The formation of flavour compounds during fermentation.

Enari, T-M., Makinen, V. and Haikara, A.

Abstract
Small scale tests were done on batch fermentations unaerated, aerated and stirred and also on a continuous process. The effects of temperature and yeast strain were studied on wort of low and high N content. Fusel alcohols, esters, diacetyl, acetaldehyde and lactic acid were determined as likely to affect beer flavour. When the temperature was raised from 10 degrees to 15 degrees C in stirred and continuous fermentations the fusel alcohol was increased; with aeration during fermentation still more fusel oil was produced accompanied by greater yeast growth. With flocculent yeast there was little difference in fusel oil between low and high N wort in unaerated stirred fermentation but aeration gave an increase of fusel oil with the high N wort. The fusel oil consisted mostly of amyl alcohols but different fermentations showed variation in the proportions of these. Continuous fermentation gave more optically active amyl alcohol while aeration of a stirred batch gave increased isobutanol. Ester formation was increased in continuous and in aerated fermentations. With powdery yeast there were only small differences in lactic acid but flocculent yeast produced less acid. With flocculent yeast the type of fermentation had little effect on diacetyl but though the powdery yeast gave a high maximum in conventional fermentation the final concentration was not excessive. Changes in brewing technique and yeast strain influence the production of higher alcohols so that determination of the amount and composition of fusel oil indicates any change in brewing methods likely to affect flavour.
Keywords: biosynthesis brewing fermentation flavour volatile compound