As I go through old posts on the forum, I think I'm understanding things better. I'd like to explain how I understand cultures, and ask those of you who really do understand them to critique what I have to say.
A main thing that makes a specific cheese, is the path it follows in acid development. When we use pH targets, we are attempting to get our cheese to follow the "right" path for that specific type of cheese. We control (or try to control) the acid-path the cheese follows through a number of different actions--selecting specific cultures whose rate of acid production can be made to accommodate our desired path, starting with a specific amount (number of active bacteria) of that culture, and controlling the amount of time the milk/cheese spends at a given temperature.
Because different bacteria have different reproduction and acid production rates, we frequently use a blend of them in an effort to get an acid production curve that matches or can be manipulated to what we'd like to see our cheese do. (?--I envision that the blends I use have one kind of bacteria that gets me a good start at a low pH, then another that takes over where the first one slows down, and so forth. The total acid output of the blend of bacteria is simply the sum of the acid output of each different kind, weighted, of course, by each kind of bacteria's concentration. I have seen Linuxboy--whose name is also Pav?--ask about the pH curves of the starter people are using...he asked me once at least, too, though I have no idea where to get that information from unless I just make it myself.)
We also select certain bacteria for their participation in flavor development (?--which they contribute by exploding. Their enzymatic guts reacting with stuff in the cheese to make flavors we like...mmmm dead germ guts).
We control the amount of starter culture we use by trying to add a specific number of active bacteria to our milk. New folks like me just use measuring spoons and packets of stuff we got from the internet because we don't know any better. This is an imprecise method because those DVI packets are sold by activity which is not necessarily tied to volume. More precise methods do a better job at adding a specific number of active bacteria. These methods include using bulk starter/mother culture (which is just milk that has already been innoculated and allowed to reach a stable population of bacteria--from what I understood of Sailor's photo essay) at a recipe-specified percent of mass, and using the specific units of starter activity provided by the manufacture of the DVI (like DCU for Danisco brand starters).
The rate at which the bacteria reproduce and convert lactose into lactic acid depends on the temperature. By controlling the amount of time we hold the inoculated milk at a given temperature, we control how rapidly the bacteria reproduce and how much acid they make. There is an optimum temperature for each kind of bacteria and by going above or below it, we slow them down in the effort to get them to produce the right amount of acid the cheese's acid-path calls for.
That's about all I understand. It seems that I've seen a few places where people have mentioned that pressing the curds increases the rate of acid production, but I don't understand why that would be the case. Do bacteria behave differently under pressure? I suppose I do...
If you've read this far, thanks for doing so! I vaguely remember something from chemistry a long time ago that pointed out that pH and the concentration of an acid are not the same thing (the word molar comes to mind), but I haven't seen much discussion about the concentration of acids--just pH--here, so I'm guessing I don't need to worry about that distinction. If I do, and I confused them above, sorry about that.