Temperature Effects on Produce Degradation 617
20.3.2.2 Hydrocooling
Hydrocooling is accomplished by moving cold water around the produce with a
shower system or by immersing the produce directly in cold water.^23 Because water
is a better heat transfer medium than air, hydrocoolers reduce temperature faster
than forced-air coolers. For most products, cooling water is kept at 0 to 0.5°C.^3 If
the produce is simply immersed in the cold water, the water in contact with the
produce will heat up and cooling will be slowed. Therefore, to be effective, a
hydrocooler must cause the water to move over and contact all surfaces of the
produce. The water must also be free of disease-causing organisms.
There are two types of hydrocoolers.^3 Shower-type coolers rain water down over
the produce. They can be built with a slowly moving conveyor for continuous flow
operation, or they can be batch-operated. Immersion coolers are best suited for dense
products that sink. They usually cool slower than shower coolers because the water
flows past the product at a slower rate.
Cooling time in hydrocoolers depends on the diameter of the produce and water
flow rate.^3 With the same flow rate, small-diameter products, such as cherries,
radishes, and asparagus, cool faster than large items, such as melons. Asparagus
spears, being long and narrow, can be hydrocooled in about 2 min, whereas large
globular capsicums require 10 min.^20
Hydrocoolers do not cause moisture loss during cooling and, in fact, can rehy-
drate slightly wilted produce.^23 Because hydrocooler water can spread spoilage
organisms, the water should be obtained from a clean source and treated (usually
FIGURE 20.7Response of a typical commodity to airflow rate during cooling. Ta is the
beginning temperature of the product; Tb is the air temperature in the cooling room. The time
required to lower the produce temperature to 7/8 of the difference between Ta and Tb drops
as airflow rate is increased. (From Boyette, M.D., Wilson, L.G., and Estes, E.A. Forced air
cooling, Publication AG-414-3 The North Carolina Agricultural Experiment Station, Raleigh,
NC, 1989. Use with permission.)
A7/8 CoolingA Still Air
B 1/2 cfm/lb
C 1 cfm/lbTemperatureRelative Time12345678910TbTaB
C