Comparative fermentation trials are described in which wort from a single brew was divided into four batches, which were pitched at about half the normal rate (12.8 million cells/ml), a rate within the normal range (30.3 million cells/ml), double rate (53.8 million cells/ml) and triple rate (74.9 million cells/ml) respectively. Samples were collected twice daily from each fermenter for yeast cell counting, viability determination and chemical analysis. It was found that higher pitching rates led to more rapid attenuation, but that although the maximum cell count was highest in the triple pitched batch, the net yeast growth (i.e. the difference between the maximum number of cells per unit of volume and the number added at pitching) was about the same in all four batches, implying that the yeast crop from a fermentation using a higher pitching rate is likely to contain more old cells. This may explain why the percentage of viable cells in the yeast crop from the double and triple pitched fermentations was lower than in the other two. It was also found that higher pitching rates led to lower bitterness unit levels in the resulting beers (possibly due to the uptake of bitter substances by yeast cell walls and/or to increased foaming during fermentation) and lower free amino nitrogen (FAN) utilization rates, leaving higher residual FAN levels in the beers, as well as higher levels of ethanol and fusel alcohols and lower levels of esters and other volatile organic compounds, with consequent differences in beer flavour. Lower pitching rates were found to result in higher intensities of the overall aroma and of hoppy and fruity aroma notes in the finished beer. It is therefore evident that a consistent pitching rate is essential to maintain the consistency of beer quality.
Keywords : aroma beer brewers' yeast composition fermentation flavour growth performance pitching rate quality viability