When I was 7 years old and still living in Taiwan, I went with my mother to buy milk on a Sunday morning. It was September and the temperature rose from a soothing 25 degrees Celsius to a sultry 40 degrees Celsius in just a few hours. For reference, the human body tries to stay at a constant 37 degrees Celsius, so 40 degrees felt unbearable. Because my mom was a busy yet forgetful woman, she continued to add errands to our list of errands. The milk run turned into a grocery trip, turned into a quick visit to a friend, turned into a pit stop at the office; by the time we got home, the milk had been sitting in the car for a full 8 hours.
Long story short, I complained about my sour Ovaltine milk the next morning; but always the don’t-complain-to-me type, my mom refused to believe me, so I had to choke down the entire cup. I came home that afternoon to a profusely apologetic mother. Sometime during the day, she realized the entire gallon had soured and had forced her only baby to drink it. I still wrote her a nice card for Christmas though. No hard feelings.
Why does milk sour? Why does it develop lumps, chunks, and curds? The answer is simple: bacteria. Specifically, lactobacillus acidophilus. This harmless bacteria can be found on and in many different organisms, including cucumbers, cabbages, humans, and cows. In the correct conditions, lactobacillus grows, consumes all oxygen, goes through the fermentation process, and produces beautiful lactic acid as a byproduct. Lactic acid gives pickles, sauerkraut, and yogurt its distinctive sour flavor. It’s why your mouth develops a sour taste after eating chocolate or other foods high in sugar. It’s what rots your teeth, makes your vaginal secretions acidic, and promotes good digestion in your gut.
Though most milk is pasteurized, this doesn’t mean that all bacteria are destroyed. This is why pasteurized milk and other pasteurized products need to be refrigerated. In my mother’s hot car, the small percentage of lactobacillus that was not killed in the pasteurization process grew for 8 hours before being put back in the cold. That 8 hours was enough to sour the milk, but not curdle it. Had we gone on a 3 day errand, we’d have returned to some cheese and questionable milk protein-fluid sitting in the car.
What’s with the curdling, you ask? Milk is made out of protein and fats suspended in water. Acidic compounds such as lactic acid unfold the protein. Unfortunately, since most of the structure of milk protein does not like water, it clumps with the fat and other unraveled milk proteins, creating the infamous lumps and chunks in milk-gone-bad. Good news though, this is how we get cheese. I’d write a blog post about cheese, but for authenticity’s sake, it’d have to be in French.