Last night I got the most insightful e-mail from Diana at Off the Bone that answered the questions I had about making ricotta, yogurt, farmer's cheese, and other fresh cheese variations. I asked Diana if it would be ok to share her explanation with my readers and she graciously agreed to be my guest blogger.
Here is what she wrote...
There are two things that need to happen when you make cottage cheese
1) the milk needs to curdle - i.e. proteins need to coagulate and separate from water in the milk
2) lactic fermentation needs to occur
1) gives it the texture / consistency you want, and 2) gives it the tang.
The two are related, in that lactic fermentation will make the milk curdle, but they do not inevitably go together - milk can curdle without lactic fermentation. This is what happens when you add lemon juice to warm milk - it curdles, but does not become tangy. You can curdle the milk with anything acidic - vinegar, wine, citrus juice, hibiscus tea, whatever. But curdling, as I mentioned, is not the only thing we are after. We are also after lactic bacteria.
Lactic bacteria, of course, are everywhere. So, if you let the milk sit without refrigeration they find it, and curdle it. That's how Russian grandmothers make cottage cheese.
Your Russian friends are absolutely right in that supermarket milk is really hard to get to curdle. In fact, I find that it spoils before it curdles if you just let it sit at room temperature (and trust me, the difference is very obvious by smell - clean + acidic vs disgusting). Lactic bacteria just don't like it. So what you should do is introduce some lactic bacteria and give them a head start (by creating conditions they like). Once lactic fermentation begins - i.e. lactic bacteria begin breeding - other bacteria are very unlikely to breed in the same environment, because lactic bacteria outcompete them. You get lactic bacteria from buttermilk or "live cultures" plain yogurt (they are different strains, so the taste will be a bit different, but probably not different enough to care what you use for the first batch), and they like it warm. Like, 120F or so (they slow down below 98 and start dying above 130).
So, start them off warm - heat up the milk, add yogurt / buttermilk, take the pot off heat and wrap it in a blanket or something to keep it warm for a while. As it naturally cools the fermentation will slow down, but that's ok - the milk at this point is pretty safe from all the other bacteria, and you aren't racing the clock.
The fermentation is done when the milk sets - wobble the pot, or look out for a thin film of whey on top of the set "jelly."
That's it. Strain it. The more you break up the curds, the rougher / grainier the texture of the cottage cheese. If you are super gentle, though (spooning the curds into the strainer sort of thing), you'll end up with no texture at all - basically a greek yogurt.
I haven't experimented with it very much, but I'd try cutting the curd like you do when you make cheese. Take a butter knife or a chopstick, cut in a cross pattern (like you're drawing a grid on top of your pot, except the chopstick goes all the way to the bottom), and then stir a bit with a spatula to get the curds to start releasing whey and forming tighter chunks. Then strain.
A good solution in terms of milk is to get Shaw Farm milk (it's sold in Kickass Cupcakes in Davis Square, in Wilson Farms in Lexington, possibly elsewhere, too). It's noticeably better than the supermarket kind. They even sell unhomogenized, I believe, if you're into that.
Thank you, Diana, for such a good explanation. I'll give it another shot and hopefully will end up with farmer's cheese.