WrongWhey, it's hard to say what's causing your problems. Basically from the start of the cheese making process, when you add the culture you entire goal needs to be to get the curds in the mold at the right acidity level and the right structure. If you get that right, then everything else is easy. So you have to measure the culture correctly, ripen the milk appropriately, add the right amount of rennet, wait the correct amount of time, cut the curds the right size, let the curds heal, stir appropriately, raise the temperature (if you are) at the right speed, and judge when to stop. Get any of those things wrong and it makes everything else more difficult. If you get something wrong and don't notice it, then it all starts to snowball so that by the time you get to pressing, it's impossible.
The biggest places I think most people go wrong are:
1. Usually too much culture. This causes the milk to acidify too quickly and by the time you are in the mold, everything's gone wrong.
2. Not renneting correctly. Rennet is rated by strength. "Single strength" rennet is usually about 200 IMCU per ml, but some producers are as low as 150 and others are as high as 220. For "double strength" it's even worse. Usually 300 all the way up to 900 are typical values (4.5x typical "single strength"). You want to use between 35 and 45 IMCU per liter of milk (lower values for raw milk). Normally recipes are "x teaspoons", which are absolutely useless instructions. For normal single strength rennet, use 4 drops per liter of milk and you will be quite close.
3. Not coagulating to the right level. Your milk will coagulate at a speed depending on: the amount of rennet, the temperature of the milk and the acidity of the milk. More of anything makes the milk coagulate faster. Measure how fast it's going because it's important. This is called the flocculation technique: float something like a wine or soda bottle cap on the milk right after you add the rennet. Time how long it takes before when you try to spin it, it doesn't spin (I wait until it leaves a bit of an indentation in the milk when I remove it). Normally that's about 12 minutes. The total time you wait is a multiplier of that. For most cheeses it will be between 2.5 and 3.5. So if you are going for 3.0, then total waiting time should be 36 minutes.
If it's taking you a long time to coagulate, then there is a problem either with your milk or your rennet. If you are using pasteurised or raw milk, then the problem is *probably* your rennet. There are a couple of other concerns if you are using fresh milk from a single cow, but if you aren't you can ignore that. If you are using pasteurised milk, then you almost certainly must add calcium chloride to the milk. If you are buying the liquid from the same place you bought the rennet then use 5 drops per liter of milk.
The most common problem with rennet is usually diluting it with water that contains chlorine. You often see advice of boiling water and letting it cool, or just letting it sit out or blah, blah, blah. For lots of technical reasons all of those things can fail. Go to the store and buy a bottle of water. Those are disinfected with ultra violet light. Use that for diluting rennet. You can also use distilled water.
If you are sure your rennet is fine, then you may have added too little culture. Extending your time is fine (because the milk is acidifying slowly).
4. Wating for a "clean break". That term is essentially nonsense in the context it's usually used. Wait for the multiplier for the cheese you are using. If you don't know, then 3.0 is a good default.
5. Homogenised milk. Basically the fat is all screwed up and coating the protein. The rennet can't get to it. This means the curds shatter when you try to stir. I use this technique when using homogenised milk: Cut at a multiplier of 2.0 about twice the size you ultimately want, but don't stir. Stir once at a multiplier of 3.0. Wait until a multiplier of 4.0. Stir once. Very slowly stir about once every minute for the rest of the time. This will stop your curds from turning into mush/powder. You just want to stir to prevent matting. Because the curds are fracturing all the time, they are draining quickly. You don't have to stir all the time with homogenised milk.
6. Case hardening your curds. Some recipes raise the temperature while stirring (sometimes called "scalding"). If you raise the temperature too quickly, you cause the outside of the curds to harden quickly, trapping the whey on the inside. His causes lots of problems. When it says to raise the temperature by 6 C over 30 minutes, it really *does* matter if you miss your goal by a single degree. Basically, If I'm doing something like this, I want to raise 1 degree every 5 minutes. With my lowest temperature setting, I will increase 1 degree in 1 minute and then turn off the heat and wait 4 more minutes. I do that 6 times. This works well for me. But if you crank it up those 6 degrees in 2-3 minutes (which is really easy to do), you *will* have problems.
7. Not knowing when the curds are "done". For most cheeses the curds are "done" when you hit the right acidity level and the structure is even all the way through the curd. Take a curd. Break it apart with your fingers. Does it have an even texture all the way through? If yes, you are done. If it's "juicy" inside, you are not done. If it's hard and bouncy, then you have gone too far. For acidity, you should be thinking about your total time since you added the culture and the temperature. If you are adding extra time because your curds don't have the right texture, then you risk over acidifying the curd. It's crucial to cut and stir the curds correctly so that the text is ready at the right time. I usually err on the side of acidity, though. Even if my curds aren't quite done, I'll go into the mold early if I think I'm going too slowly. Judge this mostly from your flocculation time.
8. Mishandling the curds when getting them in the mold. Draining the curds into a colander and then putting that in the mold is not usually the best technique. It's fine for some cheeses, but not good for others. It adds air between the curds and cools them off too quickly. Your default technique should be to let the curds fall to the bottom of the pot and then ladle off the whey. Gently compress the curds into a mass (called a "tomme") and do your best to get that into your cheese cloth. Sometimes people put the cheese cloth in the pot first, manage to get it in underneath the curds and then drain the whey (or lift the curds out with the cheese cloth). I usually just take it out in 2 or 3 big clumps.
The big advantage to this technique is that there is no (or very little) air between the curds. When the whey drains out, the curds are sucked together and close with absolutely no need to press them. There was never any air between them in the first place, so there is very little way air can get in to make a gap.
9. Over pressing too early. Pressing the cheese squeezes the whey out. However, it squeezes the whey out of the outside of the cheese first. If you press it too hard, the outside of the cheese closes and leaves your cheese a bit like a water baloon. You want to close the rind in about 2 hours, and no earlier. As soon as 2 hours are up, feel free to put blocks of uranium on it to close it.
Most cheeses need very little weight. I've closed some tommes with no weight as all (but usually I end up putting about the same amount of weight at the cheese -- in my case about 5-600 grams). I don't think I put more than 2 kg (about 5 lbs) on *any* cheese that I make. You need to up that for larger cheeses, but if you are putting more than 3x the weight of the cheese on most cheeses, you know that you have a problem in your make. The exception is anything you've cheddared. That needs a truck full of trucks on it. Even Parmesan cheese, made professionally, puts less than half the weight of the cheese on when pressing. The vast majority of cheeses made at home don't really need a press.
I hope this book of a post helps :-)
