Mechanisms of Food Additives, Treatments, and Preservation Technology 329
10.2.3.3.2 Chilling and Freezing
Metabolic and enzymatic activities, as well as the rate of microbial growth, generally
decrease when the temperature is lowered. Normally, the generation time for micro-
organisms is doubled for every 10°C reduction of temperature. The sensitivity of
various microorganisms to temperature is different. The majority of spoilage and
pathogenic microflora do not grow below 5°C. However, some psychrotrophic patho-
gens such as Listeria monocytogenes, Yersinia enterocolitica, Clostridum botulinum
types A and E, Aeromonas hydrophyla, (enterotoxigenic), and E. coli are able to
multiply slowly in refrigerated foods. Refrigeration thus appears to be the only hurdle
in the preservation of high-moisture or intermediate-moisture food products, unlike
fruit and vegetable products that require more than one treatment (Bibek, 1996).
When the temperature of food is reduced below –2°C, free water in fruit or vegetables
starts freezing and forming ice crystals. The preservation effect of freezing also
results in subsequent damage of the microbial cells by extra- and intracellular ice
formation and concentration of extra- and intracellular solutes (Lund, 2000). Frozen
fruit and vegetable products are more stable than refrigerated products for long
periods of time if the texture is unaffected by the very low temperature. Frozen foods
are relatively safe. The few outbreaks of food-borne illness associated with frozen
foods indicate that commercial freezing processes kill some, but not all, human
pathogens. Archer (2004) evaluated freezing preservation methods. Freezing is more
likely to damage the texture of the produce than is refrigeration. Compared to
refrigeration, the freezing process damages the structure and processes in the plant
tissues. Upon thawing of frozen food items, microbiological, enzymatic, and chem-
ical changes are accelerated. Blanching prior to freezing will reduce the extent of
enzymatic changes that occur in thawed frozen foods. Formation of ice crystals and
subsequent damage of cells also decreases the microbial stability of the thawed
products. The microorganisms present on the surface of frozen fruit or vegetables
grow in the released juice without any barrier of the former intact fruits. The extent
of undesirable changes depends on the number and size of ice crystals. The ice
formation should be as fast as possible to minimize the food’s structural damage.
Undesirable changes of fruits and vegetables can be prevented by high-pressure
freezing, the use of cryoprotectant and cryostabilizers, dehydrofreezing, and appli-
cations of antifreeze protein and ice nucleation protein.
The thawing process can be optimized by high-pressure thawing, microwave
thawing, ohmic thawing, and acoustic thawing to shorten thawing time, reduce drip
loss, and improve product quality (Li Bing, 2002). These methods reduce the time
required for frozen materials to thaw and the risk of overheating. High-pressure
freezing is a process that involves subjecting the material to be frozen under pressure
and cooled below the freezing point. When the product is sufficiently cool, the
pressure is released and the product immediately freezes, allowing formation of a
high number of very small ice crystals with minimal damage to texture. During the
rapid phase change the bubbles of air are trapped in tissues. Pressure-frozen fruits
have softer textures and could be eaten without thawing (LeBail et al., 2002). During
storage the structure of frozen products may change due to recrystallization, espe-
cially when the storage temperature is not constant. High-pressure thawing of fruit