Reading Titratable Acidity Through a Make
pH meters get the attention, but the older measurement — titratable acidity — is the one many cheese recipes are actually written around, and it tells you something a pH reading does not.
Two measurements, two questions
pH measures the concentration of free hydrogen ions right now: how acidic the milk is at this instant.
Titratable acidity measures how much base it takes to neutralise all the acid present, bound and free. It answers how much acid has been produced.
Milk is buffered, so those diverge. A make can produce a substantial amount of lactic acid while the pH barely moves, because the milk's proteins and phosphates are mopping it up. TA sees that; pH does not, until the buffering capacity is used up and the pH then falls quickly.
Which is why recipes written for TA are tracking the culture's actual work rather than the current state.
The measurement and the arithmetic
The classic dairy titration takes a 9 mL milk sample, adds phenolphthalein indicator, and adds 0.1 N sodium hydroxide drop by drop until a faint pink persists. The volume of NaOH used is the titre, and:
%TA (as lactic acid) = (titrant mL × normality × 9) ÷ sample mL
With the standard 0.1 N and 9 mL sample that reduces neatly to %TA = titrant mL ÷ 10. The acidity calculator handles non-standard normalities and sample sizes too, and also reports degrees Dornic, an older scale still printed on some cultured-dairy labels where 1°D = 0.01% lactic acid.
| Titrant used | % lactic acid | Degrees Dornic |
|---|---|---|
| 1.4 mL | 0.14% | 14°D |
| 1.6 mL | 0.16% | 16°D |
| 1.8 mL | 0.18% | 18°D |
| 2.0 mL | 0.20% | 20°D |
| 2.5 mL | 0.25% | 25°D |
| 6.0 mL | 0.60% | 60°D |
Fresh cow milk typically sits around 0.14–0.16%. A reading meaningfully above that in milk you believed was fresh means it has already been fermenting — which is information worth having before you commit it to a make.
Where it fits in a make
The useful checkpoints are the same ones the recipe uses:
- Fresh milk, before anything is added — your baseline, and a check on the milk's condition.
- After ripening, before rennet — confirms the culture is alive and working. A reading that has not moved is the single most useful early warning there is.
- At renneting — the acidity at set influences how the curd behaves.
- At draining — usually the most important target in the recipe, because it largely determines the finished texture.
The difference between readings matters more than any single value. A rise of a few hundredths of a percent over a ripening period tells you the culture is working at roughly the expected rate; no rise tells you something is wrong while you can still do something about it.
The culture check that saves a batch
The most valuable single use of TA for a home maker is confirming, twenty minutes into ripening, that the culture is active.
Freeze-dried cultures lose viability with age and with moisture ingress, and a packet that has been opened and re-sealed several times may deliver a fraction of its stated activity. There is no way to tell by looking. A baseline reading and a second one after ripening costs a few minutes and catches a dead culture before you have added rennet to twenty litres of milk.
Acidity is doing safety work too
Falling pH is not only about texture and flavour. It is one of the hurdles — alongside salt and reducing moisture — that makes a cheese progressively inhospitable to organisms you did not culture on purpose.
So a make that fails to acidify is not merely disappointing. It has lost one of its principal defences, and that is the reason to treat a flat TA reading as a stop-and-reassess signal rather than a "carry on and hope" one. Under-acidified cheese is the case where discarding a batch is the right call.
Practical cautions with the titration
- Sodium hydroxide is caustic. Even at 0.1 N it will irritate skin and is genuinely dangerous to eyes. Eye protection, no eating or drinking near the bench, and store it away from where food is prepared and out of reach of children.
- The endpoint is a faint persistent pink, not a strong one. Overshooting is the most common technique error and reads as higher acidity than is really there.
- Standardised NaOH degrades over time, particularly if the bottle is left open, absorbing carbon dioxide from the air. An old solution reads low.
- Sample size must be accurate. The formula divides by it, so a sloppy 9 mL is a proportional error in every result.
Do you need it at home?
Honestly: not for a first cheese. A recipe followed carefully with fresh culture and decent milk will work without any titration at all, and a calibrated pH meter is easier day-to-day if you only want one instrument.
TA earns its place when you are trying to make the same cheese repeatably, or diagnosing why a make behaved differently from last time. It converts "the curd seemed slower today" into a number you can compare with last month's, and that is the point at which the technique stops being laboratory theatre and starts being useful.