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Food Tests

Food Tests. Indicators are substances that detect the presence , absence or concentration of certain chemicals (food substances). It may show degree of reaction between two or more substances by means of a characteristic change , especially in color . USE OF INDICATORS.

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Food Tests

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  1. Food Tests

  2. Indicators are substances that detect the presence, absence or concentration of certain chemicals (food substances). It may show degree of reaction between two or more substances by means of a characteristic change, especially in color.

  3. USE OF INDICATORS

  4. Summary Table

  5. Summary: • What are indicators and how do they show a chemical reaction?

  6. Chemical Digestion Food Tests • Test for Starch • Iodine (2 – 3 drops) • Negative – Iodine remains brown/orange • Positive – Iodine changes orange to black

  7. Chemical Digestion Food Tests • Test for Glucose • Benedicts solution (about 2cm3) • HEAT • Negative – stays blue • Positive – from blue to orange (brick red)

  8. Chemical Digestion Food Tests • Test for Protein • Biuret’s solution – 2cm3 • Negative – Biuret’s stays blue • Positive – Biuret’s changes to lavender

  9. Starch Test Amylose in starch is responsible for the formation of a deep blue color in the presence of iodine. The iodine molecule slips inside of the amylose coil. Starch amylopectin does not give the color, nor does cellulose, nor do disaccharides such as sucrose in sugar.

  10. The Biuret Reagent Test The principle underlying the test can be demonstrated with the chemical compound biuret which, just as proteins, is able to complexcopper (II) ions. It detects the peptide bond between the urea molecules or between amino acids. The blue reagent turns violet in the presence of proteins, and changes to pink when combined with short-chain polypeptides. Not all biuret tests actually require the Biuret reagent. Rather, the term "biuret test" is a generic term for the testing of proteins by using copper (II) sulfate solution in an alkaline environment. Thus, the process of testing for proteins in a solution by first adding a small amount of sodium hydroxide, and then adding copper (II) sulfate drop by drop, is also known as a biuret test.

  11. Benedict's reagent Benedict's reagent is used as a test for the presence of reducing sugars such as glucose, fructose, galactose, lactose and maltose. Benedict's reagent contains blue copper(II) sulfate (CuSO4) which is reduced to red copper(I) oxide (Cu2O) by aldehydes, also oxidizing them to carboxylic acids. The copper(I) oxide is insoluble in water and so precipitates. Chemical test To test for the presence of reducing sugars in food, the food sample is dissolved in water and about a sample is added to the reagent. The mixture is heated in a boiling water bath, and any precipitate formed is recorded as a positive result for the presence of reducing sugars in the food. Sucrose (table sugar) is a non-reducing sugar and thus does not react with Benedict's reagent. Sucrose can produce positive results with Benedict's reagent if heated with dilute hydrochloric acid prior to the test. Doing so hydrolyses the glycosidic bond to give the monosaccharides glucose and fructose.

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