This is a simple and practical way to get students to discuss sugar content of foods, and leads in to discussion of nutritional content of foods, “healthy” foods etc. It can be run as a competition (either individual or group) in which students have to guess the sugar levels in advance.
You will need a hand-held refractometer, either 0-20, 0-30 or 0-50 Brix or similar (cost approximately £100), plus a lemon squeezer and/or juicer. A pH meter or universal indicator paper can also be used.
Suggested foods/drinks: Oranges, lemons, tomatoes, fruit juices, Coca-cola, Diet Coke, standard baked beans, reduced sugar baked beans. NB other fruits/vegetables can be included.
Prepare juices from fruits
Measure and record Brix levels (very similar to % sugar) of all juices, drinks and sauce from beans
As an option you may also wish to measure pH values.
Another option is to demonstrate the principle of the refractometer – this can simply be done by preparing one or more sucrose solutions (e.g. 30 and 60%) and observing the refraction when a spoon or spatula is dipped into these compared to water.
Some points of interest/discussion include:
- Coca-cola contains less sugar than many fruit juices
- Diet coke really does contain virtually no sugar!
- Lemon juice usually has a surprisingly high sugar content – note pH
- How much less sugar do reduced sugar beans contain compared to standard beans (not that much)
- Tomatoes contain a lot of water!
The science behind it all
The refractometer measures the amount of sugar based on the Brix scale. The Brix scale gives a direct reading of grams of sugar per 100 grams of pure water. If a soft drink gives a measurement of 12 Brix, that means that there are 12 grams per 100 millilitres. So in a 330ml can there would be 70.92 grams of sugar.
Some of your findings might be quite surprising. Your test results should show that Coca-Cola contains less sugar than many fruit juices and diet Coke really does contain virtually no sugar. Lemon juice usually has a surprisingly high sugar content –although note how the pH levels compare to the rest of your samples. Sugar reduced beans don’t actually contain that much less sugar than the standard ones and tomatoes contain a lot of water!
It just goes to show that some foods that we assume have less sugar than others actually have more.
Beyond the science
Sugar can be used for more than just making things taste sweet. Certain types of sugar are popular in the processed food industry because they not only add taste but also add colour, bulk and thickness. They can also be used a kind of preservative to stop mould from forming on some foods.