Method 712a Determination of starch in potatoes by hydrolysis and autoanalysis

Reagents (extraction).

• Benzoic acid, saturated solution

Procedure (extraction). Weigh 0.100 g of the dried (oven dried at 90°C, or freeze-dried) and milled (to 1 mm) sample into a 50-ml beaker; at the same time, weigh a sample for moisture determination at 110°C. A standard potato starch (Merck) sample should be included as a control. Add 10 ml of 10% ethanol and swirl to mix; allow to stand for 30 min to dissolve any dextrins, sugars and tannins, which could amount to about 12% by weight in DM. Centrifuge at 1500 g for 5 min, then wash the residue into a McCartney bottle, previously graduated at 15 ml, with 1 M HCl, and make up to the mark. Tightly screw on the cap, which has been lined with thick PTFE tape to prevent corrosion. Heat in an oven at 106°C for 40 min to hydrolyse the starch to glucose. Cool, and wash the contents with water into a 150-ml beaker containing 50 ml water. Adjust the pH to 3.0 ±0.2 dropwise with 1 M NaOH, transfer with beaker washings to a 100-ml volumetric flask, make up to the mark and mix. Allow any residue to settle, then pipette 10 ml into a 50-ml volumetric flask, and make up to the mark with saturated benzoic acid solution and mix. Analyse the sample by autoanalysis using the method given for water soluble carbohydrates (Method 7.14 below).

Calculation. The percentage starch in sample DM is given by the equation:

where F = fructose concentration in sample solution (pg ml-1); W = sample moisture content, and 0.9 is the correction factor to compensate for the conversion of C6H12O6 back to the C6H10O5 residue units in starch. A further factor of x 0.98 corrects for hydrolysis of potato cell walls. If required, the cell walls from the neutral detergent procedure may be used to find the correction factor for other plant species.

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