Global: Heat-Stressed Crops Raise Sugar-End Risk for Fry Processors
Research shows soil temperatures around 23 °C and water stress during early bulking trigger sugar ends. The defect is irreversible in storage and darkens fries.
Hot, dry seasons leave a hidden defect in many processing crops. It is called sugar-end disorder. The basal, or stem, end of the tuber stores less starch and more sugar. When fried, that end turns dark brown.
A 2008 review in the American Journal of Potato Research described the problem. The authors were Thompson, Love, Sowokinos, Thornton and Shock. They linked sugar ends to high soil temperatures, short periods of water deficit and unbalanced nitrogen.
When the damage happens
The critical window is early. Tubers are most sensitive from tuber initiation to early bulking. Mike Thornton of the University of Idaho told Spud Smart in 2019 that the risk rises when soil reaches the mid-70s °F. That is about 23 °C or higher.
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Stress effects add up. Several short episodes of heat or drought cause more damage than one. The review explains the mechanism. Stress switches the tuber end from building starch to breaking it down. Sucrose then accumulates in the basal tissue.
Storage cannot fix it
Sugar ends are irreversible, according to the Spud Smart report. Warmer storage does not turn the sugars back into starch. Cold exposure below about 10 °C makes symptoms show more clearly.
Fry colour is graded on the USDA Munsell colour chart for frozen French fries. A score of No. 3 or darker counts as a defect.
Varieties differ. Russet Burbank and Ranger Russet are susceptible. Clearwater Russet is a newer variety with better resistance. The review notes that resistance is highly heritable, which helps breeders.
What growers can do
The Spud Smart report lists research-based steps: plant early enough to close the canopy before heat; keep soil moisture steady during early tuber growth; use light, frequent irrigation in heatwaves; avoid excess nitrogen. It also advises wound healing at 10–13 °C after harvest, then slow cooling.
Why it matters
The 2026 season brought heatwaves to many potato regions in Europe. Growers delivering to fry processors face a risk that is invisible at harvest. Sugar ends appear only after frying tests, often weeks later. Processors should test incoming lots by stem-end fry colour. Growers should document irrigation during early bulking, as this is the stage that decides fry quality.
Photo: French fries in a deep fryer (illustrative). Jamain / Wikimedia Commons, CC BY-SA 3.0
