When you get your order of french fries in a fast food restaurant or open up a potato chip bag while enjoying a football game, the first thing you see is the color of the products. Those with light cream to gold-brown color make you feel like you want to finish them all. In fact, color is the No. 1 attribute for desirable processing quality. Tubers yielding discolored products may be rejected and therefore cause financial loss to the grower. For potato products, dark color is mostly caused by the reaction between reducing sugars (glucose and fructose) and free amino acids during frying under high temperature. This process is called the Maillard reaction.
Sugar end defect, a typical discoloration problem for fry processing potatoes, and stem-end chip defect, a color issue for chipping potatoes, are two major physiological disorders leading to devaluation of tuber marketable quality. Sugar end defect is a smeared discolored area usually on the stem end of french fries, while stem-end chip defect is characterized by a dark-colored region along the vascular tissue or “mustache”-shaped discoloration on the stem end of potato chips.
With regards to the causes of the two defects, heat with high day and night temperatures for a couple weeks coinciding with tuber initiation and early tuber bulking can result in sugar end defect development. In comparison, researchers have observed that under greenhouse conditions, stem-end chip defect can be caused by a short period of heat stress with high day temperature (higher than 95 degrees Fahrenheit) and night temperature (higher than 84 degrees) during either the early or late tuber bulking stage.
An extreme symptom of sugar end defect is jelly end rot, where starch accumulation is severely disrupted and tuber tissue is collapsed. Jelly end rot can provide openings for pathogen infection during storage. So far, no translucent tissue or secondary pathogen infection associated with stem-end chip defect has been observed on chipping potatoes. Instead, during the growing season, infection of the fungus Verticillium dahliae specifically to the plant vascular tissue associated with the early dying complex can exacerbate the problem after tubers are harvested and put into storage.
Below are tables listing similarities and differences between sugar end defect and stem-end chip defect.