Well, you also have the case where some acids are stronger than others. You can have a fairly low TA and a low pH if the acids are fairly strong (malic acid has a pKa of 3.4, citric 3.1 and tartaric of 2.98, pKa is the pH when half of the acid is deprotonated so it is a measure of the acids strength)
Lifting Gear for Carboysfor Jack Keller's Wine Blog. Search down for March 17th, 2005. There he discusses the Carboy Lifter Martin Benke invented for Jack. There are pictures and the pictures are accurate. I...
Acetic acid has a pKa of 4.76. So the higher the pKa the stronger the acid is, because more of the H ions will be in solution at that pH. It is kind of hard to get your mind around. It is kind of opposite of what you would think.
But the thing is, the total amount of acid put into the must also affects yeast activity. You can get a low pH with a small amount of somethign like malic acid. Which is common in meads (low tartaric acid content as opposed to wines).
Another reason that pH affects yeast activity is that it disrupts their metabolic rate (it is hard to do the chemical reactions if the pH is not just right). But that does not mean that it will not go on at all.
