Why does Milk go Sour?

As we can see from the com­po­si­tion, the lac­tic medi­um con­tains var­i­ous types of bac­te­ria.

In nor­mal form they are not dan­ger­ous for the or­gan­ism, but over time (es­pe­cial­ly un­der the in­flu­ence of ac­cel­er­at­ing fac­tors) they start to mul­ti­ply, trans­form­ing lac­tose into lac­tic acid.

As a re­sult, the acid­i­ty of the milk in­creas­es, which in its turn caus­es the pro­tein to fold and liq­uid to sep­a­rate into whey and a vis­cous mass.