pH (= - log[H+]) takes only H+ ions into account, it is weakly influenced by weak acids (tartaric, malic, citric, etc.). Total acidity takes all acids into account: dissociated acids (strong acids) as well as weakly dissociated acids. Therefore acidity is a priori different from pH [Moorhead in V4]. Strong acids are 10 times as acidic as weak acid but they make only 1 % of acids so they do not contribute much to the total acidity. Roughly, pH measures the concentration of H+ and it is important to chemical reactions and micro-organisms (yeasts and bacteria) and total acidity measures weak acids and matters for taste.
So the word acidity will be used to talk about the sourness in taste. All acids do not modify pH and acidity the same way. If their acidities are the same, tartaric acid has a lower pH than malic [V9].
After V9, the pH increases and acidity slightly decreases during fermentation. This is also what figure 4 seems to show. H4 says that the pH goes down.
What are the effects of acidity and of pH?
Should one add acids?
The pH of honey is naturally low (3.9 on average). At the end of the fermentation one would like the pH to be low for the stability of mead while aging and some acidity for a well-balanced taste. But during the fermentation, the pH should not be low to the point where yeast activity could be reduced (a pH between 3.7 and 4 seems adequate). Furthermore, citric acid added at the beginning of fermentation will be metabolized by yeasts (this can also be the case of malic) and tartaric acid may create instabilities against coldness. One may therefore prefer to add acid after the fermentation.
If fruits are also added, they may bring in enough acid and it may not be necessary to add any more. If this happens to be needed, should one choose the acid linked to the fruit or to the honey (adding citric acid to orange blossom honey for instance)?