The Origin of the Medium Chain Length Fatty Acids Present in Beer

The medium chain length fatty acids that are excreted during fermentation are produced by synthesis and not by degradation. The fermentation of a wort supplemented with propionic acid (C3) or valeric acid (C5) leads to the excretion of nonanoic acid (C9) in addition to the usual even chain acids. C9 acid was not detected in the beer when the inoculated yeasts contained a high proportion of pentadecanoic acid (C16) and heptadecanoic acid (C17) or when the C17 acid was added to the wort, demonstrating that a degradative route is unimportant. The content of the medium chain length fatty acids in beer varies directly with their content in yeast; thus the fatty acid composition of the beer reflects changes in the content of these acids in yeast brought about by alteration in the supply of oxygen or by the addition of C3 acid to wort.