Some have argued that the risk of mutations of the yeast cells as they divide is a drawback to feeding the stored yeast. This is a concern in very large breweries because of very large yeast cell masses and constant repitching, which may eventually yield a mutation that affects brewing characteristics. Several factors make it rare for home brewers to observe mutations in a yeast strain. DNA mutation rates are very low, and a high percentage of DNA mutations in yeast have no effect on the yeast cell. Two proper ways for brewers to obtain an adequate quantity of healthy yeast include obtaining yeast from a clean brewery pitched within three days after cropping.
Breweries use cropped yeast to pitch their next brew.
Lager yeast should not be used if successively cropped more than six times, and ale yeast should be used after multiple croppings only if the brewery is impeccably clean, which is not always the case. To minimize yeast autolysis, yeast should be stored in the refrigerator and used within three days after cropping. Storing yeast for slightly longer periods of time is possible if the liquid from the top of the yeast slurry is decanted and replaced with fresh, clean wort. Cropped yeast is superior to cultured yeast because glycogen and sterol levels are highest after cropping. Unfortunately for home brewers, brewery yeast may not be accessible or convenient for them to obtain.