When someone makes cheese, the curdled milk is divided into the cheese and the whey. The latter can be drunk fresh but (unless it is of sour milk cheese) it spoils within a few hours. There is a patented procedure to conserve it, that's why this product which is produced in surplus, is sold so expensive.
Fresh whey is a healthy drink, and on bathng it is good for the skin. But what can one do with hundreds of litres of whey from the professional cheese production if they cannot be preserved? Frequently they are used to fatten pigs. So pork is a byproduct of cheese production. The met of the pigs that have lived on Alpine meadows drinking whey, has a good reputation and does not have much in common with the meat of pigs that have passed their lives in factories without ever seeing the daylight.
It is important to give the pigs only either sweet or sour whey, that is not to change between the two. As it is difficult to avoid that some of the given sweet whey turns sour, it is easiest to give sour whey.
Whey and its properties (in Spanish)