Given the background description of some cooking related query, summarize the question into a title

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.
Using whey from yogurt to make ricotta