Mechanisms of Food Additives, Treatments, and Preservation Technology 305
moderate temperatures for a period prior to low-temperature storage, which is effec-
tive for some fruits such as grapefruit (Hatton and Cubbebedge, 1983). The other
methods are intermittent warming, heat treatment, controlled atmosphere storage,
treatment with calcium or other chemicals, waxing, film packaging, and application
of ethylene, abscisic acid, methyl jasmonate, polyamines, or other natural com-
pounds. Low-temperature conditioning and intermittent warming maintains high
levels of phospholipids, increases the degree of unsaturation of fatty acids and the
amount of spermidine and spermine, and stimulates the activities of free radical
scavenging enzymes. Heat treatment induces heat-shock proteins, suppresses oxida-
tive activity, and maintains membrane stability. Methyl jasmonate can activate
lipoxygenase gene expression and induce synthesis of abscisic acid and polyamines.
Polyamines may act as free radical scavengers and membrane stabilizers. All of
these processes can enhance chilling tolerance of tissues and alleviate chilling injury
of fruits and vegetables (Wang, 2000).
In addition to chilling stress, some commodities are susceptible to heat stress.
High-temperature injury has been more of a concern during the growth and devel-
opment of plants than in postharvest handling, because it should not occur during
the normal handling of fruits and vegetables (Shewfelt, 1986).
10.2.1.2 Atmosphere
The effect of storage or packaging atmosphere composition is described above and
also in other chapters of this book. Prevention of undesirable changes necessitates
storing produce within its “safe” range below or above the critical limits for carbon
dioxide and oxygen, respectively. Reducing the concentration of oxygen and/or
increasing the concentration of carbon dioxide in the storage atmosphere surrounding
fresh produce reduces the rate of respiration of fresh fruits and vegetables and also
inhibits microbial and insect growth. It is usually combined with chilling. The
composition of the surrounding atmosphere can be controlled either during the
storage of the produce or within its packaging. These methods are used for storage
and packaging of the both respiring and nonrespiring produce. Modified atmosphere
storage (MAS) and modified atmosphere packaging (MAP) use gases to replace the
air around produce (or also involve sealing packages of fresh produce without gas
modification) without further control of the produce. When respiring produce is
stored or packaged under these conditions, gas equilibrium is established due to the
properties of the packaging material. Changes in gas composition during storage
depend on the respiration rate of fresh produce and hence on the temperature of
storage, and, in the case of the MAP, also on the permeability of the packaging
material to gases and water vapor, the internal and external humidity, which affect
the permeability of some films, and on the surface area of the packaging in relation
to the amount of food it contains. For MAP of fresh produce the terms equilibrium-
modified atmosphere (EMA) or passive atmosphere modification (PAM) are used.
In controlled atmosphere storage (CAS) or controlled atmosphere packaging (CAP)
the composition of gas around respiring produce is monitored and constantly con-
trolled. Recent advances in both systems now make the distinction between MAP
and CAP less clear (Fellows, 2000c; Gorris et al., 1994). The packages for CAS or