The growth‐regulating effects of fermentation temperature, medium composition, addition of ergosterol and introduction of trace amounts of oxygen have been studied for six yeasts. This investigation demonstrates some important experimental details which frequently impair the reproducibility of small‐scale fermentations. The necessity of a balanced composition of growth media and control of the content of ergosterol and of trace amounts of oxygen is stressed. Variations in the yield obtained in defined media, with added minimum amounts of essential growth factors only, were in the range of 30–100% of optimum growth. Commercial yeast extracts tested seemed not to contain any compounds with a stimulating effect similar to that of ergosterol.

The main yeast fermentation process can be characterized by two separate phases, the acetoin-producing phase (phase I) and the acetoin-reducing phase (phase II). The time at which yeast changes from production to removal is here designated as the 'point of metabolic change'. The rise and fall in acetoin concen tration was affected by composition of the wort, inoculum size, yeast strain, presence of traces of oxygen and addition of ergosterol, whereas temperature seemed to be of somewhat less importance. A high and positive correlation between the maximum level of acetoin and the concentration of ethanol measured at the 'point of metabolic change' has been demonstrated. The appearance of the 'point of metabolic change' is discussed in light of the level of acetoin in finished beer. The absolute threshold for acetoin was determined as 17 ppm.

During sampling and determination of diacetyl, 2-acetohydroxy acids are easily converted to vicinal diketones. A simple procedure for gas chromatographic deter* mination of diacetyl, 2-acetolactate, acetoin and the homologous compounds is given.By careful sampling, less than 0-01 ppm of diacetyt was detected during the main fermentation in one brewery, whereas another strain of brewer's yeast yielded a maximum of 1-7 ppm of diacetyl. When samples of fermenting liquids are exposed to air at 60°C, complete conversion of 2-acetohydroxy acids takes placein less than one hour. The possibility that part of 2-acetolactate may be converted to acetoin, how ever, cannot be excluded. In finished beer 2-acetolactate levels of 0-2-0-5 ppm were observed. During the main fermentation the values ranged from 0-5-2-5 ppm. IntroductionResults have recently appeared, however,

The distribution of metals in wort seems to be as important for the brewing process as the metal levels alone. Malt samples from the Norwegian EBC trials have been analysed for Ca, Mg,and Zn.The same elements have been analysed in worts made from these malts. The degree of extraction of metals from the grain is calculated, and this information is compared with available analytical data for the malt or wort samples.A constant ratio between metal extracted and the total metal content in malt was established only for Wig, and not for the other two metals. The comparison with other variables did, however, result in significant correlations. A high correlation was found between Zn and amino acids in wort, and between Ca and polyphenols. This indicates that the amino acids and the polyphenols may take an active part in the regulation of these two metals.

Results are given for the production of diacetyl, 2,3-pentandione-l-acetolactate, 2-acetohydroxy butyrate, acetoin and 3-hydroxy-2-pentanone for 13 yeasts belonging to the genus Saccharomyces. The data presented indicate that in general the yeasts do not produce significant amounts of vicinal diketones during unstirred laboratory fermentations. The observed maximum levels of diacetyl ranged from 0*02-0*32 ppm averaging 0-09 ppm (0*07 ppm if S. carlsbergensis (strain H) is omitted). During active fermentation all the yeasts tested produced 2-acetohydroxy acids. The 2-aceto hydroxy acids and the 3-hydroxy-2-ketones are determined gas chromatographically by stepwise conversion of the four compounds to diacetyl and 2,3-pentandione. The maximum amounts of 2-acetolactate produced by the yeasts tested averaged 0*7 ppm, ranging from 0*08-1*65 ppm.