Here’s why some fruit and plant tissues go brown when cut.
Plant cells have various compartments, including vacuoles and plastids, which are separated from each other by membranes.
The vacuoles contain phenolic compounds, which are sometimes colored but usually colorless, while other compartments of the cell house enzymes called phenol oxidases.
In a healthy plant cell, membranes separate the phenolics and the oxidases.
However, when the cell is damaged, by cutting into an apple, for example, phenolics can leak from the vacuoles through the punctured membrane and come into contact with the oxidases.
In the presence of oxygen from the surrounding air these enzymes oxidize the phenolics to give products that may help protect the plant, favoring wound healing, but also turning the plant material brown.
The browning reaction can be blocked by one of two agents, both of which are present in lemon juice.
The first is vitamin C, a biological antioxidant that is oxidized to colorless products instead of the apple’s phenolics. The second agents are organic acids, especially citric acid, which make the pH lower than the oxidases’ optimum level and thus slow the browning.
Lemon juice has more than 50 times the vitamin C content of apples and pears. And lemon juice, with a pH of less than 2, is much more acidic than apple juice, as a quick taste will tell you. So lemon juice will immediately prevent browning.
You could also prevent cut apples from browning, even without lemon juice, by putting them in an atmosphere of nitrogen or carbon dioxide, thus excluding the oxygen required by the oxidases.
An excellent vegetable for observing browning is celeriac.
It is possible to cut a large, relatively uniform slice of this root tissue and then lay several small filter paper disks on the cut surface, each soaked in a different solution such as lemon juice, apple juice, vitamin C, other antioxidants, citric acid, other acids, and suchlike.
A disk soaked with an agent that blocks the action of oxidases will leave a white circle on an otherwise brown surface.
Polyphenol oxidase (PPO) was discovered in mushrooms in 1856 by Christian Schoenbein.
It is widespread in nature and found in humans, most animals, and many plants. In plants its function is to protect against insects and microorganisms when the skin of the fruit is damaged.
The dark brown surface formed by the skin is not attractive to insects or other animals, and the compounds formed during the browning process have an antibacterial effect.
In some foodstuffs made from plants this browning effect is desirable. For example, in tea, coffee, or chocolate it produces their characteristic flavor.
However, in other plants or fruits such as avocado, apples, and pears, browning is an economic problem for farmers, because brown fruit is not acceptable to consumers and it doesn’t taste good.