| Trans fatty acids are fatty acids (TFAs) containing at least one trans double bond, but only polyunsaturated fatty acids with methylene interrupted double bonds. Natural trans fatty acids mainly exist in ruminant products. It also produced during the procedure of oils and fats hydrogenation, refining and food processing. Studies found that large intake of trans fatty acids will cause a variety of adverse effects on health. But 11tC18:1 in the ruminant products metabolized into 9c 11t C18:2 has been confirmed in the human body, this isomer of conjugated linoleic acids, has physiological activities good for health. Therefore, not all trans fatty acid isomers have the same physiological effects which may vary in different isomers. Accordingly, it's necessary to separat and determine different isomers of trans fatty acids. Researches on TFAs were mainly focused on ruminants and bakery foods recently, but little attention was paid on TFAs of other foodstuffs. Thus, the understanding of the presence of trans fatty acids in foodstuffs has grate theoretical and practical value for the study on their physiological activities and the calculation of Chinese residents' TFAs daily intakes.The main contents of this study are: (a) the establishment of isolation, qualitative and quantitative detection technology on TFAs analysis: Fatty acid methyl esters are pre-separated with silver-ion thin layer chromatography (Ag~+-TLC), then detected with gas chromatography (GC). (b) TFAs contents and compositions of 17 types of foodstuffs were determined with Ag~+-TLC/GC. (c) Daily intakes of TFAs was calculated reference to the data of "China nutrition and health survey" and representative food consumptions. (d) Analyze the TFAs contents of traditional processed foods, and also gain the existence status of TFAs. Main results are as follows:1. Fatty acid methyl esters were pre-separated withAg~+-TLC, then detected with GC, equipped with 100 m specific highly polar capillary column CP-Sil 88. Thus, the overlap of trans and cis C18:l isomers between C18.0 and C18:2 n-6 was avoided, which can not be totally resolved by gas chromatography. The relative standard deviations of trans fatty acid isomers are 0.29%～0.48%; recoveries are 92.52%~ 106.66% in this method.2. TFAs are common in foodstuffs, mainly in 9t C18:1,trans C18:l and 9t12t C18:2. Trans C18:1 account for a large portion of total TFAs contents in various foodstuffs; not only ruminant products are abundant in 11t C18:1, 9t C18:1, 10t C18:l and 11t C18:l but also other foodstuffs. A few 9t C16:l and 9/12t C18:2 are found in certain foodstuffs.3. Chinese daily intakes of TFAs showed an increasing tendency in the years of 1982, 1992 and 2002. Daily intakes of TFAs in the year of 2002 reached 0.555 g/ (day per reference man), 70.8% higher than that in 1982. Urban residents' TFAs intakes was 0.729 g/ (day per reference man) in the year of 2002, salience higher than rural residents'. The main sources of TFAs in daily diet were animal foods and vegetable oils. TFAs from animal foods were 58.4% in total, and vegetable oils were 22.3%. The incidence and mortality of cardiovascular disease raised in the same period, which has positive correlation with TFAs intakes.4. Because of the wide use of edible oils and high temperature of frying and baking, some traditional processed foods are high in TFAs. TFAs contents of these foods have positive correlation with PUFA (9/12t C18:2 not included) and cw-MUFA, the correlation coefficients are 0.77 and 0.47 respectively. Therefore, a part of TFAs may peroduced by the isomerizations of PUFA or MUFA during the thermal processing. |
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