How fresh are your fruits? – A gas measurement exercise

By Richard Needham

This is a simple exercise involving measuring gases, which reveals some surprising secrets. It can give rise to interesting discussions around what we mean as scientists by the term ‘living’.

You can do this activity at any level in school – the data you collect could be used:

• Key Stage 1 – as a way of classifying fruits

• Key Stage 2 – what we mean by living and non-living

• Key Stage 3 – measuring respiration rates

• Key Stage 4 – planning investigations and photosynthesis

• Post-16 – investigating the effect of variables on biochemical reactions

I collected the data shown here at home in the kitchen one wet Sunday, so no special facilities are needed. I used a plastic box with two holes in the lid – one for a carbon dioxide sensor and one for an oxygen sensor. Mrs N kindly provided a couple of unripe avocado pears, which just fitted inside the box. I was surprised at how much carbon dioxide they released within a few minutes.

Measuring the respiration of an avocado

The graph below shows just how quickly the atmosphere inside the box changes. Note the gradients of the lines are difficult to compare as oxygen is measured as a percentage, whilst carbon dioxide is measured in parts per million.

Changes to oxygen gas (%) and carbon dioxide gas (ppm) over 60 minutes with single avocado

So are avocados unique? No they are not, as all the fruits I tested absorbed oxygen and gave out carbon dioxide. Some are far more active than others. For example my bananas absorbed oxygen three times faster than avocados. Grapes were slowest, again a result that surprised me as they have a huge surface area, and thin skins. When I measured gas exchange in grapes there was hardly any change shown after ten minutes. In the chart below, the data point for grapes is partially hidden by ‘apples’.

Gas exhange comparisons in fruits

Equipment used

1. Vernier bio chamber, with gaseous oxygen and carbon dioxide sensors attached to a laptop via a Labquest mini USB interface.

1. Vernier Logger Pro software for data collection and initial analysis, and then used Google docs to produce the comparative graph of results.

You should get similar results with any oxygen and carbon dioxide sensors providing they measure gaseous concentrations rather than dissolved gas. A sandwich box or ice cream container could be used in place of a proprietary bio chamber.

Questions and opinions that arose in discussions with colleagues

• Surely fruits are dead once they are picked from the tree?

Several colleagues assumed that once picked, a fruit becomes ‘dead’. Thinking about the ripening processes that continue this cannot be the case. However, some people assumed that the gas exchange shown in the data was a consequence of microorganisms and rotting processes.

•  But a mushroom is not a fruit.

To a biologist a mushroom cannot be a fruit as it is not a plant and does not possess the organs required for fruit development. Sometimes mushrooms are described as fruiting bodies (of basidiomycete fungi) and so I have no hesitation in justifying their inclusion in this investigation. Interestingly the data point for the mushrooms lies well below the line expected if these results are to be explained by aerobic respiration alone.

•  Why don’t green ones produce oxygen?

I initially thought that green fruits would be photosynthesizing, and produce very different results to those shown here. There is no way to tell if photosynthesis did influence the results, unless some further work is carried out, such as repeating at different light intensities.

•  How did you make it a fair test?

Good question. I am against the notion that scientific investigations always need to be fair tests. I did use similar masses of fruit in each case (about 420g) but did not attempt to control the effects of surface area.

•  Why would you want to do this?

I enjoy finding out things that are not readily explained by the science we teach in school. It challenges me to think deeper about what I am teaching, and leads to better understanding through discussion with others. Using sensors is one way of uncovering new information, and often turns up results that are unexpected.

And of course: Well how do you explain the results?

I will leave that question for you to answer.

Some useful links to help with your answers or planning

SAPS – Respiration and photosynthesis made easy

STEM elibrary- photosynthesis

Comments

I would be interested to hear your explanations for these results.

I would also like to know if these results are repeatable.

Also, do you have any examples of other ways that sensors can turn up unexpected results?