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
1. Add 5 ml of starch solution into a test tube. PROCEDURE a. Starch 1. Add 5 ml of starch solution into a test tube. 2. Then, add a few drops of iodine solution. Observe any color changes. Record results. b. Glucose 1. Add 5 ml of glucose solution into a test tube. 2. Then, add 10-15 drops of Benedict’s solution. 3. Place the test tube into a hot water bath for 2-3 minutes. (Use a test tube holder to hold test tubes). 4. Carefully remove the test tubes from the water bath and observe any color changes. Record results. c. Protein 1. Put on your goggles. (Note: Biuret reagent is caustic to the skin and clothing. Rinse immediately upon contact and notify your teacher about any spills. 2. Add 5 ml of protein solution into a test tube. 3. Then, add 10-15 drops of Biuret reagent to the test tube and observe any color changes. Record results.
Summary Table Reagent/ Substance Used to test for Food Substance Control Before After Biuret reagent Protein Lugol’s iodine Starch Benedict’s solution Glucose
What are indicators and how do they show a chemical reaction? Reagent/ Substance Used to test for Food Substance Control Before After Biuret reagent Protein Lugol’s iodine Starch Benedict’s solution Glucose Summary: What are indicators and how do they show a chemical reaction?
Chemical Digestion Food Tests Test for Starch Iodine (2 – 3 drops) Negative – Iodine remains brown/orange Positive – Iodine changes orange to black
Chemical Digestion Food Tests Test for Glucose Benedicts solution (about 2cm3) HEAT Negative – stays blue Positive – from blue to orange (brick red)
Chemical Digestion Food Tests Test for Protein Biuret’s solution – 2cm3 Negative – Biuret’s stays blue Positive – Biuret’s changes to lavender
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.
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. 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.
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.