Minnesota Science
Vol. 43, No. 2
Potato Research Finds Clue to Dark Chips
by Larry Etkin
When you think of potato chips you probably think of "plain or rippled" or perhaps onion or herb dip to eat them with.
But to Minnesota potato farmers, processors, and snack food distributors, potato chips are more than just a snack food that goes great with dip; they are an important economic product. Over two hundred and fifty million pounds of last year's Minnesota potato crop were processed into potato chips.
In recent research, University of Minnesota Agricultural Experiment Station plant pathologist Ernest Banttari, cooperating with Paul Orr of the USD Potato Research Laboratory in East Grand Forks, Minnesota, showed that an old disease of potato is responsible for some new problems to the chip industry. They linked chip discoloration to a disease organism known as purple top.
Purple top, which is descriptive of the disease in some potato plants, is caused by an organism known as aster yellows mycoplasma which can infect over 300 plant species. Potato chipping cultivars such as Monona and Norchip are quite susceptible.
The disease can cause a variety of symptoms in different cultivars. These can include the "purple top" foliage discoloration, or some cultivars may develop yellowed and curled leaves, stunted growth, and occasionally inedible "aerial tubers."
The last severe outbreak of purple top was in 1981, although there is no good estimate on how much damage it did. "No firm estimate exists of the amount of loss from that outbreak because no surveys were made;" Banttari says. "There were numerous questions concerning dark chips in the 1981-82 processing season. At that time we did not have the answer."
Now we know the problem is associated with infestations of potato fields with aster leafhopper, an insect that can't overwinter in Minnesota, but is often carried in from southern states on strong winds. "A low percentage of the leafhopper may actually carry the mycoplasma but there can be enormous numbers – millions, billions of insects," Banttari explains. "Therefore, you don't need a high percentage of infective leafhoppers to cause problems."
Purple top can infect many cultivars, but Banttari and Orr focus on its effects on those normally grown for chipping. Infected tubers appear normal at harvest. The problem shows up after the potato has been processed and fried.
The disease causes an abnormal conversion of starches to "reducing sugars" which caramelize during frying. The result is a severe darkening and discoloration, which affects the appearance and taste of finished potato chips. (Brown spots occasionally found on potato chips might or might not be the result of this plant mycoplasma. Discoloration of chips can also result from other common causes, such as when potato tubers are bruised or over chilled.)
Banttari is developing an assay,, to detect aster yellows mycoplasma in leafhoppers, that can be completed fast enough to alert growers to potential control of leafhopper infestation by applying an insecticide. "At present, the only control measure possible is to control the insect," Banttari says. The current method of detection involves exposing healthy indicator plants to insects suspected to be infected, and then observing the plants for weeks to see if symptoms develop.
Banttari is working on a simple, accurate and quick seriological assay that should return results within a day or two. This diagnostic test could not only be used on plants, but also to screen out heavily infected lots of potatoes before they are placed in storage. He says this would be a great help because "we have some evidence that chip discoloration intensifies when storage of infected tubers is longer."
PHOTO CUTLINE: Darkened chips (right) due to purple tap infection of tubers.
On the left, chips from healthy tubers.