Determination of starch in cereal grains by acid hydrolysis method

Objective

To determine of starch in cereal grains by acid hydrolysis method

INTRODUCTION

• Cereal grains usually contain as high as 70% of starch. Starch is a chief calorific constituent of food.
• The manufacture of malt liquors, alcohol, distilled liquors and vinegar, so far as these products are made from cereal grains, the quality is dependent on the starch content of the raw material.
• The value of the raw materials used, other nutritional parameters being equal, is proportional to the starch content.

PRINCIPLE

• Upon treatment with acid, the components of starch (amylose and amylopectin) are hydrolyzed progressively via dextrin and maltose to the final product (glucose).
• To effect complete hydrolysis, the sample needs to be defatted and the acid hydrolysis process is carried out by refluxing for at least 2.5 h.

REQUIREMENTS

• Reagents
• Diethyl Ether, AR grade
• Ethyl Alcohol (10 %, v/v).
• Dilute Hydrochloric Acid (2.5 %): Prepared by mixing 20 ml of concentrated
• hydrochloric acid ( sp gr 1.16 ) and 200 ml of water.

• Sodium Carbonate Solution (20 % m/v) .
• Stock Solution of Dextrose –Weigh accurately 10 g of anhydrous dextrose into a one-litre graduated flask and dissolve it in water. Add to this solution 2.5 g of benzoic acid, shake to dissolve benzoic acid and make up the volume to the mark with water. (This solution should not be used after 48 hours.)
• Standard Dextrose Solution – Dilute a known aliquot of the stock solution of dextrose (with water containing 0.25 %, m/v of benzoic acid to such a concentration that more than 15 ml but less than 50 ml of it will be required to reduce all the copper in the Fehling’s solution taken for titration. Note the concentration of anhydrous dextrose in this solution as mg/100 ml (Prepare this solution fresh everyday).
• Methylene Blue Indicator Solution – Dissolve 0.2 g of methylene blue in water and dilute to 100 ml.
• Fehling’s Solution (Soxhlet modification) – Prepare by mixing immediately before use, equal volume of solution A and solution B.
• Solution A – Dissolve 34.639 g of copper sulphate (CuSO4 .5H2O) in water, add 0.5 ml of concentrated sulphuric acid of sp gr 1.84 and dilute to 500 ml in a graduated flask.
• Filter the solution through prepared asbestos.
• Solution B – Dissolve 173 g of potassium sodium tartrate (K-Na-C4 H4 O6.4H2O) and 50 g of sodium hydroxide in distilled water, dilute to 500 ml in a graduated flask and allow the solution to stand for 2 days. Filter this solution through prepared asbestos (washed).
• Carrez-I (Zinc Acetate Solution) : Dissolve 21.9 g of zinc acetate [Zn (C2H3O2)2.2H2O] and 3 ml of glacial acetic acid in water. Dilute to 100 ml.
• Carrez-II (Potassium Ferrocyanide Solution) : Dissolve 10.6 g potassium ferrocyanide in distilled water and make the volume to 100 ml.

PROCEDURE

• Standardization of Fehling’s Solution
• Pour the standard dextrose solution into a 50 ml burette.
• Find the titre i.e., the volume of the standard dextrose solution required to reduce all the copper in 10 ml of Fehling’s solution corresponding to the concentration of the standard dextrose solution.
• Pipette 10 ml of Fehling’s solution into a 300 ml conical flask and add from the burette, standard dextrose solution required for reduction of all the copper, so that not more than 1 ml will be required later to complete the titration.

• Heat the flask containing the mixture over wire gauze. Gently boil the contents of the flask for 2 minutes.
• At the end of 2 minutes of boiling, add 1 ml of methylene blue indicator solution without interrupting boiling.
• While the contents of the flask continue to boil, begin to add standard dextrose solution (one or two drops at a time) from the burette till the blue colour of the indicator just disappears.
• Note: The titration should be completed within 1 minute, so that the contents of the flask boil altogether for 3 minutes without interruption.
• Note the titre.
• Multiply the titre (obtained by direct titration) by the number of milligrams of anhydrous dextrose in 1 ml of the standard dextrose solution to obtain the dextrose factor.

Preparation of the Solution

• Weigh 5 g of ground material, add 220 ml of 2.5 % dilute HCl.
• Attach to reflux condenser and reflux for 2 ½ hours.
•  Transfer the contents to 500 ml volumetric flask.
• Add 5 ml Carrez-I and 5 ml of Carrez-II. Make up the volume to 500 ml and filter using Whatman No. 40 filter paper.
• Discard first few drops. Take 50 ml of the filtrate in 250 ml volumetric flask, add about 150 ml water and neutralize. Make up the volume to 250 ml.
• Complete titration in the same manner as for the standard.

CALCULATION

RESULTS AND INFERENCE

• The mean of the results of two determinations should be reported.
• The difference between the results of two determinations carried out simultaneously or in rapid succession by the same analyst (repeatability) shall not exceed 0.5 % by mass.
• Cereal grains intact with bran have lower starch content as compared to refined and processed grains

PRECAUTIONS

• The sample should be ground to powder so as to pass through 40 mesh size (<0.5 mm) before analysis to aid in complete hydrolysis.
• During hydrolysis, avoid frothing by regulating the heat intensity so that contents just keep simmering.
• Ensure neutralization of acid with alkali before titration.
• Titration should be performed in such way that it completes within 1 min. thereby minimizing the oxidation due to air.