Peroxide value , measurement stability measurements

Rancidity measurements are taken by determining the concentration of either the intermediate compounds, or the more stable end products. Peroxide values (PV), thiobarbituric acid (TBA) test, fatty acid analysis, GC volatile analysis, active oxygen method (AOM), and sensory analysis are just some of the methods currently used for this purpose. Peroxide values and TBA tests are two very common rancidity tests however, the actual point of rancidity is discretionary. Determinations based on intermediate compounds (PV) are limited because the same value can represent two different points on the rancidity curve, thus making interpretations difficult. For example, a low PV can represent a sample just starting to become rancid, as well as a sample that has developed an extreme rancid characteristic. The TBA test has similar limitations, in that TBA values are typically quadratic with increasing oxidation. Due to the stability of some of the end-products, headspace GC is a fast and reliable method for oxidation measurement. Headspace techniques include static, dynamic and solid-phase microextraction (SPME) methods. Hexanal, which is the end-product formed from the oxidation of Q-6 unsaturated fatty acids (linoleate), is often found to be a major compound in the volatile profile of food products, and is often chosen as an indicator of oxidation in meals, especially during the early oxidative changes (Shahidi, 1994). 

The maximum shelf-life of bulk packaged WMP containing 3% moisture is about 6 months at 30°C (Kjaergaard Jensen, 1988). The oxidation of WMP, as measured by peroxide value, is dependent on the moisture content of the powder, van Mil and Jans (1991) reported that under similar storage conditions, the peroxide value of WMP increases more rapidly for powder containing 3% moisture than in powder containing 2.4% moisture. The water activity (aw) range for WMP is usually 0.13 0.20, with a typical value from 0.16 to 0.18 (Wewala, 1990). Stapelfeldt et al. (1997) found that the quality of WMP is maintained best at aw between 0.11 and 0.23, whereas the quality of the powder decreases when stored at aw of 0.31 at 45°C. However, the critical aw for improved oxidative stability of WMP stored at 40°C for one year is 0.21 0.24 at a moisture level of 3.4% (Wewala, 1990). 

AOM Stability. The active oxygen method (AOM) is the most commonly used analytical method for measuring oxidative stability of fats and oUs products. AOM employs heat and aeration to accelerate oxidation of the oil by continuously bubbling air through a heated sample. Periodic peroxide value analyses are performed to determine the time required for the oil to oxidize under the AOM conditions. This method requires close attention to detail to produce reproducible results, and even then, the variation between laboratories is 25 for a 100-hour AOM sample. 

Peroxide value is the most widely used method (AOCS methods Cd S-53 and Cd 8b-90) to determine the quality of the oil. The primary oxidation products of oils and fats are the hydroperoxides. They can be quantitatively measured by determining the amount of iodine liberated by their reaction with potassium iodide. The peroxide content is expressed in terms of milliequivalents of iodine per kilogram of fat. However, when these hydroperoxides start breaking down to produce off-flavor compounds, correlation to the quality and stability of the oil will no longer be valid. Freshly deodorized oil should have zero peroxide value. In most cases, for the product to have acceptable storage stability, the peroxide value of oils used should be less than 1.0 meq/kg fat at the point of use. 

Figure 16. Effects of processing and storage temperature on oxidative stability as measured by peroxide value. (A) 1-qt container held for 7 weeks in storage. (B) Refined and bleached oil in 1-qt size container.

Lipid hydroperoxides can be measured by peroxide value (PV) and 2-thiobarbituric acid (TBA) tests. The resistance of a fat or an oil to oxidative rancidity can be measured by the Schaal oven test, Swift test, and oil stability index (OSI) analysis. 

Wang and Johnson (2001) reported on test measurement methods that were major indicators of soybean oil quality. These tests included peroxide value, anisidine value, FFA content, phospholipid content, total tocopherol content, oxidative stability index, color, and moisture content. For soybean meal, they reported on urease activity, protein dispersibility index (PDI), rumen bypass or rumen undegradable protein, trypsin inhibitor activity, moisture content, residual oil content, protein content, fiber content, color, amino acid profiles, and protein solubility under alkaline (KOH) conditions. 

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... 

The Rancimat method has been used to measure the antioxidant activity of synthetic and natural antioxidants (2-/>2d) and has correlated well with oil stability measured by the Active Oxygen Method (27) and peroxide value measurement (28). Our study showed that using the Rancimat method to study the antiphotooxidative effect of carotenoids on the soybean oil was in agreement with the results using the headspace oxygen depletion method (16) and the peroxide value method (16,JT). 

The oxidative stability of food lipids has been evaluated by a variety of methods using a wide range of conditions and techniques for measuring oxidation. The choice of appropriate methods and oxidation conditions is critical in the interpretation of stability data. Thus, interpretation of data pertaining to lipid oxidations should take into account the limitation of the methodology used. The oxidative stability of unsaturated lipids can be estimated by determining the extent of oxidation produced under various defined and standardized conditions. The susceptibility of the samples to oxidation can be measured reliably if the samples are initially fresh (with a peroxide value approaching 0) and the conditions of oxidation are sufficiently mild and... 

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. 

We have seen earlier that in the presence of an antioxidant the rate of hydroperoxide formation is related to the ratio of lipid concentration to that of the antioxidant concentration, which is dependent on the rate of inhibition reaction (4). The effectiveness of an antioxidant is generally based on the balance between the inhibition rate (k ) of reaction (4) and the transfer reactions ( ), (9) and (13). Therefore, the effect of antioxidants on hydroperoxide decomposition reactions (10) and (11) is an important property that needs to be evaluated. However, most studies of antioxidant actions measure initial events of lipid oxidation based on oxygen absorption, hydroperoxide formation, and peroxide values (Chapters 5 and 7). Very few studies have measured the effect of antioxidants on decomposition products of hydroperoxides, such as aldehydes and carbonyl compounds. Yet these volatile decomposition aldehydes are most relevant to the development of rancidity and to the ultimate quality and stability of food lipids. 

No data have yet been published on the stability of CLA in capsules. Observations on polymers and volatiles in capsules are reported above. In a stability test program according to International Conference on Harmonization (ICH) guidelines on a free fatty acid product, the content of total CLA was not significantly reduced after 24 mo at 25°C/60% relative humidity. In this test, CLA was measured by GC. Peroxide value (PV) did not develop in the capsules (data not published). 

AOM or Swift Stability Test. This is a method for measuring the oxidative stability of oils and fats. The induction period is determined from a plot of peroxide value against time when air is bubbled through at 98+0.2°C and is based on the time required to reach a specific peroxide value (e.g. 100). Standard methods are described by AOCS [Cd 12 57(89)]. 

Industry uses a number of analytical methods to characterize oils and fats, in terms of a number of parameters which include moisture, titer (solidification point), free fatty acid, unsaponifiable material, iodine value, peroxide value, and color. Moisture content of the oils and fats is an important measure for storage stability at elevated temperature because it facilitates hydrolysis which in turn impacts odor and color quality. Titer is a measure of the temperature at which the material begins to solidify, signifying the minimum temperature at which the material can be stored or pumped as a fluid. Free fatty acid is a measure of the level of hydrolysis the oils and fats have undergone. Increased fatty acid content usually negatively impacts product color stability because fatty acids are more susceptible to oxidation. Unsaponifiable material is a measure of the nontriglyceride fatty material present, which affects the soap yield of the material. The iodine value is a measure of the amount of unsaturation present in the oils and fats. Peroxide value is a measure of the... 

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]. 

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