SOP: Determination of Methoxyl Content of Pectin

GIFSQ — Global Institute for Food Safety & Quality

Principle

The methoxyl content, which expresses the degree of esterification, is found by saponifying the pectin and titrating the liberated carboxyl groups. It is an important factor in controlling the setting time of pectins, their sensitivity to polyvalent cations, and their usefulness in the preparation of low solid gels, films and fibres.

Apparatus required

• Stoppered flask

Reagents

a. 0.25 N and 0.1 N standard sodium hydroxide.

b. 0.25 N standard hydrochloric acid.

Procedure

To the neutral solution titrated for equivalent weight, containing 0.5 g of pectic substance, add 25 ml of 0.25 N sodium hydroxide

↓

Shake thoroughly; allow to stand for 30 min at room temperature in a stoppered flask

↓

Add 25 ml of 0.25 N HCl (or an amount equivalent to the base added)

↓

Titrate with 0.1 N NaOH to the same end point as before

Calculation

Methoxyl content % = (ml of Alkali × Normality of alkali × 3.1) / Wt of sample

Where,

• ml of Alkali = volume of 0.1 N NaOH used in the titration (ml)
• Normality of alkali = normality of the sodium hydroxide solution used
• Wt of sample = weight of the pectic substance taken (g)

Precautions

• Allow the saponification to stand for the full 30 min at room temperature in a stoppered flask.
• Titrate to the same end point as in the equivalent-weight titration.

References

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

Owens, H.S., R.M. McCready, A.D. Shepherd, S.H. Schultz, E.L. Pippen, H.A. Swenson, J.C. Miers, R.F. Erlandsen & W.D. Maclay, Methods used at Western Regional Research Laboratory for Extraction and Analysis of Pectic Materials, AYC-340, Western Regional Research Laboratory, Albany, California (1952) — cited in the book.

Gee, M., E.A. McComb & R.M. McCready, Food Res., 23, 72 (1958) — cited in the book.