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pH versus titratable acidity

Why do cider makers measure both pH and acidity?

In short

Because the two answer different questions. Titratable acidity counts how much acid is in the juice, by neutralising it with alkali and measuring how much was needed. pH measures the concentration of free hydrogen ions, which is what determines chemical and microbial behaviour.

Perceived sourness follows titratable acidity fairly closely. Microbial risk, sulphite effectiveness and protein stability follow pH. A juice can be high in one and unremarkable in the other.

The reason they diverge is buffering. Apple juice contains salts, chiefly of potassium, that resist changes in pH, so two juices with the same acid content can sit at noticeably different pH values.

Same pH, twice the acidTwo juices reading pH 3.6 on the same meter, one carrying twice the titratable acidity of the other. Buffering is the reason, and it is why a cider maker measures both.Juice A4 of 8 acid units4.0 g/LJuice B8 of 8 acid units8.0 g/LBoth read pH 3.6 on the meterBuffering is whyMalic acid is weak, so most of it sitsundissociated; its potassium salts absorbadded hydrogen ions. The titration findsevery acid group. The meter finds only free ones.
Two juices reading pH 3.6 on the same meter, one carrying twice the titratable acidity of the other. Buffering is the reason, and it is why a cider maker measures both.
Described in full
Shape
Two panels side by side, each representing a juice. Each panel contains eight small squares in two rows, standing for units of acid: the left panel has four filled and four drawn in dashed outline, the right has all eight filled. Beneath each is its titratable acidity — 4.0 and 8.0 grams per litre. A single wide band below both, with arrows leading into it from each panel, states that both read pH 3.6.
What the squares count
Acid groups available to a titration. The titration finds every one of them, dissociated or not, because the alkali drives the equilibrium until all of them have reacted. That is what titratable acidity measures: how much acid is present.
What the meter finds
Only the hydrogen ions actually free in solution at that moment. Malic acid is a weak acid, so most of it sits undissociated at cider pH, and the free fraction is a small part of the total.
Buffering
The juice also contains the potassium salts of its own acids, and those salts absorb added hydrogen ions. A juice rich in potassium therefore holds a higher pH than its acid content alone would predict, which is how two juices with very different acid loads arrive at the same meter reading.
Why it matters
Perceived sourness follows the amount of acid; microbial risk and sulphite effectiveness follow pH. A maker who measures only one of the two is blind to whichever question the other answers, and the two questions have different answers on the same juice.

What each measurement is

Titratable acidity is determined by adding a standard alkali solution to a measured volume of juice until it reaches a defined end point, and calculating from the volume used. The result is conventionally expressed as grams per litre of the dominant acid — for cider, as malic acid. It is a count of acid groups available for neutralisation, whether or not they have dissociated.

pH is measured with an electrode and reports the negative logarithm of the hydrogen ion activity. Because it is logarithmic, a change of one pH unit is a tenfold change in hydrogen ion concentration, and small-looking differences are large.

The relationship between them is mediated by two things. The first is that malic acid is a weak acid, so only a fraction of it is dissociated at any moment. The second is buffering: the juice contains the salts of its acids, and those salts absorb added hydrogen ions, holding pH steadier than the acid content alone would suggest.

What each one governs

Which properties follow titratable acidity and which follow pH.
PropertyFollowsWhy
Perceived sournessTitratable acidity, mostlyAcid is buffered in the mouth by saliva, so total available acid matters more than the free hydrogen ions in the glass
Risk of bacterial spoilagepHLactic and acetic bacteria are inhibited progressively as pH falls; below about pH 3.3 most struggle
Sulphur dioxide effectivenesspH, very stronglyThe active molecular form is a pH-dependent fraction of the free sulphite, and that fraction falls sharply as pH rises
Colour and browning ratepHPhenolic oxidation and the colour of the products are pH-dependent
Protein and colloidal stabilitypHProtein charge, and therefore aggregation behaviour, depends on pH
Fermentation vigour at the extremespHVery low pH stresses yeast, particularly in combination with high alcohol

Why buffering makes the difference

Buffering capacity is the juice’s resistance to a change in pH when acid or alkali is added, and in apple juice it comes largely from potassium salts of the organic acids. Fruit from a well-fertilised or high-potassium site carries more, and its juice sits at a higher pH for the same titratable acidity.

This has a practical consequence that catches people out. Adding malic acid to a heavily buffered juice raises titratable acidity as expected and moves pH much less than expected. A cidermaker aiming to bring pH into a safer range may need considerably more acid than a calculation from acid content alone would suggest, and the addition will also make the cider taste sharper than intended.

It also means that a target range for one measure does not translate into a target range for the other. Working practice quotes both — a pH low enough to inhibit spoilage and give sulphite something to work with, and a titratable acidity high enough that the cider tastes fresh and low enough that it does not taste aggressive — and the two are managed separately, most often by blending sharp fruit into low-acid juice, which moves both at once.

How fermentation and malolactic conversion move them

Fermentation itself changes acidity modestly. Yeast produces succinic and other acids, and precipitation of some salts during fermentation removes a little; the net movement is usually small and can go either way.

Malolactic conversion moves both substantially and in the same direction. Losing a carboxyl group from each malic acid molecule reduces titratable acidity, and the pH rises correspondingly. That pH rise is the reason malolactic conversion is a microbiological decision as well as a sensory one: a cider that finishes malolactic at a higher pH is less protected against everything else.

This is the practical argument for building acid into the blend at the juice stage. A blend that includes sharp fruit starts at a lower pH, ferments more safely, retains protection through any malolactic conversion, and needs less sulphite to achieve the same effect.

Why the same number is quoted differentlyTitratable acidity is expressed as malic acid in most cider literature, as sulphuric acid in some French practice, and as tartaric acid in wine convention. The same juice therefore carries three different numbers. Any figure is meaningless without knowing which convention it uses.

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