Mechanisms of Food Additives, Treatments, and Preservation Technology 299
Pectinolytic enzymes pectins in plant foods are attacked by a series of enzymes.
Pectin esterases, which occur widely in plants and microorganisms, demethylate
pectin to pectic acid. The other group of pectolytic enzymes are hydrolases and
lyases, which attack the glycosidic bond in polygalacturonides. Polygalacturonases
occur in plants and microorganisms; pectin and pectate lyases are only produced by
microorganisms. During processing pectinolytic as well as other enzymes exude
from bruised cells and may diffuse into inner tissues and contribute to softening and
other modifications of texture. Pectin esterase is responsible for cloud flocculation
in citrus juices. Pectinolytic enzymes and other enzymes (amylolytic and
celullolytic) are produced by microorganisms (molds) during the microbial deteri-
oration of fruit or vegetables. These enzymes are also used for pulp-enzyming before
pressing of fruit and vegetable juices or to clarify the squeezed juices (Belitz and
Grosch, 1999).
Peroxidase is widely present in plants and catalyzes the peroxidatic reaction:
ROOH + AH 2 ↔ H 2 O + ROH +A.Peroxidase is very important in the ripening process, although its role is not fully
understood. It is believed to contribute off-flavors and off-odors in some food
products (Robinson, 1991). Peroxidase is still understood as a potential source of
deterioration of plant products. Peroxidase activity may result in oxidative actions
that involve hydrogen donor components. For some fruits (e.g., cantaloupe melon)
peroxidase activity could be related to tissue response to increased oxidative stress
in cut fruit (Lamikarna and Watson, 2000). Peroxidase mainly contributes to changes
in fresh produce. Residual peroxidase is often used as a measure of effectiveness of
processing methods in a number of processed fruits and vegetables. In some cases,
such as in frozen fruits and vegetables, residual peroxidase activity may be less
important for quality assessment than residual lipoxygenases. In such cases, blanch-
ing optimization may be linked to residual lipoxygenases (Sheu and Chen, 1999).
10.1.3 CHEMICAL CHANGES
Chemical changes in fruits and vegetables occur mainly during processing and
storage. At the beginning stages, after harvest, these changes are not so important
compared with enzymatic or physiological deterioration. Two principal chemical
changes occur during the processing and storage of fruit and vegetables that lead to
a deterioration in sensory quality: lipid oxidation and nonenzymatic browning
(within those also hydrolytic reactions). Chemical reactions are also responsible for
changes in the color and flavor of foods during processing and storage. Oxidation
of polyunsaturated lipids is responsible for production of off-flavors and off-odors.
The course of the reaction is influenced by light, oxygen concentration, high tem-
perature, the presence of catalysts (generally, transition metals such as iron and
copper), and water activity. Fruit and vegetable products containing seeds or pro-
cessed products in which the oil is in contact with oxygen, such as dried, unpeeled
fruits and dried strawberries containing seeds, are quite susceptible to lipid oxidation
during storage. Nonenzymatic browning is also a major cause of deterioration during