Mechanisms of Food Additives, Treatments, and Preservation Technology 301
These complexes usually differ in color from the original anthocyanins, and they can
indicate contamination of products with metal ions (e.g., from the packaging and/or
processing equipment). Products of nonenzymatic browning reactions accelerate
destruction of anthocynanins^ (Clydesdale, 1997; Belitz and Grosch, 1999). Betalaines
are nitrogenous red or yellow glycosidic, water-soluble pigments present in vegetables
(red beets and other beets). Unlike the anthocyanins, the hue of the betalaines is not
dependent on pH, but they are more sensitive to heat and oxygen^ (Clydesdale, 1997).
The carotenoids are a group of mainly lipid-soluble compounds responsible for
many of the yellow and red colors of plant and animal products. Carotenoids are
more stable than other pigments. During heat treatment they undergo cis/trans
isomeration. Cis isomers are less stable to oxidation. Oxidation is usually a complex
reaction with mechanisms similar to autoxidation of polyunsaturated lipids. Isomer-
ization and oxidation of carotenoid pigments are accelerated by the products of other
degradation processes (e.g., nonenzymatic browning reactions). The degradation of
carotenoids contributes to off-flavor formation and to undesirable color changes
(e.g., during the production and storage of tomato paste or tomato ketchup)
(Clydesdale, 1997). Undesirable production of new pigments may occur, particularly
when fruits and vegetables are exposed to light. The formation of brown or black
pigments by the reaction of quinones produced by the enzymatic browning reaction
or the formation of dark melanoidines at the final stages of nonenzymatic browning
were mentioned above. Other examples of undesirable color changes include the
formation of pink pigment in pickled, cut white cabbage, cauliflower in acid brine,
onion purées, postcooking blackening of potatoes, and formation of green-blue
pigments in garlic paste and other products. The blackening that may occur after
cooking potatoes is the formation of a complex between chlorogenic acid present
in the peridermal layers of tubers and ferric ions during cooking. These complexes
are oxidized, and based on the pH of the medium they form green, blue-grey, or
black products^ (Hugghes and Swain, 1962; Friedman, 1997). Mechanisms of color
changes during the processing of garlic are not sufficiently clear. The tendency to
turn green is higher when the cloves are processed in their physiological active stages
(i.e., after harvest or at the end of the dormancy). The rate of greening depends on
the activity of allianase and content of propenylcysteine sulfoxide (Velísˇek, 1999;
Ahmed and Shivhare, 2001). The other mentioned changes have not been explained
yet but also correspond with physiological activities at the time of processing and
are dependent on the concentration of potential precursors, which are formed in
processed tissues by physiological or enzymatic reactions. The aim is to optimize
processing in accordance with the physiological state of the produce and minimize
undesirable enzymatic changes in processed tissues.
Softening and texture changes, cutting, peeling, and, especially, slicing result in
dramatic losses in firmness of fruit tissues. The tendency to soften is dependent on
the physiological state. Softening itself is a complex process that includes physio-
logical, enzymatic, chemical, and physical changes in the tissue. Pectolytic and
proteolytic enzymes exude from the bruised cells and diffuse into inner tissues
(Varoquaux et al., 1990). Protopectin is transformed to water-soluble pectin, cellulose
crystallinity decreases, and cell walls become thinner. Sugars and other components