Determination of the flour acidity

Acidity of a flour - the important indicator of its quality which testifies to its freshness. Acidity of a flour is predetermined by presence of fibers which have sour reaction, presence of free fat acids and different connections of phosphoric acid. In a flour storage time its acidity increases. It is a consequence of splitting of fats by enzymes to free fat acids and glycerine, hydrolysis of fibers to amino acids and to formation of sour phosphates in the course of disintegration phosphatides.

Acidity of a flour express in degrees. For degree of acidity of a flour take quantity of cm³ of 1 n solution of sodium hydroxide necessary for neutralisation of acids and sour salts in 100 g of flour. Acidity of a flour depending on a kind and a grade changes within 3-5 °.

By the standard it is provided determinations titrated acidities of a flour for the (water-flour suspension). The method owing to adsorbtion abilities of starch and fiber to sodium hydroxide yields something the overestimated results. Besides, there are other methods of determination of acidity the torments connected with extraction of soluble substances of hydrogen, ethanol or an aqueous alcohol solution.

More exact results concerning quantity of substances which react with alkali, can be obtained via using a water-alcoho mix for extraction. Substances, soluble in water and spirit are passing to extragent.

Devices, laboratory ware and reactants: technical scales, conic flasks vol. of 100-150 cm³ and 300-500 cm³, the measuing cylinder, a measured flask vol. 250 cm³, a pipette vol. 25 cm3, solution sodium hydroxide 0,1 n, the indicator phenolphtaleine.

Course of determination of acidity of the flour by suspension:

5 g, weighed to within 0,01 g, transfer to a dry conic flask vol. of 100-150 ml and add 50 ml of the distilled water. Flask contents mix to disappearance of lumps of a flour and add three drops of phenolphtalein (for a rye flour - 5 drops). Titrate with 0,1 n solution of sodium hydroxide to occurrence of pink colouring which does not disappear throughout 20-30 s. At the moment of colouring disappearance in specified time add to 3-4 more drop of phenolphtalein. Occurrence of pink colouring testifies to the titration termination.

Acidity of a flour, in acidity degrees, define under such formula:

Where A - quantity 0,1 n a solution of sodium hydroxide used for titration, sm; m - weight charge torments, g; 1/10 - factor of recalculation 0,1n solutions sodium hydroxide on 1,0 n.

Course of determination of the flour acidity in water extract:

Flour charge 25 г, weighed to within 0,01 г, contain in a conic flask vol. of 300-500 cm3, add of 250 cm3 from a measured flask of the distilled water, carefully mix and leave for 2 h. For extraction of soluble substances in water. Then filter in a dry flask and with pipette transfer 25 cm3 of obtained filtrate with pipette into a conic flask vol. of 100 sm, add 3-4 drops of phenolphtalein and titrate with 0,1 n a solution sodium hydroxide.

Acidity of the flour, in acidity degrees, calculates by such formula:

Where: А₁ - quantity 0,1 n a solution of sodium hydroxide, 25 ml of a filtrate used for titration, cm³; and - weight of a flour which answers 25 ml of a filtrate, г;

Where m - weight charge torments, г; A - the quantity of the water taken for extraction, cm³

Example

For titration of 25 ml of the filtrate obtained from charge 25 g of wheat flour in 250 ml of water 1,0 ml 0,1 n a solution sodium hydroxide have gone. Acidity of a flour:

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