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36 POTATO GROWER | JULY 2019 Evaluating harvest and transport practices to avoid blackspot bruise Battered & Bruised Diggin' In Diggin' In HANDLING | By Rabecka Hendricks, Nora Olsen & Mike Thornton Blackspot bruising is a quality concern no matter what market of the industry a potato is destined for. A blackspot bruise occurs when a tuber is impacted by an external force. This force may not break the skin, but instead damages the cells underneath. Blackspot bruise forms when the damaged cells' phenolic compounds are oxidized by the enzyme polyphenol oxidase (PPO), which in turn produces quinones (pink color), and then transforms into dark pigments (melanin). After the potato is peeled, the damaged cells appear as a dark or black spot. Development of the blackened area is not immediate and takes time to visually appear. The darkening is irreversible and considered a defect both for fresh and processing potatoes. An impact is needed to cause a blackspot bruise to form. Ideally, the sooner you are aware of conditions or equipment causing an impact, the sooner you can make changes or adjustments to reduce the potential for blackspot bruising. Peeling potatoes to identify the proportion with blackspot bruise symptoms can be a great tool to identify areas to adjust for bruise control. To do this, growers and shippers can simply sample potatoes at various locations at harvest, handling in and out of storage, or after packing or transport. A good rule of thumb is to hold potatoes at 70 degrees Fahrenheit for 24 to 48 hours, then rate for blackspot bruise. Shorter periods of time (six to 12 hours) at higher temperatures of 80 to 90 degrees have been used to speed up blackspot bruise development for earlier detection. These recommendations are based on the time and temperature needed for maximum blackspot discoloration. The question remains, Can you get a fairly good idea of blackspot bruise level (e.g., less than maximum coloration) in a shorter amount of time to make equipment or handling modifications sooner? A recent study by the University of Idaho set out to determine the possibility of evaluating bruises sooner than 24 hours and without using high temperatures. The main objective was to address the development of blackspot bruise over a five-hour period and compare to bruise development after 24 hours in Russet Burbank and Russet Norkotah. An impact pendulum was set at a drop height of 7 or 12 inches and delivered four uniform bruises on each potato. Tubers were bruised, stored at 70 degrees, then visually evaluated for blackspot bruise at one, two, three, four, five and 24 hours after impact. Discoloration—primarily pink—began to develop within the first hour after impact. For example, Russet Burbank impacted with a 12-inch drop showed 50 percent of the impact areas discolored pink after one hour. The change in bruise color from pink to brown primarily occurred two to three hours after impact,