Use of Solar Energy to Produce High Quality Dried Vine Fruit. 2018-08-29آ  in drying grape varieties

  • View

  • Download

Embed Size (px)

Text of Use of Solar Energy to Produce High Quality Dried Vine Fruit. 2018-08-29آ  in drying grape...

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


    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:


    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


    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

  • 4

    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.


    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(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

  • 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.

  • 7

    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 * Sultana ** & M12



    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 pe


View more >