Titratable Acidity Titration

Fruit juice contains a mixture of organic acids, not a single one. Rather than report each acid separately, routine titratable acidity work converts the titration result — the volume of alkali needed to reach the endpoint — into a figure as though the entire acid content were the one acid that dominates in that commodity. Because different acids have different molar-equivalent weights, the same titration data converts to a different number depending on which acid is used for the conversion.

A pH meter detects the titration endpoint objectively, by monitoring the sample's pH as alkali is added and stopping at a defined pH value prescribed by the applicable method. It works reliably regardless of the juice's colour or clarity, which is why it is the preferred detection approach in laboratory practice. A colour indicator instead relies on a visible colour change at the endpoint, which is simpler to carry out without a calibrated pH meter but becomes difficult or impossible to read in a deeply coloured juice, such as red grape or many berry juices, or in a turbid sample where the colour change is masked. Because the two approaches do not fall at exactly the same point in every juice, results produced by a colour-indicator endpoint and a pH-meter endpoint are not strictly interchangeable.

Titratable acidity and pH measure genuinely different things. Titratable acidity totals the acid available to be neutralised by an added alkali, including acid held in buffered or undissociated form. pH measures the activity of free hydrogen ions in the juice at the moment of testing — the intensity of acidity as it presents itself right now. Because juices are buffered, two juices at the same pH can carry substantially different titratable acidity, and vice versa. Using a titratable acidity figure to answer a question that is actually governed by pH — most consequentially, the acid barrier that determines microbial safety in a preserved or processed product — is a documented and real source of error, not a theoretical one.

Dissolved carbon dioxide compounds this distinction further: in wine and in fermenting or carbonated juice, dissolved carbon dioxide titrates as though it were acid and inflates a titratable acidity result unless it is removed from the sample before titration begins, an interference that does not affect a pH reading in the same way.

• The number of fruit sampled and how representative they were of the lot set the reliability of the reported figure, exactly as for any destructive laboratory test on a variable population.
• The juice extraction method — how thoroughly and from which tissue juice is expressed — changes the titratable acidity figure obtained, which is why standardised protocols specify sample preparation.
• A titratable acidity figure should only be combined with a soluble solids reading into a sugar-acid ratio when both were taken from juice prepared the same way; mismatched preparation methods propagate directly into a misleading ratio.
• The alkali's concentration must be standardised against a known reference before use; drift in the alkali's actual concentration shifts every titration result taken with it.