Dear friend, thou seem'st to wrong my soul too much, Thinking that Studioso would account That fortune sour which thou accountest sweet; Not any life to me can sweeter be, Than happy swains in plain of Arcady. PHILOMUSUS.

Thus the bacteria lactis act upon the milk sugar present in milk, and convert it into lactic acid, thus bringing about the souring of milk.

The reason for this undoubtedly lies in the fact that the boiling kills foreign germs, and thus prevents early souring or putrefaction.