At breakfast the other day, my daughter told me the milk I'd poured her smelled sour.
Oops.But don't worry, she's fine.
I lacked the culinary savvy to make use of it, so I just dumped the rest.
And rather than cry over soured milk, I decided to look into what happened.
What's Milk Got?
The main nutrients of milk are lactose, (the milk sugar some people are intolerant of), proteins of various sorts, and butterfat, all sloshing around in about 85 to 90% water. It total, milk has about 100,000 different compounds in it, and you could probably find a microbe that likes every one.
In the old days, if you got milk, you could also get Typhoid and Scarlet fever, Diphtheria, Tuberculosis or various diarrheal diseases. Mmmm. Pasteurization is named for Louis Pasteur who, in 1859, showed how heating up wine or beer would keep it from going cloudy because of microbes. Franz Von Soxhlet, a German agricultural chemist with impressive sideburns and a great interest in milk, was actually the first to suggest sterilizing milk for infants in 1886. Nowadays, it is usually heated to 71 C and then rapidly cooled to kill off bacteria and other microbes.
Bacteria tend to get a lot of bad press. But lactic acid bacteria occur naturally on plants like grasses and can easily end up in milk. They ferment lactose at room temperature and turn it into lactic acid. More acid means more hydrogen ions, a lower pH and a sour taste. Lower pH change the structure of proteins like casein, which gives milk its whiteness, so it curdles into clumps called curds. The leftover yellowy liquid is whey. Apparently, little Miss Muffet was just having unpasteurized milk that had been left out for a while. Microbes like Lactococcus lactis (nominated the Wisconsin "State Microbe") get added back to the milk after pasteurization to make dairy products like yogurt and cheese.
Making Scents of a New Weird Odour
So the sourness is one thing, and curdling is another. Smell is yet another milk phenomenon. After the milk container has been opened, mystery microbes can get back in. And if you leave it out, they can reproduce more quickly. Other bacteria break down milk proteins that result in smelly byproducts. Still others can break down the butterfat that smells bad. And some molds can use lactic acid, break the proteins or attack the fat and give off unappetizing aromas.