What is That Liquid on Top of My Yogurt?

Know what I'm talking about?  You open your container of yogurt and there's a thin layer of liquid on top?  It also happens with other dairy products like sour cream and cottage cheese.  What is that and why does it happen?

There are two main reasons:

1. A process called syneresis.  This is a fancy schmancy name that means "watery proteins leaking out of the product".  Yogurt, sour cream, and cottage cheese are all products that are made from milk proteins forming a gel with water.  Some of the proteins are very sticky (like casein) and some are not as sticky (like whey).  As these foods sit on the grocery store shelf and then in your refrigerator, the less sticky proteins leach out over time and collect on the surface.  If it's a large container with multiple servings, it keeps happening each time you take a serving and put it back in the frig because now you've disrupted that gel.  As the gel settles down to the bottom of the container, more watery whey proteins leach out.  

What to do about it?  I've blogged about whey before, but just as a refresher, it's a milk protein that is similar in composition to human muscle, so it's great for your body.  Just mix it back in with the yogurt, sour cream, or cottage cheese, and feel good about yourself.

2. The other reason only applies to products that have already been opened and put back in your frig.  This will not make you feel as good.  

Many dairy products contain a small amount of carbohydrate (especially sweetened yogurts and sour cream).  Human saliva contains enzymes that digest carbohydrates within the mouth.  Imagine a scenario where you take a spoonful of yogurt, stick it into your mouth, and then back into the yogurt.  You decide not to eat the entire container of yogurt and stick it back in the frig.  All of sudden, those lucky enzymes from your spoon are swimming around in their favorite food, digesting it along the way, leaving behind a trail of digested carbohydrates that draw out water. Imagine a scenario where it's not your enzymes in the food you're about to eat, but someone else's.  EWWW!    

Best to never contaminate your food with your saliva-ridden spoon, don't you think?  Here endeth the lesson.  :)