Food tests and enzymes Food Tests

Food testing in biology is a standard set of procedures which all GCSE students will need to complete, and which is revisited at A level. The tests are for the presence of carbohydrate (simple and complex), protein, and fat in food.

Generally, it is easy and uncomplicated to set up and can be as complicated or easy for the students as the teacher wants it to be, depending on the foods chosen to be tested.

WHAT TO TEST FOR?

• Carbohydrates (complex: starch): test with iodine solution.

• Carbohydrates (simple, reducing: glucose): test with Benedict’s solution (and heat).

See our recipe for making starch solution on page 40

• Carbohydrates (simple, non-reducing: sucrose): break bonds with hydrochloric acid then neutralise before testing with Benedict’s solution as for simple sugars.

• Protein: test with biuret solution.

• Fats: test with ethanol, Sudan III or greaseproof paper.

FOODS TO USE FOR THE TESTS:

Biological molecule under test

Easy lesson

Complex carbohydrate Starch solution*

Simple carbohydrate (reducing) Glucose solution

Simple carbohydrate (non-reducing)

Sucrose solution

Longer (messy) lesson

Potato/bread

Bread/juice

Any sucrose flavoured drink/squash

Protein Milk Cheese/protein shake

Fat Oil

Butter/cheese

Enzymes

Ah! The dreaded call for an enzyme practical can chill the heart of many a dedicated technician. Enzymes are biological catalysts that speed up the rate of many reactions in the body, they are substrate specific, meaning that they will only work on certain substances.

Not only that, but individual enzymes also have specific temperatures and pHs that they work best at; add to this the fact that they will denature at high temperatures (the shape of the enzyme will change and will not fit the substrate, so they will not be able to speed up the reaction) and you have a very temperamental activity to deal with.

For example, amylase enzyme will break down starch into glucose, and it should work in the temperature range 20°C - 60°C, but optimum pH can vary from a minimum of 3 to a maximum of 8, depending on the type of amylase you have.

Always make up enzymes in distilled/deionised water. Check the pH and temperature conditions required and ensure that your diluted enzyme is within those ranges. Always test the enzyme with the chosen substrate before use.

Once you have a successful enzyme/substrate combination you should be able to use it many times over about a week (if the enzyme solution is kept in the fridge).

Make sure you do a quick test before each use as ANY contamination of substrate solution with the enzyme solution (even a contaminated pipette tip) will mean the experiment will fail as the enzyme in the substrate solution will break it down (so the test will not work). And yes, we mean as little as one drop in one litre!

For a full list and troubleshooting tips visit our Resource Hub and download our handy guide.