Deterioration of fats

Fats deteriorate in two ways. One is normally a chemical process the other is normally enzymatic. They are oxidative rancidity and lipolytic rancidity. In the former, oxygen (normally in the form of a free radical) adds across double bonds. As this is a zero activation energy process it is 

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Coe, M. R., Deterioration of Fats and Oils, Quartermaster Corps Manual 17-7, Army Serv-... 

The thiobarbituric acid (TBA) test was proposed over 40 years ago and is now one of the most extensively used methods to detect oxidative deterioration of fat-containing foods (41). During lipid oxidation, malonaldehyde (MA), a minor component of fatty acids with 3 or more double bonds, is formed as a result of the degradation of polyunsaturated fatty acids. It is usually used as an indicator of the lipid oxidation process, both for the early appearance as oxidation occurs and for the sensitivity of the analytical method (42). In this assay, the MA is reacted with thiobarbituric acid (TBA) to form a pink MA-TBA complex that is measured spectrophotometrically at its absorption maximum at 530-535 nm (Figure 2) (9,43,44). The extent of oxidation is reported as the TBA value and is expressed as milligrams... 

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). 

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. 

Figure 3. Stages of progressive deterioration of fats through oxidation (5).

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. 

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. 

Processes used in recovery and refining and subsequent storage conditions are the main factors affecting the quality of edible fat or oil. A number of analytical methods are available for assessing the quality and deterioration of fat or oil. 

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