SOP: Quantitative Estimation of Starch by Diastase Method

GIFSQ — Global Institute for Food Safety & Quality

Principle

Spices like chilli, allspice and clove contain little or no true starch, for which acid hydrolysis gives unduly high results; the starch is therefore converted to sugars by digestion with malt extract (diastase) at 55 °C, and the resulting glucose is hydrolysed with acid and estimated by the Lane and Eynon method, with a blank correction for the malt extract.

Apparatus required

• Beaker
• Reflux condenser
• 250 ml and 500 ml volumetric flasks

Reagents

a. Ethyl ether.

b. 10% alcohol (500 ml for washing).

c. Malt extract: use good quality new barley malt having good diastatic activity; make into powder as and when required — for every 80 ml of malt extract required, add 100 ml of water to 5 g of powdered malt and store at room temperature for 2 hr with occasional shaking (if stirring is done with an electric stirrer, 20 min is sufficient); filter and refilter the first portions to get a clear infusion.

d. Concentrated HCl: 20 ml for hydrolysis.

e. 10% NaOH solution and Na₂CO₃ solution (for neutralization).

Procedure

Extract 4 g of the sample with ether, and then with 500 ml of 10% alcohol as in the acid hydrolysis method

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Before proceeding to the estimation of starch, ensure the residue is free of sugars by testing a few drops of the washing with α-naphthol and sulphuric acid (see page 27 of the book)

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Transfer the residue along with the filter paper to a beaker with 50 ml of water

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Immerse the beaker in boiling water and heat for 15 min with continuous stirring to gelatinize the starch

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Cool to 55 °C, add 20 ml of malt extract and incubate for 1 hr at 55 °C

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Heat again to boiling for a few minutes, cool to 55 °C, again add 20 ml of malt extract and incubate for 1 hr

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Repeat the procedure, if need be, until the residue treated with iodine solution shows no blue tinge when examined microscopically

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Cool the contents, make to 250 ml with water, and filter

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To 200 ml of the filtrate in a flask, add 20 ml of concentrated HCl, connect with a reflux condenser, and heat in a boiling water bath for 2.5 hr

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Cool, nearly neutralize with 10% NaOH solution, and exactly with Na₂CO₃ solution

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Transfer the solution to a 500 ml volumetric flask, make to volume, filter, and determine the glucose in an aliquot by the Lane and Eynon method (see Chapter 1)

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Carry out a blank determination using the same volume of malt extract as used with the sample

Calculation

% Starch = (% Glucose by weight in sample − % Glucose by weight in the blank) × 0.90

Where,

• % Glucose by weight in sample = glucose determined by the Lane and Eynon method in the sample aliquot
• % Glucose by weight in the blank = glucose determined in the blank determination

References

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

Official Methods of Analysis, Association of Official Analytical Chemists, Washington DC, 12th edn., 1975, p. 580.