Acid-set cheese
Cheese whose curd is formed by acid rather than rennet, as in paneer, chhena, ricotta and quark.
Also: acid-coagulated cheese, heat-acid cheese
An acid-set cheese is one in which the milk protein, casein, is made to clump by lowering the acidity of the milk instead of by adding rennet. There are two routes. In heat-acid cheeses, hot milk is soured quickly with lemon juice, vinegar, citric acid or sour whey, and the curd separates at once: this is how paneer, chhena, queso blanco and ricotta-style cheeses are made. In slow lactic cheeses, bacteria turn milk sugar into lactic acid over many hours at room temperature, as in quark, tvorog and many fresh goat cheeses. Acid-set curds do not melt and stretch the way rennet curds do, which is why paneer holds its shape in a hot curry. Most are eaten fresh.