Precooling is a technique used to lower the temperature of fruit prior to cold storage or shipping. Precooling began in the early 1900s to prepare fruit for rail shipment or export. The importance of pre-cooling depends on the fragility of the product and the storage life expectancy. The choice of the precooling temperature depends on temperature at harvest, sensitivity of the fruit to chilling, physiology of the product, and potential postharvest storage life. The rate of cooling depends on various factors: (1) rate of heat transfer from the fruit to the surrounding medium, (2) temperature difference between the fruit and the cooling medium, (3) nature and physics of the cooling medium, and (4) thermal conductivity of the fruit. Various methods to cool harvested fruit fall into passive or active categories. Passive techniques include shading, placing fruit in a cold room, or simply adjusting harvest time to early or late in the day to avoid the highest midday temperatures. Active techniques include hydrocooling or forced-air cooling. Most tree fruit are cooled by the cold-air method. However, stone fruit can be rapidly and effectively cooled by hydrocooling, which involves a cascading cold-water drench.
Hydrocooling requires the most handling but is the quickest form of heat removal. The cooling medium, water, comes in direct contact with the entire fruit surface, and heat is given up to the liquid water, which has a higher heat transfer coefficient than air. The target temperature for water in a hydrocooler is 0°C. The rate at which hydro-cooling will lower fruit temperature depends on several factors:
(1) temperature differential between the fruit body and the water,
(2) velocity of water cascading over the fruit, (3) relative surface area being covered by the cool water, (4) thermal conductivity of the water, and (5) total volume of water being used. Active versus passive system efficiency is illustrated in peaches by the time required to lower the temperature by 90 percent of the difference between the initial fruit temperature and the cooling medium temperature. Hydro-cooling can achieve the temperature goal in approximately 20 minutes, while forced-air cooling requires three hours, and passive room cooling requires 18 hours. In addition to handling considerations, hydrocooling involves potential contact with liquid-carrying pathogenic spores from other infected fruit and an increased risk for decay. Sanitation of the water drench is important and can be managed through the use of products such as chlorine or chlorine dioxide. Monitoring and maintaining these disinfectants at effective levels is important to minimize decay losses.
Some temperate fruit do not lend themselves to the direct water contact of hydrocooling techniques, and thus air cooling must be used to remove field heat. For forced-air cooling to be effective, care ful planning is essential. Refrigeration capacity, container design and placement, along with fan placement, size, architecture, and speed, govern the rate and efficiency of cooling. The goal of forced-air cooling is to keep cold air moving at 61 to 122 meters per minute across the fruit. Rapid cooling in this fashion relies on maximum air movement around individual fruit rather than around containers. Large containers can significantly reduce the rate of air penetration and therefore increase cooling time for fruit that comprise the center core of a bin. Vented bulk containers used in forced-air cooling should allow rapid airflow and disbursement of the flow to come in contact with as much fruit surface area as possible. Tight stacking of bins in cold rooms is essential for maximizing cooling rates. Without tight stacking or with misalignment of bins, boxes, or crates, airflow patterns can be disrupted and result in short cycling and some produce not being cooled actively. Bin or carton stacking into the configuration of a tunnel, with fans placed directly against the container stacks, provides a configuration that promotes a negative pressure gradient and draws rather than pushes cold air across and around fruit from spaces between and vents within containers. Tarps can be used to seal openings that otherwise would allow the air to short-circuit and not come in contact with fruit. Serpentine cooling is another form of forced-air cooling in which barriers are placed over alternating forklift openings, thus forcing air to move either up or down through the fruit as it moves toward the fans.
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