Notes on a short talk about the different grape cooling systems given by our MD Mr Gerhard Potgieter last year



The main purpose of cooling table grapes is to lower their respiration rate and thereby increase their life span after harvesting.

Because respiration rate increases exponentially with temperature of the grapes, it is very advantageous to lower the temperature to below 20C as soon as possible after picking and to below 10C as soon as possible thereafter. At picking temperature of 30C grapes respire 8 times faster than at 10C and at packing temperature of 20C 4 times faster than at 10C.

How one achieves this reduction in temperature is very dependent on the overall system which is being followed from packing to market.

A producer with own cooling facility and containerising his product on the farm is able to use systems which would fail if attempted by a producer delivering to a central cold store which in turn dispatches to a secondary cold store for storage and thereafter via conventional ships to overseas ports where a plethora of handling and storage facilities is available.

There is no "ONE SIZE FITS ALL" refrigeration system which is ideal for all applications. Take cognisance of the entire handling and storage logistical chain and plan the associated refrigeration system requirements accordingly.

Changes are taking place in packaging design and marketing (new marketers, new market countries, new systems, etc). Under these circumstances one must try to build as much flexibility into new facilities as you can so that changed conditions can be accommodated later.

Attempt to do a cost vs benefit analysis before spending a disproportionate amount of capital on refrigeration systems. Ensure that spending is in proportion to benefit and to the expenditure on the other aspects of your table grape production. Refrigeration cannot improve the inherent condition of poor quality products.

Some features of the main refrigeration systems follows:

COOLING SYSTEMS FOR PRIOR TO PACKING

  1. Evaporative cooling
    1. Limited to areas with low RH in midsummer.
    2. Long retention times generally necessary.
    3. Cooling air has high RH.
    4. Little cooling on days with high RH.

     

  2. Mechanical refrigeration
    1. Short retention times possible (20min 60min).
    2. Can increase humidity by:
        • - combining with evaporative cooling

          - fog spray

          - wet coil

    3. Relatively high cost compared to evaporative cooling.
    4. Could have other uses on farm.
    5. Cost will vary depending on existing infrastructure.

      Typical operating cost for larger installations is 25 50c/carton.

 

POST PACKING COOLING SYSTEMS

  1. "Traditional RSA"
    1. Generally multi-purpose cold rooms with forced air cooling tunnels.
    2. Designed for "slow" cooling rate related to unperforated plastic bags.
    3. RH 85 90% - not important for unperforated bags.
    4. RH can be increased.
    5. Pallets remain in position during p/c and storage.

     

  2. "Delmonte"
    1. Already covered by previous speaker.
    2. Uses perforated plastic bags, high air volumes and high RH.
    3. Purpose designed tunnels fruit must be transferred for equalisation of temperatures and storage.

     

  3. Filacell
    1. High humidity cooling air
    2. low desiccation of naked fruit

      no advantage for unperforated bag packaging

    3. Uses well ventilated carton
    4. no plastic bag in carton

      shrink-wrap complete pallet once cold

    5. Purpose designed tunnels fruit must be transferred for storage.
    6. Minimum practical fruit temperature +C.
    7. Very useful for high surface:volume ratio products eg lettuce, flowers, etc.

     

  4. Horticold
    1. Cartons have open plastic bags on continuous belt. Cooling by low temperature high volume cooling air.
    2. Naked fruit in contact with cooling air fast cooling rate (45min).
    3. Used to cool to 10C followed by forced air cooling.
    4. Balance of precooling done in another facility such as 1 or 2.
    5. Refrigeration equipment utilised only 50% of packing day.

     

  5. Hypobaric
    1. Existing technology (vacuum/refrigeration).
    2. Not generally used in fruit industry.
    3. Investigations for application in South Africa under way.
    4. Possibly too "complicated".

Click to go back to Materials Handling and System Development