For these reasons, yeast stored without feeding on wort may have little remaining energy stores, resulting in slow sterol synthesis and possible long lag times at the start of fermentation. Long lag times give wort-spoilage bacteria a chance to grow. It is therefore important to use a fresh, healthy yeast culture. 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.