Whey is the liquid remaining after milk has been curdled and strained. SWEET WHEY Use rennet and sometimes rennet with a bacterial culture to make sweet whey. The culture has little effect for the first hour so the whey stays sweet. Don't store sweet whey containing culture bacteria because it will soon become sour. Sweet whey is high in fat and low in lactic acid. Sweet whey can be used to make ricotta with or without extra milk added. ACID WHEY (or sour whey) Use a bacterial culture to separate the milk. This takes up to 24 hours. Or use citric acid, lemon juice, vinegar or another acid to curdle the milk. Acid whey is low in fat and high in lactic acid. Acid whey can not be used to make ricotta because the yield is tiny. Surplus whey of either type can be used in bread making, soups, sauces, gravies, brining meats and with fermented vegetables.