Sweet or sour – what’s in our supermarkets?

Super-sweet broccoli. Super-sweet tomatoes. Even super-sweet Brussels sprouts.

We’re seeing increasing numbers of vegetables and fruit bred to have high sugar contents. But at the same time, newspaper headlines are proclaiming the evils of sugar.

So what’s really going on? In this project starter, you’ll discover some of the science behind fruit and vegetables. We’ve also got a short protocol that you can use to test out the supermarkets’ claims for yourself.

Super sweet?

Super sweet?

Get inside the science

Fruits originally evolved as a means of dispersing the offspring of the plant (the seeds) well away from the parent plant. For plants that produce succulent fruit this requires the cooperation of animals (especially birds and mammals) which are tempted to eat the fruit. They then carry the seeds – on their feathers, fur or within their intestines – to new habitats to be colonised.

It is the attractive taste that rewards the cooperative animal, as well as the energy rich meal provided by the flesh of the fruit.

However, it is essential that the fruit avoids ‘premature’ consumption. This might lead to the immature seeds being destroyed, or they may be too immature to germinate once they have been dispersed. Avoiding this premature consumption, may be the reason why the attractive flavour of the fruits develops only when the fruit is ripe. In fact, unripe fruit may even carry distasteful chemicals as a deterrent.

Vegetables are mostly storage organs used by plants to provide a source of nutrition during the next growing season. Few are sweet in themselves, though many have large reserves of carbohydrate polymers, such as starch and inulin. During the mobilisation of these reserves, these polymers are broken down to produce mobilisable sugars such as sucrose and fructose.

The process of cooking helps to remove some of the chemical deterrents that plants use to deter herbivores consuming these tissues.

Domesticated fruits and vegetables have all been selectively bred so that they contain a particularly attractive taste for humans. Skilful greengrocers and cooks know how to tempt their customers with subtle combinations of flavours, and food manufacturers use panels of tasters to determine which flavours are best.

It is reasonable to suppose that the improved flavour of a ripe fruit, compared to an unripe one, is at least partly due to increased levels of sugars. It is also possible that these sugars will have been liberated from storage, in the form of starch granules or some other polysaccharide. Other changes in flavour may result from changes in the pH of the juice of the fruit, linked to changes in the concentrations of organic acids such as ascorbic acid (vitamin C), and commonly citrate and malate. A desirable sugar-acid balance makes an important contribution to a ‘pleasant taste’ in ripe fruits. In addition, volatile chemicals released from the fruit indicate its state of ripeness and add to our enjoyment when eating fruits.

We are seeing increasing numbers of vegetables and fruit bred to have high sugar contents. Is this necessarily a good thing? What are the implications?

Stained cross section of a Brussel sprout, showing the cells in extreme close-up.

Stained cross section of a Brussel sprout, showing the cells in extreme close-up.

Practical Investigations – step-by-step protocols

We’ve put together step-by-step protocols for you to use for a practical investigation on this topic.