1. Food tests: Required Practical 4
Sunday, 09 December 2018
Food tests: Required Practical 4
Key Questions
What is the basic structure of carbohydrates, lipids and proteins?
What are the roles of carbohydrates, lipids and protein in the body?
How can we test for carbohydrates, lipids and protein?
Starter: What is the function of the following molecules in the body?
Carbohydrate
Fat
Protein
Challenge: name a disease caused by having either too much or too little of each of these nutrients. 

2. Carbohydrates Contain carbon, hydrogen and oxygen
Made up of sugar units
Substrate for respiration – energy release
Simple Sugars
One or two sugar units
Immediate energy
Complex carbohydrates
Long chains of sugar units
Slow release energy
Sucrose
Glucose
Fructose

3. Testing for Carbohydrates
Glucose
Blue Benedict’s solution turns orangey-red when it is boiled with glucose.
Starch
Orangey-brown iodine turns blue-black when it reacts with starch.
chopped up food
hot water
drop iodine solution onto the food
heat
black = starch

4. Lipids
Contain carbon, hydrogen and oxygen Fats (solids) and oils (liquids)
Glycerol +3 fatty acids (chain length and saturation varies)
Energy store, cell membranes, hormones, nervous system
Insoluble in water

5. Testing for Lipids
A red stained oil layer, floating on the water surface is produced if lipids (fat) is present
filter
grind food
Add distilled water then stir
add 3 drops Sudan III
Red stained floating layer = fat

6. Protein Contain carbon, hydrogen, oxygen and nitrogen
Long chains of amino acids folded into a specific 3D shape
Structural (form tissues) or metabolic (enzymes, hormones, antibodies)

7. Testing for Protein In the Biuret test the blue solution turns purple.
potassium hydroxide
copper sulphate
shake
purple = protein
chopped up food

8. Plenary: For each food, decide on the chemicals found in them.
8A What’s in food?
Plenary: For each food, decide on the chemicals found in them.
Food sample
Test
Result
Iodine solution
orangey-brown
Benedict’s test
blue
Biuret test
purple
Ethanol test
cloudy white
Plenary
Protein
Fats
Starch
Glucose

9. Food sample Test Result Iodine solution blue-black Benedict’s test
8A What’s in food?
Food sample
Test
Result
Iodine solution
blue-black
Benedict’s test
blue
Biuret test
Ethanol test
clear
Protein
Fats
Starch
Glucose

10. Food sample Test Result Iodine solution orangey-brown Benedict’s test
8A What’s in food?
Food sample
Test
Result
Iodine solution
orangey-brown
Benedict’s test
orangey-red
Biuret test
blue
Ethanol test
clear
Protein
Fats
Starch
Glucose

11. Food sample Test Result Iodine solution orangey-brown Benedict’s test
8A What’s in food?
Food sample
Test
Result
Iodine solution
orangey-brown
Benedict’s test
orangey-red
Biuret test
blue
Ethanol test
clear
Protein
Fats
Starch
Glucose

12. Exam question
A student is analysing the nutrient content of egg whites.
Fully describe an investigation that the student could carry out to find out if protein is present in a sample of the egg whites. (6 marks)

13. Mark scheme
Level 0 – There is no relevant information. (no marks)
Level 1 – There is a brief description of how to carry out the investigation. (1 to 2 marks)
Level 2 – There is some description of how to carry out the investigation but some details are missing. (3 to 4 marks)
Level 3 - There is a clear and detailed description of how to carry out the investigation. (5 to 6 marks)
Here are some points your answer may include:
Grind up a sample of the egg white using a pestle and mortar.
Put the sample into a beaker and add some distilled water.
Stir well with a glass rod to allow some of the food to dissolve in water.
Filter the mixture through a funnel lined with filter paper.
Transfer 2 cm3 of the filtered solution into a clean test tube.
Add 2cm3 of Biuret solution and gently shake the test tube.
If the food sample contains protein, the solution will change from blue to pink to purple.
If no protein is present, the solution will stay bright blue.