Using whey from yogurt to make ricotta

Question

My girlfriend and I make a large batch of yogurt every week, and have done so for more than a year now - we just eat a ton of the stuff. THis leaves a lot of leftover whey which we used to just throw out. This past week I took my first swing at making mozzarella, and noticed that the recipe suggested not throwing out the whey, but rather using it to make ricotta by boiling it. I ended up throwing out the whey from the mozzarella anyways because it was a tiny batch, and we had to make yogurt anyway.

So, we made the yogurt, and my girlfriend saved they whey for me. I've been simmering it for a few minutes now, and it doesn't appear to be working. Nothing is curdling. I'm wondering if its a function of not having anything acidic in it? I tried adding a quarter teaspoon of citric acid, but still nothing is curdling. At this point I'm curious to play with it, so I may try simmering it all the way down into gjetost.

So really i guess my question is, outside of being less acidic, is whey from making yogurt different from whey from cheese making?

Also as a note, we use 1% milk.

Answer

Yogurt whey cannot be used to make ricotta.

With most cheeses, including mozzarella, the milk isn't boiled. The casein proteins are bound up with some of the lactose and almost all the fat to make the curd. The whey for such cheeses contains the rest of the lactose, tons of vitamin B, and almost all the albumin.

The albumin proteins are water soluble when they haven't been denatured. They also denature at a higher temperature than the casein does. Therefore, they almost all wash out of the curd with the whey. Bringing mozzarella whey to a (almost) boil denatures the albumin which precipitates out and can be strained to make ricotta.

When making yogurt the milk is heated to 190 or higher and then cooled. This is precisely to denature the albumin. The extra protein gives the yogurt a lot more structure.

However it means that there is no protein left in the whey- mostly lactose, lactic acid, and vitamin B.

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