Storage Quality
When it comes to potato storage there are two things every farmer needs to know. First, no crop, no matter how good it was when harvested, is going to improve in storage. Quality is either maintained or lost, but never improved. Second, potatoes are living organisms that will continue living while in storage. Like all living things, they will interact with the environment around them.
To maintain quality in storage, producers need both quality storage facilities and quality potatoes to put into storage. Storage maintenance is key throughout the year. Not only that, but how the crop is handled will change from year to year.
In storage, the factors that affect performance include the environment, agronomy, varietal traits, disease and harvest and management practices, said Todd Forbush, postharvest potato storage expert at Techmark, in Lansing, Michigan.
The most important factor is variety.
Varietal traits give us a starting point for understanding the strengths and weaknesses of the crop going into storage,” Forbush said.
When choosing a variety, Forbush recommends looking at yield, specific gravity, sugar profile in storage, bruise resistance and disease resistance.
He also reminds growers to be cognizant of weather at the time of harvest. While growers have no control over the environment, they do have control over soil conditions and agronomy. Take preharvest samples to get a more complete picture of the tubers’ maturity and storability, Forbush said.
“What you’re trying to do is figure out how those four main pieces came together in any given season,” Forbush said. “And then determine what the best storage practices are given the way those four variables impacted the crop’s genetic expression.”
While the first step to maintaining storage performance is making sure there are quality potatoes to store, without a quality storage facility none of that matters. To evaluate a current structure, Forbush said, consider its location, the structure itself, its insulating system and its ventilation system and controls.
To minimize transportation time, storage facilities should be situated near the production point, preferably on a road that has 12-month access. It should also be located in proximity to an adequate power supply, specifically three-phase power, Forbush said.
As potatoes are being moved into storage, take note of the climate at the time, Forbush said. Store one variety in each room, and fill the bin in no more than three days’ time. Finally, sell the crop in less than three weeks, he said.
“If it takes you too long to fill that bin of potatoes, you’re going to have a lot of diversity as far as the harvest conditions and those are going to come back and cause losses in storage,” he said. “Losses in storage are losses right across the bottom line.”
When evaluating a facility, be sure to take a look at the insulation system as well. Insulation requirements state that a system should have an R-value of 45 to 60 in the ceiling, an R-value of 35 to 40 for the sidewalls and high to low-density materials from the inside out.
“You don’t want to trap moisture in those walls,” Forbush said. “If you let moisture get into the insulation system it’s going to condense at some point after. If it condenses near structural material, it can compromise it. It’s important that once that moisture moves into the wall that it can move out.”
Good ventilation is arguably one of the more important factors in potato storage.
Ventilation will create a uniform environment and maintain temperature. It will also supply oxygen for respiration and remove carbon dioxide, the product of respiration. Poorly designed ventilation systems can cause air to be pulled from and directed to the wrong places in the bin. The result is non-uniform ventilation.
Forbush reminds growers that air takes the path of least resistance. To learn how air in a bin is moving, measure it. A well-designed ventilation system will include these attributes:
- Adequate airflow 1.25 to 1.5 cfm/cwt for table stock and process potatoes (25 to 30 cfm/ton)
- Inlet and exhaust maximum of 1,400 ft/min
- Main air plenum maximum of 900 ft/min
- Lateral duct maximum of 1,000 ft/min
- Slot duct max of 1,200 ft/min
To determine airflow, collect the following information: fan size, horsepower and manufacturer; bin capacity (L x W x potato pile height); plenum area (the chamber prior to the potatoes); fresh air, return air, exhaust air and humicell area; duct area (pipe or flume size and quantity); and slot area (type of slot guarded or not).
The most commonly missed number is the number that determines whether or not the air is moving at the correct speed through the pile. Even airflow is key, says Forbush, and that often comes down to air duct and slot sizing.
“If you don’t have uniform back pressure, you don’t have uniform airflow,” Forbush said.
Potatoes can be greatly affected by disease in storage. Producers have experienced great losses to diseases like fusarium dry rot, pythium leak, pink rot, soft rot and silver scurf. Disease can be prevented through better management, said Phil Nolte, University of Idaho Extension seed potato specialist.
Nolte said there are two factors that, if controlled, will make potatoes less favorable for disease: skin set and pulp temperature. It’s important to let the potatoes sit between two and three weeks after vine kill. That’s the amount of time it takes for the skins to set, he said. If harvested early, the skins will be thin and weak, making them more susceptible to damage.
Potatoes are very picky, he said. They should not be harvested if temperatures are 42˚F or below or 65˚F or above. Under colder conditions, starches and cell walls are more brittle, he said. And warmer temperatures provide the perfect environment for increased disease pressure.
“The potato tuber does have some ability to defend itself from invasion by pathogens provided that you’re within a temperature range where you favor the potato over the pathogen,” Nolte said. “When you start getting up above 65˚, the progress of the invader is so much more rapid than that potato’s response that you greatly favor some of these storage disease issues.”
At the time of harvest, it’s important to handle the crop with care, Nolte said. Even in the absence of disease, subtle damage – black spot bruise or shatter bruise – can occur, rendering the tuber unsuitable for processing or fresh consumption.
“Pretty much everything you see as a storage problem with diseases requires a wound to get into the tuber,” Nolte said. “Keeping that to a bare minimum is another good way to ensure that the crop you store will come out in good shape.”
Finally, between-season management is equally important to management practices implemented throughout the season. Nolte suggests cleaning both storage facilities and handling equipment between seasons. “It’s a three-step process,” he said:
- Using a shovel and a broom, thoroughly clean the facility of debris.
- Wash with water and detergent, making sure to break down dirt and plant sap.
- Using a disinfectant solution, clean all surfaces, keeping it wet for no less than 10 minutes. This will ensure that all pathogens are killed.
