Linolenic acid, oxidative stability

Oxidation Stability (OSI or Rancimat) has been introduced in many ways. In EN14214 it exists as OSI and as a maximum acceptable Iodine Value, or maximum level of linolenic acid or of poly-unsaturated fatty acids. Oxidation stability is of importance when it comes to polymerization and oxidation during storage as well as during use in the engine. Oxidation is directly related to the presence of unsaturated bonds in the FAME, and probably because of this the EN 14214 includes a cap on Iodine Value. 

Odor and color stability problems were also related to the alkyl chains used for SAI. These could be traced to the oxidation of unsaturated carbons, such as oleic acid (Ci8 fatty acid with a single double bond between carbon 9 and 10, i.e. bond position 9 counted from the carboxyl carbon), linoleic acid (Cis fatty acid with two double bonds at position 9 and 12), and linolenic acid (Cis fatty acid with three double bonds at position 9, 12, and 15). Natural coconut fatty acid contains about 6% oleic acid, about 3% linoleic acid, and less than 1% linolenic acid. Tallow fatty acid contains nearly 44% oleic and about 6% of other unsaturates [20]. Partial hydrogenation of the coconut fatty acid used in the manufacture of SCI served to eliminate linoleic and linolenic acids for improved odor stability, while not eliminating oleic acid, which is important for good lather. 

The oxidative stability of an oil depends on the fatty acid (FA) composition and triacyl-glycerol (TAG) structure, as well as on non-TAG components, such as tocopherols, carotenoids, ascorbic acid, citric acid, free fatty acids, and sterols, which may either prevent or promote oxidation. Several investigations have reported correlations of FA composition, TAG structure, and oxidative stability (135-140). For example, the oxidative stability of purified TAG from soybean oil (SBO) in air in the dark at 60°C is correlated positively with a greater concentration of oleic acid (O) and lower concentrations of linoleic (L) and linolenic (Ln) acids of SBO TAG. 

If that does occur, then the present system of classification of oils may be impossible to police, and a modified system may become necessary. Perhaps the sale and perceived value of oils will necessarily become dependent on the performance, not the source of the oil. With bulk oils such as palm, peanut, sunflower, safflower, sesame, soya, rapeseed, com, fish, and animal fats and oils, the fatty acid composition will obviously be important for health reasons. If the oil is to be used for frying then the frying properties will be important. In the case of palm products the physical properties and minor components such as carotenoids will be defined. Similarly animal fats will be judged mainly on physical behaviour and effect on the product in which they are used. In all cases the oxidative and stability of the oil will have to be defined. Sesame is a very stable oil, and thus its stability, together with its low level of linolenic acid, would be its major attribute, except for toasted sesame, which would probably be classed as a specialist oil. Already most baking fats sold to the public are blends developed to give the best performance, with no mention on the pack as to the source. If a bulk oil of this type had the desired chemical composition, stability and cooking behaviour, then perhaps the source would not be a matter of concern. 

Is the Iodine Value found in EN 14214 based on science It is certainly not so in an absolute way, but some relation cannot be denied. Frankel (2005) for example clearly states that oxidation stability is a function of two things the number of double bounds, and their position towards one another in the fatty acid. Oleic acid with one double bound oxidizes 40 times slower than linoleic acid with two double bounds, and one bis-allylic position in-between both. Linolenic acid with three double bounds separated with two bis-allylic positions oxidizes only 2.5 times faster than linoleic acid. Oxidation is a radical driven reaction, and the bis-allylic positions are a much more favorable point of attack than the allylic positions next to the double bound. 

In neutral oils and fats, the fatty acids are not usually randomly distributed among different positions on the glycerol backbone and are associated in particular patterns. As an example, saturated fatty acids such as palmitic and stearic acids are associated with the sn- and sn-3 positions of soybean oil, albeit at higher proportions in the sn- position. However, the reverse is observed at high content of saturated fatty acids. Linoleic acid is preferably in the sn-2 position, whereas oleic acid is randomly distributed among the three positions. Linolenic acid is primarily at sn-2 followed by sn- and sn-3 positions. The stereospecific distribution of fatty acids has a marked effect on the oxidative stability of the resultant oils, and their presence at the sn-2 position helps their stability (19). 

The stability of canola oil is limited mostly by the presence of linolenic acid, chlorophyll, and its decomposition products and other minor components with high chemical reactivity, such as trace amounts of fatty acids with more than three double bonds. These highly unsaturated fatty acids can possibly be formed during refining and bleaching (52). The presence of 7% to 11% of linolenic acid in the acylglyce-rols of canola oil places it in a similar category with soybean oil with respect to flavor and oxidative stability. The deterioration of flavor as the result of auto -and photo-oxidation of unsaturated fatty acids in oils and fats is referred to as oxidative rancidity. 

The low total polyunsaturation of canola oil, about 30% versus 58% for soybean oil, along with the high content of monounsaturates, about 60% versus about 25% for soybean oil, are responsible for the good flavour stability of this oil, despite the presence of linolenic acid. Additional minor, but important reasons, for better oxidative stability of canola oil compared with soybean oil are as follows ... 

Canola Rapeseed Oils with Modified Fatty Acid Composition Since the introduction of standard canola, there has been considerable plant breeding efforts to produce canola oils with modified fatty acid compositions. These efforts were primarily to improve oxidative stability, or crystallization properties, or even produce lauric acid-containing oils and, more recently, canola oil containing gamma linolenic acid (11). The following is a list of these developments ... 

Anisidine Value. Anisidine value is a measure of secondary oxidation or the past history of an oil. It is useful in determining the quahty of crude oils and the efficiency of processing procedures, but it is not suitable for the detection of oil oxidation or the evaluation of an oil that has been hydrogenated. AOCS Method Cd 18-90 has been standardized for anisidine value analysis (103). The analysis is based on the color reaction of anisidine and unsaturated aldehydes. An anisidine value of less than ten has been recommended for oils upon receipt and after processing (94). Inherent Oxidative Stability. The unsaturated fatty acids in all fats and oils are subject to oxidation, a chemical reaction that occurs with exposure to air. The eventual result is the development of an objectionable flavor and odor. The double bonds contained in the unsaturated fatty acids are the sites of this chemical activity. An oil s oxidation rate is roughly proportional to the degree of unsaturation for example, linolenic fatty acid (C18 3), with three double bonds, is more susceptible to oxidation than linoleic (C18 2), with only two double bonds, but it is ten times as susceptible as oleic (C18 l), with only one double bond. The relative reaction rates with oxygen for the three most prevelent unsaturated fatty acids in edible oils are ... 

The fatty acid composition of the new crop has been modified, and the level of linolenic acid has been reduced from over 50% to 2% (6). This greatly improves oxidative stability of the oil, which by fatty acid composition is very close to sunflower and soybean oils (Table 2). Linola has been found to be more resistant to oxidation than regular flax oil, and its stability is comparable with soybean, canola, and sunflower oils (Przybylski, unpublished data). 

According to the composition indicated by the Codex Alimentarius (Codex-Stan 210-1999), the saturated fatty acid content of regular sunflower oil is lower than that in corn (maximum 22%), cottonseed (maximum 32%), peanut (maximum 28%), and soybean (maximum 20%) oils, and higher than the saturated content of safflower (maximum 12%) and rapeseed (maximum 12%) oils. The linolenic acid content (18 3) of regular sunflower oil is fairly low (always lower than 0.3%), giving the oil a good oxidative stability. 

Oxidative stability of the oil depends primarily on its polyunsaturated fatty acid content. This includes linolenic and linoleic acids. Linolenic acid-containing... 

Linolenic acid content must be low in order to provide maximum oxidative stability to the oil. This is why soybean and canola oil, which contain about 8% linolenic acid in the natural state, are hydrogenated to reduce their linolenic acid content to less than 2% determined by the capillary GC Method (2). Poor frying stability in sunflower oil comes primarily from the high level of hnoleic acid. Therefore, sunflower oil must also be hydrogenated to reduce its linoleic acid content to 35% or lower for industrial frying. Table 1 lists the analyses of the most commonly used industrial frying oils. 

The high degree of unsaturation, particularly the significant level of linolenic acid, of soybean oil limits its food application due to its low oxidative stability. Partial hydrogenation is used to increase the melting temperature and, at the same time, to improve the oxidative stability of soybean oil. 

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