Can leftover yogurt be successfully frozen?
To find out, we froze plain yogurt—whole milk, low-fat, and nonfat—for two nights, after which we defrosted it in the refrigerator. We used the thawed yogurt in plain muffins and uncooked yogurt sauce and compared them with samples prepared with never-frozen yogurt. All of the muffins were identical in appearance, taste, and texture, but the previously frozen yogurt produced sauces that were markedly runny and slightly broken in appearance.
Here’s why: When milk is heated to make yogurt, its proteins coagulate to form a weak gel that traps the water and fat. But as ice crystals form during freezing, water is drawn away from the protein network, causing the gel to weaken or even collapse. Once thawed, the protein network does not re-form, explaining our thin, separated results.
Some brands of commercial yogurt contain added stabilizers such as pectin that reinforce the protein network and, as a result, may fare better when frozen. But to avoid any problems, we recommend that you use thawed yogurt only for baking applications.
Freezing breaks down yogurt, making it unusable in uncooked applications like this sauce. (It's fine for baking.)