Ever peel back a yogurt lid, mouth salivating, ready to spoon yourself into a creamy dreamy abyss, only to be greeted with a thin layer of watery liquid sitting atop your yogurt?
You're not alone, and don't worry, you're not in any danger (unless the yogurt is expired, in which case find another snack).
What is it?
The watery substance you see on top of your yogurt actually isn't water at all; more likely, the cloudy liquid is whey, a natural protein found in dairy products.
Whey is rich in milk-protein and can also contain some of the milk's nutrients, including calcium and vitamin D. Some people consume it in things like Quest bars, while others avoid it completely if they stay away from animal-proteins.
Why does it end up on top of my yogurt?
In many milk products, whey can separate over time or due to storage conditions. Yogurt-making occurs in two steps: first milk is heated and partly cooked, and then the warm milk is fermented. During heating, the whey protein is denatured, and can cluster with other proteins, thickening the yogurt.
Next the milk is cooled and bacteria are added for fermentation. The added bacteria and natural lactic acid from the milk proteins help create a gel; rapid gelling creates a firmer texture, but one that is also more likely to leak whey.
Yogurt making is a complicated process, and keeping the mixture homogenous over travel and time is a challenge. Sometimes whey oozes from the yogurt gel structures and—due to its lesser density—collects at the top.
This is even more common in strained yogurt styles, like Greek and Icelandic, and also why some yogurts (like Fage), have a little parchment paper on top—it helps absorb the extra liquid whey that may naturally rise to the surface.
What can I do about it?
Some manufacturers add emulsifiers, gelatin, gums, or thickeners as an attempt to avoid the problem, which is not one of safety, but one of customer's perceived quality.
So how do you fix the separation? Well, you can give the yogurt a good stir, or if you're grossed out by it, don't care about its potential nutritional contribution to the food, and/or want a thicker sensory experience you can pour it off and keep on spooning.
Spoons up, yogurt lovers.