Use of Solar
Energy to Produce
Dried Vine Fruit .
DRIED FRUITS RESEARCH COUNCI L -
DAV 39F Use of Solar Energy to produce high qua lity
dried vine fruit.
Organi sation: Department of Agriculture and Rural
Project Supervisors: M. Schache,
Sunraysia Horticultural Centre,
Food Research Institute,
1989 - 1990 .
1 . To evaluate a commercial-sized prototype "solar drying
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
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 .
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
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.
- unglazed collector replacing the glazed
- 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 .
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.
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
Buckets of fruit were set aside from each load picked. Bunches of grapes
were taken from these buckets and placed into preweighed, labelled V(ire-
mesh 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
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
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.
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
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 *
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
* Sodium metabisulphite was sprayed on 21/3 to retard mould growth.
** Extended rain pe