Water and Its Relation to Fresh Produce 281
leading to a rapid loss in quality. Thompson et al. (2001) reported that cooling delays
cause reduced product quality for three reasons: (1) they allow respiration and
associated normal metabolism to continue at high rates, consuming sugars, vitamins,
and other constituents; (2) they foster water loss; and (3) they increase the development
of decay. Methods used to cool fresh produce include cold air circulation (room cooling,
forced-air or pressure cooling), cold water treatment (hydrocooling), icing, and evapo-
ration of water (evaporative cooling, vacuum cooling) from the produce.
Room cooling is the most common and simplest procedure used to initially cool
fresh produce packaged in boxes or bulk containers. The produce is exposed to cold
air in a normal cold storage environment. Thompson et al. (2002) stated that slow
cooling and possibly excessive loss of water are the main disadvantages. Pressure
(air-cooling) cooling, performed by forcing air through packages, is more efficient
than room cooling because it cools the produce rapidly. Some of the common
methods of air-cooling include tunnel-type, forced-air cooling, cold wall, serpentine
cooling, forced-evaporative cooling, and container venting, as well as cooling during
transportation. In general, the forced-air cooling method is much faster because it
causes cold air to move through containers (Perez et al., 1998).
Water has a higher heat capacity than air, and it is used to cool fresh produce
in a system referred to as hydrocooling. Water maintained at 0°C rapidly cools the
produce as it passes under a shower or is soaked in a cold bath. Hydrocooling reduces
the likelihood of water loss and may even add water to slightly wilted products,
especially leafy vegetables (Wills et al., 1998; Cantwell et al., 2001; Thompson et
al., 2002). Issues that arise due to hydrocooling include microbial recontamination,
inability of the produce to withstand cold wetting, and chemicals such as chlorine
used in water treatment.
Ice (crushed or as flakes) placed in contact with the produce is often used as a
cooling agent, especially during transportation. The ice in direct contact with the
product rapidly cools the product, and the melting ice contributes to high humidity,
resulting in reduced water loss. Perrin and Gaye (1986) concluded that the quality
of broccoli in terms of retarding yellowing, retention of chlorophyll, and retention
of nutritional value was greatly retained when a bed of ice was placed beneath the
broccoli heads in combination with mechanical refrigeration. Some leafy green
vegetables that have a high surface area-to-volume ratio can be cooled by evaporation
of water through vacuum cooling under low atmospheric pressure. Fresh produce
in sealed chambers is subjected to reduced pressure, and as the water boils, the latent
heat of vaporization cools the produce. Loss of water during vacuum cooling can
be minimized by spraying the produce with water prior to sealing the chamber
(Thompson et al., 2002). Dry air cooled by blowing across a wet surface can also
be used to cool products.
9.7.2 ELEVATED RELATIVE HUMIDITY
Increasing the relative humidity of the environment in which the fresh produce is stored
is one of the major steps in preventing water loss and maintaining quality of because
humid environments reduce water transpiration. Watada et al. (1996) noted that due to
the large surface area, especially in fresh-cut products, there was a great potential to