Food science is a fascinating topic that dabbles in nutrition but predominantly delves in to the science behind food – its make up, its functions, chemical processes and so on. This blog will be taking a look at fruit and answering some common questions.
Why does fruit turn brown?
This is due to the phenolic compounds found in fruit – also known as tannins. These compounds are the cause of browning and bruising that occurs to fruit as it ripens.
Phenolic compounds play a role in enzymatic browning along with two other substances – oxygen and polyphenol oxidase enzymes [1]. The polyphenol oxidase enzymes oxidize phenolic compounds which turns them brown from a once clear state. The brown segments are called melanins and are perfectly safe to consume [1]. Oxygen will enter the fruit cells when it is either bruised or cut – this is why cut fruit goes brown so quickly.
Why does lemon prevent browning?
This is due to the ascorbic acid content of lemon which makes the pH of lemon very low (acidic). Enzymes, such as pholyphenol oxidase, are very sensitive to extremes in pH. An extreme pH will disrupt their structure and can lead to them denaturing or it can inhibit their activity (the ideal pH for polyphenol oxidase to function is 7.0) [1]. Denaturing is the process whereby a protein loses its shape/ structure e.g. tertiary structure, which can lead to cell death or disruption of cell activity. The very acidic pH of ascorbic acid can denature the polyphenol oxidase enzyme which is needed to catalyse (cause) the enzymatic browning process in fruit [1]. Orange and lime juice will also produce the same effect.
Therefore, the decreased activity of pholyphenol oxidase enzymes will lead to a decrease in oxidation and as a result, browning.
Additional ways to slow enzymatic browning
- Reduce the pH (as discussed above)
- Blanch fruit – this denatures enzymes but can impact texture and taste
- Lower temperature – this will slow the browning process by slowing down the activity of the enzymes
- Submerging cut fruit in water or coating with sugar – This prevents oxygen exposure, though I do not recommend coating your fruit with sugar or syrup!
Why is fruit sometimes bitter?
Have you ever noticed a bitter taste in a piece of fruit or an astringent mouth feel after eating it? This is also due to the phenolic compounds (tannins) which are largely found in under-ripe fruit [1]. Some examples of fruit that contain phenolic compounds are; apples, bananas, strawberries, grapes, pears, nectarines and persimmons.
Why does fruit become more colourful as is ripens?
The colour pigments in fruit consist of carotenoids, flavonoids (betalains, anthoxanthins, anthocyanins), and chlorophylls. The colour of fruit becomes increasingly more vibrant with the aging process – this is due to the breakdown of chlorophyll [1]. As the chlorophyll breaks down, the pigments are exposed more so [1]. Further colour changes can be brought about by a change in pH or ethylene gas, which encourages the ripening process [1].
Fruit acidity
Organic acids are found in fruit and can create a tart flavour. As fruit becomes riper, the acidity tends to decrease [1]. Common acids in fruits include:
- Citric acid
- Malic acid
- Tartaric acid
- Benzoic acid
- Oxalic acid
Some of these acids are volatile and some are non-volatile. The volatile acids will vaporise during heating. The non-volatile acids do not vaporise but they can leach out in to the water that fruit is being cooked in [1].
Oxalic acid present in some fruits can bind with calcium in the intestine, forming calcium oxalate [1]. This is an insoluble complex and can occasionally lead to kidney stones.
As a result of the above acids, the majority of fruit has a pH under 5.0. Fruit that has a higher pH tends to be sweeter and act more like a vegetable e.g. cucumber, squash [1].
Fruit, drupes and pomes
Fruit – Fruit develops from the flower of the plant and is edible.
Drupes – A drupe is a fruit that contains its seeds inside a pit, for example, apricots and plums.
Pomes – Pomes do not contain their seeds inside a pit. Rather they are clustered in the centre of the fruit e.g. apples.
References
- Brown A. Understanding Food Principles and Preparation. 5th Stamford: Cengage learning; 2014.
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This article was written by our dietitian Belinda Elwin who is a Dietitians Association of Australia member and Accredited Practising Dietitian and Nutritionist.
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