Water and Its Relation to Fresh Produce 279
9.7.1 TEMPERATURE
Although the relationship between humidity and water exchange is easily explained,
that between temperature and water is more complex. Kays (1997) explains three
thermal parameters that have a significant effect on moisture exchange in storage:
(1) the actual temperature, (2) the differential in temperature between product and
environment, and (3) the fluctuations in storage temperature. If the weight of water
vapor in air is held constant, lowering the temperature decreases the maximum
amount of water the air will hold and the relative humidity increases. Similarly, the
water vapor deficit between the produce and its immediate surroundings will
decrease at a given humidity with decreasing temperature. It is therefore important
to lower produce temperature to decrease its rate of water loss. A produce temper-
ature that is higher than the temperature of its environment, especially in succulent
produce, results in an increased vapor pressure difference between the gas atmo-
sphere within the produce and the surrounding cooler air (Burton, 1982). Conse-
quently, the gradient created results in increased water loss from the produce to the
environment. It is therefore strongly recommended that harvested produce be cooled
as quickly as possible to minimize water loss prior to refrigeration. Table 9.4 shows
TABLE 9.3
Daily Water Loss Rate, Days of Shelf Life, and Maximum Water Loss
in Some Selected Fruits and VegetablesProduceWater Loss of
Original Fresh Weight
(% loss d–1 mbar wvpd–1)Days of
Shelf Life
at 20°CMaximum Water Loss
Before Becoming
Unsalable (%)Bean 1.8–3.6 4–6 6
Brussels sprout 2.8 3–5 8
Cabbage 4–11 7–10
Carrot 3–7 4–8
Cauliflower 1.9 3–4 7
Cucumber 0.4 4–7 5
Lettuce 7.5 2–3 3
Onion 7–10 10
Potato 0.05–0.5 10–15 7
Pea 1.3 3–5 5
Green Pepper 3–5 7
Spinach 11.0
Sweet corn 1.4 2–4 7
Tomato 2–5 7Blackberry 2–3 6
Raspberry 1–4 6
Strawberry 0.7 2–3 6
Source:Robinson, J.E. et al., Ann. Appl. Biol., 81, 399, 1975. With permission.