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Pitching

The inoculation or pitching of yeast into fermenter is one of three primary control factors in brewery fermentations. The other two, oxygen and temperature, are considered below (see section on ‘fermentation'). As noted above, brewery fermentations are peculiar in typically transferring yeast between successive fermentations. The efficiency of fermentation (i.e. ethanol production vs. new biomass) is indirectly managed by limiting new yeast growth through a combination of a relatively high pitching rate and availability of the nutrient oxygen. Achieving the desired balance of the two parameters is central to delivering fermentation that is of the required rate and extent and, importantly, is consistent.

Yeast pitching rate is broadly based on the maxim of 1 x 106 viable cells/ml for every 1° Plato ofwort (O'Connor-Cox, 1998b). So at a wort gravity of 10° Plato the pitching rate is 10 x 106 viable cells/ml, 15 x 106 viable cells/ml at 15° Plato, and so on. Although by no means carved in stone, this approach - together with management of wort oxygen - provides a good base for optimal fermentation. Expectations include (i) new yeast growth (crop is 3- to 4-fold the amount of yeast pitched), (ii) required fermentation rate and extent (including diacetyl rest) (iii) desired flavour and aroma profile, and (iv) management of fob and cooling demand.

Increasing pitching rate has often been explored (e.g. Edelen et al., 1996; Erten et al., 2007; Ver- belen et al., 2009). As ever, the experimental approach, scale, and pitching ‘multiple' vary but with consistent oxygen concentration, the process outcomes are broadly consistent. As would be anticipated, increasing the pitching rate results in faster fermentations with correspondingly higher and earlier peak yeast cell counts. The quantity of new yeast growth was unchanged irrespective of pitching rate as this is determined by the (fixed) concentration of oxygen. The combined (pitched + new) biomass obviously increased, resulting in greater losses of bitterness ‘bound' to the yeast cell wall. Analysis of beer volatiles from such trials provides no clear, repeatable insights as to the impact of enhanced pitching rate. Given the lack of an experimental baseline, this is no great surprise.

 
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