SOP: Quantitative Estimation of Starch by Direct Acid Hydrolysis

GIFSQ — Global Institute for Food Safety & Quality

Principle

Alcohol-insoluble carbohydrates are hydrolysed with hydrochloric acid to reducing sugars, which are estimated by the Lane and Eynon volumetric method and calculated as crude starch. Dark digests that interfere with the Fehling titration are clarified with neutral lead acetate before titration.

Apparatus required

• Funnel with Whatman No. 2 filter paper
• 500 ml conical flask
• Reflux condenser
• 500 ml volumetric flask

Reagents

a. Ethyl ether.

b. 10% alcohol and absolute ethyl alcohol (for washing).

c. Concentrated HCl: 20 ml for hydrolysis.

d. 2.5 N NaOH (for neutralization).

e. Saturated neutral lead acetate solution: 30 ml for clarifying dark digests.

f. Solid potassium oxalate (for removing excess lead).

Procedure

Weigh 4 g of the prepared sample and transfer to a funnel containing Whatman No. 2 filter paper

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Extract the sample with 5 successive 10 ml portions of ethyl ether

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Allow the ether to evaporate from the residue, wash with 150 ml of 10% alcohol, and then with 15–20 ml of absolute ethyl alcohol

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Transfer the insoluble residue to a 500 ml conical flask using water; add distilled water to make the total volume 200 ml, and then add 20 ml of concentrated HCl

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Connect the flask to a reflux condenser and reflux for 2.5 hr

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Cool, and add 2.5 N NaOH until the solution is almost neutral to indicator paper (pH 6–7) — the solution must not be alkaline at any time

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Make up the solution to 500 ml in a volumetric flask and determine the reducing sugars by the Lane and Eynon volumetric method as given in Chapter 1

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With some spices (e.g., black pepper), the resulting solution after HCl digestion would be very dark in colour, which interferes with the Fehling titration — to overcome this, transfer the neutralized digest after cooling to a 500 ml volumetric flask, add 30 ml of saturated neutral lead acetate solution, make to volume and filter; remove the excess lead from the filtrate using solid potassium oxalate; refilter through a dry filter paper into a dry conical flask and use the clear filtrate for titration with Fehling solution

Calculation

% Starch = % Reducing sugars × 0.90

Where,

• % Reducing sugars = reducing sugars determined by the Lane and Eynon volumetric method

Precautions

• The neutralized solution must not be alkaline at any time before the titration.

References

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