Swiss cheese is the generic name for several related varieties of cheese, originally made in Switzerland. It has a distinctive appearance, as a block of the cheese is cratered with holes (although not all kinds of Swiss cheese has this feature; see below).
The 450 known Swiss cheeses are classified in 5 categories: extra-hard, hard, semi-hard, semi-soft, and soft. Cowmilk is used in 99% of the cheeses produced, the remaining share is mostly made up of sheep and goat milk.
Three types of bacteria are used in the production of Emmentaler cheese: Streptococcus thermophilis, Lactobacillus, and Propionibacter shermani. In a late stage of cheese production, P. shermani consumes the lactic acid excreted by the other bacteria, and releases carbon dioxide gas, which forms bubbles that appear to be "holes" when the cheese is sliced. The cheese industry calls these holes or tunnels "eyes". Swiss cheese without eyes is known as "blind".
