Effect of the Timing of Oxygenation on Very High Gravity Brewing Fermentations¹

1 Presented at the 55th Annual Meeting, Denver, CO, April 1989.

Abstract

Yeasts require small quantities of oxygen to synthesize unsaturated fatty acids and sterols, both of which are needed for continued anaerobic growth and cell division. As the fermentation rate is proportional to the number of metabolically active yeasts present, rapid wort attenuation is therefore related to the availability of oxygen. The effects of limited oxygen supply are compounded in high gravity or very high gravity (VHG) wort fermentations. Alterations in the quantity and the timing of the provision of an oxygen supply were evaluated in 28 g dissolved solids/100 ml VHG brewing fermentations. The objective was to ascertain the optimal supply time, postinoculation, that would give the most rapid attenuation. For the commercial Saccharomyces uvarum lager yeast employed, oxygen was most stimulatory when it was added between 10 and 14 hr postpitching. Yeast dry weight and viability were maximized, and the rates of attenuation, nitrogen assimilation, and ethanol formation were the most rapid. Conversely, earlier oxygen application (2, 4, or 6 hr postpitching) was not as effective as aerating the wort before pitching; oxygen supplied more than 14 hr after inoculation was similarly not as stimulatory. Excess amounts of dissolved oxygen supplied at 12 hr (25 mg/L dissolved oxygen) further stimulated the attenuation rate but not yeast growth. The practical extensions of the research are discussed with relevance to current industrial practice.

• Very high gravity
• Oxygen timing
• Yeast growth
• Fermentation rate