Oxidation indices peroxide determination

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)" . ... 

In this chapter an environmental friendly extraction process of com germ oil based on the use of supercritical CO2 (SC-CO2) is presented. The effect of important operating parameters in supercritical fluid extraction (SEE) processes such as pressure, temperature and flow rate on the extraction kinetics and the quality of the extracted oil is discussed. As for many SC-CO2 extractions of vegetable oils, extraction curves of com germ oil present an initially linear part with a slope close to the oil solubility value in CO2. Then, a second section of the extraction curve is determined by the diffusional resistance in the solid matrix. Characterization of supercritical cmde com oil is presented by showing some properties reported in the literature such as physical parameters, fatty acid composition, neutral lipids, content of tocopherols, acid index, peroxide value, antioxidant capacity and the oxidative stability. 

Chemical Antioxidant Systems. The antioxidant activity of tea extracts and tea polyphenols have been determined using in vitro model systems which are based on hydroxyl-, peroxyl-, superoxide-, hydrogen peroxide-, and oxygen-induced oxidation reactions (109—113). The effectiveness of purified tea polyphenols and cmde tea extracts as antioxidants against the autoxidation of fats has been studied using the standard Rancimat system, an assay based on air oxidation of fats or oils. A direct correlation between the antioxidant index of a tea extract and the concentration of epigallocatechin gallate in the extract was found (107). 

The determination of F2-isoprostanes, oxidation products of arachidonic acid, has been proposed as a more reliable index of oxidative stress in vivo, overcoming many of the methodological problems associated with other markers. The isoprostanes have emerged as a most effective method of quantifying the potential of antioxidants to inhibit lipid peroxidation. However, one drawback of this method is that quantification of F2-iP requires sophisticated techniques, in particular GC/MS and HPLC/MS... 

The time to reach a certain PV may be used as an index of oxidative stability for food lipids. The effects of antioxidants and food processing on fats are often monitored in this way. Thus, a longer time period to reach a certain PV is generally indicative of a better antioxidant activity for the additive under examination. However, a low PV represents either early or advanced oxidation the breakdown of peroxides to secondary oxidation products will result in a decrease in PVs during the storage period. For determination in foodstuff, a major disadvantage to the classical iodometric PV assay is that a 5-g test portion is required it is sometimes difficult to obtain sufficient quantities of lipid from foods low in fat. Despite its drawbacks, PV determination is one of the most common tests employed to monitor lipid oxidation. 

Active Oxygen Method for Fat Stability (AOM) (Cd 12-57) determines the time (in hours) for a sample of fat or oil to attain a predetermined peroxide value (PV) under the conditions of the test. The method is used to estimate the comparative oxidative stability of fats and oils. The method has been placed in surplus, in favor of Cd 12b-92 (Oil Stability Index), but retains official status and is still used in domestic industry. p-Anisidine Value (AV) (Cd 18-90) determines the amount of aldehydes (principally 2-alkenals and 2,4-dienals) in animal and vegetable fats and oils. These are degradation products of peroxides, which are not removed by bleaching. Some fats and oils chemists propose increased use of this method in purchase specifications. Bleaching Test for Soybean Oil (Cc 8e-s63) determines the color of a sample of soybean oil after treatment with a specified bleaching earth. Specific methods exist for other oil species. 

The TBA test is perhaps the most widely used method for determining lipid peroxidation. The representative adduct of lipid peroxidation, malondialdehyde, forms a 1 2 adduct with TBA that can be measured by spectroscopy or fluorometry. The general procedure, of which there are numerous variations, simply involves heating a small quantity of the test substance for a defined period of time in an aqueous acidic solution of TBA, and then measuring the absorbance (535 nm) of the red color which is produced in the TBA reaction. It should be considered as an index of oxidative stress that represents primarily lipid peroxidation. ... 

Because of its convenience, the TB A method has become a common assay to determine the degree of peroxidation and oxidative susceptibility of a wide range of biological materials, including LDL. However, the validity of the TBA determination as an index of lipid peroxidation in biological samples has been a matter of considerable debate in the literature. The determination of TBARS inherently lacks specificity, and is subject to interference by many compounds including materials that are not due to lipid peroxidation (see Chapter 5.E). This method is also flawed by analytical artifacts, and is affected by the same factors as lipid peroxidation. 

Ahlblad and co-workers [37] determined the oxidation profiles of polyamide 6,6 using FT-IR spectroscopy and imaging chemiluminescence. FT-IR analysis provides carbonyl index depth profiles and imaging chemiluminescence gives a peroxide depth profile. 

Among the most common methods to measure thermal and oxidative behavior of oils are the classical oxidative stability analysis used by industries, based on the active oxygen method (AOM), which determines the number of hours required for the oil to reach a peroxide value of 100 meq/kg O and the oxidative stability index (OSI), which can be considered as automated AOM with an apparatus that simulates the events under specific atmospheres, usually with the use of high temperatures. The OSI method measures the changes in water conductivity when the oxidation compounds are formed [17]. 

Other parameters that are usually determined to assess the oil stability and quality are the acid index (Al) and the peroxide value (Ca 5a-40 and Cd 8-53 AOCS Methods). The oxidative stability is also an important parameter in the characterization of fats and oils that can be determined among other methods by using the Rancimat test. 

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