A practical brewer's view of diacetyl.
For the suppression of diacetyl production in beer the yeast should contain less than 0.01% of bacteria. This can only be detected by acitidione plating and low bacterial levels can be secured by regular acid persulphate washing. The yeast strain should be relatively free from respiratory deficient types as these are less able to reduce diacetyl. Diacetyl is formed during the early stages of fermentation and more active yeast action will minimize formation and encourage the later reduction by yeast alcohol dehydrogenase so it is advisable to maintain full yeast rate and to increase pitching temperature which can be safely done when the bacterial content is low. Slow separation of yeast in deep vessels gives time for the reduction of diacetyl and this may be encouraged by arresting fermentation about a degree S.G. above final attenuation and holding the temperature before cooling. The addition of valine or of foreign reductases to suppress diacetyl is not commercially practicable but if necessary yeast activity may be stimulated by the addition of nutrilites in a "yeast food". On transfer to storage tanks, plant sterility must be observed to ensure freedom from lactic bacteria and absorption of air prevented as this will cause residual yeast to revert to the primary fermentation metabolism when diacetyl is produced.
Keywords: brewing diacetyl survey