New 'smart' bottle cap could tell you if your milk is bad

Aug 12, 2015 at 2:23 p.m. ET

Figuring out if your milk has expired can be tricky, and it usually involves a risky test — the sniff check.

If your milk is good, it's no problem, but if it's gone bad? You're left with a noseful of sour, funky badness. Other times, seemingly odorless milk comes out of the container chunky or sour-tasting. There just doesn't seem to be a great way to figure out if milk is still good.

A new way to see if milk is fresh

That's why researchers at the University of California Berkeley decided to create a "smart" bottle cap that can detect if your milk is still good.

"You just flip over and let milk get trapped a little bit, I mean in the smart cap," research specialist Chen Yang told ABC News. If the milk has gone bad, a radio signal sent through it by the cap will show a drop in the electrical signal (versus regular milk). "We believe that's because the growth of the bacteria is gradually changing the electrical property of the milk," Yang said.

The caps currently take nine hours each to make via 3-D printing, so they aren't very viable for mass production. Until they are, how are we supposed to figure out if our milk has gone bad?

More: Do food expiration dates really matter?

How to tell if milk has gone bad

If your milk has an expiration date, you should throw the milk away after that date, Peter Bonet of the Lincoln Culinary Institute told Lifehacker. But if it has a sell-by date, that just means the grocery store needs to stop selling the milk by that day, not that it will be undrinkable.

Currently, the FDA says that an open container of milk can last up to one week in the fridge.

But it really depends on when your milk came out of the cow and how far away the sell-by date is. According to Lifehacker, there's a way to calculate how long your milk will last if it doesn't come with an expiration date: "If you can store your milk for about five days after the sell-by date, subtract the current date from the sell-by date, add five, and you have the amount of days this milk will be good for. For example, if I buy milk today that has a sell-by date of Aug. 27, it will be good for 15 days, or a little over two weeks. Or, if there's an expiration date on the carton itself, just follow that."

This can all get complicated (that's quite a bit of mental math when you're standing in the middle of the grocery store), and chances are you'll sometimes wind up with milk that goes bad sooner than you'd like anyway. What then?

More: How long does it really take for food to go bad?

What to do with sour milk

If your milk is only a few days past its expiration date and slightly sour, you can use it in several different ways (though if it's curdled or discolored, you should throw it out).

You can try using it to make cheese. Simmer the sour milk in a double boiler until the curds separate from the whey, then strain through cheesecloth. Add cream, salt and pepper to taste for cottage cheese, or let drain in the cheesecloth overnight in your fridge for a thicker, more spreadable cheese.

You can bake with sour milk. Its high acidity acts in a similar way to buttermilk. Try using it in everything from Boston Brown Bread to griddle cakes.

You can even use sour milk in the garden. Pour it around plants to deter deer or feed it to your rose bushes.

While an interactive cap telling us when milk goes bad would certainly be handy, in the meantime, we can get by with these tips and sour milk recipes.

More: How long do eggs stay good in the refrigerator?