Weight loss behaviors of straight-chain unsaturated fatty acids [8 kinds of monoenoic :
cis- (
c-) and
trans-(
t-) 9-hexadecenoic (C
16 : 1), 9-octadecenoic (C
18 : 1), 11-icosenoic (C
20 : 1), and 13-docosenoic (C
22 : 1) acids ; and all
cis-11, 14-icosadienoic (C
20 : 2), 8, 11, 14-icosatrienoic (C
20 : 3), and 5, 8, 11, 14, 17-icosapentaenoic (C
20 : 5) acids] and their characteristics were studied by thermogravimetry (TG), derivative thermogravimetry (DTG), and differential scanning calorimetry (DSC).
1) The TG curve for monoenoic acids in a nitrogen flow showed a simple weight loss pattern. In an air flow, the weight loss pattern in the temperature range from initial to 50% weight loss was essentially the same as that in the nitrogen flow. Following 50% weight loss, the pattern was more complicated than that in the nitrogen flow. The initial temperature of weight loss on TG and first peak on DTG curve in air and nitrogen shifted to the higher-temperature side with increase in carbon number.
2) On the TG curve for polyunsaturated fatty acids in the air flow, a weight gain due to oxidation was observed at 120 °C. It was significant, indicating that oxidative sensitivity could be calculated as the rate of weight gain. The weight loss pattern after initiation became more slowly with increase in the numbers of double bonds. On the TG curve in the nitrogen flow, the rate of weight loss with rise in temperature after 50% weight loss decreased with increase in the numbers of double bonds.
3) Volatile and residual substances at 25% weight loss of unsaturated fatty acids were analyzed by gas chromatography (GLC). In the nitrogen flow, the proportion of original fatty acid in the volatile and residual substances was above 57%. The proportion of original fatty acid in the air flow clearly decreased with increase in unsaturation. Icosapentaenoic acid in the volatile substances was about 18%, while none could be detectected in the residual substances.
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