Acid whey is generally a liquid whey. There are two kinds of liquid whey. The production of cottage cheese yields acid whey. Its pH is about 4.0. Sweet whey is a by-product of the production of swiss, cheddar or mozzarella cheese. Its pH is approximately 6.0. The two kinds of whey differ somewhat in their usefulness. Sweet whey is less stable than acid whey during storage. It will lose up to 40% of it's nutritional value if stored longer than 24 hours. Acid whey has a lower nutritional quality than sweet whey. Liquid acid whey can be successfully fed to pigs, but additional protein will be needed if good performance is expected. Acid liquid whey differs from sweet liquid whey in that it cannot replace supplemental protein in the diet.
Whey powder is a dried whey. Dried whey is a by-product of cheese manufacturing and a kilogram contains about the same amount of nutrients as 14 kg of liquid whey. Dried whey is routinely added to diets of pigs weaned at an early age (21 days or less) as a source of milk protein and sugar (lactose). Numerous studies have demonstrated improved performance of pigs fed whey in starter diets.
While levels of 10% to 30% whey are commonly used in starter pig diets, some research has indicated that dietary levels of 30% to 45% dried whey can be added to the diet without depressing performance. A routine level of whey in pig starter diets in the U.S is 20%.