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Why must breweries carefully control hygiene when reusing yeast?

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Six small glasses of craft beer in varying amber and golden hues are arranged on a metal tray atop a wooden bar counter in Brussels, Belgium. — Beer Glasses of beer in a bar in Brussels.

Brewing is unique among beverage fermentation industries because yeast from one fermentation is reused to "pitch" the next batch. Because this process involves recycling the same biological agent, breweries must maintain strict hygienic conditions and rigorous quality control.1

These measures are essential to ensure that the yeast culture remains pure, specifically by maintaining a high proportion of live cells while ensuring the culture is free from contamination by bacteria or other unwanted yeast strains. Because brewers place great value on specific, selected yeast strains to maintain the unique character and flavor profile of their beer, protecting these cultures from contamination is vital to consistent production.12