Submitted to: Common Tater
Publication Type: Trade Journal
Publication Acceptance Date: August 11, 2012
Publication Date: September 1, 2012
Citation: Bethke, P.C. 2012. First things first. Common Tater. 64:16-17. Technical Abstract: They all rotted. That wasn’t supposed to happen, but soft rot has a way of showing up where it isn’t welcome. In this experiment, tubers were dipped in either a suspension of soft rot bacteria or in water. A few days later, all tubers were melting, even those that we didn’t inoculate. Such is the power of soft rot that serious problems with decay can appear rapidly under environmental conditions conducive to growth. In this case, bacteria on the non-inoculated tubers had waited nine months for warm and wet conditions to appear. The first few days that tubers are in storage have a disproportionate effect on how successfully the crop will be stored. An immediate goal is to create an environment that favors tuber wound healing and final skin maturation, but does not favor growth of the pathogens that promote spoilage of the stored crop. Appropriate conditions are temperatures of 50-55°F with no free water on the tubers. Warmer or wetter, for even a few days, puts the harvested crop at risk. Soil contains soft rot (Pectobacterium carotovorum) and pink rot (Phytophthora erythroseptica), and tubers arrive at the storage along with these unwanted organisms. Removing field heat and eliminating free water quickly are essential for successful potato storage. As is often the case when it comes to potato storage, success depends on what happens in the field. When tubers arrive with pulp temperatures of 55°F or less and without free water, the storage manager can focus on optimizing crop quality during the wound healing and preconditioning periods. When tubers arrive wet and warm, the storage manager has to perform triage in order to prevent disease from taking hold. This involves using the ventilation system to rapidly dry and cool the tubers. Unless wet soil is restricting airflow through the pile, this may succeed in preventing large-scale disease. The downside is that it will likely increase tuber water loss. Potatoes are most vulnerable to water loss during the first few days in storage. Wounds and skinned areas lose water very rapidly, especially when they have not had long to heal. Every effort should be made during the first few weeks of storage to minimize water loss. The payback for this is reduced rates of pressure flattening and pressure bruise on unloading the storage, since these increase as tuber hydration decreases. Rapid cooling and drying of freshly harvested tubers, however, result in accelerated water loss during this critical early period. It may seem surprising that removing free water from the tuber surface promotes water loss from inside of tubers. This wouldn’t happen if all of the tubers in a pile dried at the same rate, but surface moisture is typically removed from tubers near the bottom of the pile first, and later from tubers near the top. This happens because the ventilation air picks up moisture as it moves up the pile and evaporation in a drier environment is faster than in a moister environment. As a result, tubers on the bottom of the pile may be dry while tubers near the top still have free moisture. Subsequent efforts to dry the top of the pile will remove internal water from tubers on the bottom of the pile. Humidification of the ventilation air will slow both the rate of water loss from tubers on the bottom and the rate of drying from tubers on the top. Individual circumstances will dictate the amount of humidification needed to minimize losses caused by disease while maintaining tuber quality. Removing field heat means cooling tubers while simultaneously removing heat produced by respiration. Since respiration rates increase with increasing temperature, the amount of cooling capacity needed increases disproportionately as pulp temperature going into storage increase. Cooling air always removes moisture from tubers, and rapid cooling removes moisture at a faster rate than slow cooling. Thus, when disease is kept under control by rapidly removing field heat, a penalty is paid in terms of increased water loss.