Spontaneous oxidation in milk

Off-flavor due to spontaneous oxidation of milk fat is a troublesome issue because the process and its prevention are not well understood and it tends to occur in otherwise well-managed, high-yielding dairy herds 

Spontaneous milk is capable of developing oxidized flavor within 48 h of milking without the presence of contaminating iron or copper. Bruhn et al, (1976) reported that 12-20% of raw milk samples are in this category. 

Susceptible milk does not oxidize spontaneously but does develop oxidized flavor following contamination with iron or copper. Use of noncorrodible dairy equipment has reduced the incidence of copper contamination. 

Non-susceptible milk does not oxidize even in the presence of iron or copper. 

Spontaneous oxidation of milk fat, which has been known for over 60 years (Corbett and Tracy, 1943), is influenced by heredity, stage of lactation and feeding practices (Shipe, 1964). Some cows consistently produce spontaneous milk, others occasionally, and others not at all (Parks et al, 1963). Differences between milk from the different quarters of the same cow may occur. 

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In milk, as in many odier foods, oxidative reactions are a prime cause of flavor deterioration and loss in nutritional quality (1). Spontaneous oxidation in milk giving rise to oxidized flavor (off-flavor) is a well-described phenomenon (2 3), i diich is supposed to proceed dependii on factors inherent in die milk itself. These inherent factors include fatty acid conq>osition, content of low-molecular weight antioxidants, pro- and anti-oxidative enzyme systems, and transition metal ion content. Moreover, external factors such as handling, agitation, tenq>erature, exposure to light, and contamination by metals and microorganisms are known to trigger additional deteriorative oxidative reactions in milk. 

Furthermore, these hydroperoxides ean start a radical chain reaction, leading to additional aroma-active fat degradation products and enhanced autooxidation. The hydroperoxide degradation can happen spontaneously and may also be catalyzed by enzymes. Finally it should be emphasized that XO contributes to lipid oxidation in milk fat only if an appropriate substrate is present. 

Hopkins (1938) has shown that lactoflavin in presence of light catalyses the oxidation of ascorbic acid, and thus is responsible for its spontaneous destruction in milk. 

XO, which can excite stable triplet oxygen (302), is a pro-oxidant. Milk which undergoes spontaneous rancidity contains about 10 times the normal level of XO, and spontaneous oxidation can be induced in normal milk by the addition of XO to about four times normal levels. Heat-denatured or flavin-free enzyme is ineffective and the susceptibility of unsaturated fatty acids to oxidation increases with the degree of unsaturation. 

With the advent of noncorrodible dairy equipment, oxidative deterioration in fluid milk as a result of copper contamination has decreased significantly, although it has not been completely eliminated (Rogers and Pont 1965). However, the incidence of spontaneous oxidation remains a major problem of the dairy industry. For example, Bruhn and Franke (1971) have shown that 38% of samples produced in the Los Angeles milkshed are susceptible to spontaneous oxidation Potter and Hankinson (1960) have reported that 23.1% of almost 3000 samples tasted were criticized for oxidized flavor after 24 to 48 hr of storage. Significantly, certain animals consistently produce milk which develops oxidized flavor spontaneously, others occasionally, and still others not at all (Parks et al. 1963). Differences have been observed in milk from the different quarters of the same animal (Lea et al. 1943). 

Aurand, L. W., Woods, A, E. 1959. Role of xanthine oxidase in the development of spontaneously oxidized flavor in milk. J. Dairy Sci. 42, 1111-1118. 

King, R. L. and Dunkley, W. L. 1959B. Relation of natural copper in milk to incidence of spontaneous oxidized flavor. J. Dairy Sci. 42, 420-427. 

Parks, O. W., Keeney, M. and Schwartz, D. P. 1963. Carbonyl compounds associated with the off-flavor in spontaneously oxidized milk. J. Dairy Sci. 46, 295-301. 

Rajan, T. S., Richardson, G. A. and Stein, R. W. 1962. Xanthine oxidase activity of milks in relation to stage of lactation, feed, and incidence of spontaneous oxidation. J. Dairy Sci. 45, 933-934. 

Creamy flavors in butter have been associated with 4-cis heptenal produced for autoxidation of isolinoleic acid (Begeman and Koster, 1964). Drier flavor in foam spray-dried milk has been associated with 6-rra x-nonenal, which has a flavor threshold in fresh milk of 0.07 pg/kg (Parks et al., 1969). Bassette and Keeney (1960) implicated a homologous series of autoxidation-derived saturated aldehydes, together with products of Maillard browning, in cereal-type off-flavors in powdered skim milk. Staleness in dry whole milk may be associated with saturated and unsaturated aldehydes (Parks and Patton, 1961). 2,4-Decadienal has been reported to be the principal compound responsible for the off-flavor associated with spontaneously oxidized milk (Parks et al., 1963). Oxidized flavors in sunlight-exposed milk are commonly related to C6 to Cn alk-2-enals... 

Clearly, more research is required to clarify the somewhat confused picture regarding the role of enzymes in the oxidation of milk lipids. However, the key factor affecting the susceptibility of milk to oxidation appears to be its relative content and distribution of pro-oxidants and antioxidants. Bruhn and Franke (1971) reported that spontaneous oxidation is directly proportional to the copper content and inversely proportional to the a-tocopherol content of milk. Charmley et al. (1991) showed that intramuscular injection of cows with a-tocopherol may overcome a spontaneous oxidized flavor problem caused by low levels of a-tocopherol in milk. In general, milk from pasture-fed cows is less susceptible to oxidation due to a higher content of tocopherols than milk from cows given dry feed (Bruhn and Franke 1971 Urbach, 1989, 1990). 

St. Laurent et al. (1990) investigated the effects on milk flavor of a-tocopherol supplementation (0, 700 or 3000 IU/day) to a feed consisting of grain mix, hay and pasture in herds with a chronic spontaneous oxidized flavor problem. a-Tocopherol supplementation resulted in improved milk flavor but no relationship was apparent between milk a-tocopherol levels and the extent of flavor improvement. In this study, the flavor problem decreased significantly when the cows subsequently got access to spring pasture. 

The endogenous copper in milk is derived via the bloodstream from the cow s feed (Haase and Dunkley, 1970). ft is unclear to what extent the copper content of the feed influences the copper content of milk (Mulder et al., 1964 Riest et al., 1967 Dunkley et al., 1968a). However, the total level of endogenous copper in milk does not appear to be the key factor in spontaneous oxidation. King and Dunkley (1959b) and Samuelsson (1966) reported that oxidation may occur irrespective of copper content above a threshold value of 0.06 pg/kg. 

The feasibility of increasing the a-tocopherol concentration of milk by supplementation of the feed has been investigated in many studies (Dunkley et al., 1966, 1967 King et al., 1966 St. Laurent et al., 1990 Barrefors et al., 1995 Focant et al., 1998 Granelli et al., 1998). These studies showed that when feed was supplemented with varying levels of a-tocopheryl acetate, the a-tocopherol content of the milk was increased with consequent increased resistance to spontaneous and copper-induced oxidation. King et al. (1967) reported that when feed was supplemented to achieve an intake of 1 g a-tocopherol per day per cow, oxidation was effectively controlled in milk... 

Bruhn, J.C., Franke, A.A., Goble, G.S. 1976. Factors relating to development of spontaneous oxidized flavor in raw milk. J. Dairy Sci. 59, 828-833. 

Granelli, K, Barrefors, P., Bjorck, L., Appelqvist, L.A. 1998. Further studies on lipid composition of bovine milk in relation to spontaneous oxidized flavor. J. Sci. Food Agric. 77, 161-171. 

St. Laurent, A M., Hidiroglou, M., Snoddon, M., Nicholson, J.W.G. 1990. Effect of a-tocopherol supplementation to dairy cows on milk and plasma a-tocopherol concentrations and on spontaneous oxidized flavor in milk. Can. J. Anim. Sci. 70, 561-570. 

It has been proposed that spontaneous milks have a high content (10 times normal) of xanthine oxidase (XO). Although addition of exogenous XO to non-susceptible milk induces oxidative rancidity, no correlation has been found between the level of indigenous XO and susceptibility to oxidative rancidity. The Cu-ascorbate system appears to be the principal pro-oxidant in susceptible milk. A balance between the principal antioxidant in milk, a-tocopherol (Chapter 6), and XO may determine the oxidative stability of milk. The level of superoxide dismutase (SOD) in milk might also be a factor but there is no correlation between the level of SOD and the propensity to oxidative rancidity. 

Oxidation occurs initially in the polyunsaturated phospholipid fraction associated with the fat globule membrane-water interface, followed by the main triacylglycerol fraction. Different milks vary widely in susceptibility to oxidation, and oxidative deteriorations in milk are classified empirically as i. spontaneous, developing oxidized flavors within 48 hr after milking without added metal catalyst ii. susceptible, developing oxidized flavors within 48 hr after contamination with copper, and iii. resistant (or non-susceptible), not developing oxidized flavors after 48 hr even after addition of copper or iron. 

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