Experimental Conditions Affecting Yeast Growth

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,

Section: Methodsmentioning

confidence: 97%

The Influence of 2-Acetohydroxy Acids on the Determination of Vicinal Diketones in Beer and During Fermentation

Haukeli¹,

Lie²

1971

“…Earlier we have reported that stirring at 60°C may accelerate the conversion rate of a-acetolactate. 4 We have, however, not been able to reproduce these results, indicating that stirring is of less im portance for the spontaneous conversion of a-acetolactate. Our present results demonstrate clearly that temperature and pH have the greatest practical effect on the conversion rate.…”

Section: Resultsmentioning

confidence: 83%

“…The fermentations were carried out as described earlier. 5 Reagents. a-Acetolactate was prepared fresh each day by hydrolysis of a-acetolactic acid ethyl ester acetate obtained from Oxford Organic Chemicals by the method of Ronkainen et a/.12 The a-acetolactate solution was kept at 4°C and pH 7 until used.…”

Section: Methodsmentioning

confidence: 99%

CONVERSION OF Α-Acetolactate AND REMOVAL OF DIACETYL a KINETIC STUDY

Haukeli¹,

Lie²

1978

Self Cite

Brewers' yeast does not form diacetyl during fermentation, but a-acetolactate which spontan eously is converted to diacetyl. This non-enzymic reaction is mainly dependent upon pH, temp erature and substrate concentration. The present kinetic studies demonstrate that temperature and pH have a dominating effect on the spontaneous conversion of a-acetolactate during primary fermentation. From the reported kinetic studies it is possible to predict the conversion rate of a-acetolactate present in the fermenting liquid, and to calculate the time necessary to convert a given amount of a-acetolactate to diacetyl.As long as yeast is present in the fermenting liquid, diacetyl can not be detected analytically due to the high diacetyl-reducing activity of the yeast cells. The specific activity is strain-depen dent and decreases gradually during fermentation. The diacetyl-reducing activity of the yeast will, however, normally be high enough to compete with the diacetyl formed from a-acetolactate.

“…IB arc in good accordance with our earlier observations. [10][11][12][13][14][15][16][17][18] The laboratory fermentations demonstrated in Fig. IB were all prepared with small inocula and will therefore deviate strongly in fermentation velocity owing to variable growth yield. Of the three examples presented here, one was a very fast fermentation, one medium and one an extremely slow fermentation.…”

Section: Resultsmentioning

confidence: 99%

Effect of Lipids and Oxygen on Yeast Growth and the Biosynthesis of Acetoin During Fermentation

Haukeli¹,

Lie²

1976

Self Cite

Brewery yeast needs traces of oxygen for the biosynthesis of unsaturated fatty acids and ergosterol. Owing to the increase in cell mass during primary fermentation the concentrations of these essential lipids decrease and thereby affect the physiological condition of the yeast. When the sterol concentration of whole cells has decreased to 0-2 to 0-3 mg per 100 tng dry yeast, the yeast changes its metabolism. This metabolic change is revealed by a decrease in acetoin con centration. The absorption of wort nutrients and consequently the efficiency of growth is at this point also greatly reduced. The ratio between yeast growth and mole ethanol formed (i.e. the molar growth yield) decreases greatly during wort fermentation. A close correlation between molar growth yield and the change in acetoin metabolism can be observed. This metabolic change occurs when the ratio between yeast growth and ethanol formed is in the range of 8-3 to 9-1, averaging 8-7 g/mole.