There is an oft-repeated misconception that we have no way of judging the acidity or workings of the culture without using some means of measuring the acidity.
This of course, is a myth.
Traditional cheesemakers have made the world's finest cheeses for hundreds of years without ever once measuring the pH. While the technology has been around for centuries, it still has not yet found its way into many old world cheeses.
Throughout the process of making cheese, there are a wide number of subtle indicators that tell us how the cheese is acidifying. There are certain things that happen in the cheese that an experienced observer can use to time important parts of the cheesemaking process. A little bit of the technical side of cheesemaking can be put away in exchange for a lot of art.
The number one most important measure we have of the acidity of the cheese is the character if the whey. I was taught by a fine old Swiss Cheesemaker that you proceed to cut the curd not when the whey has reached a certain pH, but once it has become 'schon gälb' (pretty yellow). In other words, I was taught to watch the whey until it took on a precise nature of color and clarity. Then it was time for the next step. I am doubtful that he had any knowledge that it was really the acidity that we were looking out for.
If you observe the whey during the process of the make, you will notice changes occurring throughout the process. When you first cut the curd (VERY slowly, mind you) then it should be a milky white, almost like skim milk, maybe with a slight yellowish tint (if it is right away yellow, then your are too acidic). Here is where the speed of cutting is important. If you stir too fast, you disturb the whey and mess up the acidification.
When making Alpkäse, the curd is first cut to great big coarse pieces and then stirred for about ten minutes. In this time period, the whey changes color. Once it is 'Schon Gälb' then it is time to cut it to final size. Same thing when brewing the curd, once it has deepened to the proper shade of green we know the cheese is at the right stage in its development to start cooking the curd.
What is occurring here is simple, the whey when we first cut it has a lot of proteins and sugars suspended in it, but not very much acid. As the process continues, the acid reacts with the solids still in the whey to turn it first yellow and eventually to a pale green. This is caused both by acid developing in the whey itself and by acid being dispelled from the curd. This is why we don't ever work too fast, if you stir the curd too roughly, you cause it to loose too much acid which makes it difficult to judge the whey. The acid is produced by the culture, and is a direct indicator of how the culture is developing.
Now all of this comes with experience. You don't know when to move on if you don't know just what color it is you are aiming for in the whey. The differences are pretty small, and inexperience will cause you to most likely overshoot -going for too much yellow or green which means too much acidity.
This is where pH comes in handy for the hobbyist cheesemaker. You don't need the experience to judge the whey, all you need is a simple test.
I think most of us here understand the importance of acidity. We can give a recipe telling you precise times to proceed to the next step, but what is important is that we proceed to the next step when the culture is ready for it.
And what about adding the rennet? how do we know if the culture is working, and we can proceed with this steP? After all, we don't have any whey at this step to judge, do we?
Actually, we do.
Before adding the rennet, our culture is happily working away to produce yogurt. It is already producing acid, causing the cheese to coagulate very slowly. What we have, then, are small pockets of liquid separating from the milk. When we see little tiny specks of green liquid on the surface of our cheese, we know the culture is working and we can go ahead and add the rennet.