Apples to Apples

Ever wonder why some apple varieties are quicker to go brown once sliced than others? It's got to do with varying PPO (polyphenoloxidase) levels. Horticulture professor at Cornell University, Susan K. Brown and NY Times Science writier C. Claiborne Ray explained the brown apple phenomenom in this week's Q&A:

Cutting an apple releases compounds from within cells that interact with oxygen in the air in a process called oxidation, said Susan K. Brown, professor in the department of horticulture at Cornell University.

“The browning is primarily because of a chemical reaction catalyzed by an enzyme called polyphenoloxidase, or PPO,” she said. Phenolic (nutritional) compounds in the fruit are oxidized into slightly colored compounds, called quinones, which then change to form darkly colored pigments, she said.

“Different apple cultivars vary widely in their PPO levels, total phenolic content and rates of browning,” Dr. Brown said. A variety may have less browning because of low PPO, low phenolics or some combination of the two.

Many popular cultivars, like McIntosh and Fuji, are fairly prone to browning, she said, but Cornell has developed some varieties that brown much less than most on the market. Other factors affecting the rate of browning include the ripening stage of the fruit, the length of time in storage and any topical treatments that have been applied.