Unit 2 - Module 1 - Food Tests Flashcards
What indicates a positive result for starch?
The solution changes from brown to blue-black.
What is the test for starch?
Add a few drops of iodine solution.
What is the test for a reducing sugar?
Add Benedict’s solution and heat to 80oC in a water bath.
What indicates a positive result for a reducing sugar?
An orange-red precipitate forms.
What is the test for a non-reducing sugar?
If the reducing sugar test is negative, boil with HCl, cool and neutralise with sodium hydrogencarbonate solution or sodium carbonate solution. Repeat Benedict’s test.
What indicates a positive result for a non-reducing sugar?
An orange-red precipitate forms on the second test.
What is the test for a protein?
Add biuret reagent.
What indicates a positive result for a protein?
The solution turns from blue to lilac.
What is the test for a lipid?
Add ethanol to extract lipid and pour alcohol into water in another test tube.
What indicates a positive result for a lipid?
A white emulsion forms near the top of the water.
How can you estimate how much reducing sugar is present in a solution?
The more reducing sugar present the more precipitate is formed, and the more Benedict’s solution will be used up. If the precipitate is filtered out, then the concentration of the remaining solution can be measured.
How can you compare solutions with a colorimeter?
Place the solution in a sample chamber between the light and the photoelectric cell in a small plastic container called a cuvette. The more. The more copper sulphate solution that is used up, the less light will be blocked out and the more light will be transmitted. This means the readings taken give a measure if the Benedict’s reaction.
What is a colorimeter?
It is a device that shines a beam of light through a sample. A photoelectric cell picks up the light that has passed through the sample. It will give you a reading showing how much light has passed through.
