Role of Oxygen in High-Gravity Fermentations in the Absence of Unsaturated Lipid Biosynthesis. E. S. C. O'Connor-Cox, E. J. Lodolo, and B. C. Axcell, South African Breweries, Research and Development Department, P.O. Box 782178, Sandton 2146, RSA. J. Am. Soc. Brew. Chem. 51:0097, 1993.

The effects of dissolved oxygen were evaluated in high-gravity lager fermentations in the measured absence of unsaturated fatty acid and ergosterol biosynthesis. Under these conditions, dissolved oxygen was found to enhance yeast growth and the rate of assimilable nitrogen uptake. The yeast formed more acetaldehyde, SO₂, and dimethylsulfide when unsaturated lipid biosynthesis was blocked. These results indicate that dissolved oxygen has a multifaceted role in yeast metabolism. The depletion of intramitochondrial energy supplies through the inclusion of bongkrekic acid drastically decreased yeast growth and fermentative performance and interfered in lipid metabolism. Such fermentations were also characterized by inadequate assimilable nitrogen uptake and excessive production of acetaldehyde, SO₂, dimethylsulfide, and vicinal diketones. The inhibition of the plasma membrane ATPase did not affect the fermentation or yeast metabolism to the same degree as did bongkrekic acid. These effects were not due to energy provided by oxidative phosphorylation but rather to an intramitochondrial energy supply provided by the mitochondrial adenine nucleotide translocase system. The results indicate that minimal mitochondrial development and energy supply are critical to fermentative performance as well as to finished beer quality. The practical implications to the brewer are discussed.

Keywords: Ergosterol, Fermentation, Growth, Inhibitors, Mitochondria, Oxygen, Unsaturated fatty acids