Webinar offers tip for post-harvest potato storage
- 5 hours ago
- 9 min read
By Tamara Botting
Potatoes are a staple for Canada’s agricultural industry. In 2024, there were over 127 million hundredweight of potatoes produced across the country.
Of course, growing the produce is only the first part of the ‘farm to table’ process; oftentimes, it’s several months before the potatoes grown in Canada will be eaten by consumers.
To help make sure the produce product hitting the market is the best it can be, Fruit and Vegetable magazine presented the Fresh from the Field: Post-Harvest Best Practices for Carrots and Potatoes webinar for producers in November 2025, sponsored by Wyma Solutions.
While broadly speaking, some of the practices are similar between the two crops, the particulars are unique to each.
Scott Graham, Mid-West Regional Raw Agronomy Manager – Manitoba and North Dakota with JR Simplot Company, was one of the speakers; his portion of the presentation focused on potatoes.
Graham emphasized the importance of properly preparing the storage bins for potatoes before it’s put back into use each harvesting season.
All of the organic matter, such as potatoes and soil, need to be removed from the storage area.
Then, the whole storage area needs to be cleaned – this includes the walls, beams, flume boards, insulation, cement, plenums, pipes/ culverts, ventilation and humidification systems. Producers can do the initial cleaning with soap and hot water or steam, using a high pressure washer, and then rinse.
It’s also necessary to disinfect the space. Graham noted that when the disinfectant is applied, it should be left wet on the surfaces for 10 to 15 minutes – he reminded audience members that disinfectants can vary in their chemistry and application use, so each producer should be sure to check the label to confirm that the disinfectant they’re planning to use is appropriate for their potato storage.
Graham noted that the combination of hydrogen peroxide and peracetic acid is very popular with many growers, as it’s well proven to be effective, in that it breaks the biofilms down and kills any bacteria or fungus that may be lingering.
Yearly inspections and recalibrations need to be done on the temperature, humidity sensor and humidification systems.
“If you’ve got lower humidity than what you’re intending, your profit is literally flying out the window – or in this case, out the back louvers of your storage,” Graham said.
The processes for these checks are fairly straightforward.
To double check your storage control panel is functioning accurately, take a thermometer into the storage plenum and make sure that it’s reading the same temperature as what’s showing on the panel.
With humidification systems, it’s important to check the header tube to ensure that it’s not clogged, and check for broken or disconnected tubing, while also making sure the pads are in the correct place.
Also, check the humidification system inspecting the wet bulb/dry bulb box ensuring the wick is clean, and if it needs to be replaced. Similarly, he recommended only using distilled water in your wet bulb/ dry bulb box.
If you need to calibrate your relative humidity sensor, remove the wick from the wet bulb sensor, and allow it to stabilize for 20 minutes. Then, compare the wet and dry bulbs – if they’re reading the same temperature, then there’s nothing more that needs to be done. If not, though, then you’ll need to recalibrate the system.
Electronic humidity and temperature sensors should be cleaned and checked regularly to make sure the readings you’re getting are accurate. Also, check the humi-cell annually to make sure the cardboard insert isn’t clogged – these will need to be replaced every three to five years at least, but possibly more often, as hard water can cause scaling due to heavy mineralization, and plug a humidification system.
Graham noted that a clean 6 ft x1 ft x 1ft. humi-cell media weighs approximately eight pounds; any media over 12 pounds should be replaced. Back pressure from a plugged humi-cell can reduce fan performance by 12-20 per cent or more, based on the blockage. This might reduce the fan speed (meaning a lower cubic feet per minute (CFM)), and/ or lower the humidity in the storage space.
When it comes to their heaters, some farmers will remove them in the off season, as they can get rusty. Before you need to use them, though, it’s a good idea to inspect and test your heaters to make sure they’re working correctly.
Louvers and doors should also be inspected: control arms should be greased so that they can work properly, the door needs to be plugged in for the winter months, and the louvers need to be correctly in place and free swinging.
Finally, be sure to check your exhaust cages, making sure that they’re intact so that no birds or animals can get in.
After potatoes are harvested, they take in oxygen and release carbon dioxide (CO 2), heat and moisture.
“This is a lot of energy that’s going in the storage (space),” Graham said. This is why it’s so important to have all of the temperature, humidity and airflow control systems working optimally – they need to push that energy up and out.
He noted that the three things that govern the respiration rate of potatoes are temperature, maturity, and tuber health.
· Temperature: The lowest rate of respiration for potatoes is at about 45 degrees F (7 degrees C); the rate of respiration is higher when the temperature is above or below that mark.
· Maturity: Immature potatoes have higher respiration rates than mature potatoes.
· Tuber health: Potatoes that are stressed (whether too hot or chilled, or those that are diseased) will have higher respiration rates. Also, fungal and bacterial diseases respire on their own, which adds more CO2 and heat to the storage space.
“Right from the day those potatoes are planted right up until the day of harvest, keep your scouting notes, know your fields, flag any problems,” Graham said.
He urged producers to pay attention to their crops, because the plants will communicate what’s happening. Things like the vine canopy health, the skin set, and the sucrose levels will all help to paint the picture.
“It’s very critical to take sugar samples pre-harvest; that’ll assist you with your digging date order and it’ll confirm other factors that have happened throughout the summertime as well,” Graham said. Fairly common occurrences like a later planting season, hail, or several severe heat days stacked in a row could all have an impact on that year’s potato crop.
He reminded the producers that optimal storage practices start in the field.
Ideally, the pre-harvest irrigation should be applied two or three days ahead of time, to hydrate the tubers; depending on your soil type, this could be four to six tenths. While hydration is important, it’s also best to avoid harvesting wet spots.
Graham urged producers to record their tuber pulp temperatures hourly at least – though he knows some farmers will do it after every load, just to be sure. Harvesting should only be done when the pulp temperatures are between 45 to 65 degrees F (7 to 18 degrees C); if the temperatures are lower than 45, there will be an increase in shatter bruises; above 65 will mean an increase in pathogens and pressure bruises.
At harvest time, there can be many different types of rots, like pink rot and pythium leak, etc.
Graham reminded producers that pre-conditioning removes excess sugars, as it’s a combination of respiration and reconversion to starch, and curing is essential for wound healing; the development of a good skin reduces water loss and minimizes the danger of pathogens.
With potatoes, the storage temperature should be at 52 to 55 degrees F (11 to 12 degrees C) for two to four weeks after the harvest (longer if needed), while the relative humidity should be at 95 per cent, which promotes suberization and quick wound healing, minimizes dry rot infections and prevents shrink and pressure bruises.
Graham noted that the majority of weight loss for potatoes happens in the first 30 days in storage, making early storage management the optimal time to minimize water loss and pressure bruises.
Ventilation should be continual, to remove CO 2 and provide the needed oxygen.
“The huge thing is, get that energy out of the pile. You want uniform conditions throughout your pile,” Graham said.
He added that when it comes to CO 2 management, the levels should be below 2,500 parts per million (PPM) for most French fry varieties. Sugars can accumulate if the CO 2 level in the storage is too high, and that will have an impact on fry colour. Sensors help prevent the CO 2 levels from being too high for too long.
Graham reminded everyone, “It’s very critical when it is cold to check your return air door and make sure that they’re not frozen up.”
He noted that with more being learned about the sugars of potatoes, “a lot of growers have been able to reduce their preconditioning time, from six to eight weeks down to two or three, if they know that their crop is mature.”
Graham added, “The sugars are telling us we can go .4 degrees a day after three to four weeks of preconditioning. We can be a little more aggressive to get down to 50, and then do the .2 down to your holding temperature.”
For the storage ramping period, the advice given was:
· Check sugars and fry colour two to three weeks after harvest
· For shifting from ramping storage temperature to holding temperature – rapid cooling may cause sugar accumulation and fry colour to go dark or pressure bruise; instead, it’s recommended to go slowly, at rates of .2 to .4 F per day
· For non-refrigerated storage, wait to ramp until the outside conditions are suitable
· Use ramping only when a minimum of 16 hours of cooling air is available each day
Meanwhile, for storage holding and reconditioning periods:
· The holding temperature is very dependent on your potato variety and delivery timeframe – communicate with your processor on the time of delivery. If, for instance, you know you’re going to be shipping in December, is there a need to take it down to 47 or 48? (Keep your fry colour in check, because you might not need to go that low)
· Utilize variable frequency drives to maintain a .5 to 1 degree F differential between the top and bottom of the pile
· Reconditioning or conditioning refers to increasing your storage temperature to reduce the sugars for better fry colour (this has been a more common practice in recent years); in mid November to the start of December, it’s a good idea to cool the pile off. (Graham also noted here that in the winter, the temperatures are going to be that much colder, so there may be more dripping condensation happening – if you can drop the temperature to 47 or 48, that will reduce the amount of pathogen activity via condensation)
· Increase your storage temperatures four to six weeks prior to delivery
Throughout the storage cycles, managing condensation needs to be an ongoing concern.
Condensation forms when warm, humid air is cooled to the dew point, meaning that when warmer air – which can hold more moisture than cooler air – is cooled, it will condense and form water droplets.
Graham suggested that producers consider adding a heater above the pile at the front of the storage space, because combined with fans, a heater can further reduce the likelihood of condensation, keeping the ceiling lining dry.
Speaking of fans, Graham suggested suspending them from the ceiling, or placing them on the top of the pile to improve air circulation; if the fans are slightly offset and going in different directions, this will maximize air circulation. The air movement mixes the cooler air near the ceiling with the warmer air from the pile, and reduces the risk of condensation.
While not necessary, Graham noted that he has seen some farmers install light timers in their storage spaces. This has several benefits, including that keeping potatoes in the dark helps prevent them from greening, and it reduces overall energy use.
He also said that he’s seen some farmers replacing their old wooden flume boards with composite boards, because they last longer and it helps to eliminate the issue of foreign materials. If you are using wooden boards, Graham reminded everyone to make sure they’re flush with the floor, “because we don’t want any splinters coming into the factory.”
As far as other possible upgrades to your storage facility, before you do any modifications, it’s best to conduct an engineering review first, Graham said. If the existing storage is still structurally sound, upgrading the insulation and ventilation should be a priority. It’s important to make sure the work is being done properly, so you’re getting the right amount of airflow.
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