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As reported in various studies (Ferguson & Stanley, 2003; Heyes et al., 2010; Manolopouplou & Papdopoulou, 1997), lowering the respiration rate of fresh fruits is essential to preserving market quality. The most important technology for lowering respiration rates is proper cooling of produce within hours of harvest. Proper cooling preserves the product quality by inhibiting the growth of decay-producing micro-organisms; restricting enzymatic and respiratory activity; inhibiting water loss; and reducing ethylene production. In general, harvesting done in the early morning hours minimizes field heat and its exposure to the sun. Cooling produce to storage temperatures before packaging and transportation is important. Refrigerated loading and unloading should be used. Transportation trucks should be cooled before loading, and load pallets should be loaded towards the center of the truck. Insulating plastic trips should be used in the truck and in the loading docks. Produce should be moved rapidly to the storage area, at the appropriate temperature, and displayed at the appropriate temperature range (Hellickson, 2003).

All fresh horticultural crops are living organisms, even after harvest, and they must remain alive and healthy until they are either processed or consumed (Hellevang, 2003). The energy needed for living comes from the food reserves in the product itself. The process by which

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the reserves are converted into energy is known as respiration. Heat energy is released during respiration, but the rate varies depending on the type and variety of the product, the level of maturity, the amount of injury and the product temperature. Product temperature has the greatest influence on respiratory activity. Rapid, uniform cooling after harvest lowers the respiration rate which in turn reduces the rate of deterioration and help provide longer shelf life. Lowering the temperature also reduces the rate of ethylene production, moisture loss (wilting or shrivelling) and growth of micro-organisms. The resulting economic loss exceeds the increased cost of expedited handling of produce by more frequent deliveries from the field to the cooling facility for initiation of forced air cooling. This is not true for all crops, but is especially true for highly perishable produce in hot weather (Ezeike & Hung, 2009). Room-cooling means produce is simply placed in a cold storage room and cools slowly and non-uniformly, mainly through conduction and natural convective contact with refrigerated air. However, cold room is normally used to store previously cooled produce and does not have the capacity to remove the heat from the un-cooled produce. Most cold rooms will increase in temperature after each fresh batch of warmer produce is added. Forced-air cooling can quickly remove field heat from freshly-picked produce. High capacity fans are used to pull refrigerated air through the produce (Ezeike & Hung, 2009). Rapid and uniform cooling is achieved by the forced-convective contact of the high speed, refrigerated air with the warm produce. Pulling air rather than blowing it through is preferable, because air flow is more uniform using this method. With proper container design and orientation, produce can be rapidly and uniformly cooled in baskets, boxes, bins, or bags. Forced-air cooling simply does a better job with refrigerated air in cold storage (Ezeike & Hung, 2009).

Hydro-cooling occurs by flowing chilled water over the produce and rapidly removing heat. It is usually at least ten times faster than forced-air cooling in removing heat from the produce, but is less energy efficient. This cooling is not suitable for produce that is delicate and sensitive to wetting such as most berries. The other method of cooling is top-icing, in which crushed ice is placed over the produce in boxes or containers, where liquid icing injects slurry of water and ice into the produce packages (Ezeike & Hung, 2009). This is an effective method for dense produce such as broccoli that cannot be cooled easily by forced-

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air cooling. Vacuum cooling can be obtained by placing produce inside a vacuum chamber and applying vacuum, causing water to evaporate from the produce surface and hence lowering the produce temperature. It is an effective method for produce with a high surface- to-volume ratio, such as leafy vegetables. Wilting due to moisture loss during vacuum cooling can be prevented by pre-spraying produce with water (Thompson., 2003a).

Field heat must be removed quickly from kiwifruit after harvest as the fruit can lose water rapidly. After 3-4% water loss, the fruit may exhibit noticeable shrivelling, predominately at the stem end. As with all fresh fruit, the rate of water loss is directly related to the vapour pressure difference between the fruit and its environmental (Thompson., 2003a). Temperature and relative humidity control this vapour pressure difference. In warm field temperature with low relative humidity common during harvesting, the rate of water loss can be 25 to 50 times greater than at the recommended 0°C (32°F) and 95% relative humidity in a kiwifruit storage room. Thus, one hour in the field after harvest may result in as much as water loss as 1 or 2 days in storage room (Hasey et al., 1994). Another reason to rapidly cool

kiwifruit after harvest is based on the kiwifruit’s propensity to rapidly soften after harvest.

The softening process in kiwifruit is temperature dependent. For example, fruit softens three times faster at 5°C (41°F) than at 0°C (32°F). Rapid heat removal helps minimize flesh softening during subsequent 0°C storage if the flesh is not exposed to ethylene during cooling (Thompson., 2003a). Hydro cooling of kiwifruit is not recommended as prolonged wetting of the fruit reportedly worsens the occurrence of decay. Forced-air cooling, widely used on other fruits is preferred for rapid cooling of kiwifruits. This involves creating a slight pressure difference on opposite sides of bins or pallets to cause cold air to flow through side ventilation openings in the container to rapidly cool the fruit. The intimate contact between cold air and warm fruit results in rapid heat removal (Hasey et al., 1994). Many factors influence the rate of cooling which include:

1. the density of produce in the container (less dense the produce pile, the faster the cooling);

2. the container type, orientation and venting (if air passes uniformly and evenly around the produce, cooling is faster);

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3. the volume and surface area of the produce (lower the ratio, faster is the cooling, example; cherries cool quicker than melons);

4. the travel distance of the cooling air (shorter the distance, faster the cooling of the overall pile); and

5. the capacity of airflow (the higher the air flow, faster the cooling).

The ‘7/8’ cooling time is standard industry term that describes the time required to remove

seven-eighths (87.5%) of the temperature difference between the starting produce temperature and the temperature of the cooling medium (refrigerated air, in the case of forced-air cooling). It is a convenient method of indicating when the produce has come as close as practical to the temperature of the cooling medium (Ezeike & Hung, 2009). The cooling of kiwifruits should be in a similar manner as cooling of plums. Crops with high respiration rates (asparagus, broccoli, leaf lettuce, spinach, sweet corn, mushrooms) at harvest temperature must be cooled rapidly and quickly (less than 90 minutes) after harvest. Crops with high respiration rates at harvest temperatures (blueberries, raspberries, strawberries, sweet cherries, cauliflower, snap beans, head lettuce) should be forced-air cooled as quickly as it is practical (less than 3 hours) after harvest. While crops with low or moderate respiration rates at harvest temperatures (apples, kiwifruit cabbage, cantaloupes, celery, peaches, plums, peppers and squash) can be cooled within 4-5 hours of harvest to avoid quality loss (Ezeike & Hung, 2009).