How do microorganisms help make yogurt and cheese?

Microorganisms play a fundamental role in the production of yogurt and cheese by facilitating the fermentation of milk. This process primarily involves the conversion of lactose into lactic acid by specific bacteria, such as Streptococcus and Lactobacillus.1

Yogurt Production

Yogurt is created by inoculating milk with a culture of Lactobacillus bulgaricus and Streptococcus thermophilus. When incubated at warm temperatures, these bacteria ferment the milk sugar, producing lactic acid that causes the milk proteins to coagulate into a smooth, custard-like curd. Some manufacturers also include Lactobacillus acidophilus for potential health benefits.23

Cheese Making

In cheese production, fermenting microorganisms—often referred to as a cheese-starter—are added to milk to initiate acidification. This acid, often working alongside the enzyme rennet, causes casein to clump and form curds.45

During the ripening stage, various microorganisms and their enzymes break down proteins and fats, which is essential for developing the unique flavors, textures, and appearances of different cheeses. For example, Propionibacterium shermanii produces carbon dioxide to create the characteristic "eyes" in Swiss-type cheeses, while Penicillium molds contribute to the distinct qualities of blue-veined and surface-ripened varieties.6