Oxidative deterioration of fats and oils

Edible oils in contact with air deteriorate eventually owing to oxygen uptake and the formation of primary and secondary oxidation products of unsaturated FAs. The kinetics of the formation and the identity of these oxidation products have been the subject of several monographs and reviews (Chan, 1987 Hamilton, 1989). 

The first intermediates in the chain reaction leading to volatile secondary oxidation products are hydroperoxides. They react further in a very complex way to give a diverse spectrum of short-chain secondary reaction products pertinent to oxidative rancidity. Grosch (1987) isolated 

The first stage of autoxidation of a pure oil is easily traceable by applying conventional wet chemistry, for example determination of the peroxide value. Peroxides are known to be heat labile compounds, which undergo breakdown at elevated temperatures to form simple hydrocarbons. Thus ethane and pentane are the predominant breakdown products of linolenate and linoleate peroxides, respectively (Evans et al., 1967). Scholz and Ptak (1966) and Evans et al. (1969) proposed a gas chromatographic method to measure rancidity in edible oils whereby the undiluted oil is injected directly into the hot injector (250°C). At these temperatures lipid peroxides are 

Source LFllrich, F. and Grosch, W., Flavour deterioration of soya-bean oil identification of intense odour compounds formed during flavour reversion, Fat Science Technology, 90 932-6, 1988 

Fairly recently the idea of directly injecting an oil sample has been picked up and applied to the determination of organophosphorous insecticides (Grob, Biedermann and Giuffre, 1994) and to discriminate between conventionally processed edible oils and mildly deodorized oils (Grob et aL, 1994). The injector insert employed is shown in Fig. 3.7. Grob et aL have termed this sampling technique injector-internal HS analysis . 

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During lipid oxidation, it is often observed that PV first rises, then falls as hydroperoxides decompose (38). PV and p-AnV reflect the oxidation level at early and later stages of oxidation reaction, respectively. Totox value measures both hydroperoxides and their beakdown products, and provides a better estimation of the progressive oxidative deterioration of fats and oils (38). However, Totox value has no scientific basis because it is a combination of two indicators with different dimensions (7). Recently, Wanasundara and Shahidi used TEA values and defined TotoxjBA as 2PV + TEA using the TEA test in place of the p-AnV assay (60). 

Trace metals, in particular iron, copper, and nickel, accelerate the oxidative deterioration of fats and oils. Cmde oil should not be in contact with iron, copper, or brass valves. One brass valve in a soybean oil refinery can lower the flavor stabihty of deodorized products. For proper handling, only stainless-steel tanks and valves should be used. 

Five factors contribute to the oxidative deterioration of fats and oils (1) oxygen or air (2) heat (3) light (4) pro-oxidant metals and (5) time. 

Hydrolysis, the reaction of fats and oils with water, and oxidation, the chemical reaction in which oxygen combines with another substance with the liberation of heat, are the two basic processes that result in the deterioration of oils and fats. Oxidation is mostly responsible for much more of the deterioration of fats and oils than hydrolysis. 

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. 

Storage of fats and oils leads to rancidity, a largely oxidative deterioration that causes development of unpleasant tastes, odors, and toxic compounds.239 Similar chemical changes account for the "drying" of oil-based paints and varnishes. These reactions occur... 

A relationship between peroxide value and anisidine number that is used to measure the rancidity level of fats and oils. It is defined as (2 X PV) + AN. It reflects total oxidation to date. Considered an impurity. High levels of moisture in an oil can lead to deterioration in storage. Soap can be formed when moisture is present in the crude oil and reacts with the free fatty acids and a catalyst (alkali ion), or it can result from incomplete removal of soap from washed refined oil. 

Shipment by tanker is the most economical means of ocean transport of fats and oils. A typical shipment ranges from 1814 to 9072 metric tons (2000 to 10,000 short tons). Tankers are compartmentalized and inert-gas blanketing or sparging is not employed because the gas can escape. Rather, the compartments are filled to capacity to minimize headspace air and reduce oxidative deterioration. Heating is not required to unload liquid oils, but to facilitate unloading of palm, coconut, and tallow, heating is required. 

Oxidative deterioration is a major problem associated with the processing, storage and usage of fats and oils. The susceptibility of fats and oils... 

ANTIOXIDANTS. Usually an organic compound added to various types of materials, such as rubber, natural fats and oils, food products, gasoline, and lubricating oils, for the purposes of retarding oxidation and associated deterioration, rancidity, gum formation, reduction in shelf life, etc. 

Increased reducing power, i.e., increased antioxidant activity or more negative redox potential, is also one of the symptoms of the Maillard reaction (see Chapter 1). This has considerable significance, because, as far as foods are concerned, one of the main ways in which they deteriorate chemically is through oxidation, particularly of unsaturated fats and oils, leading to oxidative rancidity. The topic has considerable physiological significance as well (see Chapter 8). 

Chapter 4 presents the chemistry and technology of edible oil, fat, and wax processing including refining, recovery, crystallization, interesterification, and hydrogenation. The key oxidation reactions of lipids leading to quality deterioration of processed and unprocessed foods, and the mechanism of... 

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... 

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). 

Hydrogenation is the addition of hydrogen to the carbon-carbon double bonds in the presence of a metal catalyst, which results in the conversion of liquid oils to hard or plastic fats and soft fats to firmer products. It also results in improved resistance of the fats and oils to deterioration through oxidation. 

Antioxidants are frequently added to unsaturated fats and oils in order to protect these against oxidative deterioration. For this reason, they are also added to a variety of food products containing unsaturated lipids. Antioxidants frequently applied are esters of gallic acid, butylated hydroxyanisole (BHA), butylated hydroxytoluene... 

The unsaturated fatty acids in all fats and oils are subject to oxidation, a chemical reaction which occurs with exposure to air. The eventual result is the development of an objectionable flavor and odor. The double bonds and the adjacent allylic functions are the sites of this chemical activity. Oil oxidation rate is roughly proportional to the degree of unsaturation for example, linolenic fatty acid (18 3) with three double bonds is more susceptible to oxidation than linoleic (18 2) with only two double bonds, which is ten or more times as susceptible as oleic (18 1) with only one double bond. Oxidative deterioration results in the formation of hydroperoxides, which decompose into carbonyls, and dimerized and polymerized gums. It is accelerated by a rise in temperature, oxygen pressure, prior oxidation, metal ions, lipoxygenases, hematin compounds, loss of natural antioxidant, absence of metal deactivators, time and ultraviolet or visible light. Extensive oxidation will eventually destroy the beneficial components contained in many fats and oils, such as the carotenoids (vitamin A), the essential fatty acids (linoleic and linolenic), and the tocopherols (vitamin E). 

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