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 pentadecanoicacid (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 com position of the beer reflects changes in the content of these acids in yeast brought about by altera tion in the supply of oxygen or by the addition of C3 acid to wort.

The purification of Methanobacterium thermoautotrophicum from a culture contaminated with a heterotrophic organism is described. A defined inorganic medium under H2/CO2 (80:20 v/v) has been developed to support growth of M. thermoautotrophicum up to a concentration of at least 1.7 g dry weight/l. In a conventional medium iron and nitrogen sources were found to be growth-limiting factors. Throughout most of the culture period the rate of transfer of hydrogen or carbon dioxide from gas to liquid was the factor which controlled the growth rate. The growth yields of bacteria were in the range of 0.6-1.6 g dry weight/mole CH4.

The addition to wort of lipids derived from malt spent grains had a pronounced effect on yeast metabolism. The lipids allowed the fermentation of de-oxygenated wort and also stimulated yeast growth and the corresponding rate and extent of fermentation of air-saturated wort by yeast strains having a high oxygen requirement. The lipids increased the fusel alcohols content of beer and de creased the content of esters and medium chain-length fatty acids.The yeast incorporated sitosterol and unsaturated fatty acids from the spent grain lipids and the unsaturated fatty acids changed the pattern of fatty acids and sterols synthesized by the yeast. The fatty acids were present in the spent grain lipids mainly as triglycerides, free fatty acids and phospholipids. Using pure lipid compounds it was shown that the triglycerides were inactive and that the spent grain lipids exerted their effect on fermentation through the synergistic action of free unsaturated fatty acids, sitosterol and phospholipid. Phospholipid could be replaced by the detergent, Triton X-100. The effect of the lipids on the synthesis of esters, fusel alcohols and medium chain fatty acids could be explained solely by their content of unsaturated fatty acids.

D‐values of a heterofermentative beer spoilage lactobacillus were measured at 55°C, 60°C and 65°C in beers containing <0·05% to 4·4% v/v ethanol. Z‐values for the different beers varied between 9·17 and 12·13°C. At each temperature an increase in ethanol reduced the measured D‐value. The maximum, 5·01 min was observed in alcohol‐free beer (<0·05%) at 55°C and the minimum, 0·31 min, at 60°C and 65°C in beer containing 4·4% ethanol. D‐values could be increased by prior growth in the presence of ethanol. They could be reduced by adding ethanol to alcohol‐free beer or by increasing its hop extract content. The implications for the pasteurization of low‐alcohol beers are discussed.

D‐values of a heterofermentative beer spoilage lactobacillus were measured at 55°C, 60°C and 65°C in beers containing <0·05% to 4·4% v/v ethanol. Z‐values for the different beers varied between 9·17 and 12·13°C. At each temperature an increase in ethanol reduced the measured D‐value. The maximum, 5·01 min was observed in alcohol‐free beer (<0·05%) at 55°C and the minimum, 0·31 min, at 60°C and 65°C in beer containing 4·4% ethanol. D‐values could be increased by prior growth in the presence of ethanol. They could be reduced by adding ethanol to alcohol‐free beer or by increasing its hop extract content. The implications for the pasteurization of low‐alcohol beers are discussed.