Mass balance

The gross changes which occur during the course of a typical high-gravity lager fermentation are illustrated in Fig. 3.1. In this case the initial temperature was 11°C. This was allowed to increase to 12°C and held at this value throughout primary fermentation. There was an initial lag phase, which lasted for 12-24 hours. During this period there was little or no observable change in specific gravity, yeast count and ethanol concentration. However, the oxygen concentration rapidly decreased, falling to undetectable levels within the first 24 hours. The concentration of free amino nitrogen began to fall as soon as the fermentation started and this was accompanied by a rapid decline in pH.

The fermentation rate, as judged by decline in wort specific gravity, gradually accelerated and reached a maximum after 24-36 hours. The decline in specific gravity was inversely related to increase in ethanol and yeast biomass. The minimum value of specific gravity was achieved after approximately 100 hours and this coincided with maximum observed values for ethanol and yeast biomass. Patterns of decline in free amino nitrogen and pH mirrored each other and minimum values of each were achieved after about 80 hours, some 20 hours before full wort attenuation was achieved. During the phase of active fermentation, total yeast biomass and the suspended yeast count were coincident. The maximum suspended yeast count was reached after about 100 hours and after this time this parameter declined with the deceleration in fermentation rate. This is a reflection of the lack of mechanical agitation in the vast majority of brewery fermenters.

During the course of the fermentation approximately 50° of gravity were fermented. This was accompanied by an increase in yeast dried biomass from roughly

Fig. 3.1 Some features of the progress of a typical high-gravity fermentation (Boulton, unpublished data).

lgl 1 to just over 4g 1 1. The terminal ethanol concentration was approximately 50gl

A crude mass balance for a fermentation of the type described above may be written as follows:

Sugars + Free amino nitrogen + Yeast + Oxygen -> (150 gl1) (150 mgl1) (lgl1) (25mgl1)

Ethanol + C02 + Yeast

Conversion of sugar to ethanol is approximately 88% of the theoretical, the shortfall being that proportion which is utilised by the yeast to generate additional biomass and to a lesser extent the formation of other metabolic by-products of yeast growth. Ethanol and carbon dioxide are, of course, produced in equimolar amounts; however, the yield of the latter is slightly reduced since a small proportion is utilised by the yeast cells for anabolic carboxylation reactions (Oura et al., 1980). The quantity of oxygen supplied at the beginning of fermentation is a key determinant of the regulation of the proportion of wort sugars used for new yeast biomass and that which is dissimilated to ethanol (Section 3.5).

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