Review of Food Tests

USE OF INDICATORS

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.

Chemical Digestion Food Tests 1.Test for Starch –Iodine (2 – 3 drops) –Negative – Iodine remains brown/orange –Positive – Iodine changes orange to black

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.

Summary Table Reagent/Subs tance Used to test for Food SubstanceControl BeforeAfterBeforeAfter Biuret reagent Protein Lugol’s iodine Starch Benedict’s solution Glucose BileFat Brown paperFat

Chemical Digestion Food Tests 2.Test for Protein –Biuret’s solution – 2cm 3 –Negative – Biuret’s stays blue –Positive – Biuret’s changes to lavender

The Biuret Reagent Test The principle underlying the test can be demonstrated with the chemical compound biuret which, just as proteins, is able to complex copper (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.biuretcomplexcopperionspeptide bondproteinspolypeptides 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.sodium hydroxide

Summary Table Reagent/Subs tance Used to test for Food SubstanceControl BeforeAfterBeforeAfter Biuret reagent Protein Lugol’s iodine Starch Benedict’s solution Glucose BileFat Brown paperFat

Chemical Digestion Food Tests 3.Test for Fat (a. paper test) –Smear oil/fat onto paper –Paper turns translucent –Negative – stain dries (not translucent) –Positive – stays translucent

Summary Table Reagent/Subs tance Used to test for Food SubstanceControl BeforeAfterBeforeAfter Biuret reagent Protein Lugol’s iodine Starch Benedict’s solution Glucose BileFat Brown paperFat

Chemical Digestion Food Tests 4.Test for Fat (b. bile) –Add bile to oil –shake –Negative – separation of the bile –Positive – bile emulsifies the fat Emulsification – breaking up a large blob of fat into smaller fat bubbles

Summary Table Reagent/Subs tance Used to test for Food SubstanceControl BeforeAfterBeforeAfter Biuret reagent Protein Lugol’s iodine Starch Benedict’s solution Glucose BileFat Brown paperFat

Chemical Digestion Food Tests 5.Test for Glucose –Benedicts solution (about 2cm 3 ) –HEAT –Negative – stays blue –Positive – from blue to orange (brick red)

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.reducing sugars glucosefructosegalactoselactosemaltosecopper(II) sulfatecopper(I) oxidecarboxylic acidsprecipitates 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.Sucrosesugar

Summary Table Reagent/Subs tance Used to test for Food SubstanceControl BeforeAfterBeforeAfter Biuret reagent Protein Lugol’s iodine Starch Benedict’s solution Glucose BileFat Brown paperFat

Chemical Digestion Food Tests 6.Test for Liver (Catalase) –Hydrogen Peroxide (about 1cm 3 ) –Negative – no bubbles –Positive – foam / bubbles / gas

Peroxidase speeds up the breakdown of hydrogen peroxide, a waste product of cell metabolism that can be toxic in high concentrations. The breakdown that occurs is as follows: 2H 2 O 2  2H 2 O + O 2 Hydrogen Peroxide Water Oxygen Peroxidase, or catalase, an enzyme found in animals and plants, and in high concentration in the cells of the liver. Peroxidase has an active site that fits perfectly, like a lock and key, with hydrogen peroxide. When peroxidase, found in the liver, is exposed to hydrogen peroxide, large numbers of oxygen bubbles are produced, as well as water; therefore, a positive test for liver peroxidase will produce large numbers of oxygen bubbles when the liver is exposed to hydrogen peroxide.