Food tests: glucose, starch and gelatine solutions: I have to make up solutions for a prac where students are working out if select foods are a protein, carbohydrate or sugar. Can you please advise about the concentrations for the mixtures?
Submitted by sat on 03 August 2018
Answer reviewed 18 Jan 2023.
Food testing is routinely carried out in senior biology classes. Students test a variety of food samples for carbohydrates, such as sugar and starch; lipids; proteins and vitamin C.
In response to this question we have developed an information sheet. See Laboratory Notes: Food Tests
Submitted by on 14 August 2018
Positive Test Results for Test 1: Glucose When you add Bendeict’s solution, the solution turns blue. As it heats the colour changes from Blue Green Yellow Tomato Red at the end NOTE: When starch is heated it also breaks down to a simple sugar and so shows a positive result. Test 2: Fat/Oils When the oil has been rubbed into the brown paper, if you hold it up to the light it should be translucent. Test 3: Starch After the Iodine - I2/KI has been added the starch solution should be coloured anywhere from purple through to black. Test 4: Protein No colour change should occur when the sodium hydroxide – NaOH is added. After the copper sulfate – CuSO4 is added it should change from light blue to dark blue to purple. Foods: To avoid contamination make sure the students cut each piece of food up in a different area on the cutting board. Also make sure they wash the knife after each food. Preparations of Solutions: Keep in fridge until required for use. Then return when finished. Glucose: Dissolve 1 teaspoon of glucose in 200ml of water. (Do not use sugar) Protein: Dissolve ¼ teaspoon of gelatine in 200ml of water. Heat to nearly boiling to dissolve. Starch: Dissolve 1 teaspoon of powdered starch in 200ml of water. Heat to boiling to dissolve. If you do not heat, the solution will remain cloudy. Egg White: Use raw. Beat egg white with 100ml water. Store in small dropping bottles.