SOP: Extraction of Gums from Fruit and Vegetable Products

GIFSQ — Global Institute for Food Safety & Quality

Principle

Proteins are removed from the food extract by selective precipitation with tannic acid (precipitates proteins only) or trichloroacetic acid (precipitates proteins and alginic acid); the gums and related polysaccharides are then precipitated from the protein-free filtrate with alcohol, and the precipitation of the mixture with ammonia followed by acid indicates gums, starches or dextrins.

Apparatus required

• Blender
• Centrifuge
• Filter

Reagents

a. Water.

b. 20% Tannic acid solution.

c. 40% Trichloroacetic acid.

d. Alcohol (for precipitation — 5 volumes).

e. Ammonia solution.

f. Concentrated HCl.

Procedure

Grind solid samples in a blender; weigh 50 g of the sample (to solid or semi-solid samples add an equal quantity of water)

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Filter or centrifuge and decant the supernatant liquid

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To about 10 ml add 20% tannic acid solution, and to another 10 ml add 40% trichloroacetic acid; compare the volume of precipitate formed in the two cases — tannic acid precipitates only proteins, while trichloroacetic acid precipitates proteins and alginic acid

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Depending on the results, treat the rest of the supernatant liquid with either tannic acid or trichloroacetic acid; allow to stand for 15 min and centrifuge for 5 to 10 min

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Test a portion of the supernatant liquid for complete precipitation with tannic acid or trichloroacetic acid; if precipitate forms, add more, recentrifuge and filter

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To the filtrate, add 5 volumes of alcohol with constant stirring and allow the solution to stand for 5 to 10 min

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Add ammonia, drop by drop, until the mixture is alkaline; allow to stand for 5 min

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Add 10 drops of concentrated HCl with vigorous stirring — a precipitate at this time indicates the presence of gums, starches, dextrins, etc.

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Allow the mixture to stand overnight, filter, wash the precipitate thoroughly with alcohol, and dissolve the residue in 30 ml of boiling water — boil, if necessary, to make into solution

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Use this solution for the identification tests

References

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

Jacobs, M.B., The Chemical Analysis of Food and Food Products, D. Van Nostrand Co. Inc., Princeton, New Jersey, 3rd edn., p. 476 (1958); Jacobs, M.B. & L. Jaffee, Ind. Eng. Chem., Anal. Edn., 3, 210 (1931); Mantell, C.L., Water-soluble Gums; Hart, F.L., J. Assoc. Offic. Agric. Chem., 20, 527 (1937) — as cited in the book for the separation of gums from foods.