Keeping Apples Cool to the Core

By Erin Peabody
October 29, 2004

This harvest season's perfect apple is crisp, juicy--and hopefully hasn't suffered a nasty sunburn. That's right. A major concern of fresh apple growers across the country is sun scald, which causes unwanted bronzed or bleached spots on the fruit's skin.

Now an Agricultural Research Service scientist has figured out a way to enhance water conservation while protecting fruits from blistering heat. ARS agricultural engineer Robert G. Evans has developed a model that may one day tell growers precisely when to turn on their sprinkler systems to cool overheated apples, and when to turn them off once the fruits have cooled to an optimal temperature.

His approach relies on evaporative cooling provided by overhead water sprinklers. As water droplets cover an apple's surface and dry, the fruit's temperature drops--similar to the cooling effect we experience when stepping out of a pool on a sunny day.

Growers have used evaporative cooling for years, to encourage the deepening of an apple's rosy color, for instance, and for cooling fruits in the summer heat. But up to now, there's been no easy way to target apples' ideal temperature--and conserve water.

In regions that experience hot sun throughout the day, growers might run their sprinkler systems from early in the morning to late at night. Using Evans' findings, they could save more than half of the water they're currently using to keep heat-sensitive apple varieties, like Jonagold and Fuji, adequately cool.

Key to Evans' research was running thermal conductivity studies to determine apples' optimal core temperature, which turns out to be about 92 degrees Fahrenheit for many varieties.

More controlled sprinkling sessions, punctuated by drying-out periods, could also help prevent some of the foliar fungal diseases that plague apple trees.

Evans' study appeared in a previous issue of the Transactions of the American Society of Agricultural Engineers. He works at the agency's Northern Plains Agricultural Research Laboratory in Sidney, Mont., but conducted much of this work while at Washington State University in Prosser, Wash.

ARS is the U.S. Department of Agriculture's chief scientific research agency.