Milk typically contains about 5% lactose by weight, but more processed dairy products like cheese, butter, and whey protein contain much less. This is because, with cheese, bacteria ferment lactose into lactic acid during the fermentation process. Butter is an almost pure source of dairy fat, and the fat content has been separated from carbohydrates (lactose) and protein. Whey protein is a concentrated dairy protein, and much of the carbohydrate (lactose) and fat has been removed. Dairy products like yogurt, quark, kefir, sour cream and other fermented products also contain less lactose than milk.