Dependence between sterols synthesis and the method of beer yeast oxygenation

Oxygen is necessary for yeast to synthesize membrane components (unsaturated fatty acids and sterols), but its high content in the medium during fermentation increases the concentration of cell oxidative metabolism products. This slows down beer maturation process and impairs its quality. The alternative way is to aerate the inoculum to accumulate sterols in cells and reduce the cells’ requirement for oxygen. The author studied the effect of inoculum preparation conditions and oxygen content in the fermentation medium on the formation of sterols by the brewer’s yeast Saccharomyces cerevisiae. Pre-fermentation treatment involved a short aeration of the inoculum (for 30 min) in water, beer wort or young beer with further exposure in an anaerobic environment (for 1–3 hours). The content of sterols was evaluated by means of spectrophotometry, chromatography-mass spectrometry, thin-layer chromatography (TLC), and gas-liquid chromatography (GLC). The article reveals that when yeasts are aerated in young beer, cells synthesize by 16% and 73% more sterols than in water and wort, respectively. This is due to the presence of carbon sources in beer which are effective for sterols synthesis. After application of any method for providing yeast with oxygen (at culture preparation or wort fermentation stage) six components were detected in the unsaponifiable fraction using TLC: ergosterol, ergosta-5,7-diene-3β-ol, ergosta-7,22-diene-3β-ol, fecosterol, zymosterol, lanosterol. GLC revealed five compounds: squalene (39–54%), lanosterol, 24 (28) -dihydroergosterol, ergosterol (23–35%) and an unidentified component which according to mass spectrometry was 24-methylene-24,25-dihydrolanosterol. An increase in the oxygen level in the fermentation medium from 4.0 to 16.0 mg/l contributes to the decrease in sterols accumulation per unit of oxygen consumed by the yeast. Preliminary aerationallowed yeast to multiply regularly at oxygen concentration in the fermentable wort of 4.0 mg/l and ferment the extract of the medium at the level of the sample where oxygen content was 8.0 mg/l. This shows the advantage of using yeast pre-fermentation aeration and conducting beer wort fermentation process without additional saturation with oxygen.