SOP: Determination of Titratable Acidity

GIFSQ — Global Institute for Food Safety & Quality

Principle

The free organic acids in the juice are neutralized by titration with standard sodium hydroxide. For routine work the end point is taken to a faint pink colour with phenolphthalein, or electrometrically to pH 8.1 with a glass electrode for dark-coloured juices of low acidity that cannot be diluted. The result is expressed in terms of the predominant acid of the juice.

Apparatus required

• Burette
• Conical flasks or beakers
• Glass-electrode pH meter or electrometric titrimeter (for dark-coloured juices)

Reagents

a. Standard 0.1 N sodium hydroxide solution.

b. Phenolphthalein indicator solution.

Sample preparation

For carbonated beverages: expel the carbon dioxide by boiling, cool, and then titrate.

Procedure

Take a measured volume of the juice (or a known weight, diluted as needed)

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Titrate with standard 0.1 N NaOH, swirling constantly, to a faint pink colour with phenolphthalein — or, for dark-coloured juices of low acidity, titrate electrometrically to pH 8.1 with the glass electrode

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Note the titre

Calculation

Acidity, % (w/v or w/w) = (Titre in ml × Normality of NaOH × Equivalent weight of the acid × 100) / (Weight or volume of sample × 1000)

Where,

• the acidity is expressed in terms of the predominant acid present in the juice
• the equivalent weights of the common organic acids in juices are: anhydrous citric acid = 64; citric acid monohydrate = 70; tartaric acid = 75; malic acid = 67; lactic acid = 90

Precautions

• For carbonated beverages always expel the CO₂ first — dissolved gas consumes alkali and inflates the result.
• For dark-coloured juices of low acidity, dilution masks the colour but also dilutes the acid; use the electrometric end point instead.
• The book directs the reader to the acidity expression on its page 10 (Chapter 1); the calculation above follows the standard form used in the GIFSQ fruit-products acidity SOPs.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.