Autoxidation deterioration

Another potential area of application of FTIR spectroscopy is in the determination of the oxidative status or stability of an oil. Autoxidation is a major deteriorative reaction affecting edible fats and oils, and it is of major concern to processors and consumers from the standpoint of oil quality, as the oxidative breakdown products cause marked off flavours in an oil. A wide range of end products are associated with the autoxidative deterioration of fats and oils, the most important being hydroperoxides, alcohols, and aldehydes. Moisture, hydrocarbons, free fatty acids and esters, ketones, lactones, furans, and other minor products may also be produced, with the free fatty acids becoming more important in thermally stressed oils. In addition, there is significant cis to trans isomerisation and conjugation of double bonds in the hydroperoxides formed as an oil oxidises. 

Thermal Oxidative Stability. ABS undergoes autoxidation and the kinetic features of the oxygen consumption reaction are consistent with an autocatalytic free-radical chain mechanism. Comparisons of the rate of oxidation of ABS with that of polybutadiene and styrene—acrylonitrile copolymer indicate that the polybutadiene component is significantly more sensitive to oxidation than the thermoplastic component (31—33). Oxidation of polybutadiene under these conditions results in embrittlement of the mbber because of cross-linking such embrittlement of the elastomer in ABS results in the loss of impact resistance. Studies have also indicated that oxidation causes detachment of the grafted styrene—acrylonitrile copolymer from the elastomer which contributes to impact deterioration (34). 

Free-radical chain inhibitors are of considerable economic importance. The term antioxidant is commonly appUed to inhibitors that retard the free-radical chain oxidations, termed autoxidations, that can cause relatively rapid deterioration of many commercial materials derived from organic molecules, including foodstuffs, petroleum products, and plastics. The chain mechanism for autoxidation of hydrocarbons is ... 

Ideally samples from living organisms should be extracted without any delay to prevent autoxidative or enzymic deterioration of their lipid constituents. If this is not feasable the sample should be frozen immediately and stored at — 20 °C in a glass container under nitrogen. Often lipids will be extracted into an organic solvent and during this and subsequent steps in the analytical procedure minimal exposure of the lipids to air, light and heat is very important to prevent oxidation or destruction of the lipids. 

Lipid autoxidation in fluid milk and a number of its products has been a concern of the dairy industry for a number of years. The need for low-temperature refrigeration of butter and butter oil, and inert-gas or vacuum packing of dry whole milks to prevent or retard lipid deterioration, in addition to the loss of fluid and condensed milks as a result of oxidative deterioration, have been major problems of the industry. The autoxidation of milk lipids is not unlike that of lipids in other... 

The literature dealing with the autoxidation mechanism involved in lipid deterioration has been concerned with investigations on pure unsaturated fatty acids and their esters. The reactions involved, however, are representative of those occurring in lipids and lipid-containing food products. 

In addition to the previously mentioned chemical tests, methods based on the carbonyl content of oxidized fats have also been suggested (Henick et al 1954 Lillard and Day 1961) as a measure of oxidative deterioration. The procedures determine the secondary products of autoxidation and have been reported to correlate significantly with the degree of off-flavor in butter oil (Lillard and Day 1961). The methods, however, are cumbersome and are not suited for routine analysis. 

Parks, O. W. 1974. The lipids of milk Deterioration, Part II. Autoxidation. In Fundamentals of Dairy Chemistry. B.H. Webb, A. H. Johnson and John A. Alford (Editors). AVI Publishing Co., Westport, Conn., pp. 240-263. 

Autoxidation can lead to deterioration of food, drugs, cosmetics, or polymers, and inhibition of this reaction is therefore an important technical issue. The most important classes of autoxidation inhibitors are radical scavengers (phenols, sterically demanding amines [65, 66]), oxygen scavengers (e.g. ascorbic acid), UV-light absorbers, and chelators such as EDTA (to stabilize high oxidation states of metals and thereby suppress the metal-catalyzed conversion of peroxides to alkoxyl radicals) [67]. 

The autoxidation reaction is difficult to control, so it is not often used for synthetic purposes. However, it is a very important natural process. The slow deterioration of organic materials, such as rubber, paint, and oils, and that of many foods, such as butter... 

There are a number of analytical measures of oxidative deterioration of oils and fats. The most widely used are the peroxide value (PV) (15), which measures the hydroperoxide content by iodine titration and the anisidine value (AV) (15), which detects aldehydes by a color reaction. As an oil suffers damage because of autoxidation, the hydroperoxide content, and PV rise but do not do so indefinitely. As the hydroperoxides break down, the concentration of aldehydes and AV increase. Oxidation is better assessed by a combination of PV and AV, the Totox value... 

Autoxidation is the most common process leading to oxidative deterioration and is defined as the spontaneous reaction of atmospheric oxygen with lipids (3). The process can be accelerated at higher temperatures, such as those experienced during deep-fat frying, which is called thermal oxidation, with increases in free fatty acid and polar matter contents, foaming, color, and viscosity (4). Lfnsaturated fatty acids... 

As oxidation normally proceeds very slowly at the initial stage, the time to reach a sudden increase in oxidation rate is referred to as the induction period (6). Lipid hydroperoxides have been identified as primary products of autoxidation decomposition of hydroperoxides yields aldehydes, ketones, alcohols, hydrocarbons, volatile organic acids, and epoxy compounds, known as secondary oxidation products. These compounds, together with free radicals, constitute the bases for measurement of oxidative deterioration of food lipids. This chapter aims to explore current methods for measuring lipid oxidation in food lipids. 

Some volatile aldehydes formed by autoxidation of unsaturated fatty acids are listed in Table 1. The aromas of aldehydes are generally described as green, painty, metallic, beany, and rancid, and they are often responsible for the undesirable flavors in fats and oils. Hexanal has long been used as an index of oxidative deterioration in foods. Some aldehydes, particularly the unsaturated aldehydes, are very potent flavor compounds. Table 2 fists aroma characteristics of some common aldehydes found in fats and oils (8). 

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