Nisin is a natural food preservative which can inhibit harmful bacteria growth and reproduction in yogurt, prevent the decay of yogurt, and extend the shelf life of product. Because of water activity of yogurt is Aw＝0．9－0．95, bacteria, mould, yeast are all easy to grow and reproduce, the first microorganism that lead to decay of yogurt is bacteria which is the most active, the most universal factor, playing the leading role. Nisin inhibits the reproduction of bacteria that will effectively extend expiration date of yogurt.
Yogurt contains a plentiful of lactobacillus, even though store in refrigerator to sell, acidity can also slowly rising affects edible quality of yogurt. If lack of the support of cold chain, yogurt continues to ferment, lactic acid produced dramatically, acidity will increase excessively, taste badly. As one enterprise indicated that because of the increasing air temperature, acidity of finished yogurt products increased sharply from 70°T to 120°T that lead to inedible. Nisin can inhibit the production of lactic acid which is metabolites of lactic acid bacteria, effectively solve yogurt after acid increase. In conclusion, nisin is also a kind of excellent quality improver.