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When Good Milk Goes Bad

New technologies aim to help consumers figure out when to drink and when to toss the white stuff.

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Sensors in a recently developed high-tech milk cap may oneday replace traditional caps, making it easier to tell whether your milk is spoiled before you take a swig.


Spoiled milk can do a disgusting number on a cup of coffee, bowl of cereal, or strawberry smoothie, and an unexpected gulp of it is a definitely off-putting way to start the day. To date, the best bet for steering clear of bad milk is the whiff test—that is, take a sniff and if the milk in your carton smells funky, don’t drink it. The problem is that sense of smell isn’t always a reliable guide, and bad milk periodically outwits the human nose.

Spoiled-milk surprises, however, may soon be a thing of the past, thanks to a technological innovation developed at the University of California, Berkeley, and the National Chiao Tung University in Taiwan. Teams from these institutions have developed a 3-D-printed “smart cap” impregnated with microelectronic sensors capable of telling consumers at a glance if their milk has gone off the rails. Just slosh a little milk into the cap and a series of integrated circuits and wireless sensors will instantaneously let you know whether your milk is good to go or due to be poured down the drain.

Milk inevitably spoils because it’s not squeaky clean to begin with. Most milk today is pasteurized, a technique that uses heat exposure—commonly 15 seconds at 72 degrees C (162 degrees F)—to kill pathogenic (disease-causing) bacteria. (See Did You Know? Weird Facts About Milk.) Pasteurization, however, doesn’t do in all bacteria, and it’s these sturdy survivors that eventually cause milk to go bad.

Spoilage is a microbial joint endeavor. Milk is about 87 percent water, plus proteins, fats, minerals, and lactose (milk sugar), this last the root of the problem for most people unable to digest milk. Lactose is a feast, however, for residual Streptococci bacteria, which gobble it up and convert it into lactic acid. As lactic acid accumulates, the pH of milk increases to the point where Streptococci can no longer grow. A second class of bacteria, Lactobacilli, then steps up to the plate, similarly converting lactose to lactic acid until it, too, stewing in its own acidic juice, ceases to multiply. The rising acidity literally sours the milk, and causes the milk proteins to coagulate.

Casein, the major protein in milk, is the compound that—suspended in minuscule particles in water— makes milk look white. Acid causes the casein molecules to clump together (curdle), forming soft lumps called curds. The remaining thin yellowish liquid is known as whey. Under happier and more controlled circumstances, curds and whey are the raw material of cottage cheese, the dish the arachnophobic Miss Muffet was consuming while she sat on her tuffet. In bad milk, the pair taste awful.

If the now steadily spoiling milk is not discovered and thrown out, yeasts and molds can then begin to multiply in the acid environment, and finally Bacilli begin to feed on the milk proteins, further boosting the pH and spewing out smelly ammonia-containing by-products. Milk, by this point, has gone from merely bad to worse. And if you drink it, it can make you sick.

The smart cap isn’t the first device to attempt to finger bad milk before unsuspecting consumers use it to douse their Cheerios. In 2012, GE and product development network Quirky sponsored a contest to come up with ways to make everyday objects “smarter with software.” The winning idea was a proposal for a smart milk jug. “Give me a container that I put a gallon of milk in, that tells when it’s REALLY going bad!” the contestant wrote.

The result was the Milkmaid, a streamlined quart bottle sitting on a metal base (the Smartbase) containing an array of pH and temperature sensors. Based on information from the sensors, a microcontroller in the base slowly changes an LED from reassuring green to alarming orange, as the milk segues from good to gicky.

The base also contains a weight sensor, capable of sending a text message to the owner’s cell phone alerting him or her to low milk quantity and the need to buy more. Though the Milkmaid sounds like a techie milkshake-lover’s dream, it never got off the ground commercially, possibly because it was just too complicated for the average milk drinker. Apparently nobody wanted to worry about changing the milk-bottle battery.

Hopefully we’ll have better luck with the smart cap.

Because bad milk is … really bad.