Page 286 - Natural Antioxidants, Applications in Foods of Animal Origin

Application of Natural Antioxidants in Dairy Foods 265
VetBooks.ir subsequent storage. Thus, the knowledge of the principal oxidants and anti-

oxidants occurring in the foodstuff is indispensable for understanding the
oxidative reactions that may occur in the food components and their implica-
tion on food quality and safety. Moreover, it is must to have the information
pertaining to enzymatic and non-enzymatic catalysts of oxidation.
The various oxidants present in the dairy products include PUFA attached
to triglycerides, phospholipids, cholesterol esters, riboflavin, proteins, and
so forth. Phospholipids are considered to be pro-oxidant because of the pres-
ence of monounsaturated and PUFA group attached to diglyceride or sphin-
gosine in it (Table 7.1). However, Chen and Nawar (1991) reported that
phospholipids can act as either pro-oxidant or antioxidant in dairy products
depending on the pH and ratio of water to phospholipid species in it. Simi-
larly, the thiol groups (-SH) in milk are reported to act as pro-oxidant or anti-
oxidant depending upon the conditions. Yee and Shipe (1982) observed in a
model system, free thiol groups in the presence of copper promoted oxidation
of emulsified methyl linoleate, while it acted as antioxidant component in
the presence of haem. Milk and milk products may contain metallic compo-
nents (such as transition metal ions- cupric, ferric, and haem proteins) that
normally act as pro-oxidant by catalyzing the decomposition of preformed
hydroperoxides to initiate the new oxidation chain reactions (Labuza, 1971;
Korycka-Dahl & Richardson, 1980; Pokorny, 1987). The ferri-porphyrin
proteins, together with their juxtaposition with lipids in the milk fat globule
membrane (MFGM) have powerful pro-oxidative properties (Kendrick &
Watts, 1969; O’Connor & O’Brien, 2006). Juxtaposition of copper-protein
complex with the phospholipids of the MFGM is also an important factor
in development of oxidized flavor in liquid milk (Samuelsson, 1966). The
water-soluble vitamin, that is, riboflavin, is a potent photosensitizer and
is associated with photo-oxidation of milk. Besides riboflavin, porphyrins
and chlorophylls are also reported to be involved in the photo-oxidation
of certain milk products like cheese (Wold et al., 2005). It is reported that
concentrations of ascorbic acid above those in normal milk (~20 mg/L)
could provide antioxidant protection; however, at the normal concentra-
tions in milk, ascorbic acid acts as a pro-oxidant (O’Connor & O’Brien,
2006). Caseins are more effective as antioxidant than whey proteins. The
antioxidant activity of casein may be attributed to the hydrophobic nature
of the same (Taylor & Richardson, 1980; Eriksson, 1982) and orientation
of potential antioxidant side-chains of constituent amino acids at the lipid
interface. Among the whey proteins, lactoferrin has also been reported to
inhibit peroxidation induced by Fe by chelating it (Gutteridge et al., 1981;
2+
Allen & Wrieden, 1982).