نماذج اختبارات

نموذج اختبار مادة تغذية
DETERMINATION OF TANNINS IN TEA

Tannins are complex phenolic compounds that precipitate proteins and responsible for the sensation of astringency. Many food products contain tannins in their consumable forms e.g.:

• Tea, Cocoa.

• Unripe fruits (apples, cherries, strawberries, bananas)

• Walnuts

Astringency is the contracting or drying taste, which results from coagulation of the proteins of saliva and the mucous epithelium of the mouth, causing a reduced lubricant action.

Tannins are water-soluble so they are extracted from tea by boiling with

water.

Principle:

Ferric chloride reagent gives a color with tannins under acidic conditions.

The color is measured spectrophotometrically and compared with the color

obtained with a standard tannins solution

Procedure:

1- Weigh accurately 0.5g of tea.

2- Add 75ml of water and boil for 30 min.

3- Filter in a 100ml-measuring flask and complete to volume with water.

4- Take 1ml of solution; add 1ml of ferric chloride reagent and 8ml of water.

5- The standard is prepared by adding in a tube 1ml of standard, 1ml of

reagent and 8ml of water.(conc. of std. =1mg/ml)

6- Read the absorbance of unknown and standard against blank at 540 nm.

7- Calculate the concentration of unknown using the following formula and considering the dilution you made.

Conc. of unknown = Abs. of unknown /Abs. of standard x Conc. of standard

8- Give full report on your work.

Best wishes