Fatty acids hydroperoxide determination

Localization of double bonds in unknown compounds has frequently been determined by ozonolysis. Unsaturated fatty acids of biological membranes are susceptible to ozone attack, but there are some important differences from autoxidation reactions. These include the fact that malonaldehyde is produced from linoleate by ozonolysis (53) but not autoxidation and also that ozonolysis does not cause double bond conjugation as judged by absorption at 233 nm (52). Reactions with the polyunsaturated fatty acids produce several possibilities for toxic reactions direct disruption of membrane integrity and toxic reactions caused by fatty acid hydroperoxides, hydrogen peroxide, and malonaldehyde. 

Peroxide Value. Oxidation of oils is a major cause of their deterioration. Hydroperoxides are the primary products formed by the reaction between oxygen and the unsaturated fatty acids. Hydroperoxides have no flavor or odor but break down rapidly to form aldehydes, which have a strong, disagreeable flavor and odor. The peroxide concentration, usually expressed as peroxide value (PV), is a measure of oxidation or rancidity in its early stages. PV measures the concentration of substances, in terms of milliequivalents of peroxide per 1000 grams of sample, that oxidize potassium iodide to iodine. AOCS Method Cd 8-53 (103) is the official method for peroxide value determinations. 

C -Aldehyde formlng-actlvltv. Cg-Aldehyde formation from C g-fatty acid was determined by the procedure used for hydroperoxide lyase activity except for the addition of C g-fatty acids Instead of the 13-hydroperoxldes. 

Hydroperoxide formation by the ene reaction path may lead to formation of conjugated double bonds in polyunsaturated fatty acids (see Section V.A) this reaction is concurrent with POV increase. An increase of the CDV, as measured from the absorbance at 233 nm, therefore indicates oxidation of polyunsaturated lipids. A strong correlation exists between CDV predicted from the absorbance in the 1100 to 2200 nm NIR region and CDV determined by the Ti Ia-64 AOCS official method , by UV spectrophotometry at 233 nm. The method was applied to determine CDV for oxidized soybean oil. A secondary absorption maximum of lesser intensity appears in the 260-280 mn range, and is assigned to ketone dienes . 

Mechanistic studies of autoxidation have concentrated on methylene-interrupted fatty acids, but many of the observations are valid for other compounds. Conjugated fatty acids such as CLA also oxidize through an autocatalytic free radical reaction, with the predominant hydroperoxide determined by the geometry of the conjugated diene system (45). Other groups with activated methylenes may be susceptible to oxidation, for example, the ether methylenes of ethoxylated alcohols used as surfactants (46). 

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. 

Figure 18 FTIR spectrum of a mixture of varying amounts of OH-containing components, such as moisture, monoglycerides, and free fatty acids, which may potentially interfere with the determination of hydroperoxide content (PV) in oxidised oils.

AOCS has a recommended practice (Cg 3-91) for assessing oil quality and stability (AOCS, 2005) for measuring primary and secondary oxidation products either directly or indirectly. For example, peroxide value analysis (AOCS method Cd 8-53) (AOCS, 2005) determines the hydroperoxide content and is a good analysis of primary oxidation products. To determine secondary oxidation products, the procedure recommends p-anisidine value (AOCS Method Cd 18-90, 2005) volatile comlb by gas chromatography (AOCS Method Cg 4-94, 2005) and flavor evaluation. (AOCS Method Cg 2-83, 2005). The anisidine value method determines the amounts of aldehydes, principally 2-alkenals and 2, 4-dienals, in oils. The volatile compound analysis method measures secondary oxidation products formed during the decomposition of fatty acids. These comlb can be primarily responsible for the flavors in oils. The... 

Another example is the production of green note compounds using lipoxygenase pathway enzymes. Lipoxygenase and hydroperoxide lyase are the determinant enzymes for the conversion of fatty acids into natural food flavor components. However, these compounds present in natural sources at very low levels and because of their instable nature it has been difficult to purify these compounds. As a result, considerable efforts have been made to clone these two enzymes for commercial uses in the production of natural flavor components. Many plant lipoxygenase from different plants have been cloned and expressed in E. coli or yeast [5],... 

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