All-malt Wort Research Articles

LIPID METABOLISM AND THE REGULATION OF VOLATILE ESTER SYNTHESIS INSACCHAROMYCES CEREVISIAE

The specific rates at which ethyl acetate and iso-amyl acetate are produced by yeast increase markedly at that point in fermentation where syntheses of lipids (i.e. saturated fatty acids and squalene) stop. An increase in the acetyl-CoA: CoASH ratio, or a reduced availability of substrates (fatty acyl-CoAs) for acyl transferases, are possible reasons for such a stimulation of ester synthesis. Increased rates of ester production are not sustained but contribute significantly (ca. 30% for ethyl acetate) to the total concentration of acetate esters in beer fermented from 1·040 all-malt wort. Addition of linoleic acid (50 mg litre−1) suppresses the induction of ester synthesis and reduces overall formation of both ethyl and iso-amyl acetates by ca. 80%. Possibly, linoleic acid exerts these effects either by directly inhibiting the activity of an ester-synthesising enzyme, or by allowing alternative use of acetyl-CoA for the synthesis of saturated fatty acids.

EXPERIMENTS ON TOP FERMENTATION: PART VI: THE NITROGEN REQUIREMENTS OF YEAST IN FERMENTATION

Although it is difficult to obtain precise evidence, it appears probable that for a number of years, and particularly in the last few, the average nitrogen content of British barleys has fallen. At the beginning of this century there were fears that the nitrogen content of barleys was too high and that, as one consequence, amongst others, bacterial instability resulted in the beer. Recently the suggestion has often been made that the decline in barley nitrogen content may have proceeded too far, so that there may even be nitrogen deficiency in worts. It was considered desirable to attempt a contribution towards the solution of this question. The method of experiment has been to ferment malt worts and, in comparison with them, portions of the same wort with which had been mixed increasing proportions of sugar solution of the same gravity, thus progressively reducing the proportions of yeast nutrients other than sugars. Two types of fermentation have been used. In one the wort was continuously shaken with yeast as in an attenuation limit test. In the other type of test, small scale fermentations were employed, so designed as to simulate brewery fermentations as closely as possible. Attenuation was taken as the criterion of satisfactory nutrition, incomplete attenuation indicating deficiency of nutrients other than sugar. In both types of fermentation, additions of assimilable nitrogen compounds and of bios were made to test if these were responsible for the deficiencies which were found to occur. In the attenuation limit tests, using pure culture yeast at very low pitching rates, sudden and marked deficiency occurred at some point as the sugar proportion was increased. Nitrogen and bios shortage were found to occur together at this point but nitrogen shortage had the more pronounced effect. There appeared to be sufficient nitrogen in all-malt worts to allow progressive increase of the sugar proportion up to 50 per cent. In the small scale brewery type fermentation tests, dilution of the wort by sugar gave effects which set in more gradually but which were evident with 20 per cent. of the wort replaced by sugar solution and were inferred to commence at the nitrogen level of a normal all-malt wort; so that it appears that the utilisation of nitrogen here is less efficient. This suggests that in practice the nitrogen requirements for attenuation will lie between the amounts found in all-malt and half-malt worts, and thus near the amounts normally found in brewery worts; which may, in some cases, be deficient in nitrogen.