Where does “syřidlo” come from?

syřidlo (Czech) comes from Czech sýr, from Proto-Slavic syrъ, from Proto-Balto-Slavic sū́ˀras, from Proto-Indo-European súHros — sour, acidic, salty.

syřidlo (Czech): rennet an enzyme used as the first step in making cheese, to curdle the milk and coagulate the casein in it