Rennet cheese is a cheese that uses rennet in the cheesemaking process to coagulate the milk and to separate the curd from whey. Rennet is a mix of enzymes that causes the casein proteins in liquid milk to clump together, forming a solid curd that is eventually transformed into cheese. Traditionally, rennet was taken from the stomachs of young ruminant animals like calves or lambs. Today, many cheesemakers use rennet produced from vegetable sources or microbes or from the genetically modified genes of ruminant animals.
Some cheeses use acids like vinegar or lemon juice rather than rennet to curdle the milk. Queso blanco, cream cheese, mascarpone, quark, and ricotta are a few of the cheeses made without rennet.