Temperature Effects on Produce Degradation 601
those items that have the shortest storage life respire at high rates (most leafy vegeta-
bles), are harvested ripe (berries), or are chilling-sensitive (bananas and cucumbers).
The most important factor in controlling respiration rate is temperature.^4 Sweet
corn, fresh summer squash, and lima beans respire about 8 times faster at 21°C than
at 0°C; broccoli respires 15 times faster and spinach 11 times faster.^5
Van’t Hoff’s rule states that the velocity of a reaction increases two- to threefold
for every 10°C rise in temperature. The temperature quotient for a 10°C interval is
called the Q 10.^6 This value is useful since it allows calculation of the reaction rate
at one temperature from a known rate at another temperature. The Q 10 can be
calculated by dividing the reaction rate at a higher temperature by the rate at a 10°C
lower temperature, i.e., Q 10 = R2/R1.
The Q 10 values are useful in predicting quality of fresh fruits and vegetables;
however, they have limitations.^4 Because produce respiration rates do not follow
ideal behavior, Q 10 values of fresh fruits and vegetables are usually given within
specified temperature ranges. The Q 10 values of fresh fruits and vegetables are
generally lower in higher storage temperature ranges (greater than 10°C) than at
lower temperatures, and the Q 10 concept is not valid at low temperatures for chilling-
sensitive products. Values are dependent on time, the physiological state of the
material, and the continuing physiological activity of the material.^7
Typical figures for Q 10 are shown in Table 20.3. These typical Q 10 values make
it possible to construct a table like Table 20.4 showing the effect of different
temperatures on the rates of deterioration and relative shelf life of a typical perishable
FIGURE 20.1The many positive effects of lowered temperatures lead to extensive use of
cool storage to restrict deterioration without causing abnormal ripening or other undesirable
changes. (Photo courtesy of Western Precooling Systems, Fremont, CA.)