The fresh yeast sold in grocery stores, Saccharmyces cerevisiae, is safe for use as long as it remains viable. It must be kept refrigerated until it is used. Yeast manufacturer Red Star recommends use within 10 days of purchase.
Fresh or cake yeast consists of live yeast cells mixed with corn starch and pressed into small cakes. A 2-ounce cake leavens 12 cups of flour, making it the equivalent of three packets of active dry yeast. It is not suitable for bread machine recipes calling for rise times of less than an hour. These recipes require instant yeast, which works in about half the time required for fresh or active dry yeast.