Lipid oxidation: measurement methods

Flavor Components of Fats and Oils, Lipid Oxidation Measurement Methods, Marine Mammal Oils, Modification of Fats and Oils via Chemical and Enzymatic Methods, Novel Separation Techniques for Isolation and Purification of Fatty Acids and Oil By-Products, Quality Assurance of Fats and Oils, Tree Nut Oils. 

Ying Zhong Memorial University of Newfoundland, St. John s, Newfoundland, Canada, Antioxidants Regulatory Status , Citrus Oils and Essences , Lipid Oxidation Measurement Methods , Marine Mammal Oils. 

The oil stability index (OSI) method provides results in a matter of hours instead of months (required for studies done at ambient temperatures). These OSI results are useful as comparative measures of oxidative stability, i.e., to determine the effect of a treatment or antioxidant compared to a control sample. Meaningful predictions of the actual shelf lives of specific commodities require that such shelf life studies be performed at ambient conditions. If only accelerated tests are to be performed, two or more tests based on different principles of lipid oxidation measurement should be conducted the effect of accelerated storage temperature should also be investigated. 

Decomposition of the primary products of lipid oxidation generates a complex mixture including saturated and unsaturated aldehydes such as hexanal. Hexanal is the most commonly measured end product of lipid oxidation, and both sensory and physicochemical methods are used for its determination. Where other antioxidant activity tests may be nonspecific, physicochemical measurement of hexanal offers the advantage of analyzing a single, well-defined end product. 

A simple method for assessing lipid oxidation is measuring the headspace concentration of hexanal by capillary GLC. Also, the total volatiles appearing in the chromatogram up to hexanal can be taken as oxidation index. The method was applied to determine the amounts of lipid peroxides present in rat liver cells. Enhancement of the hexanal concentration can be achieved on adding ascorbic acid (22), that reduces Fe(ni) present in the matrix to Fe(II), which catalyzes decomposition of hydroperoxides to aldehydes. Significant correlations are found between hexanal concentrations and various oxidation indices, such as TBARS (Section IV.D.2)" . ... 

Various methods have been employed to measure the extent of autoxi-dation in lipids and lipid-containing food products. For obvious reasons, such methods should be capable of detecting the autoxidation process before the onset of off-flavor. Milk and its products, which develop characteristic off-flavors at low levels of oxidation, require procedures that are extremely sensitive to oxidation. Thus methods of measuring the decrease in unsaturation (iodine number) or the increase in diene conjugation as a result of the reaction do not lend themselves to quality control procedures, although they have been used successfully in determining the extent of autoxidation in model systems (Haase and Dunkley 1969A Pont and Holloway 1967). 

Other accelerated methods for measuring lipid oxidation include the oxygen bomb test and the Schaal oven test. These methods are described by Wan (1995). 

Lipids are susceptible to oxidation and, as such, require analytical protocols to measure their quality. As described in vnitd2.i, autoxi-dation is one of the chief processes by which lipids degrade. The primary products from this reaction are hydroperoxides. These odorless and colorless transient species break down by various means to secondary products, which are generally odoriferous by nature. Being able to measure secondary oxidation products by simple spectrophotometric means is important for the food scientist so that he or she is able to characterize the extent of lipid oxidation. However, the researcher should be cautioned that one assay (e.g., TBA test) does not provide all the answers. To get a better picture of the story, both primary and secondary products of lipid oxidation should be assessed simultaneously by the different methods available (unitdu). 

Pikul, J., Leszczynski, D.E., and Kummerow, F.A. 1989. Evaluation of three modified TBA methods for measuring lipid oxidation in chicken meat. J. Agric. Food Chem. 37 1309-1313. 

Salih, A.M., Smith, D.M., Price, J.F., and Dawson, L.E. 1987. Modified extraction 2-thiobarbituric acid method for measuring lipid oxidation in poultry. Poultry Sci. 66 1483-1488. 

From the above description of a molecular species absorbing in the UV wavelength range, it appears that the UV test is not wholly specific for substances produced in lipid peroxidation. Therefore other methods are needed to detect and evaluate lipid oxidation. Among the variety of methods available in the literature, iodometry is the chosen official method, although it fails when hydroperoxides are present in low amounts. Note also that iodometry will measure the peroxides present in the oil, but not their decomposition products. 

Measurement of hydroperoxides is the classical method for quantifying lipid oxidation and a variety of assay procedures are available. The oxidation of ferrous to ferric iron by hydroperoxides in the presence of ammonium thiocyanate to produce ferric thiocyanate, which can be quantified spectrophotometrically at 505 nm, has been used extensively to study lipid oxidation in milk (Loftus-Hills and Thiel, 1946). Newstead and Head-ifen (1981) recommend that extraction of fat from whole milk powder be carried out in the dark when using this procedure to avoid artefactually high... 

Other traditional methods available for monitoring the extent of lipid oxidation include the Anisidine value, the Kreis test (Mehlenbacher, 1960), methods based on the carbonyl content of oxidized fats (Henick et al., 1954 Lillard and Day, 1961), and measurement of oxygen uptake either by manometry or polarography (Tappel, 1955 Hamilton and Tappel, 1963). 

In recent years, modern instrumental methods have been developed to monitor lipid oxidation in biological samples, including dairy products. These include use of electron spin resonance (ESR) spectrometry, direct measurement of secondary oxidative products such as malonaldehyde, static and dynamic GC/MS methods. ESR spectrometry permits detection of free radicals formed in the very early stages of oxidation prior to the formation of peroxides. The method has been applied successfully to dairy products such as milk powders and processed cheese (Nielsen et al., 1997 Stapelfeldt... 

In particular, this chapter wiU stress the need to look beyond the classic radical chain reaction. Lipid oxidation mechanisms have been proposed based on kinetics, usually of oxygen consumption or appearance of specific products (e.g., LOOK) or carbonyls (e.g., malonaldehyde), assuming standard radical chain reaction sequences. However, when side reactions are ignored or reactions proceed by a pathway different from that being measured, erroneous conclusions can easily be drawn. The same argument holds for catalytic mechanisms, as will be shown in the discussion about metals. In the past, separation and analysis of products was laborious, but contemporary methods allow much more sensitive detection and identification of a broad mix of products. Thus, multiple pathways and reaction tracks need to be evaluated simultaneously to develop an accurate picture of lipid oxidation in model systems, foods, and biological tissues. 

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. 

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