296 Produce Degradation: Reaction Pathways and their Prevention
exposed to high temperatures the deterioration rate is increased. Exposure to solar
radiation is an example; produce left in the sun after harvest may reach temperatures
as high as 50°C. It will achieve a high rate of respiration and, if packed and
transported without cooling or adequate ventilation, it will become unusable. Long
exposure to tropical sun will cause severe water loss from thin-skinned root crops
such as carrots and turnips and from leafy vegetables (Dauthy, 1995).
10.1.1.3 Loss of Water
Fresh produce continues to lose water after harvest, but unlike the growing plant it
can no longer replace lost water from the soil and so must use up its water content
remaining at harvest. This loss of water from fresh produce after harvest is a serious
problem, causing shrinkage and loss of weight. When the harvested produce loses
5 or 10% of its fresh weight, it begins to wilt and soon becomes unusable. To extend
the usable life of produce, its rate of water loss must be as low as possible.
10.1.1.4 Undesirable Effects on Respiration
Fresh produce belongs to the “respiring” group of food. Respiration, the basic
reaction of all plant material, also continues after harvest until consumption, pro-
cessing, or decay. The extent of gas exchange depends on various factors, such as
temperature, O 2 and CO 2 concentrations, presence of ethylene, and degree of
mechanical damage of plants. Conditions that accelerate respiration are usually
undesirable because they usually accelerate senescence and decay of the produce.
Similarly excessive reduction of the respiration rate can be also undesirable. Such
an effect is caused, for example, by a reduction of the oxygen concentration in the
surrounding atmosphere below the critical limits, or by an increase in the carbon
dioxide concentration above tolerable levels. When the air supply is restricted and
the amount of available oxygen in the environment falls to about 2% or less, there
is a risk that anaerobic respiration (anoxia) and fermentation instead of respiration
will occur. Fermentation breaks down sugars to ethanol and carbon dioxide, and the
alcohol produced unpleasant flavors in produce and promotes premature aging
(Rudell et al., 2002). Similar situations may occur when ventilation is inadequate.
A restricted air supply leads to the accumulation of carbon dioxide around the
produce. When the concentration of this gas rises to between 1 and 5% in the
atmosphere, it will quickly ruin produce by causing bad flavors, internal breakdown,
failure of fruit to ripen, and other abnormal physiological conditions (Kader et al.,
1989). Maximum levels of carbon dioxide and minimum levels for oxygen for
storage of selected fruits and vegetables are given in Table 10.2.
10.1.2 ENZYMATIC CHANGES
Physiological and enzymatic changes caused by endogenous enzymes are difficult
to adequately classify. In this approach the physiological changes are more complex
than the individual reactions previously described. Individual enzymatic reactions
that are involved in postharvest senescence and spoilage belong to physiological
changes with many parallel processes. Examples of undesirable enzymatic changes