300 Produce Degradation: Reaction Pathways and their Prevention
storage of dried and concentrated foods. Nonenzymatic browning includes the Maillard
reaction (reaction of reducing sugars with α-aminocompound, e.g., aminoacid),
which can be divided into three stages: (1) early Maillard reactions, which are
chemically well-defined steps without browning; (2) advanced Maillard reactions,
which lead to the formation of volatile or soluble substances; and (3) final Maillard
reactions, which lead to insoluble brown polymers. The Maillard reaction is usually
a limited part of a series of very complex chemical reactions of the compounds
present in the food products. Other nonenzymatic browning reactions include acid
degradation of sugars (inversion of sucrose, formation of 5-hydroxymethyl-2-furan-
carbaldehyde (HMF) and 2-furancarbaldehyde (F) from hexoses and pentoses). The
nonenzymatic browning reaction is important during processes such as frying, dry-
ing, concentration, and long-term storage of preserves at ambient or elevated tem-
peratures, and these reactions contribute to the overall deterioration of the products
(color changes, off-flavors, off-odors, usually as a contribution to damaging the fresh
appearance and flavor of the processed fruit or vegetables).
Color changes, pigment modifications in foods are mainly physiological. These
include the development of color in fruits such as tomatoes and bananas during
ripening after harvest, or physiological degradation of chlorophyll and other pig-
ments at the end of senescence and in early phases of decay. Chlorophylls are green
plant products containing two types of chlorophyll pigment, a and b, in the ratio of
about 3 to 1. Almost any type of food processing or storage causes some deterioration
of the chlorophyll pigments.
Chlorophyll decomposition is a two-step process. In the first step, magnesium
is removed from the molecule, producing dull, olive-brown pheophytin (phenophy-
tinization); in the second step, pheophytins are converted to pheophorbids (Schwarts
et al., 1981). Pheophytination is the major color change during processing and
storage of green plant materials. Heat accelerates the rate of the change and it is
acid-catalyzed. López-Ayerra et al. (1998) observed the degradation of chlorophylls
in raw, frozen, and canned spinach and found that about 15.9% of chlorophylls a
and b were lost during the freezing process and 99.9% after canning as a consequence
of the heating used in industrial processing. Magnesium in the chlorophyll molecule
can be displaced by other metal ions (e.g., copper or zinc); the product is more stable
and has a different hue. The reaction with contaminating copper ions during the pro-
cessing is a potential cause of undesirable color changes. Different stability of chloro-
phylls a and b is used for the evaluation of age of the product (frozen storage, etc.).
Anthocyanins are a group of more than 150 reddish, water-soluble pigments that
are very widespread in the plant kingdom. The rate of anthocyanin destruction is
pH-dependent and is greater at higher pH values. For example, according to Wesche-
Ebeling et al. (1996) preserved plums after 90 days of storage at room temperature
contained 77% of their original anthocyanin at pH 2.95 but only 29% at pH 3.45
and 8% at pH 3.95 as a result of anthocyanin degradation. The reaction proceeds in
two steps. The glycosides anthocyanins are hydrolyzed to anthocyanidins and sugar;
production of aglycons can increase the intensity of color, but aglycons are less
stable and undergo other degradation reactions. Some anthocyanins are able to form
complexes with metals such as Al, Fe, Cu, and Sn and are important for packaging.