Managing storage temperature is an enormous step to ensuring top quality product.
The vast majority of potato producers have a good to excellent understanding of storage management guidelines. Still, every winter an unfortunate high number of producers face unnecessary stored tuber losses due to incorrect temperature and moisture management.
For now, the best scientists are able to offer are strong and repeated reminders about the importance of managing one’s storage conditions. In the longer term, however, research may point toward ways to minimize and mitigate temperature-induced losses.
After devoting enormous time and input expense to produce a crop of potatoes, it seems intuitive for producers to follow storage recommendations to the letter to maintain the condition of their investment. Yet, says Dr. Khalil Al-Mughrabi, a researcher at New Brunswick Department of Agriculture, Aquaculture and Fisheries’ Potato Development Centre, this isn’t always the case.
“There is always room for improvement. Farmers can produce the best crop, but if they manage their storage poorly, stored potatoes can deteriorate very quickly and end up a complete loss to the farmer.”
Temperature plays a critical role at every stage of storage. The first step to maintaining tuber quality over long-term storage is to cure tuber skins and seal wounds inflicted during harvest. Many of the most costly storage diseases (including dry rot, leak and pink rot) are caused by soil-borne pathogens that are carried into storage on the tuber surface. To minimize entry of pathogens into tuber wounds and to cure tuber skins to prevent moisture loss, stored tubers should be cooled or warmed slowly (no more than two to three degrees per day) to between 13 C and 15.5 C, maintaining a relative humidity of 92 to 95 per cent. While suberization occurs much faster at higher temperatures, this moderate temperature recommendation best manages the risk of disease spread. Tubers destined for seed or fresh markets should be held at suberization and curing temperature for two weeks; tubers destined for processing may require as long as four to five weeks at this temperature to acquire sufficient colour.
If tubers enter storage in poor condition due to frost or significant disease damage, it is sometimes necessary to skip the suberization and curing period in favour of dropping temperature quickly. While rapid cooling may minimize further losses, these tubers will not store well.
To control disease while supporting the drying process, Al-Mughrabi recommends a two stage cool-down for severely damaged tubers: first cool the pile to 8 to 9 C for three to four weeks, then cool further to 3 to 4 C. If the damaged tubers are intended for processing, he recommends talking to a consultant to analyze whether the low colour associated with low early storage temperature or the increased disease risk associated with higher temperature is the lesser of two evils.
Once suberization of good condition tubers is complete, storage temperatures should be dropped as soon as possible. Long-term storage is best achieved when processing potatoes are held to 7 to 10 C, fresh (table) potatoes at 4 to 5.5 C, and seed potatoes at 3 to 4 C.
At the end of the storage period, tubers should be gently warmed a few degrees prior to handling to limit shatter bruise and cracking.
Over the last decade, the potato industry has seen an increasing number of producers who invest in heaters. Though Al-Mughrabi says a potato pile produces enough heat to keep the crop sufficiently warm in most modern and well-insulated storage facilities, heaters can be useful to speed drying to manage disease.
“When dealing with rot, heaters above the pile will warm up the return air to the point that the control system must bring in extra fresh air to maintain the correct plenum temperature. The extra fresh air will typically reduce the relative humidity (assuming the humidifier is off) by three to four per cent, which can cut drying times in half,” he explains.
“A heating system of this size will cost $5 to $10 per hour to operate, so they are not a cheap option, but well worth it when dealing with rot.”
Storage losses have declined substantially over the years as storage facilities and storage technologies like ventilation systems and temperature sensors improve. Basic storage management recommendations themselves, however, have remained relatively unchanged for many years. Dr. Dmytro Yevtushenko, the University of Lethbridge’s new potato research chair, hopes that reality may soon change.
“Generally it’s true that the recommendations in place, at least for processed potatoes, are the same recommendations we’ve had in place for many, many years. We need the temperature at 8 C, otherwise too much sugar is produced; we need relative humidity at about 95 to 98 per cent, good ventilation, darkness. But is there still room for research? Yes. Storage recommendations were established a long time ago and they are a general rule. But, there are always ways to improve.”
Improving storage management is one of the highest priority research areas requested by the grower stakeholders who partially fund Yevtushenko’s position. As such, Yevtushenko plans to devote a significant portion of his near-term research capacity towards analyzing ways to better control the physiological age of potatoes during storage.
“We want to look at how the balance of endogenous growth regulators (phytohormones) inside tubers changes during long-term storage, and whether that can affect starch metabolism. Based on that research, we will try to figure out if we can drop storage temperatures while still maintaining proper sugar content,” Yevtushenko explains. “Obviously, it would be nice to reduce the temperature to 4 C because cooler temperatures mean less pathogens.”
Yevtushenko has spent his first year in the University of Lethbridge’s newly created research chair position talking through research priorities with stakeholders and applying for grant funding. Once his research team and funding are finalized over the next few months, he anticipates conducting cutting edge research into tuber storage temperatures and much more.
Until Yevtushenko and other experts provide a new scientific answer to maximizing stored tuber quality, Al-Mughrabi has just one vital piece of advice about managing stored tubers: “Be vigilant. Vigilance [of storage conditions] is very important.”