Page 289 - Natural Antioxidants, Applications in Foods of Animal Origin

268 Natural Antioxidants: Applications in Foods of Animal Origin
VetBooks.ir on oxygen or sulfur (on proteins) and oxygen excited-state species ( O )

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generated by heat, light, or metal catalyzed reactions target double bonds
and generate reactive forms of these substrates (homoradicals). Subsequent
cross-reactivity of intermediate species produces complex product matrices
including auto-oxidative molecular products (Bennett et al., 2013).
In a mixed food system, lipids are considered to be most vulnerable to
oxidation and also to initiate attack on non-lipid substances. In milk and
dairy products, lipids, besides serving as the precursor of certain flavorful
compounds like methyl ketones (Kinsella, 1969a) and lactones (Wadhwa
& Jain, 1985), also induce the formation of few undesirable compounds
that cause off-flavor defects via hydrolytic and oxidative rancidity. The
oxidative flavors are products of autoxidation of unsaturated fatty acids of
triglycerides, phospholipids, and cholesterol esters, which are essentially a
free-radical chain reaction involving initiation, propagation, and termina-
tion stages. Unsaturated fatty acids oxidize to form unstable odorless and
tasteless hydroperoxide that degrade to yield flavorful carbonyls and other
compounds (O’Connor & O’Brien, 2006). It is assumed that during autox-
idation, initially oxygen attaches itself in loose linkage to a double bond
of the unsaturated fatty acid (Gunstone & Hilditch, 1945). This results in
an activation of the adjacent (α-) methylene group, from which a hydrogen
atom or proton gets loosened leading to drawing the oxygen atom towards
it to form hydroperoxide and not peroxide, the process creates a new double
bond at a different place; which has a trans configuration as opposed to
the cis configuration of natural unsaturated fatty acids. Thus, the reaction
proceeds via free radical mechanism. It is reported that cow ghee has lesser
tendency in comparison to buffalo ghee for autoxidation because cow ghee
absorbs oxygen slowly in comparison to buffalo ghee (Rangappa & Achaya,
1975). The oxidized unsaturated fatty acids, particularly oleic, linoleic, lino-
lenic, and arachidonic acid lead to formation of n-alkanals, alk-2-enals, and
alk-2,4-dienals. The type of off-flavor perceived depends upon the quality
and quantity of carbonyls formed. Kinsella (1969b) reported that C –C
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alkanals possess oily and tallowy odor, C –C alk-2-enals exhibit oxidized
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painty odors, alkanals (C , C , and C ) and alk-2-dienals may exhibit nutmeg
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spicy odors. The potent flavor compound oct-1,3-diene produced by auto-
oxidation of linoleic acid is responsible for metallic flavor. Unsaturated
alcohol pent-1-ene-3-ol produced from linoleic acid is responsible for an
oily and grassy aroma (Forss, 1972).
Riboflavin has been traditionally considered as an active photosensi-
tizer occurring naturally in dairy products like, milk, cheese, butter and its
presence makes these products susceptible to photosensitized oxidation.