Cheese is made from milk. Milk contains a sugar called lactose which is very important in the formation of cheese. Now while making Swiss cheese, bacteria like S. Thermophilus, Lactobacillus and P. Shermani are added to milk.
However, they are good bacteria. Out of these bacteria, the S. Thermophilus and Lactobacillus first convert the lactose into lactic acid, producing curd. Then after undergoing certain processes, when the curd is kept for maturing, the P. Shermani bacteria consumes the lactic acid and releases carbon dioxide gas.
Being trapped, the carbon dioxide gas forms bubbles. Eventually, when we cut the cheese, the bubbles pop, resulting in holes in the Swiss cheese.
