A Basic Guideline to Cold Chain Management
Berries are one of the most delicate and highly perishable fruits. Their non-climacteric physiological characteristics dictate that they must be harvested in an essentially ripe condition. The quality of fresh berries depends on their maturity and appearance (red colour intensity and distribution, fruit size and shape, freedom from defects and decay), firmness, and flavour (determined by amounts of sugars, organic acids, phenolics, and characteristic aroma volatiles). Some of the most important factors in attaining and maintaining good quality are harvesting at the fully-ripe stage, avoiding physical injuries during all the handling steps, enforcing strict quality control procedures, prompt precooling, and providing proper temperature and relative humidity during transport and handling at destination.
Proper temperature management of strawberries begins with precooling (rapid removal of field heat) from field temperatures which can be as high as 30 degrees C. Rapid removal of field heat is critical to curb deterioration ofstrawberries. The recommendation for maximum quality retention of berries is precooling to near 0-2 degrees C within one hour of harvest and maintaining at 0-2 degrees C throughout the marketing channels. For commercial strawberry operations cooling to this ideal criterion is seldom achieved (in practice strawberries are cooled and shipped at 5-21 degrees C). This depends on various factors, including volume of strawberries handled, cooling and handling equipment availability and capability, economics, energy and market conditions.
RESPIRATION
Is the process by which the food reserves are converted to energy. Understanding of the cooling requirements of horticultural commodities requires an adequate knowledge of their biological responses. Fresh horticultural crops are living organisms, carrying on many biological processes essential to the maintenance of life. They must remain alive and healthy until processed or consumed. Energy that is needed for these life processes comes from the food reserves that accumulated while the commodities were still attached to the plant. The respiration rate varies with commodity, in addition to variety, maturity or stage of ripeness, injuries, temperature, and other stress related factors. Strawberries, for example, have a high respiration rate, 12 to 18 mg CO2/kg-h (2700-3900 Btu per ton per day) at zero degrees C. The major determinate of respiratory activity is the product temperature. Since the final result of respiratory activity is product deterioration and senescence, achieving as low a respiration rate as possible is desirable.
For each 10-degree temperature increase, respiratory activity increases by a factor of two to four. For example, the respiration of strawberries at 10 degrees is 49 to 95 mg CO2/kg-h (10800 to 20900 Btu per ton per day), four to five times greater than at zero degrees. Therefore, strawberries must be rapidly precooled to slow their metabolism (physiological deterioration) in order to provide maximum quality and storage life for shipping and handling operations. Strawberries are not a chilling-sensitive crop (crops which must be stored at temperatures generally above 10 degrees C – 50 Fahrenheit – to prevent physiological damage).
COOLING METHODS
The various cooling methods used for fresh produce include: Room Cooling (Passive Cooling) Exposure of produce in containers to cold air in a refrigerated space. Air sweeps past the outside of bins, pallets or individually spaced containers.
Example of berry post-harvest handling facility
Heat transfer from the produce within the container is primarily by conduction to the container surface. This method requires uniform air distribution, at least 200 to 400 feet/min air circulation, space stacking to permit air flow between packed units and preferably well vented containers. Room cooling is slow, too slow for the most perishable products. Forced Air or Pressure Cooling (Active Cooling) Produce is rapidly air-cooled by producing a difference in air pressure on opposite faces of stacks or pallet loads of vented containers. This pressure difference forces air through the containers and carries produce heat away primarily by flow around the individual fruit or vegetables. Speed of cooling is regulated by adjusting air flow volume and is usually done in 1/6 to 1/10 of the time required to room cool. Ultra-high humidity coolers are a special application of forced-air cooling. Hydro Cooling Produce may be cooled rapidly by contact with moving cold water.
Efficient hydro cooling requires the following: water moving over as much of the product surfaces as possible; water flows in shower-type hydro coolers of 10 to 15 gallons/minute per ft 2 of hydro cooler area; water as cold as possible without danger to the produce Most hydro coolers using a moving conveyor under a water shower, although some batch type shower hydro coolers are used. “Submersion hydro coolers” offer another option but are seldom as efficient as shower hydro coolers because of the difficulty of maintaining proper movement of water and produce. Vacuum Cooling Leafy vegetables are cooled on a large scale by enclosing them in airtight chambers and pumping out air and water vapour, thus cooling by evaporation of water from the product surfaces (at least 4.6mm mercury pressure, water boils at zero degrees C). Packed produce can be quickly and uniformly cooled in large loads by this method but cooling can be slowed by container walls or liners, film wraps or other barriers that retard water vapour escape. Vacuum cooling causes a water vapour loss in produce equal to about one percent of the produce weight for each 11 degrees of cooling.
Wetting of products before or during vacuum cooling reduces the amount of water loss from the produce. Vacuum cooling is not efficient with bulky products having a small surface to mass ratio. Package Icing (Top Icing, Slurry Icing) Packaging produce with finely crushed ice is an old cooling method that can be effective. To cool produce from 35 degrees to zero degrees requires melting of ice equal to 38% of the produce weight. Additional ice must be melted to remove heat leaking into the package from the outside. Package icing is often ineffective because the packages are not sufficiently protected from warming or insufficient ice is used. Packages must be capable of withstanding the prolonged exposure to free water that will be encountered.
High Humidity Storage
All fruit and vegetables lose water therefore weight, after harvest. Signs of this vary with the product but are generally visible when the loss is between three and eight percent. How fast evaporation occurs depends on several factors. The structure and condition of the product will affect it Surrounding air temperature is important because the higher the air temperature, the greater its drying power. Flesh temperature relative to the surrounding air temperature also plays a part and makes a fast cooling process desirable to reduce weight loss due to the temperature difference between product and air during cooling. The relative humidity (RH) of the surrounding air is however the dominant practical factor in weight loss – the drier the air the more rapidly water will move out of the product.
It is best to cool and store most fruit and vegetables under high RH conditions – in excess of 90%. But most cool stores
with direct expansion refrigeration systems operate in the region of 65 to 85% RH. This can be slightly increased by reducing the inflow of heat, avoiding overloading, adequate defrosting and good storage practices, but these are not sufficient to significantly lengthen the storage life. Checks on weight loss can be carried out to determine the storage time available before quality suffers. This is done by keeping one to five kg fruit in a string bag in the cool store and weighing it at regular intervals. However, increased storage life can be obtained and RH of 95% or more can be achieved in several ways.
Note: weight loss of produce is some eight times greater at 90% RH than at 100% RH and four times greater at 88% RH and 95% RH.
Considering all these factors, a practical cold chain system for a strawberry operation is as follows:
1. Receiving drop temp cold room
2. Receiving blast cooler
3. Receiving cold room
4. Packhorse temperature control
5. Staging and boxing room
6. Despatch blast coolers
7. Despatch cold room
Other considerations:
1. Sealing doors with curtain strips for access into refrigerated areas
2. Temperature monitoring and logging system
3. Dock seals on receiving and despatch doors.