牛奶放久了，为什么会变酸？

Fresh milk tastes gentle and sweet. 新鲜牛奶淡淡的、甜甜的。 Lactic acid bacteria love warm milk. 乳酸菌最爱温温的牛奶。 They quietly turn milk sugar into acid. 它们悄悄把奶糖变成乳酸。 More and more acid makes the milk sour. 酸越变越多，牛奶就变酸。 The milk also starts to clump together. 牛奶还会结成小块块。 That is how sour milk is born. 酸奶的前身就是这样来的。