SOP: Detection and Determination of Malic Acid in Vinegar

GIFSQ — Global Institute for Food Safety & Quality

Principle

Cider vinegar is distinguished by its malic acid content. A quick lead acetate test shows whether the sample is a cider vinegar at all; the malic acid is then separated as calcium malate, the precipitate is ignited, and its ash is boiled with a measured excess of standard acid whose remainder is back-titrated — the acid taken up is equivalent to the malic acid.

Apparatus required

• Filters
• Ignition vessel

Reagents

a. Lead acetate solution (for the qualitative test).

b. 10% calcium chloride solution.

c. Ammonia (for making the solution alkaline).

d. Alcohol; 75% alcohol for washing the precipitate.

e. 0.1 N HCl.

f. 0.1 N NaOH.

g. Phenolphthalein indicator (for the back-titration).

Procedure

Qualitative lead acetate test: add 1 ml of lead acetate solution to a few ml of the sample; if no precipitate settles within a few minutes and the supernatant stays clear, the sample is not cider vinegar

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Calcium precipitation: to 5 ml of the vinegar add 1 ml of the 10% calcium chloride solution and make the mixture alkaline with ammonia; filter off the precipitate that forms

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To the filtrate add 3 volumes of alcohol and heat just to boiling — a flocculent precipitate settling out shows malic acid is present

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Quantitative determination: to 100 ml of the sample add 10 ml of the calcium chloride solution, make alkaline with ammonia, let the solution stand for 1 hr, and filter

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Evaporate the filtrate down to 25 ml, add 75 ml of alcohol, heat to boiling, and filter

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Wash the calcium malate precipitate with 75% alcohol, dry it, and ignite it

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Boil the ash with a measured excess of 0.1 N HCl and back-titrate the excess with 0.1 N NaOH, using phenolphthalein as indicator

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Run a blank titration with the same volume of HCl as used for the sample

Calculation

Malic acid % = [(Blank titre − Sample titre) × Normality of NaOH × 67 × 100] ÷ (ml of vinegar solution taken × 1000)

Where,

• Blank titre = ml of 0.1 N NaOH used in the blank titration
• Sample titre = ml of 0.1 N NaOH used in the sample titration
• Normality of NaOH = normality of the standard NaOH solution
• 67 = equivalent weight of malic acid
• ml of vinegar solution taken = volume of vinegar taken for the determination (ml)

Precautions

• Give the ammoniacal solution the full 1-hr standing time before filtering, so the calcium malate separates completely.
• Wash the calcium malate precipitate with 75% alcohol, not water.

References

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