What you see: Yellowish liquid pooling at the top of your yogurt
What it is: Whey!
Eat or toss: Eat! This is a natural part of the yogurt and doesn’t indicate that it’s gone bad or is about to go bad. Depending on your yogurt preferences, you can pour it off for later use, or just stir it back in.
Sometimes yellowish, often filmy, always gross, the liquid that pools at the top of yogurt always made me pause. Was it a signal of bad things happening deeper in yogurtland? Or just something harmless that happened to my yogurt after I ignored it for a little while?
Hold on to your tuffet because the answer is whey interesting! (Sorry.)
Yep, that liquid is the very whey of Miss Muffet fame, and is a natural part of yogurt. Essentially, yogurt is milk that’s been soured with lactic acid courtesy of some specialized bacteria that munch on lactose. The lactic acid changes the structure of the proteins in milk, transforming casein proteins, which predominate, into solid curds, while leaving the whey proteins liquid. The solids and liquids are OK at staying mixed together once stirred, but their tendency is to drift apart, which is why the whey eventually accumulates and, as in the image above, creates a visual something like a bleached-out mud puddle.
The whey is, of course, perfectly edible, and stirring it back in will add to the creaminess of the yogurt, and along with some calcium and B vitamins. Greek yogurt, which is thicker, has a large portion of its whey removed. But that doesn’t necessarily make Greek yogurt less nutritious—that’ll depend on your nutrition priorities. By removing so much whey, Greek yogurt becomes denser and more protein packed, and loses carbohydrates. That means that Greek yogurt will make you fuller faster, with a lower caloric cost.
All this whey removal with Greek yogurt creates some complex food waste and environmental impact issues. You see, it takes 3 to 4 ounces of milk to create one ounce of Greek yogurt. Left over from that process is a particular type of whey called "acid whey,” which is often discarded. If that acid whey gets into waterways (waterwheys, if you will), it can choke out oxygen and mess with ecosystems. With the rise in Greek yogurt’s popularity, farmers and researchers have been experimenting with a number of ways to turn this byproduct into a marketable substance. Uses range from mixing it with manure for fertilizer to separating it into more useful components (lactic acid, for example), and even marketing it as a probiotic sports drink. There are also lots of things a home cook can do with it. If you make your own yogurt or want to work with whey in your own kitchen, Bon Appetit and Salad In A Jar have some excellent ideas.
The Mysterious Liquid in Yogurt - Indiana Public Media
Dairy Ingredients for Food Processing edited by Ramesh C. Chandan, Arun Kilara
Fermenting Yogurt at Home - National Center for Home Food Preservation
Monkeys, Myths and Molecules: Separating Fact from Fiction and the Science of Everyday Life
Protein drinks and baby formula could offset Greek yogurt's dark side - The Guardian
Why the Liquid on Top of Your Yogurt Matters - Pop Sugar
Answered! What to Do with That Watery Stuff at the Top of Your Yogurt - Glamour
Acid Whey - A Dairy Ingredient You Won't Want to Waste - Arla Food Ingredients
Whey or no whey?