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Yeast supply

Although very much the norm in brewing, the recycling of yeast from one fermentation to another is unusual. The approach is used in biofuel production (Walker, 2011), but other fermentation-based processes typically adopt a ‘single trip' approach with no ambition or opportunity to recover yeast and use it again. The supply of yeast into the fermentation process in breweries can be achieved in a variety of ways that are differentiated by scale and volume. It is rare but not unheard of, to use a yeast strain without replacement for years or decades but is typically capped at so many ‘generations' (i.e. fermentation batches). There is no hard and fast rule as to the number of generations, though there is a trend down from 15-20 to 10 to five or fewer generations in recent years. The vast majority of brewers will - as policy - replace their brewing strains periodically with the supply and propagation of pure cultures. The interpretation of this approach can take different shapes where the complexity of supply and level of quality assurance is at its greatest for large brewing companies and groups. This involves the cryopreservation and storage of propriety strains and their quality assured recovery and staged laboratory and plant propagation into fermenter. Whilst this undeniably the best possible practice to achieve strain purity and yeast quality, other approaches are none the less focused on quality and hygiene. Depending on scale and size, some breweries contract with a supplier to store, assure, validate and provide liquid slurries of their strain. Beneath this and typically with the smaller volumes of new and growing craft breweries, a new market for the supply of yeast has grown. Here ‘type' yeast strains are provided as liquid cultures or as active dried yeast for pitching directly into fermenter. At around 15 hl or below it is economically viable to adopt a ‘pitch and ditch' approach, whereby ‘ready to pitch' cultures are used without recycling. Much above these volumes, cropping and re-pitching becomes an increasingly likely process outcome.

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