Introduction to Food Tests and Calorimetry
In the previous chapters, we looked at how enzymes build up and break down large molecules. But how do we actually know what is inside the food we eat? In this chapter, we will dive into the Biology Only core practicals where we identify the presence of starch, reducing sugars, proteins, and lipids (fats). We will also learn how to calculate exactly how much energy is locked inside a piece of food using a technique called calorimetry.
Whether you are aiming for a grade 5 or a grade 9, mastering these tests is essential because they frequently appear in both Paper 1 and Paper 2!
Preparing Your Sample
Before you can test a piece of solid food (like a piece of bread or a nut), you need to get it into a liquid form so the chemicals can react with it properly.
Step 1: Take a small piece of food and grind it up using a pestle and mortar.
Step 2: Transfer the ground food to a beaker and add some distilled water.
Step 3: Give it a good stir to dissolve the nutrients.
Step 4: Filter the mixture using funnel and filter paper to remove the solid "lumps." You are now left with a clear food solution ready for testing!
Core Practical 1.13B: Chemical Reagent Food Tests
You need to know four specific tests. A great way to remember these is to focus on the initial color of the reagent and the positive result color.
1. Testing for Starch (Iodine Test)
Starch is a large carbohydrate used by plants for energy storage.
The Reagent: Iodine solution.
The Method: Add a few drops of iodine solution to your food sample.
The Result: If starch is present, the color changes from orange-brown to blue-black.
2. Testing for Reducing Sugars (Benedict’s Test)
Reducing sugars (like glucose) are smaller molecules. Note: This is the only test in this list that requires heat!
The Reagent: Benedict’s reagent.
The Method: Add Benedict’s solution to the sample and place the test tube in a water bath set to \(80^\circ C\) for about 5 minutes.
The Result: This test is semi-quantitative, meaning the color tells you how much sugar is there:
- Blue (No sugar)
- Green/Yellow (Trace amounts)
- Orange (Moderate amounts)
- Brick-red (High amount of reducing sugar)
3. Testing for Proteins (Biuret Test)
Proteins are vital for growth and repair. The Biuret reagent is actually a mix of potassium hydroxide and copper sulfate.
The Reagent: Biuret solution.
The Method: Add Biuret solution to your sample and shake gently.
The Result: The color changes from blue to purple (mauve).
4. Testing for Lipids (Emulsion Test)
Lipids are fats and oils. Because lipids don't dissolve in water, we use ethanol.
The Reagent: Ethanol.
The Method: Mix the food sample with ethanol and shake. Then, pour the liquid into a test tube of distilled water.
The Result: If lipids are present, a cloudy, milky-white emulsion will form at the top.
Quick Review Tip: If the question asks about "Reducing Sugars," always remember the water bath! If you don't mention heating it, you might lose the marks.
Core Practical 1.14B: Energy in Food (Calorimetry)
Calorimetry is a method used to measure the amount of energy in a food sample by burning it and using the released heat to warm up a known volume of water.
The Calorimetry Setup
1. Measure a set volume of water (e.g., \(20 cm^3\)) and pour it into a boiling tube.
2. Measure the initial temperature of the water using a thermometer.
3. Weigh the food sample (e.g., a dried bean or crisp) using a balance and record its mass.
4. Set the food on fire using a Bunsen burner (away from the water!).
5. Quickly hold the burning food directly under the boiling tube.
6. Keep relighting and burning the food until it will no longer catch fire.
7. Measure the final temperature of the water.
Calculating the Energy Content
To find the energy in the food, we use the specific heat capacity of water. You may be asked to use this formula:
\(Energy\ in\ food\ (J) = mass\ of\ water\ (g) \times 4.2\ (J/g/^\circ C) \times temperature\ change\ (^\circ C)\)
Wait! There is one more step. Because different food samples have different weights, we must calculate the energy per gram to make a fair comparison:
\(Energy\ per\ gram\ of\ food\ (J/g) = \frac{Energy\ in\ food\ (J)}{Mass\ of\ food\ (g)}\)
Accuracy and Errors in Calorimetry
You might notice that your calculated energy value is much lower than the value printed on the food's packaging. Why?
- Heat Loss: A lot of heat energy escapes into the surrounding air instead of heating the water.
- Incomplete Combustion: The food might not have burned completely.
- Distance: The food might have been held too far from the boiling tube.
How to improve it? You could use a lid to trap heat or use a draught shield to stop the wind from blowing the flame. Professionals use a "Bomb Calorimeter" which is fully insulated and uses pure oxygen to ensure the food burns completely!
Key Takeaways for Exam Success
1. Color Changes: Memorize the "From/To" colors. (e.g., Iodine: Orange \(\rightarrow\) Blue-black).
2. Safety First: Mention safety goggles when using chemicals like Biuret or Ethanol (which is flammable).
3. Variables: In calorimetry, the independent variable is the type of food, and the dependent variable is the temperature change of the water.
4. Math Skills: Always check your units! If the question asks for Kilojoules (\(kJ\)), remember that \(1\ kJ = 1000\ J\).
Don't worry if the math seems tricky at first—just remember the "Energy per gram" formula is just the total energy divided by how much the food weighed. You've got this!