Volatile oxidation products

Films, for both mechanical and spectroscopy studies, were affixed to the specimen panels of the weatherometer. Upon completion of the UV exposure, which occurred at 37°C 1°C in the presence of air, the films were removed and kept at room temperature in the dark for at least 24 hours in order to remove any volatile oxidation products. 

Analyses. Samples of reaction mixtures were frozen and sublimed into the vacuum line (10-6 mm Hg). The vapor was passed through a wide-bore U-trap at —45°C which collected acetic acid and volatile oxidation products and allowed any unreacted hexenes to pass. After further fractional condensation the hexene was transferred to a graduated tube to measure the volume and was finally analyzed by GLC using a 12 meter X 6 mm column packed with polypropylene glycol LB-550-X on Chromosorb W. The sublimation residue was analyzed by IR spectroscopy while the acetic acid condensate was subjected to a wet separation (5, 15) to recover the oxidation products for GLC analysis. 

R. Bernstein, S.M. Thornberg, R.A. Assink, A.N. Irwin, J.M. Hochrein, J.R. Brown, D.K. Derzon, S.B. Klamo, and R.L. Clough, The origins of volatile oxidation products in the thermal degradation of polypropylene, identified by selective isotopic labeling. Polym. Degrad. Stab., 92, 2076-2094 (2007). 

When chain fission of the alkoxy radical occurs on the other side of the free radical group, the reaction will not yield volatile aldehydes but will instead form nonvolatile aldehydo-glycerides. Volatile oxidation products can be removed in the refining process... 

Gas Chromatographic Methods. Gas chromatographic methods may be used for measuring volatile oxidation products. Static headspace, dynamic headspace, or direct injection methods may be employed. Specific aldehydes may be measured as indicators for oxidative stability of oils and fats. Thus, propanal is an and as indicator for stability of omega-3 fatty acids, whereas hexanal is best for following the oxidative stability of omega-6 fatty acids. 

Oa StabUity Index. Two conductivity instruments, Rancimat and The Oxidative Stability Instmment, have been developed as alternatives to AOM Stability analysis. These instruments measure the increase in deionized water conductivity resulting from trapped volatile oxidation products produced when the oU product is heated under a stream of air. The conductivity increase is related to the oxidative stabihty of the products. These instruments provide a more reproducible measurement of oxidation stability with less technician time and attention. 

Lipid-derived volatile compoimds dominate the flavor profile of pork cooked at temperatures below 100°C. The large numbers of heterocyclic compounds reported in the aroma volatiles of pork are associated with roasted meat rather than boiled meat where the temperature does not exceed 100 C (34,35). Of flie volatiles produced by lipid oxidation, aldehydes are the most significant flavor compounds (35,36). Octanal, nonanal, and 2-undecenal are oxidation products from oleic acid, and hexanal, 2-nonenal, and 2,4-decadienal are major volatile oxidation products of linoleic acid. 

It can be seen from eqn. (109) that the rate of oxidation in films depends on the relatively low rate of oxygen supply by diffusion, and is directly proportional to the concentration of oxygen. In the case of very thin samples the oxidation rate appears to be independent of thickness. The dependence of the induction period on the pressure of oxygen in such cases indicates that oxidation is also limited by oxygen diffusion, and the observed deviation of the behaviour of very thin samples may be explained by the fact that volatile oxidation products promote further oxidation processes. 

Gas chromatographic (GC) methods have been used for determining volatile oxidation products. Static headspace, dynamic headspace or direct injection methods are the three commonly used approaches. These methods were compared in an analysis of volatile compounds in an oxidized soybean oil. It was found that each method produced significantly different GC profiles (Frankel 1985). The dynamic headspace and direct injection methods gave similar results, but the static headspace is more sensitive to low molecular weight compounds. Lee and co-workers (1995) developed a dynamic headspace procedure for isolating and analyzing the volatiles from oxidized soybean oil, and equations were derived from theoretical considerations that allowed the actual concentration of each flavor component to be calculated. 

The active oxygen method (AOM) is the most common analytical method used to measure oxidative stability of fats and oils products. AOM employs heat and aeration to accelerate oxidation of the oil by continuously bubbling air through a heated sample. Periodic peroxide values are measured to determine the time required for the oil to oxidize to a predetermined peroxide value 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 h AOM sample. Conductivity instruments such as the Rancimat and the Oxidative Stability Instrument have been developed as alternatives to AOM stability analysis. These instruments measure the increase in the conductivity of deionized water resulting from trapped volatile oxidation products produced when the oil product is heated under a stream of air. The conductivity increase is related to the oxidative stability of the products. These instruments provide a more reproducible measurement of oxidation stability with less technician time and attention. 

Oxidation studies were conducted between 140°C and 180°C for a 50 hour period with pure oxygen being circulated at 1 atm pressure with a flow rate of 15 L/hr through a 250 g sample. Volatile oxidation products were removed as formed by the gas flow and trapped downstream. At the end of the experiment the base stock was analyzed for acid content, amount of precipitates, and changes in viscosity. Oxygen consumption was measured throughout the 50 hour time period. 

Carotenoids are present in soybeans in a very low concentration (0.8—3.7 ppm), and the main forms are lutein and P-carotene. They are co-extracted with oil but are often removed or degraded by oil refining steps designed to remove the undesirable minor components that contribute to physical and chemical instability and undesirable color, such as degumming to remove PLs, neutralization to remove free fatty acids, bleaching to decompose lipid hydroperoxides, and deodorization to remove volatile oxidation products. 

The volatiles in the frying oil increase in the beginning, but then decrease as frying continues. The most important volatiles for the quality of frying oil are saturated aldehydes Cg-Cg, enals (e.g., 2-decenal), dienals (e.g., 2,4-heptadienal), and hydrocarbons (hexene, hexane, heptane, octane, nonane, and decane). The presence of volatile oxidation products formed during the frying process was discussed by Perkins (1996) and Nawar (1998). 

TGA curves in air flatten more or less distinctly at approximately 350 °C as a consequence of the formation of non-volatile oxidation products. This levelling of the curve does not occur in the case of asphaltenes. In a temperature range from 400 to 550 °C combustion takes place marked by a near lineal decrease of the TGA curve. In this range consecutive and/or simultaneous cracking and oxidation processes may take place with the formation of highly volatile and solid intermediates as well as of combustion products such as CO, SO, NO. Above 550 °C only ash remains. The DTG curves have up to ten maxima, in the temperature range from 280 to 550 °C, except for the asphaltenes, which usually only have two maxima between 450 and 550 °C. The start temperature of the oxidation reaction, marked by the temperature of the first maximum of the DTG curve, and the end of the reaction (last maximum) may be elucidated using statistics (Tables 4-73 to 4-76). 

These results do not permit positive identification of the source of the differences between the samples. This may be evaporation of some volatile parts during the mixing process losses due to the formation of volatile oxidation products or losses during evaporation of the chloroform extraction fluid. The exponential function AG =fiT) for the... 

Q < I indicates predominating evaporation of compounds in the sample. Q > I indicates the predominance of highly volatile oxidation products, whereas Q = I suggests an equilibrium between evaporation of original compounds of the sample and of volatile oxidation products. However, it may also indicate evaporation of original components both in argon and in air. With reference to Fig. 4-67 and considering the value of Q after 30 minutes test time, there is a sequence ... 

The three samples which experience weight loss at 165 °C (Q < 1) show that this loss is due to evaporation of non-oxidized parts of the sample (Fig. 4-90). At 200 °C there is no such uniformity (Fig. 4-91). For samples B80/1 and PMB/2 Q is permanently > 1.0, i.e. their considerable losses are due to volatile oxidation products. The values for the PMB/4 are still higher ... 

Thermogravimetry in air of the vacuum distillates is no different to that in argon for temperatures up to 200 °C (Table 4-166). Above 200 °C the cuiwe of the TGA in air is shifted to higher temperatures as shown by the value of AG300. Low-volatile oxidation products are probably present. 

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