ÐÓ°ÉÔ­´´

Some friends and I were drinking from a jug of water that contained wedges of both lime and lemon. All the lemon wedges were floating, but all the lime wedges had sunk to the bottom of the jug. There were enough pieces of both for us to infer this was not just coincidence, and all of us were pretty certain that we’d seen lime slices floating before. Can anyone offer an explanation?

Thanks to the anonymous correspondent who sent a link to a page on the Steve Spangler Science site (), where the answer is tested – Ed

• This is down to the interplay of two factors: air and solutes. The cells in fruit tissue typically have quite high concentrations of solutes, mainly organic acids for citrus and sugars for apples. In some types this can amount to as much as 18 per cent of the total weight. The more concentrated the solutes, the denser the cells will be and the more likely it is that the fruit will sink.

In plant tissue there are also air spaces between the cells, so the tissue is less dense than the cells are. If the air spaces are large enough, the tissue will float even if the cells alone would be dense enough to sink. Air spaces can range from as little as 1.5 per cent of the tissue’s volume in the case of a potato to more than 20 per cent for some leaves.

The effect of air spaces tends to outweigh cell sugar content, which is why a potato sinks while an apple with 15 to 20 per cent air spaces by volume will float, despite the high sugar content of its cells.

A variety of factors affect the volume of the air spaces and concentration of solutes in different fruit, including growing conditions, ripeness and storage conditions. With citrus fruit, a major factor is the peel. The inner white layer – the albedo or pith – is low in solutes and notably high in air spaces, while the edible segments are high in sugar/acid content, and low in air spaces. Peel a mandarin and put pieces of peel and segments in a bowl of water, and you will find the segments sink while the peel floats. Some lemons have a thick peel and are resolute floaters, while holding lemons in storage for a time reduces the thickness of the albedo, making the fruit more likely to sink.

So, more than anything, what determines whether your citrus slice floats or sinks in your gin and tonic is the thickness of the albedo.

Rod Bieleski, Retired plant physiologist, Devonport, New Zealand