Ratnayake W M
Health Canada, Nutrition Research Division, Ottawa, ON.
J AOAC Int. 1995 May-Jun;78(3):783-802.
An infrared spectrophotometric (IR) method for the determination of total trans unsaturated fatty acid (trans) content and a combined gas-liquid chromatographic/infrared spectrophotometric (GC/IR) method for determination of fatty acid composition of partially hydrogenated vegetable oils (PHVO) were studied collaboratively in 12 laboratories using 7 PHVO samples, including 1 pair of blind duplicates. The test samples were methylated and analyzed for total trans content by IR and for fatty acid composition by GC/IR using a capillary column coated with SP-2560 or another suitable cyanoalkylsiloxane stationary phase. From the measured IR absorption, the isolated trans content was calculated using a calibration curve of absorption versus trans content developed with 2-component calibration standard mixtures of methyl elaidate and oleate. The GC provided the levels of mono-trans-octadecadienoates (18:2t), di-trans-octadecadienoates (18:2tt) and mono-trans-octadecatrienoates (18:3t). The trans-octadecenoate (18:1t) content was calculated with the formula: 18:1t = IR trans-0.84 x (18:2t + 18:3t) - 1.74 x 18:2tt. The cis-octadecenoate (18:1c) content was obtained as the difference between total octadecenoates (18:1) and 18:1t. Reproducibility relative standard deviations (RSDR) for 15 to 35% trans content determined by IR were in the range of 8.8-11.7%, whereas RSDR for the test sample with 5% trans content was 34.6%. RSDR values for 18:1t by the GC/IR followed the same pattern as that of IR trans values: 36.4% for the test sample with 4.9% 18:1t versus 7.8-12.5% for test samples with 14.9 to 32.6% 18:1t. The content of 18:1c in the test samples varied from 24.7 to 34.5% and their RSDR values ranged from 3.8 to 10.5%. The mean values for 18:1t and 18:1c compared favorably with the absolute levels determined by a silver nitrate-thin layer chromatography/GC procedure. The IR and GC/IR methods are recommended for determination of trans content and fatty acid composition, respectively, of partially hydrogenated fats derived from vegetable oils, terrestrial animal fats or such oils and fats isolated from food products containing > 5% trans fatty acids. For samples containing < or = 5% trans fatty acids, a direct GC method (American Oil Chemists' Society Official Method Ce 1c-89) is available for determination of both trans content and fatty acid composition, because at lower trans levels, overlap of 18:1 cis and trans isomers on GC with very polar capillary columns is negligible.(ABSTRACT TRUNCATED AT 400 WORDS)
在12个实验室中，使用7个部分氢化植物油（PHVO）样品（包括1对盲样重复样品），协同研究了一种用于测定总反式不饱和脂肪酸（反式）含量的红外分光光度法（IR）以及一种用于测定部分氢化植物油脂肪酸组成的气相色谱/红外分光光度联用方法（GC/IR）。将测试样品进行甲基化处理，然后用IR分析总反式含量，用涂有SP - 2560或其他合适的氰基烷基硅氧烷固定相的毛细管柱通过GC/IR分析脂肪酸组成。根据测得的IR吸收值，利用反油酸甲酯和油酸的双组分校准标准混合物绘制的吸收值与反式含量校准曲线计算出分离出的反式含量。GC提供单反式十八碳二烯酸（18:2t）、二反式十八碳二烯酸（18:2tt）和单反式十八碳三烯酸（18:3t）的含量。反式十八碳烯酸（18:1t）含量用公式计算：18:1t = IR反式 - 0.84×(18:2t + 18:3t) - 1.74×18:2tt。顺式十八碳烯酸（18:1c）含量通过总十八碳烯酸（18:1）与18:1t的差值获得。IR测定15%至35%反式含量的再现性相对标准偏差（RSDR）在8.8 - 11.7%范围内，而反式含量为5%的测试样品的RSDR为34.6%。GC/IR测定18:1t的RSDR值与IR反式值的模式相同：18:1t含量为4.9%的测试样品的RSDR为36.4%，而18:1t含量为14.9%至32.6%的测试样品的RSDR为7.8 - 12.5%。测试样品中18:1c的含量在从文中删除的内容 varied from 24.7 to 34.5%，其RSDR值在3.8至10.5%范围内。18:1t和18:1c的平均值与硝酸银 - 薄层色谱/GC法测定的绝对水平相当。推荐分别用IR和GC/IR方法测定植物油、陆生动物脂肪或从含>5%反式脂肪酸的食品中分离出的此类油脂来源的部分氢化脂肪的反式含量和脂肪酸组成。对于含≤5%反式脂肪酸的样品，有一种直接GC方法（美国油脂化学家协会官方方法Ce 1c - 89）可用于测定反式含量和脂肪酸组成，因为在较低反式水平下，18:1顺式和反式异构体在非常极性的毛细管柱上的GC重叠可忽略不计。（摘要截短至400字） （注：原文中“varied from 24.7 to 34.5%”前的“在从文中删除的内容”表述有误，应是原文此处有缺失信息，翻译时按原文呈现了这一不完整状态）
