Why Does Milk Turn Sour?
The milk turns sour because a little microbe, known as the milk microbe gets into it, and being very fond of the sugar which is in the milk, turns this sugar into an acid.
If we could keep milk entirely away from the air after the cow is milked, it would not turn sour, but as soon as it is exposed to the air these microbes which are constantly in the air, drop into the milk. They are alive, although invisible to the naked eye. If when they drop into the milk it is warm enough for them to get in their work so to speak, they fall upon the sugar in the milk and turn it into the acid. Their attempt to sour the milk can be overcome by keeping the milk at a low temperature in the refrigerator, but as soon as the milk is taken out of the refrigerator and left out long enough to become warm, the microbe begins to work and the milk cannot be made sweet again. If the milk is boiled as soon or shortly after the cow is milked, the sugar in the milk is changed in such a way that the microbe cannot feed upon it.
Notes
3 Pictures and descriptions by courtesy of Hartford Carpet Co.