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Best practice

It is important for the propagation process to deliver. After all, the output represents the beginning of a process where yeast will be cycled from fermentation vessel to storage tank and back anywhere between 3 and 15 or more occasions. The success is built on the twin pillars of hygiene and stepwise scale-up in volume (and biomass). It is important that the ‘steps' are appropriately sized. Cutting a step and stretching the size of the yeast inoculum can backfire if then a contaminant or variant can ‘punch above its weight' and compete with the primary yeast strain. The various steps in a typical large-scale propagation process are detailed in Table 3.3. Whether the yeast is sourced on an agar slope or as a liquid culture, the laboratory and plant steps in yeast propagation are geared to produce sufficient yeast of the correct strain and cleanliness to pitch into fermenter at the normal rate. Depending on scale, provision of liquid culture is beneficial in precluding some (or all) of the laboratory steps.

Irrespective of the size of the first-generation fermenter, propagation should be semi-aerobic - or better still, fully aerobic - so as to maximize yeast count, viability, and levels (stored and free) of sterols and UFA. The propagator should be sized appropriately so that on pitching into the first generation fermenter the yeast pitching rate meets the required specification.

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