Since the 1930s, industrial cottage cheese has been manufactured using pasteurized skim milk, or in more modern processes using concentrated nonfat milk or reconstituted nonfat dry milk. A bacterial culture that produces lactic acid ("Lactococcus lactis" ssp. "lactis" or "L. lactis" ssp. "cremoris" strains such as are usually used) or a food-grade acid such as vinegar is added to the milk, which allows the milk to curdle and parts to solidify, and it is heated until the liquid reaches , after which it is cooled to . The solids, known as curd, form a gelatinous skin over the liquid (known as whey) in the vat, which is cut into cubes with wires, allowing more whey to drain from the curds. The curds are then reheated to for one or two hours. In Iowa in the early 1930s, hot water was poured into the vat, which further firms the curds. Once the curds have been drained and are mostly dry the mass is pressed to further dry the curds. The curds are then rinsed in water. Finally, salt and a "dressing" of cream is added, and the final product is packaged and shipped for consumption. Some modern manufactures add starches and gums to stabilize the product. Some smaller modern luxury creameries omit the first heating step but allow the milk to curdle much longer with bacteria to produce the curds, or use crème fraîche as dressing.