Getting to the bottom of browning, and how to solve it
Why do apples brown?
Apples turn brown when the fruit’s phenolic compounds react with oxygen. This oxidation process is driven by polyphenol oxidase (PPO), an enzyme. When an apple’s cells are ruptured – for example, by bruising, biting or cutting – the browning reaction begins when PPO found in one part of the cell is able to react with phenolic compounds found elsewhere in the cell.
Some existing apple varieties turn brown more slowly than others, which can be due to a number of factors (but make no mistake, low browning or slow browning apples can’t deliver the same clear benefits as a truly nonbrowning apple):
- The apple variety may be high in acid, such as malic and citric acids, which impedes the PPO/phenolic reaction
- It may be low in phenolic substrate
- It may have stronger cell walls, which reduces the mixing of PPO and phenolics, or
- It may have naturally low levels of PPO
Genome mapping uncovers the PPO gene family
Recent mapping of apple’s genome revealed that PPO is genetically encoded in a diverse, multi-gene family. Apples have at least eight PPO genes, in three main PPO gene families. To breed a truly nonbrowning apple, all of the active PPO genes must be silenced.
Arctic® apples have been genetically engineered to produce little or no PPO enzyme, so cell disruption doesn’t lead to browning.