Oxidation Products and Off-Flavors

Milk is characterized as having a pleasing, slightly sweet taste with no unpleasant after-taste (Bassette et al., 1986). However, its bland taste makes it susceptible to a variety of flavor defects. Autoxidation of unsaturated fatty acids gives rise to unstable hydroperoxides, which decompose to a wide range of carbonyl products, many of which can contribute to off-flavors in dairy products. The principal decomposition products of hydroperoxides are saturated and unsaturated aldehydes (Frankel et al., 1961), with lesser amounts of unsaturated ketones (Stark and Forss, 1962), saturated and unsaturated hydrocarabons (Forss et al., 1961), semialdehydes (Frankel et al., 1961) and saturated and unsaturated alcohols (Hoffman, 1962 Stark and Forss, 1966). 

In addition to those carbonyls that are theoretically possible from the degradation of the hydroperoxides of the principal unsaturated fatty acids in 

Despite the above difficulties, several specific chemicals have been associated with specific off-flavors in dairy products. Forss et al. (1955a,b) reported that -hexanal, 2-octenal, 2-nonenal, 2,4-heptadienal and 2,4-nonadienal are the principal carbonyls contributing to the copper-induced cardboard off-flavor in milk. Hall and Lingnert (1986) associated this flavor defect with n-hexanal in spray-dried whole milk. l-Octen-3-one has been associated with a metallic off-flavor in dairy products (Stark and Forss, 1962), the metallic off-flavor being reproduced by addition of l-octen-3-one to milk or cream (Bassette et al., 1986). l-Octen-3-one has a threshold concentration of 1 pg/kg in butterfat (Shipe et al., 1978). 

1- octen-3-one, the compound associated with metallic flavor, and an oily fraction containing n-heptanal, -hexanal, 2-hexenal and heptan-2-one (Forss et al., 1960a,b). Some 40 volatile compounds were identified in cold-stored cultured butter with a fishy olf-flavor, including 4-m-heptenal, 

2- tram, 4-czs-decadienal, 2-tra i,6-c/i-nonadienal, 2,2,7-decatrienal, 3-trans, 5-d.v-octadien-2-one, l-octene-3-one and l-octen-3-ol. Cucumber flavor has been associated with 2,6- and 3,6-nonadienal and mushroom flavor with l-octen-3-ol (Badings and Neeter, 1980). 

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