Breeders make progress against 'cold sweetening' in potatoes

ABERDEEN, Idaho -- Promising first-generation potato clones bred to fry with a light color after prolonged storage at low temperatures should enter the breeding program next season at USDA's Aberdeen Agricultural Research Service facility.

Cold sweetening -- the tendency of starch to break down in cold-stored potatoes and form sugars that darken when fried -- poses a burden on processors making fries or chips long after harvest.

Aberdeen breeder Rich Novy managed to breed Russet and chip clones from four of five parental crosses Wisconsin breeder Shelley Jansky developed from a wild variety naturally resistant to cold sweetening. Novy made hybridizations from Jansky's clones last year and planted the true potato seed he germinated in a greenhouse this summer to produce seedling tubers for use in next year's field evaluations.

Several years of work will be required before any of the material can be released as a variety.

"To me, if we're going to make continued gains in cold sweetening, I like to pull in new material. This material is new germplasm with wild material in their immediate background," Novy said.

Novy crossed Jansky's clones with other parental material with good cold sweetening tolerance, as well as material resistant to late blight and certain viruses.

University of Idaho Extension Potato Specialist Nora Olsen said storing at lower temperatures reduces water loss and slows sprouting and the development of diseases.

"(Cold sweetening) is a huge criteria for processing quality in general," Olsen said. "There's definitely a lot of benefit to storing at colder temperatures."

Jansky, a USDA ARS research geneticist and an associate professor at University of Wisconsin, has filled requests for her cold sweetening-resistant germplasm from breeders in Idaho, Wisconsin, North Dakota, Minnesota, Maine and Colorado.

Jansky said some processors have reconditioned potatoes by warming them up slowly to reduce sugar content before cutting them, but results are inconsistent. Breeding resistant varieties, such as the chipping spud Snowden, is the best approach, she said.

"We can do better. We can store them colder and longer with some of this newer material," Jansky said.

Her breeding was aided by genetic research done by University of Wisconsin horticulture professor Jiming Jiang, ARS plant physiologist Paul Bethke and technician Andy Hamernik, who characterized the enzyme responsible for cold sweetening, called invertase. They demonstrated the importance of invertase by genetically modifying a potato variety prone to cold sweetening to turn off the enzyme. Their spud fried lightly after cold storage.

Jansky explained knowing the enzyme responsible for cold sweetening enables breeders to retain seedlings with ideal DNA sequences, significantly speeding up their efforts.

"As we get more effective at breeding the low-invertase trait into varieties, we should be able to store them colder and longer and have fewer crosses," Jansky said.