Determination of Polyunsaturated Fatty Acids in Milk Fat

A concentrate of the polyunsaturated fatty acids of milk fat was prepared by low4emperature crystallization, and this concentrate was fraetionated by chromatography on silica gel. Comparative analyses of the fractions by alkali isomerization, lipoxidase, and gas chromatography were carried out. Gas chromatography gave high determinations for nonconjugated dienoate and trienoate compared with the other two methods. This was attributed to the presence of unconjugatable positional isomers of linoleic and linolenic acids. The alkali isomerization and lipoxidase methods agreed closely for samples in which there was not much tetraenoate and pentaenoate. Most natural milk fat samples do not contain enough tetraenoate and pentaenoate to introduce significant error into the alkali isomerization method. The best estimate for cis, cis-conjugatable-dienoate was considered to be a combination of the lipoxidase and alkali isomerization methods, which minimizes errors due to conjugatable trans isomers of octadecadienoate and changes in background absorption. The presence of linoleate in milk fat was demonstrated by derivative formation and permanganate-periodate cleavage. The presence of positional isomers of linoleate was demonstrated. Although the cis, cis-conjugatable-dienoate determined by alkali isomerization and lipoxidase is mostly linoleate, the presence of cis,cisconjugatable-octadecadienoate isomers cannot be excluded.