Spontaneous rancidity

Autoxidation is responsible for the development of the rancidity that occurs when fats and oils spoil and for the spontaneous combustion of oily rags left open to the air. 

The slow spontaneous oxidation of compounds in the presence of oxygen is termed autoxidation (autooxidation). This radical process is responsible for a variety of transformations, such as the drying of paints and varnishes, the development of rancidity in foodstuff fats and oils, the perishing of rabber, air oxidation of aldehydes to acids, and the formation of peroxides in ethers. 

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. 

Lipolysis has been classified as spontaneous or induced. This distinction is made because different measures have to be taken to correct the problem. Induced lipolysis is most frequently defined as lipolysis initiated in raw milk by some form of mechanical agitation. Traditionally, spontaneous lipolysis has been defined as lipolysis caused by the cooling of raw milk. The cooling requirement is no longer strictly adhered to, and lipolysis in raw milk is said to be spontaneous if rancidity develops without apparent mechanical agitation (Downey 1980A,B). The distinction between spontaneous and induced lipolysis is not always clear, and both may occur at the same time. 

Spontaneous Rancidity. Studies have been undertaken to determine how widespread rancidity really is. Hemingway et al (1970) examined 12 herds and reported that about 50% of the herd samples showed... 

The reason that rancidity is not more prevalent in market milk is due to the fortuitous fact that spontaneous rancidity can be prevented or reduced by mixing such milk within 1 hr after milking with four to five times its volume of normal milk (Tarassuk and Henderson 1942). Since usually only about one out of five cows in a herd produces spontaneous milk, this defect is almost automatically eliminated or reduced. It is clear, however, that farmers with only a few cows are likely to encounter spontaneously rancid milk during the lactation period. 

Downey (1980) reasoned that although milk lipoprotein lipase is present in sufficient amounts to cause extensive hydrolysis and potential marked flavor impairment, this does not happen in practice for the following reasons (1) the fat globule membrane separates the milk fat from the enzyme, whose activity is further diminished by (2) its occlusion by casein micelles (Downey and Murphy 1975) and by (3) the possible presence in milk of inhibitors of lipolysis (Deeth and Fitz-Gerald 1975). The presence in milk of activators and their relative concentration may also determine whether milk will be spontaneously rancid or not (Jellema 1975 Driessen and Stadhouders 1974A Murphy et al. 1979 Anderson 1979). 

Driessen, F. M. and Stadhouders, J. 1974A. A study of spontaneous rancidity. Neth. Milk Dairy J. 28, 130-145. 

Hunter, A.C., Wilson, J.M., Greig, G.W. 1968. Spontaneous rancidity in milk from individual... 

Some cows produce milk which is susceptible to a defect known as spontaneous rancidity - no activation treatment, other than cooling of the milk, is required the frequency of such milks may be as high as 30% of the... 

Mixing of normal milk with susceptible milk in a ratio of 4 1 prevents spontaneous rancidity and therefore the problem is not serious except in small or abnormal herds. The incidence of spontaneous rancidity increases with advancing lactation and with dry feeding. 

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. 

Fish oils are an ideal way of emiching food products with long-chain n-3 fatty acids since they contain a high percentage of DHA and EPA. However, the highly unsaturated fish oils are notoriously unstable and oxidize spontaneously if not protected. This oxidation is the major cause of rancidity in foods to which fish oils have been added, either directly or in a protected form such as a microencapsulated product. 

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