SOP: Confirmatory Tests for Individual Stabilizing Gums

GIFSQ — Global Institute for Food Safety & Quality

Principle

Each stabilizing gum is confirmed by a specific spot test: gel formation with acid (alginates, deesterified pectins), methylene blue staining of fibres (Irish moss), absence of an alcohol precipitate (methyl cellulose), iodine staining (agar, starch), copper sulphate clotting (carboxymethylcellulose), borax gelation (locust bean gum), ruthenium red swelling (karaya) and picric acid precipitation (gelatin).

Apparatus required

• Test tubes

Reagents

a. 3 N HCl (or other mineral acid).

b. 0.5% Methylene blue indicator in water.

c. 95% alcohol and saturated NaCl.

d. Tincture of iodine.

e. Iodine solution: dissolve 2.5 g of iodine and 5 g of potassium iodide in water and dilute to 100 ml; for test purposes dilute 1 to 5 with water.

f. 1 M Copper sulphate solution: dissolve 249.5 g of CuSO₄·5H₂O in 1 litre of water.

g. 4% Borax (Na₂B₄O₇·10H₂O) solution.

h. Alcohol (for precipitating the gum).

i. Ruthenium red solution: to a few ml of 10% lead acetate solution, add enough ruthenium red to produce a wine-red colour.

j. Saturated picric acid solution.

Procedure

Alginates and deesterified pectins: add 0.2 volume of 3 N HCl (or other mineral acid) to 3 to 4 ml of the sample — a gelatinous precipitate confirms alginates or deesterified pectin.

Irish moss: add 2 to 3 drops of 0.5% methylene blue solution in water to 1 ml of the sample solution — precipitation of purple-stained fibres confirms Irish moss.

Methyl cellulose: mix 5 ml of sample with 25 ml of 95% alcohol and 2 to 3 drops of saturated NaCl — no precipitate confirms methyl cellulose.

Agar: precipitate the gum from 5 ml of sample with alcohol and stain with tincture of iodine — a blue colour is formed (starch is also stained blue by the reagent).

Starch: add 1 to 2 drops of the iodine solution to 1 ml of sample — a blue or purple colour confirms starch (some samples of gum tragacanth may give a faint blue test).

Carboxymethylcellulose: add 2 ml of 1 M copper sulphate solution to 3 to 5 ml of sample solution — an opaque, slightly bluish, clotted precipitate confirms carboxymethylcellulose.

Locust bean gum: add 1 ml of 4% borax to 3 to 5 ml of gum solution — if the mixture gelatinizes, locust bean gum is confirmed (guar gum also forms a gel in similar conditions).

Karaya: precipitate the gum from 5 ml of solution with alcohol and stain with ruthenium red — if the sample swells considerably and is stained pink, karaya is confirmed.

Gelatin: add 2 to 3 drops of gum solution to 2 ml of saturated picric acid — a fine yellow precipitate confirms gelatin.

References

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

Ewart, M.H. & R.A. Chapman, Anal. Chem., 24, 1460 (1952) — as cited in the book for the confirmatory tests.