Water is found throughout the plant structure, and the cell wall functions to control the movements of
water into and out of the cell. When the plant is living, vegetable cells contain much water. This gives
the cells rigidity, which is called “turgor”, and this also contributes to vegetable texture. After harvesting
the fruits or vegetables, water is lost from the vegetable cells and the vegetable wilts, which is why
vegetables and fruits should be eaten fairly soon after harvesting.
The most important proteins found in plants that we are interested in are the pigments, which play an
important role in determining the plants colour.
As the world of plants is almost infinite, we can only examine some important cases in the kitchen.
· Fruit and vegetable browning:
If a freshly cut slice of apple or avocado is left on a plate, it will turn brown. When vegetables are
peeled or cut up, the fine structure of the vegetable or fruit cells which have been cut will be destroyed
and cell compartment will release their contents. Amongst their contents, enzymes will be released
from their special compartments and are free to find their target – the substrates.
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For example, enzymes called polyphenol oxidases are released from their special compartment, and
are free to find their substrate, the uncoloured fruit molecules called polyphenol, and greatly speed up
the reaction between this uncoloured molecule and oxygen in the air to form molecules called
quinines. The quinines formed in this way react with each other, and rearrange themselves to form
brown melanoidins– these are molecules of the same kind as ones that allows us to tan!
polyphenol + oxygen _ quinines _ melanoidins
The melanin forming reaction in the absence of enzyme (white, polyphenol and oxygen, brown,
The melanin forming reaction in the presence of enzyme (shown as green)
There are a number of ways to prevent this undesirable browning:
Firstly, remove the enzyme: The enzyme can be removed by changing temperature: - both blanching