Carbon dioxide and ethylene gas in the potato storage atmosphere and their combined effect on processing colour

Keywords: Solanum tuberosum L., carbon dioxide, ethylene, storage, processing, fry colour, chip colour, 1-methylcyclopropene The finished colour of processed potato (<em>Solanum tuberosum</em> L.) products is a very important quality characteristic which is attributable to the concentration of reducing sugars in the raw tubers. Many internal and external factors can affect the concentration of these sugars during long-term storage. Ethylene gas, produced by the tubers or from external sources such as pathogens or engine exhaust, is known to increase reducing sugars and darken processing colour. For many years, elevated CO₂ from tuber respiration or external sources was also believed to affect sugars and cause darkening, although the research was somewhat contradictory. Restricted ventilation in potato storage buildings can cause appreciable accumulation of both gases in the storage atmosphere. The effects of elevated CO₂ and depleted O₂, with and without trace ethylene (0.5 µL L^-1), in the atmosphere surrounding potatoes were investigated. Short-term studies (three or nine weeks) using cultivar Russet Burbank stored at 9 ºC were conducted during several consecutive storage terms, with evaluation of the processing colour at intervals of three weeks. No responses to elevated CO₂ or depleted O₂ were observed, whereas ethylene darkened colour slightly. However, the colour was darker when tubers were exposed to CO₂ and ethylene together than with ethylene alone. At 0, 0.5, 1.0 and 2.0% CO₂ with 0, 0.25 and 0.5 µL L^-1 ethylene in a 4 × 3 factorial design, the colour darkened as the concentration of either gas increased, except in CO₂ without ethylene where darkening was not observed. Pre-treatment with the ethylene blocking compound, 1-methylcyclopropene prevented darkening attributable to 0.5 µL L^-1 ethylene and to 2% CO₂ plus 0.5 µL L^-1 ethylene. Long-term studies (December to June) using cultivars Shepody, Innovator and Dakota Pearl in addition to Russet Burbank were subsequently undertaken, using 2% CO₂ with 0.5 or 10 µL L^-1 ethylene (trace and sprout-inhibiting concentrations, respectively) and evaluations every four weeks. There was little or no change in processing colour in Dakota Pearl, a potato chip cultivar, in response to any of the treatments throughout the storage season. In the three French fry cultivars Russet Burbank, Innovator and Shepody, processing colour was darker in response to either concentration of ethylene at four weeks after the exposure began, although recovery to a lighter colour occurred with increased duration of storage in Russet Burbank and Shepody tubers. Interestingly, darkening in response to 2% CO₂ applied alone was observed after exposure for eight weeks or more in all three French fry cultivars. Darkening attributable to CO₂ and ethylene applied together was observed from the first evaluation after the start of exposure in the three French fry cultivars. This darkening occurred whether or not the cultivar responded to ethylene or CO₂ alone, and was worse than with ethylene alone in all three cultivars, except that Shepody tubers exposed to CO₂ plus 10 µL L^-1 ethylene was the same as with 10 µL L^-1 ethylene alone. The magnitude of darkening varied slightly among cultivars, and was usually more severe as storage duration increased. This research has provided useful information to help storage managers maintain the physiological condition of stored potato tubers to ensure light processing colour.