Various variations on the principle of oxygen

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Various variations on the principle of oxygen absorption are available. Tian, Dasgupta and Shermer (2000) developed a gas-phase flow injection analysis method for the direct determination of the oxidative stability of solid high fat containing samples. It is an automated stopped-flow gas-phase system with an oxygen sensor and a programmable temperature reactor that measures oxygen consumption of samples at various programmed temperatures. The reactor containing the sample is flushed with a carrier gas of 0.1 % O 2 and 99.9 % N 2 , heated to intended temperature, and with the use of a valve system the reactor vessel is connected on-line at set time intervals with the carrier gas and the amount of oxygen uptake in the reactor vessel is measured by an oxygen sensor. Advantages of oxygen absorption methods are that they correlate well with ambient storage temperature shelf-life, have good reproducibility and are faster than OSI or AOM (Wan, 1995, O'Brien, 1998). However, despite the advantages of oxygen absorption methods they are not as popular and are seldom used in quality control or comparison of samples. According to Tian et al (2000) the close correspondence to Arrhenius behaviour makes it
30 possible to predict the relative stability of samples at temperatures different from the experimental conditions used. However, no mention has been found in literature on the use of oxygen absorption methods to predict the shelf-life of oils. Comparison of costs involved in acquiring the instruments has also not been found but the analysis costs should be similar. * Other There is a variety of other accelerated oxidative stability testing methods that are not used as routine and will be mentioned briefly. Thermal analysis can be used to follow the oxidative and thermal (e.g. frying) degradation changes under isothermal conditions (Buzas and Kurucz, 1979). Oxidative changes are followed quantitatively by weight gain of the sample and the rate thereof. Samples are dispersed as a thin film on a ceramic block under airflow (20 Ilh). The block is heated up to 400°C. Accelerated oxidation of edible oils by thin-film oxidation with UV irradiation at different temperatures (80 and 100°C) has been measured by PV, headspace volatile peak areas and UV absorbance at 232 nm (Gordon et aI, 1994). An oil sample (2.5 g) in a crucible was irradiated from a distance of3 cm with a six-Watt short wave UV lamp (200-280 nm). Tan, Che Man, Selamat and Yusoff (2002) used differential scanning calorimetry (DSC) to compare oxidative stability of oils. The technique involves oxidation of oil samples in an oxygen-flow DSC cell with the cell temperature set at four isothermal temperatures (110, 120, 130 and 140°C). Initiation of the oxidation reaction is observed by a dramatic increase of evolved heat. Extrapolation ofthe downward portion of the DSC oxidation curve is taken as the oxidative induction time.

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