SOP: Determination of Acetyl Value of Pectin

GIFSQ — Global Institute for Food Safety & Quality

Principle

Sugar-beet pectin carries an acetyl group, and other pectins may carry it too. When the acetyl group is present in pectin, it inhibits jelly formation. Analysis of this functional group by simple alkaline saponification followed by back titration does not give satisfactory results, so the modified Clark’s method for pectic substances is used: the pectin is saponified with alkali, the liberated acetic acid is steam distilled from a magnesium sulphate–sulphuric acid medium, and the distillate is titrated with standard sodium hydroxide.

Apparatus required

• 250-ml conical flask
• Steam distillation apparatus

Reagents

a. 0.05 N and 0.1 N standard sodium hydroxide solutions.

b. Magnesium sulphate–sulphuric acid solution: mix 100 g of magnesium sulphate crystals and 1.5 g of H₂SO₄, and dilute to 180 ml.

c. Phenol red indicator.

Procedure

Weigh 0.5 g of pectin into a 250-ml conical flask and add 25 ml of 0.1 N NaOH

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Stopper the flask and stir the contents until the pectin is dissolved

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Set aside for at least 1 hr or preferably overnight

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Dilute the contents to 50 ml with water

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Pipette 20 ml into the distillation apparatus and add 20 ml of the magnesium sulphate–sulphuric acid solution

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Steam distil and collect about 100 ml of distillate, keeping the volume in the distillation flask low

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Titrate the acetic acid with 0.05 N NaOH to a phenol red end point

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Carry out a blank distillation using 20 ml of water and 20 ml of the magnesium sulphate–sulphuric acid solution, and titrate the distillate in the same way; the titre should be less than 0.1 ml of the standard alkali

Calculation

% Acetyl = (Normality of NaOH × ml of NaOH × 4.3) ÷ (Wt of sample in aliquot (g))

Where,

• Normality of NaOH = normality of the standard sodium hydroxide used for the titration
• ml of NaOH = volume of standard sodium hydroxide consumed in the titration (ml)
• Wt of sample in aliquot (g) = weight of pectin present in the 20-ml aliquot taken for distillation (g)

Note: the book gives a blank distillation (titre less than 0.1 ml of standard alkali) in the procedure, but the printed formula carries no blank-correction term — the formula is reproduced here exactly as printed.

Precautions

• Stopper the flask and stir until the pectin is completely dissolved before setting it aside.
• Keep the volume in the distillation flask low while collecting the distillate.
• The blank-distillation titre should be less than 0.1 ml of the standard alkali.

References

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

Owens, H.S. et al., ibid. — cited in the book.

Pippen, E.L., R.M. McCready & H.S. Owens, Anal. Chem., 22, 1457 (1950) — cited in the book.

McComb, E.A. & R.M. McCready, Anal. Chem., 29, 819 (1957) — cited in the book.