Аналіз одержаних результатів
Студент відповідно до завдань повинен дати порівняльну характеристику методам визначення кислотності у сировині та готової продукції.
Запитання для самоперевірки
1. Дайте визначення реакції середовища
2. Дайте визначення загальної або титрованої і активної кислотності та лужності.
3. Які існують індикатори? Як їх використовують під час визначення лужності і кислотності продуктів?
4. Як працює рН-метр?
5. Які методи визначення рН ви знаєте?
Laboratory work 2
Starch, yeast and flour acidity determination. Titrated acidity.
Determination of starch acidity
Starch acidity is caused by presence at it of phosphates and the sulfuric acid used during reception of starch from corn and during bleaching of potato starch. In case of starch storage in adverse conditions its acidity increases.
Starch acidity express in degrees. One degree of starch acidity answers 1 sm 3 0,1 n a solution of sodium hydroxide spent for neutralisation of acids in 100 g of solid of starch. Norms of sizes of starch acidity for its different kinds and grades:
The corn:
The higher 20,0
The first 25,0
Amylopectinous 23,0
The potato:
"Extra" 6,0
The higher 10,0
The first 14,0
The second 20,0
Determination course:
In a conic flask vol. of 250-300 ml place 20 g the starch weighed to within 0,01 г, add of 100 ml from the measured cylinder of the distilled water and add 5-8капель of phenolphtalein. Titrate the suspension with 0,1 n solution of sodium hydroxide from metric burette before occurrence of pink colouring. In connection with adsorption ability of starch to phenolphtalein, before the titration termination add 5-6 more drops of phenolphtalein. In parallel in the same conditions titrate with 100 sm of the distilled water.
Starch acidity, in degrees, define under such formula:
Where V1 - quantity 0,1 n solution of sodium hydroxide used for titration of test, cm3; V2 - quantity 0,1 n solution of sodium hydroxide used for titration of 100 cm3 of distilled water, cm3; m - weight of starch charge, g; B - a water mass fraction in starch, %.
Example
3,4 cm3 of potato starch used for titration in weight 20 g 0,1 n a solution sodium hydroxide and 0,2 cm3 are used for titration of the distilled water of 100 cm3 0,1 n a solution sodium hydroxide.
Moisture contents in starch - 20 %
Starch acidity:
The structure of yeast includes fat acids, basically - palmitinic and stearinic acids. In a storage time of yeast, owing to ability to live of microorganisms produces acids, therefore their acidity increases. Acidity express in milligrammes of acetic acid on 100 g yeast.
Devices, laboratory ware and reactants: technical scales, a porcelain cup, metric burette, 0,1 n solution sodium hydroxide and the indicator phenolphtalein .
Determination course
From average test take 10 g yeast and weigh in a porcelain cup to within 0,01 г, add 50 ml of the distilled water, carefully mix for homogenity weight and titrate with 0,1 N sodium hydroxide solution at presence of phenolphtalein till the pink coloring occurrence.
Acidity of yeast (mg/g) calculate by such formula:
Where A - quantity 0,1 n a solution of sodium hydroxide used for titration, cm3; 6 - quantity of acetic acid which answers 1 cm3 0,1 n a solution sodium hydroxide, mg; m - weight of yeast charge taken,
Example
3 cm3 of 0,1 n solution of sodium hydroxide is used for titration of 10 g of yeast. Acidity of yeast in mg of acetic acid on 100 g of yeast:
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:
Questions for self-examination
1. Make determination of reaction of the environment of products of food manufactures.
2. Make determination of the general or titrated and active acidity and alcality.
3. What indicators do you know? How them use during determination alcality and acidities of products?
4. How the ionometer ЕВ-74 works?
5. How to define the starch acidity?
6. How acidity of yeast is defined?
7. How acidity of the flour is defined?
8. Prove the necessity of determination pH products of food manufactures.
Laboratory work 3