乳和乳制品.用甘油三酯气相色谱分析法测定乳脂肪纯度（ISO 17678-2019）

[中文适用范围]： 本文件规定了使用甘油三酯气相色谱分析测定乳脂纯度的参考方法。 该方法利用乳脂的甘油三酯指纹与其他脂肪和油的各个甘油三酯指纹的差异来确定超出乳脂通常观察到的范围的样品。 这是通过使用基于对牛奶完整性敏感的各个甘油三酯峰的归一化加权和的定义的甘油三酯公式来实现的[6][7]。 通过将这些配方的结果与之前观察到的一系列纯乳脂样品的结果进行比较，可以确定乳脂的完整性[12]。 植物脂肪和动物脂肪如牛脂和猪油都可以检测到。 该方法适用于散装奶或由其制成的产品，无论常见喂养方式、品种或哺乳条件如何变化。 特别地，该方法适用于从声称含有成分未改变的纯乳脂肪的乳制品中提取的脂肪，例如黄油、奶油、牛奶和奶粉。 由于可能会出现假阳性结果，因此该方法不适用于与以下情况相关的乳脂： a) 从除牛奶以外的牛乳中获得； b) 从单头奶牛获得； c) 从饮食中含有特别高比例植物油如菜籽油、棉花油或棕榈油等的奶牛获得； d) 从严重饲喂不足（严重能量不足）的奶牛身上获得； e) 从初乳中获得； f) 进行去除胆固醇或分级等技术处理； g) 从脱脂奶、酪乳或乳清中获得； h) 从脂肪分解增加的奶酪中获得； i) 使用 Gerber、Weibull–Berntrop 或 Schmid–Bondzynski–Ratzlaff 方法提取，或使用洗涤剂分离（例如乳制品工业局方法）。 使用 i) 中指定的提取方法，大量的偏甘油酯或磷脂可以进入脂肪相。 注 1：在自然界中，丁酸（C4）仅存在于乳脂中，可以对植物和动物脂肪中低至中等量的乳脂进行定量评估。 由于 C4 的变化较大，其大致含量范围为 3.1% 脂肪质量分数至 3.8% 脂肪质量分数，因此很难提供高达 50% 的外来脂肪与纯乳脂肪比例的定性和定量信息。 至 20% 质量分数[11]。 注 2：实际上，定量结果不能从植物脂肪的甾醇含量中得出，因为它们取决于生产和加工条件。 此外，使用甾醇对外来脂肪进行定性测定是不明确的。  注 3 由于特殊的喂养习惯，例如与 c) 和 d) 相关的喂养习惯，有时会报告来自某些亚洲地区的牛奶出现假阳性结果[15]。 此外，纯草饮食，例如山地，特别是高原牧场饲养，有时会导致假阳性结果，这可以通过共轭亚油酸（C18:2 c9t11）含量≥1.3%脂肪酸质量来证实分数[16][17]。 尽管如此，符合本文件中规定的乳脂纯度标准的结果是可接受的，即使样品无疑是在本说明中报告的条件下生产的，包括 h) 中描述的条件。 注 4：如果怀疑阳性结果是由与 c) 或 d) 相关的情况引起的，则可以采用另一种分析方法，例如脂肪酸或甾醇分析来确认结果。 由于类似或增加的限制（例如注1和注2中所述），通过其他方法获得的阴性结果不适合对比确认乳脂纯度。 [外文原描述]： This document specifies a reference method for the determination of milk fat purity using gas chromatographic analysis of triglycerides. The method utilizes the differences in triglyceride fingerprint of milk fat from the individual triglyceride fingerprints of other fats and oils to determine samples which are outside the range normally observed for milk fat. This is achieved by using the defined triglyceride formulae based on the normalized weighted sum of individual triglyceride peaks which are sensitive to the integrity of the milk [6][7] . The integrity of the milk fat can be determined by comparing the result of these formulae with those previously observed for a range of pure milk fat samples [12] . Both vegetable fats and animal fats such as beef tallow and lard can be detected. The method is applicable to bulk milk, or products made thereof, irrespective of the variation in common feeding practices, breed or lactation conditions. In particular, the method is applicable to fat extracted from milk products purporting to contain pure milk fat with unchanged composition, such as butter, cream, milk and milk powder. Because a false-positive result can occur, the method does not apply to milk fat related to these circumstances: a) obtained from bovine milk other than cow's milk; b) obtained from single cows; c) obtained from cows whose diet contained a particularly high proportion of vegetable oils such as rapeseed, cotton or palm oil, etc.; d) obtained from cows suffering from serious underfeeding (strong energy deficit); e) obtained from colostrum; f) subjected to technological treatment such as removal of cholesterol or fractionation; g) obtained from skim milk, buttermilk or whey; h) obtained from cheeses showing increased lipolysis; i) extracted using the Gerber, Weibull?Berntrop or Schmid?Bondzynski?Ratzlaff methods, or that has been isolated using detergents (e.g. the Bureau of Dairy Industries method). With the extraction methods specified in i), substantial quantities of partial glycerides or phospholipids can pass into the fat phase. NOTE 1 In nature, butyric ( n -butanoic) acid (C4) occurs exclusively in milk fat and enables quantitative estimations of low to moderate amounts of milk fat in vegetable and animal fats to be made. Due to the large variation of C4, for which the approximate content ranges from 3,1 % fat mass fraction to 3,8 % fat mass fraction, it is difficult to provide qualitative and quantitative information for foreign fat to pure milk fat ratios of up to 20 % mass fraction [ 11 ] . NOTE 2 In practice, quantitative results cannot be derived from the sterol content of vegetable fats, because they depend on production and processing conditions. Furthermore, the qualitative determination of foreign fat using sterols is ambiguous. NOTE 3 Due to special feeding practices such as those related to c) and d), false-positive results have sometimes been reported for milk from certain Asian regions [ 15 ] . Moreover, grass-only diets such as mountain and, in particular, highland pasture feeding sometimes cause false-positive results, which can be substantiated by a content of conjugated linoleic acid (C18:2 c9t11) of ≥ 1,3 % fatty acid mass fraction [16][17] . Nevertheless, results conforming to the criteria of milk fat purity specified in this document are accepted, even if samples were undoubtedly produced under conditions reported in this note, including those described in h). NOTE 4 In cases where a positive result is suspected to be caused by circumstances related to c) or d), another analytical method, such as fatty acid or sterol analysis, can be applied to confirm the finding. Due to similar or increased limitations (e.g. as described in NOTE 1 and NOTE 2), a negative result obtained by another method is not appropriate to contrastingly confirm milk fat purity.