It is insanely easy to make a fresh cheese that approximates ricotta and can be substituted for it without consequences. Technically, ricotta, which means “recooked,” is made from whey after the curds are removed to make cheese. The whey is left to cool and stand overnight, developing an acidic quality that is necessary for curdling the whey and resulting in the fine-grained ricotta.
There are recipes all over the Internet for “fresh ricotta,” a one-step process that is typically how one produces the Indian paneer or the Mexican queso fresco. You add an acidic ingredient (buttermilk, white vinegar or lemon juice) to milk, bring it to a temperature of 175 degrees, when the curds separate from the whey. You then carefully scoop out the curds and drain them in several layers of cheesecloth. The length of time in draining determines how soft or hard the cheese is, so you would adjust that depending on how you’re using it. For example, if you’re eating it fresh, the initial 15-20 minutes of draining is probably fine. If you’re using it for lasagna or pizza, I’d go for a couple of hours. I don’t add salt to the curds but you could.
Following the excellent directions on Fankhauser’s Cheese Page, I tried making ricotta from the whey left over from the fresh cheese above. While I was making small quantities, I was able to get ricotta from the leftover whey. Fankhauser’s website said that they got 1 pound of ricotta from the whey left after processing 5 gallons of milk into cheese. Since I got such good results from the fresh cheese, I figured why not just substitute it for the ricotta? (Besides, look at the additives in store-bought ricotta, mostly stabilizers like carageenan, guar gum and cornstarch. All plant based but who needs them.)
Although I used to make this with vinegar, lately I’ve been using buttermilk, since we get good buttermilk from the same dairy where we get organic milk in glass bottles. I like the tanginess. Sometime, I’ll do a taste test to see which I like better. I’ve never used lemon juice and I probably wouldn’t since the acidity varies from lemon to lemon. When I can tomatoes I use bottled lemon juice to guarantee the acidity, but that doesn’t appeal to me on the cheese front.
Also, don’t discard the whey. Even though it looks greenish (from the riboflavin apparently), it makes an excellent base for soup. I’ve made leek and potato soup and celery soup from it with great results.
Fresh Cheese a la Ricotta (makes about 1 cup, ½ pound)
1 quart milk (whole or 2% is what I use)
1 cup buttermilk
Line a small sieve set over a bowl with 4-5 levels of cheesecloth. Have a silicone spatula, a thermometer (I use a candy thermometer) and a fine-mesh strainer (I use an Asian skimmer) handy. Put the milk and buttermilk in a heavy-bottomed saucepan and place over medium heat. Cook until the curds and whey start to separate, around 175 degrees. Do not over cook but it’s ok to go to 180. Remove from the heat and slowly scoop the curds into the lined sieve. Let them drain for 15-20 minutes. If you want a harder cheese, tie the cheesecloth in a bundle and suspend the cheese over a bowl for an hour or two. It will lose more liquid and become hard. Store in the refrigerator and use within 3-4 days.
PROSPECT THE PANTRY 
I just made a few batches of paneer/queso fresco (from 1 gallon skimmed milk each). Very easy and exciting!! I read that you really DON’T need the long drain/press time. So, since I love shortcuts, I tried with great success. Without hanging or weighting down I ended up with a cheese firm enough to cut thin slices from with a cheese slicer!
Process: I put a cotton tea-towel (US = dish-towel) in a colander, poured the whey/curds in, as soon as the majority of the whey had drained through (I’m impatient so probably helped it along with jiggling/prodding/squeezing). Then placed cloth with curds onto bread board, folded corners of the cloth over the cheese, and pressed down firmly with my hands to squeeze out the last whey. Placed the resulting patty, which was already quite firm, in a tupperware container in the fridge. It was even firmer when I took it out again – probably a few hours later. (… I imagine with forcing the whey through instead of letting it drip, I’d get even more ricotta out of my “impatient” whey!… I’ll try next time)
Perhaps the low fat curds that are firmer? does the extra fat in the curdled protein make it softer?
Next gallon, I decided to try for a less firm cheese, so didn’t press at all, just mixed in salt and herbs, and put curds into a tupperware container. when I next took it out of the fridge this one was almost cutable too… the curds had sort of stuck themselves together. I had to chop it up to make the crumbles I wanted for my salad! This cheese has been great added to soup, stir fry, whatever. It doesnt break up or disintegrate in the cooking.
It was great to find out that I didn’t need some contraption for hanging, didn’t neeed special cheese cloth, and didn’t need extra draining time!
Regarding exact acidity – I’ve been very inexact, using +/- 3T of differnt types of vinegar to the gallon. Balsamic made the cheese a little brown – otherwise no big difference – what difference have you noticed with more/less acid in the batch?
Wow. Great comments. You’re inspiring me to make a new batch. I was thinking about taking a class on making fresh cheese but I’m convinced I can experiment and figure it out. Will need to research the acid question. My hunch is that it’s largely a starter, and the amount might vary the time for curdling but not the end result, other than the color and taste of course. That’s why I go back and forth on lemon vs vinegar.