Getting ahead of storage issues
What’s the best temperature for wound healing in postharvest storage? How much time does the process need? The answers start in the field.
“Wound healing is a process; it’s a process that depends on the quality of the crop,” said Gustavo Teixeira, assistant professor and potato postharvest physiologist at the University of Idaho.
Experts agree that what happens in the field sets the tone for what follows in storage.
“Most storage problems begin in the field,” said Paul Bethke, professor emeritus at the University of Wisconsin. “There are a lot of things that can go wrong.”
Harvesting too early or improperly causes more skinning. Shatter bruising varies with the variety. Mishandled potatoes enter storage with higher respiration. Moisture released from wet or damaged potatoes can jumpstart diseases. Poorly timed harvest generates more field heat.
“All of these things come together,” Bethke said.
These compounding issues highlight why harvest timing and technique are critical to storage success.
“The big challenge, and the starting point, is ensuring you have good harvesting practices before loading potatoes into storage,” said Jacob Blauer, Washington State University assistant professor and potato postharvest physiologist.
Wound healing begins immediately after injury and is a very active metabolic process. This makes wound healing heavily temperature-dependent.
A standard curing temperature is 50 to 55° F. That’s cool enough to slow pathogen activity in postharvest storage and yet warm enough for wound healing. Wound healing is much faster at warmer temperatures, but that increases the risk of disease proliferation.
If you have a heavy pathogen load or know you could have some disease issues, lower temperatures can restrain pathogen development but also slow wound healing.
“The right temperature for wound healing depends on how your crop comes from the field,” Teixeira said. “If the crop has a low risk of disease development, high temperatures can be used for curing. But if the crop has a high risk of disease development, low temperatures are recommended to reduce pathogen growth, even though a longer wound healing is expected. It is a trade-off process.”
The ideal pulp temperature at harvest is 50 to 60° F. Removing field heat becomes more difficult as harvest temperatures rise.
If the potatoes come from the field with too much field heat, it’s sometimes hard to get the storage temperature down. If the potatoes enter storage at 65° F, for example, it could take weeks to bring the temperature down to 55° F. By then, the standard recommendation of wound healing for two weeks is long past.
“You’re trying to get that environmental heat out and establish a stable temperature for wound healing, but you’re also trying to calm the potatoes down and decrease respiration,” Blauer said. “Any time you’re moving and disrupting the potatoes, it increases respiration, and that increases moisture loss and temperature.”
FACTORING IN HUMIDITY
Wound healing takes place best under high humidity environments. Injury exposes tissue to the air and there’s rapid water loss.
“This adds stress to that area trying to wound heal and slows wound healing,” Bethke said. “You need to slow that dehydration.”
Storing potatoes at 95% humidity reduces water loss, but you may want to start at a lower humidity at higher fan speeds to dry any wet potatoes. It’s imperative to stop bacteria growth, especially if you have rot issues. Be especially careful with the most susceptible varieties. You don’t want to give disease a head start.
Adjust your fan speeds and ventilation accordingly. Set the relative humidity so there isn’t any condensation on the potato surfaces or puddling on the floor.
Depending on the end use of the crop, there are chemicals, such as chlorine dioxide, that can be applied through the ventilation system to decrease pathogen growth.
“If there’s a problem area in the field, consider loading those potatoes into storage last, if they must be stored,” Blauer said. Decreasing the pathogen load going into storage can help.
“You’re essentially dealing with spores,” Blauer said. “The best way to eliminate the source is through healthy cultivation in the field and an adequate, but not excessive, soil moisture level. We do not want to have a buildup of inoculum from the field.”
Good harvest practices, as always, are part of the solution. Make sure you have a good skin set at harvest and good soil moisture management. Do your best to have a good harvest pulp temperature.
Make sure your equipment is operating well. That includes good padding, good drop height and a minimal amount of roll back on equipment and on the storage pile.
Potatoes with large gashes or those that have been caught between rollers have more exposed surface and deeper cracks, and the injuries take more time to heal.
“Growers need to regulate all of their harvesting equipment to reduce injury,” Teixeira said.
Try to minimize the amount of soil, rocks and other foreign material coming into storage. Have your inspectors on the line remove as much foreign material as they can and remove diseased potatoes.
Only put potatoes into clean storage — preferably clean and sanitized.
Try to avoid roll backs when piling. Diseased potatoes that are contaminated with inoculum will spread that onto other potatoes, and the fans will blow the spores through the pile.
Pile roll backs during loading will also spread the sources of inoculum.
THE ROLE OF GENETICS
Observed differences in wound healing times suggest there is a genetic component in wound healing.
Washington State University is trying to identify genetic markers that are related to wound healing in potato varieties. The goal is to create a predictive model.
“We’re trying to understand what the ideal wound healing conditions are for different varieties and different markets,” Blauer said. “It’s an opportunity to optimize storage practices based on variety. Knowing this may offer opportunities when you’re thinking about long-term sugar content and end use.”
The University of Idaho is studying near-infrared hyperspectral imaging (NIR-HSI) as a non-invasive way to evaluate how much wound healing has taken place.
“We’re trying to develop a non-destructive model with NIR-HSI imaging where growers can evaluate potatoes during curing,” Teixeira said.
If, in a few days in storage, the wound healing process is completed, growers don’t need to keep potatoes warm and they can start to ramp down the temperature.
In the meantime, the messages are clear:
“Better handling means fewer problems,” Bethke said. “Be mindful of the quality of the potatoes coming into storage,” Blauer said.
“The whole process starts in the field,” Teixeira said.
