occur along the rack. However, the north/south alignment of the rack allowed maximum exposure of the drying fruit to the prevailing westerly winds.

Experimental Procedure

Buckets of fruit were set aside from each load picked. Bunches of grapes were taken from these buckets and placed into preweighed, labelled V(ire­mesh trays which were lined with berry hessian. The bunches in these trays were then submerged in a commercial drying oil/potash preparation for one minute. The trays were then placed in preset positions in each bay. Each tray was covered with berry hessian to prevent extraneous berries from falling into it. The trays were removed from the rack twice daily (Sam and 5pm) and weighed. Each tray was then replaced back into its allotted position in the rack.

The moisture content (% MC) of the drying fruit was monitored closely during the middle to latter stages of drying. The solar system was activated (where applicable) once the fruit dried to 25% MC. This

6

involved activating the timer (which a utomatically turned the fan on at 9am and off at 6pm) and closing the curtains.

Daily samples of drying fruit were taken from throughout each bay to determine moisture contents. Unless rain occurred, the system remained on until the fruit was shaken from the rack.

When the fruit was deemed dry enough to shake, the trays of fruit and the berry sheet covers were removed from the rack. The curtains were tied back, berry hessians laid out in the usual manner and the rack shaken.

The fruit from the individual bays was kept separate. Samples were taken for both quality and moisture content analysis. The fruit was assessed by a grader from the Victorian Dried Fruits Board.

If rain threatened prior to the solar system being activated, the curtains were closed until the weather cleared. If it rained after the system was activated, the fan was turned off (to prevent ducting wet air into the rack space) and the curtains remained closed. If the air became very humid, but no rain threatened, the curtains were opened to promote air circulation which helps minimise fungal growth.

Rack fill dates, drying oil spray times, activation of solar system times and rack shaking dates may be found in Table 1.

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Table 1: Rack fill dates, spray dates, solar system activation dates and rack shaking dates.

Variety Filled First Second Solar Rack Spray Spray System Shaken

Activated

1989

Fill 1 H5 Sultana 14/2-15/2 16/2 20/2 24/2 3/3

Fill 2 GA 7/3 - 8/3 8/3 15/3 28/3 * Sultana ** & M12

Sultana

1990

Fill 1 H5 Sultana 19/2 20/2 23/2 24/3 8/3

Fill 2 Carina 13/3-14/3 14/3 21/3 28/3 18/4 Gordo 11/4

* Sodium metabisulphite was sprayed on 21/3 to retard mould growth. ** Extended rain periods resulted in a small gas dehydrator being used to dry the fruit to a moisture content where it could be shaken from the rack.

Trial Objectives

Fill 1, 1989. The first trial in 1989 involving the experimental rack, 'Fill 1', investigated the difference between the drying rates of the Sultana clone H5 when dried either conventionally ('control') or using the solar system. Several growers had expressed interest in utilising their black ground sheets as curtains. Therefore the bays utilising the solar system were enclosed in either black or clear curtains to determine whether curtain colour had an effect on the drying rate of the enclosed fruit.

Fill 2, 1989. Fill 2 in 1989 was used to determine whether the solar system could be successfully used to dry sultana grapes treated with Gibberellic Acid (GA) earlier in the season. GA treatment causes the production of a larger grape which, as it contains more water, is more difficult to dry than the conventional sultana grape.

Fill 1, 1990. The first fill in 1990 was used to test the effect of activating the solar system earlier than when the grapes reached 25% moisture content. The

8

solar system was therefore activated once the second drying oil/potash preparation had been applied and had dried.

Fill 2, 1990. The second fill was used to determine the effectiveness of the solar system in drying Carinas and Gordos .

Results

All varieties of grapes dried using the solar drying system dried to a lower final moisture content than those dried conventionally. These results held true for H5 sultanas, G.A.'d sultanas, Gordos and Carinas.

Table 2. Rainfall experienced during the 1989 and 1990 harvest periods.

I Date I rnm Rainfall I 1989

8/2 2.8

14/3 37.4

15/3 14.4

21/3 13.8

1990

17/2 0.4

28/2 7.6

24/3 0.4

28/3 6.6

14/4 0.8

20/4 2.0

21/4 28.8

9

Table 3. Final moisture contents and quality gradings of grapes dried on the experimental rack.

Final % Assessed Treatment Moisture Quality *

Content

1989

Control 11.4 4L, 4L, 5L, Fill 1 5L, 5L.

H5 Sultana Solar 10.9 4L, 5L, 5L. (Black curtain)

Solar 10.4 5L, 5L, 5L. (Clear curtain)

Control 21 4B H5 Sultana Solar 17.5 3L

Fill 2 Control 24 3B

GA Sultana Solar 19 5B

1990

Fill 1 Control 15 4B.

Solar 13 4B.

Control 15 5 Crown Gordo 2.5% waste

Solar 13.5 5 Crown Fill 2 2.5% waste

Control 20 4 Crown Carina 5% waste

Solar 14.5 4 Crown 2.5% waste

* Fill 1, 1989, results show the fruit quality gradings of samples taken from the individual bays of each treatment. The quality gradings of all other fills are of pooled samples taken from each treatment rather than each bay.

The equipment used to determine the moisture contents of the drying fruit will only accurately measure contents below 25 %.

10

A comparison of the drying profiles of conventionally dried fruit (control) and solar rack dried fruit from Fill 1, 1989 can be seen in FIG. 1.

FIG. 1: Fill 1, 1989. The drying rates of H5 Sultanas dried conventionally or using the solar system.

J7

J6

J5

J •

JJ

... I: J2 I) ,;; ~ J l

t J O E

~ ~

29

28

27

26

25

2 4 0 2 6 8

n uE (OAYS •FOER SYSTC:~ TURNED ON)

0 C ONT!!OL + SOI..AA

The drying profiles of grapes dried using either black curtains or clear curtains in conjunction with the solar system compared with the fruit dried using the control bays from Fill 1, 1989 can be seen in FIG. 2.

FIG. 2: Fill 1, 1989. The drying rates of H5 Sultanas due to using clear or black curtains in conjunction with the solar system compared with conventionally dried fruit.

JJ

J2 !; 0 J l ,;; 3:

i JO ;:: ; 29 II

28

27

2 6

2 5

2 • 0

0 CONT!!OL

0.5 I 1 .s 2 2.5 J J .5 • • .s 5 5.5 6 6.5 1 7.5 e e .5 9 9.5

TI IoiE (DAYS AFOER SYSTC:I.t TURNEO ON) B..ACK CURTAINS <> CLEAR CURTAINS

11

The drying rates of Gibberellic Acid treated sultanas dried conventionally or using the solar system from Fill 2, 1989, can be seen in FIG. 3.

FIG 3: Fill 2, 1989. The drying rates of gibberellic Acid treated Sultanas dried conventionally or using the solar system.

26

... e z 8

z~ j -I ra1n

:: J j,

22 I 21

20

19

16 '

17/ 4 19/ 4 21/4 2.3/ 4 2S/ 4 27/ 4 29/ 4 1/S J / 5

n wE (DAYS) -. G.A.'d SULT:SOL..<R 0 G.A.'d SULT.COrmiOL

Due to seasonal conditions the GA treated sultanas were subject to excessive shatter. This led to a layer ofloose berries up to lOcm thick on the berry hessians. Initially this was not seen as a problem as these berries were drying. However, with the onset of inclement weather, these berries soon showed signs of fungal infection. (This was prior to activation of the solar system). This thick layer of rotting fruit was removed and the fruit remaining on the rack was sprayed with metabisulphite to retard fungal growth. Several days later the solar system was activated. A prolonged period of rain, fogs and heavy dews (see Table 2. for rainfall data) led to a cessation in drying. Heated air from a small gas burning dehydrator was applied to the bays of fruit to dry it to a moisture content when it could be shaken.

12

The drying rates of H5 Sultanas dried conventionally or using the solar system from Fill 2, 1989 can be seen in Fig. 4.

Fig. 4. Fill 2, 1989. The drying rates of H5 Sultanas dried conventionally or using the solar system.

26

25

"1 ... 23 2

"' ~ 8 ~2 -u

~1 ~ a: J ti 0 l II

:: j 18

17

17/4 1!1/4 21/4 23/4 25/4 27/4 2!1/• 1/ 5 3/ 5

Ti wE (DAYS) 0 H5 SULTANA CONTROL .,. H5 SU'-TANA SOLAR

The drying rates of grapes dried using the solar system activated immediately after the application of the second drying oil spray 1s compared to conventionally dried fruit from Fill 1, 1990 in Fig. 5.

FIG 5: Fill 1, 1990. The drying rates of H5 Sultanas dried conventionally or using the solar system which had been activated immediately after the application of the second drying oil spray.

eo

70

60 ... 2

"' ... 2

8 50

w a: J

ti 0

•o l lo!

JO

20

10

o.o 2.0 4.0 6.0 e.o 10.0 12.0 14.0

0 TIME. (CAYS)

CONTRO'- + SOLAR

13

The drying rates of Carinas dried using either the solar system or conventionally from Fill 2, 1990 can be seen in Fig. 6.

FIG. 6: Fill 2, 1990. The drying rates of Carinas dried conventionally or using the solar· drying system.

36

:s•

.... 32 z w t 30

8 28 w lr :> 26 Iii ~ 24

I! 22

"~ 18

16

14 I

0.000 4.000

rain I

-l-

8 .000

ram

12.000

TII.I£ (DAYS SINCE SYSTE~ TURNED ON) C CONTROL + SOLAR

16.000 20.000

The drying rates of Gordos dried using either the solar system or conventionally from Fill 2, 1990 can be seen in Fig. 7.

FIG. 7: Fill 2, 1990. The drying rates of Gordos dried conventionally or using the solar system .

::~

22

21

.... 20

e z 19

8 w 1e lr :> Iii ~ 17

II 16

15

14

13

0.0 2 .0 4 .0 6.0 8 .0 10.0

TI I.I£ (OAYS AFTER SOLAR ACTNATION) C CONTROl + SOLAR

14

Discussion:

The weights of the grapes drying in the trays in the various bays were used to determine the drying rates. These results were then compared to the moisture contents readings determined daily for samples taken from the bays in the rack.

Fill 1, 1989. An anomaly was found between the results obtained through tray weight readings and the daily moisture readings. The tray weights showed that the control fruit dried to a lower % initial weight than solar dried fruit. Conversely, the moisture content readings showed that the solar dried fruit dried to a lower moisture content. The reason for this anomaly is unclear. However, it may have been that by chance the fruit placed in the control trays had a lower total solids content than the fruit placed into the solar system trays. This would result in the fruit in the control trays drying to a lower final weight but not necessarily a lower % moisture content. In the following rack fills , all precautions were taken to avoid sampling errors. In the subsequent rack fills the final moisture contents of the fruit drying in the trays were determined and used to calculate the drying rates. (Moisture content determination is a destructive process and was avoided in the earlier rack fill so that the fruit in the trays could be quality graded). For the remaining rack fills the drying rate results from the trays coincided with those of the daily moisture content readings.

Fill 1, 1989 was used to determine whether any differences in rack temperatures were experienced when the bays were enclosed in either clear curtains or black curtains when used in conjunction with solar system. Black surfaces are known to absorb heat well, and it was thought that the rack temperature may be elevated in those bays enclosed with black curtains. Also, as several of the growers in the district have been using their black ground sheets as curtains in the final stages of rack drying it was decided to test the effectiveness of black curtains. However it was found that the temperatures in the bays enclosed with the clear curtains were 5°C on average above those enclosed with black curtains. It is thought that as solar radiation is transmitted through the clear plastic its wavelength is altered and it is therefore trapped in the rack space as heat. Heat absorbed by the black curtains is re-radiated from the rack (while some heat energy is radiated into the rack space, some is also radiated back into the air surrounding the rack). This could account for the temperature differences experienced between bays enclosed with clear or black curtains.

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The higher temperatures experienced by the fruit in bays enclosed in clear curtains could account for its slightly lower moisture content readings (Table 3).

Fill 2, 1989. Inclement weather interrupted the second fill of 1989 and severely retarded the grape drying. Moulds began developing on the G.A.'d sultanas and the fruit in the entire rack was sprayed with sodium metabisulphite to prevent further mould growth. Because of the persistant cool, inclement weather a small gas burner was used to provide heated air for the final stages of drying. Despite this setback, the G.A.'d sultanas dried using the solar system dried to a lower final moisture content and better quality (5 Crown Brown) compared to the conventionally dried fruit (3 Crown Brown).

The H5 sultanas dried using the solar system in the second fill of 1989 also dried faster than the conventionally dried fruit (Fig. 3). The solar dried fruit reached a moisture content of 17.5%. Although fruit may be carefully shaken at this moisture content and placed on to ground sheets to finish drying in this case it was allowed to remain on the rack to observe the effects of rain on fruit drying using the solar system. At this stage the majority of the fruit on the rack was still a light colour. Rain affected some of the drying fruit in the solar system bays which turned brown. This resulted in both light and dark fruit being shaken from the solar system bays. The mottled appearance of the dried fruit led to a 3 crown light grading. The fruit in the control. bays was more exposed to the rain and resulted in a 4 crown brown grading. This was the only case where the quality grading of the conventionally dried fruit was better than that dried using the solar system.

All other rack fills resulted in equal or better quality gradings for the solar system dried fruit.

Fill 1, 1990. During good drying weather, the air blowing through the rack proves to be very good in drying grapes through the early stages of drying. However, once the grapes reach a moisture content of approximately 30%, the drying rate slows as moisture moves from the centre to the surface of the berries . Also, as the grapes dry they are more prone to reabsorbing moisture from the cool night air. This is the reasoning behind activating the solar system when the grapes have a moisture content of approximately 25%. It was thought that activating the solar system and closing the curtains earlier may have an adverse effect on the drying rate as significant natural air movement is needed to remove highly humid air from the vicinity of the berries. The solar system was activated immediately after the second drying oil spray to test this theory.

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The presence of the curtains seemed to reduce air movement through the bays. This was despite heated air being ducted to the bays and moving up through the rack by convection. The reduced air movement resulted in a slower drying rate (see Fig. 4). However, with the onset of rain, the presence of the curtains protected the berries from moisture uptake. The berries in the solar drying bays then began to dry faster than the berries in the control bays and fmished with a lower moisture content.

Fill 2, 1990. Both the Gordos and the Carinas dried using the solar system dried faster (see Fig. 5 and Fig. 6), to a lower moisture content and equal to, or better, quality than when dried conventionally. The Gordos were boxed directly from the rack, whilst the continued threat of rain led to the Carinas being shaken and finished off on ground sheets.

Whenever rain threatened, the curtains were drawn across the solar drying bays. The presence of the curtains attached to the rack made this an easy practice.

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DEMONSTRATION RACK

The demonstration rack was converted from a conventional rack to the solar system prior to the 1989 harvest. To make it easier for growers to convert their own racks several of the components used on the experimental rack were altered for this conversion.

An unglazed collector consisting of interlocking sheets of roofing iron replaced the glazed unit of the experimental rack. This collector should last for the lifetime of the rack and be much easier to install. Two continuous clear plastic curtains were used to enclose the rack. A much cheaper inflatable poly ducting was used to duct the heated air into the rack space. The fan was situated half way along the roof of the demonstration rack to more evenly distribute the heating load.

Experimental Procedure

A one bay conventional rack situated alongside the demonstration rack was used to compare the drying rates of fruit dried conventionally to those of fruit dried using the solar system. It was treated in an identical manner to the demonstration rack.

As with the experimental rack, the tiers of the demonstration rack were filled sequentially to minimise variations in the attributes of the fresh fruit from biasing the results. The bunches were also sprayed twice with a commercially produced drying oil/potash mix. The grapes from both racks were sampled daily once a moisture content of 25% was reached.

Rack fill dates, drying oil spray times, activation of solar system times and rack shaking dates may be found in Table 4.

Fill 1, 1989. For the first fill of 1989 the demonstration rack was half-filled with the variety, Merbein Seedless. The control rack was also filled with Merbein Seedless.

Fill 2, 1989. For the second fill of 1989 the rack was completely filled with H5 Sultanas as was the control rack. Following prolonged inclement weather the fruit on both racks was sprayed with sodium metabisulphite on 31/3/98 to retard mould growth.

Fill 1,1990. For the first fill in 1990, the rack was filled with H5 sultanas.

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Fill 2, 1990. The demonstration rack was again filled with H5 Sultanas for the second fill.

During this second fill, the uniformity of drying within the solar drying rack was examined. With maximum exposure to direct sunlight and air currents, the fruit drying along the edge of the tiers dries faster than the more sheltered fruit situated in the middle of each tier. This may result in the 'edge' fruit drying excessively while the 'middle' fruit may still be too moist to shake from the rack.

The heated air produced using the solar rack is directed towards the middle of the bottom tier. Although some of the heated air will be deflected towards the edges of the tiers as it rises up through the rack, heated air movement up through middle of the rack will still occur. Although this heating may be negligible, it had been noted previou~ly that the fruit situated in the middle of the rack did seem to dry faster than on the conventional rack. Fruit was taken from either the edge or the middle of the tiers of both the demonstration and control racks and tested for moisture content to determine whether the solar drying rack results in more uniform drying throughout the whole rack.

Fill 3, 1990. A third fill of the demonstration rack was attempted in 1990. The rack was half-filled with Gordos. Unfortunately, a shortfall in grapes led to the control rack being unfilled. . The experimental rack had been filled with Gordos just prior to this fill and the results from the demonstration rack have been compared to those of the control bays of the experimental rack.

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Table 4. Rack fill, spray application, solar system activation and rack shaking dates.

Grape Type Rack Spray 1 Spray 2 Solar Rack Filled System Shaken

Activated

1989

Fill 1 H5 Sultana 13/2 15/2 20/2 23/2

~erbeinSeedless 14/2

Fill 2 H5 Sultana 2/3-3/3 3/3 6/3 9/3*

1990

Fill 1 H5 Sultana 14/2 16/2 20/2 23/2

Fill 2 H5 Sultana 28/2 1/3 5/3 8/3

Fill 3 Gordos 20/3-21/3 22/3 26/3 29/3

* Sprayed with Sodium ~etabisulphite on 31/3/89.

Results

The final moisture contents and quality gradings for fruit dried conventionally or using the commercial prototype solar system for the 1989 and 1990 harvest seasons can be seen in Table 5.

1/3

24/4

26/2

19/3

10/4

20

Table 5. Final moisture contents and quality gradings of grapes dried on the demonstration rack.

Variety Treatment Final % Assessed moisture Quality content

1989

Fill 1 Merbein Control 15 3L Seedless

H5 Sultana Solar 12.4 5L

Merbein Solar 14 4L Seedless

Fill 2 H5 Sultana Control 21 4B .

Solar 18 5B

1990

Fill 1 Control 17 5L H5 Sultana 2.5% waste

Solar 12 5B 2.5% waste

Fill 2 Control 17.5 3L H5 Sultana 10% waste

Solar 12 4L 2.5% waste

Fill 3 Control 15.5 4 Crown Gordo (Research Rack) 2.5% waste

Solar 13.5 5 Crown

The drying profiles of each fill of fruit dried on the solar drying rack were very similar. Therefore only Fill2, 1989 has been graphically represented in this report to avoid repetition.

21

The drying rates ofH5 Sultanas dried in the commercial size solar drying rack compared to grapes dried in a conventional rack can be seen in Figure 8.

FIG 8: Fill 2, 1989. The drying rates of H5 Sultanas dried conventionally or using the full size commercial prototype solar system.

26

25

ratn 0 24

23 .. Zi ,.. z

22

8 21

"' (1: :J 20 t;

~ 19 M

18

17

16

15

09-llar

n wE (DAYS ROW SOUR ACTlVAnON) 0 C ONT'ROL ~ SOLAR

The drying rates of H5 Sultana drying either on the edge or the middle of the tiers dried using either the solar system or conventionally can be seen in Figure 9.

FIG. 9: Fill 2, 1990. The drying rates of fruit situated either along the edge or the middle of the rack tiers of fruit dried conventionally or using the full size. commercial prototype solar system.

26

24

\___ ram 23

22

.. z ~

21

z 20 8 .. 19 (1:

~ 18

~ II 17

16

15

14

13

07-J.Ior 08-Mor 09-Wor 1 0-Mor 11-Wor 1 2-Wor 1 3-Mor 14-Wor 15-Wor 16-Wor 17-Wor 1 8-Wor

n wE (DAYS AF'TER SOl-"" .U:TIVAT10Nl

• control mid a cont ro.J.. s~ae ~ solar mid <> solar side

22

The hourly temperature profile measured in the solar drying rack and the ambient temperature on a typical drying day may be seen in Fig. 10.

FIG. 10. The temperature profile taken from a typical day in 1990 comparing the temperature measured in the solar drying rack with the ambient temperature.

44

42

40

Ja

J6

..... J4 u 0 J2 ..., w a:: JO :l 1-

~ ... 28

0. 26 l ... 1- 24

o 1 2 J 4 5 6 1 a 9 1 o 11 12 1 J 14 15 1s 11 1 a 19 20 21 22 z.:s

Discussion · n~o~e: OF OAY (HOURS) C CONTROL -to SOLAA

Temperature increases of at least 15°C over ambient due to the solar system were regularly measured. The increased temperature experienced by the fruit in the solar drying rack resulted in faster drying times.

The results obtained from the demonstration rack showed the same trend as those from the experimental rack. The fruit dried on the solar drying rack always drying to a lower moisture content and to an equal or better quality than fruit dried conventionally.

Fill 1, 1989. In the 1989 harvest season the fruit dried using the solar drying rack was boxed directly from the rack.

Fill 2, 1989. Rain adversely affected Fil12 and therefore the grapes were dried on the solar drying rack to a moisture content where they could be shaken carefully from the rack. They were then placed on to ground sheets to finish drying.

·.·· ·

23

1990. For each rack fill in 1990, the solar sytem dried fruit dried faster than the conventionally dried fruit (Fig 8). The rack was filled three times and the fruit was boxed directly from the rack each time it was shaken. The fruit on the control rack was shaken at the same time as that on the solar rack and placed onto ground sheets to finish drying. The fruit remained on the ground sheets for a minimum of 2 days (fruit dried in Fill 2 remained on the ground sheet for over 10 days during an extended period of rain and fog).

Fill 2, 1990. Fill 2, 1990 was used to compare the uniformity of drying between solar drying rack dried fruit and conventionally dried fruit. When grapes are dried on a conventional rack, the fruit situated along the edges of the tiers are more exposed to direct solar energy and air currents. Therefore the 'edge' fruit dries faster than the fruit situated in the more shelter.ed middle of the rack. Over the initial stages of grape drying on the solar drying rack, prior to the activation of the solar system, this will still occur.

Once the curtains were closed and heated air was directed up through the centre of the rack, the 'middle' fruit began to dry faster (Fig. 9). In fact, once it reached 25% M.C. the 'middle' fruit dried faster than the 'edge' fruit did at the same moisture content. The 'middle' fruit was also less prone to reabsorption of moisture during rain than the 'edge' fruit of the solar drying rack, which in turn was less effected than the 'edge' fruit on the control rack. The fruit situated in the middle of the control rack dried to a minimum of 22% M.C. over the whole of Fill 2, but reabsorbed so much moisture during the rain that its moisture content could no longer be determined (i.e. it had a moisture content of greater than 25% M.C.). The fruit dried on the solar drying rack was of a lower and more uniform moisture content than the conventionally dried fruit. The increase in drying rate seen with the 'middle' fruit may be due to the heated air movement up through the rack or just the heat increase due to the solar system.

Maximum temperature increases of over 15 oc above ambient were regularly measured in the solar rack. (Fig. 10).

GROWER INTEREST IN, AND ADOPTION OF, SOLAR DRYING TECHNOLOGY

In 1988 a conventional rack was modified to include the solar system at SHC. In addition to this solar rack, two others were converted in 1989. One of these was situated on the Sunraysia College ofTAFE CTAFE) farm property, while the second was situated on a grower's property in Robinvale (Mr P. O'Brien, Block 96C).

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The conversion of the rack at TAFE provided the basis for a field day in December 1989 to demonstrate to growers the steps needed in constructing a solar drying rack. Vantage points were situated around the rack so that the growers could see fully the work being carried out on the roof of the rack. Pamphlets were distributed which covered the components of the solar system and their approximate prices. Approximately 120 growers attended this field day. A follow-up field day on use of the solar drying rack was held in March 1990 while the rack was in use. Approximately 70 growers attended this field day.

Similar field days on solar rack conversion and use were held on Mr. O'Brien's property in Robin vale in January and March 1990 respectively. Approximately 30 growers attended these field days.

The solar drying rack at the TAFE farm was also featured in the Department of Agriculture and Rural Affairs display at the Annual Gadget and Machinery Field Day sponsored by the ADFA.

Unfortunately, a lack of commitment by some TAFE staff and misunderstandings led to under utilisation of the solar rack. However, promising results were gained from the short period of usage of this rack.

Mr. O'Brien used his rack much more effectively, and was able to successfully dry three fills of fruit using his solar rack. On the first two occasions of use of the solar drying rack Mr. O'Brien found that the solar rack enhanced his drying rate compared to his conventional racks situated around the drying green. Pressures on rack space during the earlier part of the season led to Mr. O'Brien removing the dried grapes from the rack once they had reached a moisture content of approximately 16%. However, pressure for rack space had reduced by the time of the third fill. The sultanas dried using the solar rack reached a moisture content of 12.6% and were boxed directly from the rack. Mr O'Brien recorded temperatures of up to 50°C in his solar drying rack. An advantage that Mr O'Brien found with the rack design was the ease of installing the curtains if inclement weather threatened.

Mr. O'Brien's rack is situated on a sloping drying green, with the rack being orientated downhill. This resulted in two problems. The first was when it rained, water tended to flow down the grooves in the roof and pool above the plenum section. A system to duct water away from the roof and onto the drying green was developed to overcome this problem. The orientation of Mr. O'Brien's solar drying rack led to cool air being blown into the collector whenever there was a cool, southerly change. However, a baffle across the air intake alleviated this problem. Mr. O'Brien also developed a ducting system, whereby he could transfer heated air from the solar drying rack to a second rack situated alongside. The second rack also used the clear curtains and inflatable ducting from the solar drying rack.

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Following the construction field day at TAFE, Messrs. J. & A. Whyte showed interest in constructing their own solar drying system. They developed a portable system designed along the lines of the solar drying rack at SHC. Air was heated as it was blown through a tunnel comprised of a black ground sheet on the bottom and clear solar weave on top. Plastic ducting channelled the heated air into the rack space. Clear plastic curtains enclosed the rack, (as with the SHC solar rack), to retain the heat in the rack during the day and retard moist cool air from coming in contact with the fruit during the night.

This system can be moved from rack to rack and installed either on the drying green alongside the rack or on its roof.

Pressures of harvest meant that the system was not fully developed before harvest ended, and therefore it is yet to be tested in a commercial situation. However, trials showed that it still produced heated air up to 15°C higher than ambient late in the drying season. This solar drying system was judged to be the best exhibit at the Annual Gadget and Machinery Field Day sponsored by the ADFA.

Fill 1, 1989 of the experimental rack was used to determine whether there was any difference in using clear or black curtains in conjunction with the solar system. The results showed that the bays enclosed in clear curtains were up to 5°C hotter than in bays enclosed with black curtains. The local manufacturer of dehydrator curtains (Mr. T. Benson of Irymple Canvas) was advised of these results. He in turn advised growers who wished to order dehydrator curtains of these results and approximately twelve growers subsequently ordered clear dehydrator curtains rather than the usual opaque green ones. These growers claim there are significant temperature increases when the clear curtains are used.

One of the major suppliers of dehydration packages (dehydrator burners and curtains) has also expressed interest in changing over from the present opaque green dehydrator curtains to the clear curtains to enhance passive heating.

Several growers who already possess dehydrators have expressed interest in using solar energy (similar to the system developed by Whytes) to preheat air before it passes into the dehydrator. This would reduce the temperature difference between ambient and the dehydrator heated air which would result in less heating being needed thus resulting in savings to the heating bill.

Tentative interest has been shown by the industrial plastics section of Rheem (who developed the material used in the clear curtains) in developing a commercial collector similar to that produced by Whytes.

\ ·. ·-

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Further Developments Research is currently being undertaken at SHC to introduce a small gas burner into the fan assembly to enhance the usefulness of the solar system. This burner will be thermostatically controlled to operate in periods of cool or wet weather and should enhance the performance of the solar drying rack in poor drying conditions.

SUMMARY

.Following the promising results obtained in the 1987-88 solar drying rack trial a commercial prototype of the solar drying rack was developed, constructed and tested at the Sunraysia Horticultural Centre, Irymple.

It was extensively trialled and consistently dried grapes faster and to a better or equal quality than conventionally dried fruit. This was achieved regardless of weather conditions.

Differences in drying times between solar system and conventionally dried fruit were most pronounced during periods of inclement weather. On two occasions the fruit dried on the solar rack was shaken, boxed and transported to the packing shed while the fruit which had been dried on

. the conventional rack, and was shaken at the same time, lay on ground sheets in the rain.

In four of the five rack fills of the project the ·fruit dried using the solar drying system w9-s shaken and boxed directly from the rack. This obviated the need for time consuming and contaminant-prone ground finishing practices.

Four fills of Sultanas were dried on the solar drying rack. The last fill of Gordos showed that alternative late maturing grape varieties can be successfully dried using the solar drying system.

This last result mirrored those experienced with the fills dried on the experimental rack. Gordos, Carinas and G.A'd Sultanas were all dried to a lower moisture content and to a better or equal quality using the solar drying system than those dried conventionally.

The early activation of the solar system was shown to retard drying during the early stages of drying. However once the drying grapes reached a moisture content of approximately 30%, the drying properties of the solar system came to the fore and fruit dried using the solar drying system dried to a lower moisture content than conventionally dried fruit.

The use of clear plastic curtains resulted in a 5°C higher rack air temperature through passive heating than when opaque black curtains

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were used. This finding also has ramifications regarding the material used for the production of dehydrator curtains.

The solar drying rack has stimulated much grower and industry interest. One grower has converted one ofhis conventional drying racks to include the solar drying system and has modified it to service a second, neighbouring rack. He has also made minor alterations to suit the local conditions.

A second grower has developed his own portable solar drying system based on the principles of the solar drying rack. Although it is yet to be commercially tested, it was able to heat air by up to 15°C over ambient late in the drying season. This temperature gain, combined with the reduced material costs and its portability (which enables it to be used on a number of racks) has led to considerable grower interest in this solar drying system.

At present research is currently underway on introducing a small gas burner into the system. This will assist drying during periods of extended inclement weather. This has also generated interest within the industry.

The real value of the solar drying rack lies in accelerating the drying rates which decreases rack turnover times and shortens the drying season while also having positive effects on grape quality.

PUBLICATIONS ARISING FROM THE PROJECT

Fuller, R.J. and Redding, G.J. "Simple solar technology for drying vine fruit." Australian Dried Fruit News, October 1989.

Fuller, R.J. and Gould, LV. "Commercialising solar technology, a case study." Aust & N.Z. Solar Energy Soc. Solar Conference, 1989.

Fuller, R.J., Schache, M.J. and Kaye, D.R. "Improved technology for the solar drying of vine fruit ." Department of Agriculture and Rural Affairs Research Report No. 86, June, 1989.

Fuller, R.J. "Origin and development of the Australian grape drying rack." Australian Dried Fruit News, December, 1989.

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Fuller, R.J. "Rack curtains- an old idea shows new promise." Australian Dried Fruit News, May, 1990.

Fuller,R.J., Schache,M.J., Morey,B.G., Hayes,R.J., Gould,I.V. and Goldsmith, C.G.

"Improving traditional grape drying technology using solar technology." I.E. Aust. Conference on Agricultural Engineering, Toowoomba, 1990.

Schache, M.J . "Grower improvements to the solar drying rack." Australian Dried Fruit News, October, 1990.