The physiological effects on brewing yeast, growing in semi‐defined wort medium, of a sudden transition from aerobiosis to anaerobiosis were studied. Two yeast strains were examined, used for ale and lager fermentations respectively. The reverse transition (from anaerobiosis to aerobiosis) was also examined. Transitions were applied by changing the sparging gas during growth or in stationary phase, and the effects on the specific activities of certain key enzymes and on the viability of the cultures were examined. Neither type of transition led to significant changes in growth rate, the rate of ethanol production or the specific activities of alcohol dehydrogenase and pyruvate decarboxylase. The most significant change was in the specific activity of CuZn‐superoxide dismutase, which showed a rapid increase in activity on transition from anaerobiosis to aerobiosis, and a decrease in activity on the reverse transition. Catalase activity in the ale yeast generally followed that of CuZn‐superoxide dismutase, whereas in the lager yeast it remained unchanged by the transitions. The transition from anaerobiosis to aerobiosis caused increases in citrate synthase and Mn‐superoxide dismutase, though only after a significant lag period. Aerobic to anaerobic transitions caused a decrease in Mn‐superoxide dismutase activity, while citrate synthase remained unchanged. Anaerobically grown cells showed a rapid loss in viability on exposure to oxygen (5–7% in the first hour), while aerobically grown cells were unaffected. When anaerobically grown cells were exposed to 0·25 mM‐potassium superoxide, there was an 8% loss of viability within 10 min, whereas aerobic cells were not affected. It is concluded that the toxic effect of oxygen is due to superoxide (or a species derived from it) and that the CuZn‐superoxide dismutase (but not the Mn‐isoenzyme) plays a role in protecting the cells. The de novo synthesis of the CuZn‐enzyme is not always rapid enough to confer full protection.

Off the enzymes that may be involved in the scavenging off oxygen radicals in barley and malt, superoxide dismutase, catalase and peroxidase all increase their specific activities during malting, whereas polyphenol oxidase decreases to zero. Of these, however, only the isoenzymes of peroxidase survive (in part) in the mash, and are responsible for the oxidation of polyphenolic materials. The concentration of hydrogen peroxide normally found in wort limits their action. Addition of hydrogen peroxide to the mash or its generation via a glucose oxidase system greatly increases haze formation, decreases the polyphenol content and causes the development of a red coloration. When the same amount of the different malt peroxidase isoenzymes was added to mashes, the intensity of the red colour varied according to the isoenzyme used. The worts produced by enhanced peroxidation afford more colloidally stable beers. The effects however are limited by the dissolved oxygen concentration in the wort.

A B S T R A C TA method of purifying superoxide dismutase (SOD; EC 1.15.1.1), ,free from lipoxygenase (EC 1.13.11.12) and catalase (EC 1.11.1.6)

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