(ED--We made a mistake in the headline of this potato storage heat story we featured in the September issue of Potato Grower Magazine. Here is the story with the correct headline. Our apologies to the authors.)
Early storage management objectives are focused on removal of field heat and wound healing (curing) the potato crop. This early part of storing the crop involves managing temperature. Temperature is one of the most important environmental factors determining the postharvest life of potatoes. Removal of field heat is not an issue during ideal harvest conditions, but it does become a necessity when harvest temperatures are above 55 degrees F.
Potatoes generate a tremendous amount of heat at harvest and in storage due to field heat and high rates of respiration. It is important to remove heat and cool potatoes as soon as possible. In storage, other sources of heat besides the potatoes can increase temperature, such as the heat transfer from the outside to inside of the building and heat from operating electrical equipment (e.g., fans). The various sources of heat all add up to the total heat load that needs to be removed from the building. The warmer the pulp temperatures are at harvest, the greater the heat load in the cellar that needs to be removed. This becomes the greatest challenge: how to remove the heat in a short amount of time.
There is only so much heat you can remove in a single day and there may not be adequate outside cooling air available to get control of the heat load. Integrating in refrigeration is a great option to cool potatoes at harvest, although the system needs to have the capacity to handle the high heat load and fresh air must be provided to purge carbon dioxide (CO2) out of the building. Theoretically, in a 100,000 cwt storage, if potatoes with 65 degree F pulp temperature at harvest are put into storage with no cooling air, that 65 degree F temperature can increase to 70 degree F temperature after 12 hours from the heat of respiration alone. That 100,000 cwt storage can generate millions of BTUs per day from respiration that needs to be removed before the crop can start to be cooled.
The elevated heat load in storage reinforces the need to get air on the potatoes as soon as possible, and as much as possible, to cool the crop. Initiate a step-down process of exhausting heat, cooling the crop, and maximizing run time. Computerized control panels can be programmed to do this. Simply setting the set point to a desired curing temperature of 55 degrees F will not maximize run time. The set point needs to be closer to the temperature of the crop to maximize cooling air. The longer potatoes are at elevated temperatures, the greater the risk for disease breakdown. There are additional risks or consequences of having potatoes stay too warm for too long. Shrinkage will be higher. Elevated temperatures encourage greater evaporation of water from the potatoes plus maintains higher rates of respiration. Together they result in greater weight loss that is never recovered.
Another objective early in storage is to wound heal potatoes. This objective is to properly heal any cuts and wounds to minimize water loss and protect from pathogen invasion from Fusarium dry rot and soft rot. This objective can be tackled at the same time field heat is being removed. Wound healing starts the first day potatoes are in storage. The general curing recommendation is to hold potatoes at 50-55 degrees F (95+%RH) for 2 to 3 weeks to heal wounds. Similar to calculating growing degree days, if we calculate accumulated heat units in storage (maximum temperature + minimum temperature/2 – 32 degrees F) for the recommended wound healing time, it would equal approximately 320 heat units (55 degrees F for 2 weeks). In comparison, if the crop was harvested at 65 degrees F, cooled 0.5 degrees F/day until the temperature reached 55 degrees F, the potatoes already would have accumulated approximately 610 heat units. These heat units far exceed what would be recommended for typical wound healing heat units. The wound healing process is complete. There would be no need to add more time and cure at 55 degrees F, but instead continue ramping the crop down to the holding temperatures.
To minimize heat exposure of the crop, account for all heat accumulation of the potatoes as time spent toward wound healing, regardless of if the temperatures were above recommended curing temperatures. Ensure potatoes are under temperatures and conditions to encourage rapid wound healing, but not too long as to have an impact on weight loss, disease development, and/or other quality issues.
Olsen is a Potato Specialist and Teixeira a Potato Post-Harvest Physiologist at the University of Idaho. Both located at the Kimberly R and E Center.