614 Produce Degradation: Reaction Pathways and their Prevention
20.3.2 PRECOOLING METHODS
Heat transfer occurs by conduction, convection, or radiation.^20 The most common
method of heat transfer in precooling is conduction (i.e., heat transfer by direct
contact). Since heat moves from a warmer substance to a cooler one, contact between
the warm produce and the cooling medium results in the produce being cooled. A
product with a large mass and a relatively small surface area will cool more slowly
than one with a larger surface area since heat must move from the interior to the
surface before it is removed from the product (Figure 20.4). For example, at an
airflow rate of 1 L kg–1 sec–1, small-diameter grapes can be air cooled in about 2 h,
while large-diameter cantaloupes will require more than 5 h.3,23 With 1.7°C water
as the cooling medium, it took more than 30 min to cool a 3-inch-diameter peach
from 32 to 4.4°C, but it took about 15 min for the same temperature change with a
2-inch peach.^24
A critical factor in determining cooling rate is the difference in temperature
between the produce and cooling medium.^23 A product is considered “half cool”
when its temperature drops so that the difference between product temperature and
cooling medium temperature is half of the initial difference. After another half-
cooling period the product is “three-quarters cool.” Because half-cooling time is a
constant value for a given system, the speed of cooling appears to slow as cooling
continues (Figure 20.5). Reducing the temperature the last few degrees may take an
extended period and is of little practical importance. In comparing cooling times for
various cooling methods, the time required to lower the pulp temperature three half-
cooling cycles, seven-eighths of the difference between the initial product temper-
ature and cooling medium, is often used.
Mathematically, rate of cooling is often expressed as either the cooling coeffi-
cient, C, or the half-cooling time, Z.^2 Theoretically, Z is independent of the initial
produce temperature and remains constant throughout the cooling period. Mathe-
matically it is expressed as Z = loge 0.5/C, where C is a negative value.
FIGURE 20.4Effect of product diameter on cooling rate. (Lurie, S., Temperature manage-
ment, in Fruit Quality and Its Biological Basis, Knee, M., Ed., CRC Press, Boca Raton, FL,
2002, Chap. 5.)
0204060807/8 Cooling time (min)0246810+ Cherry+ PeachTomato + + AppleCitrus +Melon +Product Diameter (cm)