Peroxide value chromatography

Peroxide value (POV), 656, 657-64 chromatography, 672 colorimetric determination, 632 mbber and elastomers, 676 Peroxisomes, antioxidant in biological systems, 610, 611 Peroxo complexes... 

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... 

Analytical characteristics of fish oils and whale oils, 134,139 Analytical methods anisidine value, 264 AOCS, 259 dilatation, 253 fat adulteration, 270 fat content, 250 free fatty acids, 260 gas chromatography, 265 hydroxyl value, 261 iodine value, 259 lUPAC, 249 NMR spectra, 250 pancreatic lipase hydrolysis, 267 peroxide value, 263 phosphorus estimation, 264 saponification value, 260 slip point, 251 solid fat, 255 standard, 250... 

Table 3.2 Headspace analysis of oxidized soya bean oil (peroxide value 9.5) by three methods direct injection, dynamic headspace gas chromatography (HSGC) and static HSGC. OlOOH = oleic acid hydroperoxide LoOOH = linoleic acid hydroperoxide LnOOH = linolenic acid hydroperoxide.

Figure 3.11 Headspace gas chromatography analysis of volatiles of an oxidized vegetable oil (peroxide value 26.2). Trace A sample thermostatted at 60°C. Trace B sample heated to 140°C in nitrogen. Trace C sample heated to 160°C in air. Chromatographic conditions 25 mx0.32mm fused silica column coated with SE-54 , 38°C (3 min), then 6°Cmin to 170°C. Sample thermostatted at 60°C. Reproduced from Ulberth, F. and Roubicek, D., Be-stimmung von Pentan als Indikator fur die oxidative Ranzigkeit von Olen, Fat Science Technology, 94, 19-21, 1992.

Figure 3.13 Headspace analysis of volatiles of whole milk powder stored in air at 40 C (peroxide value 1.01). 1 = pentanone 2 = pentanal 3 = methyl butyrate (internal standard) 4 = hexanal 5 = heptanone 6 = heptanal 7 = octanal 8 = octanoic 9 = nonanal. Chromatographic conditions were as in Fig. 3.12. Reproduced from Ulberth, F. and Roubicek, D., Monitoring of oxidative deterioration of milk powder by headspace gas chromatography. International Dairy Journal, 5, 523-31, 1995.

To evaluate oxidative stability, different methods are used to measure lipid oxidation after the sample is oxidized under standardized conditions to a suitable end-point. In Table 7.1, different lipid oxidation tests are ranked in decreasing order of usefulness in predicting Ae stability or shelf life of a food product. Methods for sensory evaluations, conjugated diene, gas chromatography of volatiles, peroxide values and thiobarbituric acid-reacting substances were discussed in Chapter 5. 

Lipid oxidation can be evaluated in a variety of ways, such as the determination of peroxide value (PV), carbonyl compounds (malondialdehyde, hexanal, etc.) diene conjugation, and oxygen consumption. Various methods may be used to quantify the lipid oxidation such as iodometric titration (IT), UV-visible spectroscopy (ferrous oxidation method, iodide oxidation method), chromatography, CL, or fluorescence (FL) methods. 

Neff et al. (1992) oxidized purified soyabean oil TAG at 60°C in the dark and compared the oxidative stability of each molecular species of the TAG by the determination of peroxide value and volatile compounds, and by a high-performance liquid chromatography (HPLC) analysis of the oxidized TAG molecule. They found that an increased rate of peroxide value showed a positive correlation with the average number of double bonds (r = 0.81), LA (r = 0.63) or LN (r = 0.85), but a negative correlation with OA (r = -0.82). Resistance of soyabean oil TAG to oxidation decreased with an increase in LN concentration (r = 0.87), while that increased with increasing OA level (r = -0.76). More interestingly, they found that replacement of PA in... 

Gas chromatography Hexanal is a volatile closely related to extent of lipid oxidation and rancidity. It is determined in the headspace of the package and correlates with peroxide values (Pike 1994). 

During the decomposition of peroxyesters, large amounts of CO2 are formed. The value found for the activation volume can be considered when the mechanism of peroxide decomposition is discussed. The analysis of the gaseous decomposition products by gas chromatography shows large amounts of CO2, whose formation can take place during the decomposition. 

A variety of compounds such as hydrocarbons, alcohols, furans, aldehydes, ketones, and acid compounds are formed as secondary oxidation products and are responsible for the undesirable flavors and odors associated with rancid fat. The off-flavor properties of these compounds depend on the structure, concentration, threshold values, and the tested system. Aliphatic aldehydes are the most important volatile breakdown products because they are major contributors to unpleasant odors and flavors in food products. The peroxidation pathway from linoleic acid to various volatiles is determined in several researchs, - by using various techniques (Gas chromatography mass spectrometry, GC-MS, and electron spin resonance spectroscopy, ESR), identified the volatile aldehydes that are produced during the oxidation of sunflower oil. In both cases, hexanal was the major aldehyde product of hydroperoxide decomposition, whereas pentanal, 2-heptenal, 2-octenal, 2-nonenal, 2,4-nonadienal, and 2,4-decadienal were also identified. 

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