Page 291 - Natural Antioxidants, Applications in Foods of Animal Origin

270 Natural Antioxidants: Applications in Foods of Animal Origin
VetBooks.ir cleavage, and promotion of cross-linking. Protein oxidation can be classi-

fied as direct or indirect oxidation. Direct oxidation involves attack of the
radical species or hydrogen abstraction from protein leading to formation of
a protein radical or anion. However, in indirect oxidation, the protein inter-
acts with other components like secondary lipid oxidation products, such as
aldehydes or reducing sugars, which may lead to the formation of carbonyl
compounds on amino acid side chains or formation of protein carbonyl
groups (Baron, 2013). Oxidation of proteins at the side chain often leads to
development of protein carbonyls, alcohols, and peroxides. In dairy prod-
ucts, methionine sulfoxide, dityrosine and cysteic acid have been detected
as the oxidation products of methionine, tyrosine, and cysteine, respectively
(Toran et al., 1996; Østdal et al., 2000).
It is believed that metal-catalyzed oxidation of protein is a predominant
mechanism in foods, because of abundance of transition metals, such as iron
and copper in the food matrix. Østdal et al. (2000) showed that in milk,
the lactoperoxidase can transfer a radical to other milk proteins, such as
β- lactoglobulin, casein, and serum albumin and has been suggested to be a
key element in oxidation of milk proteins. Milk proteins are also reported
to be susceptible to photo-oxidation. Among milk proteins, casein is more
susceptible to photo-oxidation than the globular proteins, α-lactalbumin,
β-lactoglobulin, and lactoferrin (Baron, 2013).
Various dairy products are reported to show flavor defects due to the
occurrence of oxidation of different components in the products like fat
phase in ice cream besides the positive effects on the flavor, is also reported
to be associated with the flavor defect like cardboard, painty, metallic, or
oxidized that may be due to the auto-oxidation or lipolysis of fat (Marshall
et al., 2003). Auto-oxidation of fat is also reported in milk powders and the
dry milk deposits may self-ignite to cause explosion in milk powder facto-
ries (Walker & Jackson, 1978; Knipschildt, 1986). The oxidation of milk
fat and the reducing sugar-protein browning (i.e., Maillard browning) are
the most significant deteriorative changes that occur in dry milk products
that result in the defects in sensory and functional properties of the product
(Farkye, 2006). Stapefeldt et al. (1997) reported that low-heat powders are
more susceptible to severe oxidative changes than medium-heat and high-
heat powders during storage. The unsaturated fatty acids in whole milk
powder oxidize to generate saturated aldehydes (Boon et al., 1976) which
are responsible for stale, tallow, cardboard, and painty flavors, while polar
lipids oxidize to generate unsaturated aldehydes and ketones which are
responsible for oxidized flavor even at a very low level (Kinsella, 1969a;
Keen et al., 1976; Hall & Anderson, 1985).