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AQA GCSE Biology (9-1): Getting the most out of Required Practicals - Food Tests

Posted by James Simms on November 25, 2019

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Specification location: AQA GCSE Biology Paper 1, Section 4.2 - Organisation

Summary of method: In this investigation, different foods are tested to determine whether particular nutrients are present or not.

Foods are tested with iodine solution to determine if starch (a complex carbohydrate) is present. If starch is present, iodine will turn from orange-brown to blue-black. If starch is not present, the iodine will remain orange-brown.
The Benedict’s test is performed on foods to see if simple sugars (such as glucose) are present. Benedict’s solution starts as a bright blue and, depending on the amount of sugar present, turns green, then yellow, then red.
Foods are tested with Biuret’s solution to determine if proteins are present. If proteins are present, the solution will change from blue to purple. If no proteins are present, the colour change will not occur.
Foods are tested for the presence of lipids (the most popular method involves agitating the food with ethanol). Food is crushed, mixed with distilled water and filtered. The filtrate is then shaken with ethanol. If lipids are present, the lipids will form a milky-white emulsion.

For more detail on the set-up of this experiment, watch our high-quality required practical tutorial on YouTube.

Common issues	Potential solutions
Time	With a 60-minute lesson, it can be challenging to test a sufficient variety of foods with the full range of necessary test methods. If time is also required to explain how to conduct the tests and the safety implications of the different reagents, it may be tempting to use another hour of class time to conduct the required methods. With classes with stronger organisational and practical skills, it may be possible to reduce the time discussing the methods by presenting them with preprinted method sheets. However, safety will still need to be discussed. In my experience so far, the best solution I’ve found is for pupils to preload the investigation’s different methods through a flipped learning technique (e.g. viewing and note-taking tutorials and undertaking quizzes on https://www.theeverlearner.com/). This minimises lag time at the start through misunderstanding and also the time needed to be spent on explaining the food-test methods. A combination of flipped learning and pre-prepared sheets is probably the optimal combination here.
Student confusion over reagent names	Students frequently become confused between testing involving Benedict’s solution and the Biuret test. To aid with this, be prepared to use the names of the two reagents in context as frequently as possible.
Retention of how to perform different reagent methods	There really is no substitute for repeating the same knowledge until it sticks: this is an ideal opportunity to use interleaving to repeat the methods at regular intervals until the methods are retained. For example, consider asking students to watch a high-quality video tutorial periodically, followed by frequent low-stakes testing.

Sample food test required practical results:

	Iodine test	Benedict’s test	Biuret test	Lipid test
Potato	✔	🗴	🗴	🗴
Cheese	🗴	🗴	✔	✔
Rice	✔	🗴	🗴	🗴
Apple juice	🗴	✔	🗴	🗴

Source: The EverLearner Staff Canteen

AQA GCSE Science subject-specific terminology:

This is an ideal opportunity for students to discuss the idea of predictions and discuss which foods they think will contain which combinations of nutrients.
As the reagent tests themselves are straightforward, this required practical may be a good opportunity to discuss the difference between reproducible and repeatable experiments.