Quality Control


    [deck]Whether they’re Russet Burbanks or Ivory Crisps, this year’s storage potatoes might have quality issues that require a unique brand of management.[/deck]

    The unusually hot and dry conditions that prevailed throughout many areas this season are presenting some unique challenges for potato storage, as many growers are uncertain how different varieties will perform in storage this year.

    “There’s more uncertainty this year because we haven’t experienced a year like this,” says Todd Forbush, an engineer with Techmark Inc. “When you are facing uncertainty a definition of your position is very valuable.”

    Techmark conducted a survey of actual weather data in potato production regions in North America and found that the 2012 growing season, in general, had periods of extreme heat and drought from March through July and milder temperatures in August and September, although there were local variations.

    What’s vitally important is that growers know what has happened in their own crop throughout the season and understand what they may be dealing with in terms of storage considerations. “The most important factor for growers is to access their local weather, in some instances field by field, and determine how to respond to the benefits received and challenges presented by that local weather,” says Forbush.

    It’s already known that this year’s harvested potatoes will have lower specific gravity, which means a greater volume of potatoes will be needed for the solids required for processing into fries or potato chips. “If you have three per cent lower specific gravity than normal you are going to need three per cent more tonnes of potatoes to process,” says Forbush.

    However, because of higher production this year in general, due to increased acres and moderate, average or good yields in most areas, that shouldn’t present too much of a problem, as long as the crops store well.

    Heat Stress

    Potatoes can age faster and be subject to various problems affecting their quality and dormancy when subjected to prolonged periods of heat stress. Potato dormancy is affected by the number of heat units received. Extremely hot weather through the growing season adds more heat units to the tuber and that shortens a potato’s dormancy for storage. Some potatoes have even been sprouting in the fields.

    “A rule of thumb is that from tuber initiation, which would probably be in late June, physiological age is measured as the temperature degree days above 4 C,” says Peter VanderZaag of Sunrise Potato Storage in Ontario. “For example, if the soil temperature at the top of the hill reaches 26 C on a certain day, those tubers will have aged 26 C minus 4 C, which is the equivalent of 22 degree days of aging. The soil temperature at the top of the hill is hotter than at the bottom of the hill, so if the tubers at the bottom of the hill are at 18 C, they will have aged 18 C minus 4 C, which is the equivalent to only 14 degree days of aging,” VanderZaag explains.

    “If you start measuring degree days in that way, the physiological aging process is already well-advanced this year in the early planted material, because of the intensity and duration of the hot, dry conditions this summer, particularly for those tubers that grew near the top of the hill,” VanderZaag continues. “This has led to a highly variable aging process as the tubers from all parts of the hill are mixed together in storage. This will have major implications for how long we can store processing potatoes this year.”

    VanderZaag anticipates that he will have to maintain high humidity levels and bring the storage temperature for his earlier harvested chipping potatoes a couple of degrees lower than normal—down to 8 C—to maintain the colour for frying and hopefully avoid losing a couple of months of storage time.

    Potatoes that were planted later in the season may have avoided some of the prolonged heat. VanderZaag is glad that delayed planting of his longer storing varieties like Marcy and Lamoaka until late May and early June may have earned him some extra storage life. “They probably tuberized around mid-July, so I think there will be less impact from heat stress,” he says.

    It’s going to be an interesting year to store potatoes.

    – George Burkholder


    The timing of when potatoes break dormancy varies depending on the variety. Varieties that store best are ones with longer natural dormancy, such as Russet Burbank. Others, like chipping variety Dakota Pearl,  could store less well because of its short natural dormancy, which is probably already shortened due to the hot summer.

    Dormancy is also controlled by temperature while in the storage. “The warmer potatoes are stored, the shorter the dormancy period will be,” says George Burkholder, owner of GRB Ag Technologies, a consulting company based in Mitchell, Ont. “If you can keep them cool, they will store longer before they want to sprout. But if they are stored too cold they will develop higher sugar levels and will be off-colour, which is undesirable for chipping potatoes, so you have to determine the lower limit of your temperature by monitoring the sugars carefully and frequently.

    Processing potatoes are usually stored at temperatures around 6 C to 9 C and will maintain dormancy at those temperatures for a certain period of time depending on the variety. Russet Burbank, for example, will maintain dormancy on average for 175 days at 6 C and for 130 days at 9 C.

    Once there is a risk of potatoes breaking dormancy, sprout inhibitors are often used to attempt to extend dormancy. “If a grower has a crop that is beyond peak maturity or over-mature, [the potatoes] will tend to break dormancy quicker than that variety will in a less stressful year,” says Forbush. “This means that growers need to be aware of and on top of their sprout control programs.”

    Sugar Ends

    Sugar ends can be another problem associated with stress caused by early-season drought, and under these circumstances, dryland producers may be especially at risk. Sugar ends are caused by excessive glucose in the stem end of the potato and can represent a problem for either chip or french fry processors, because affected potatoes become undesirably dark on one end. Some varieties, such as Russet Burbank, are especially susceptible to sugar ends. But sugar ends can also be caused by disease in the plant that inhibits the transfer and storage of sucrose produced in the plant to the tuber. So it’s important to first identify whether sugar ends are being caused by disease or are weather-related, and sample to determine how prevalent the problem is in the crop.

    Patrick Morris, lab manager at Techmark Inc., preparing potato juice for sucrose and glucose analysis.
    Patrick Morris, lab manager at Techmark Inc., preparing potato juice for sucrose and glucose analysis.

    Disease or early season-related sugar ends may leave producers with no option but to sell the crop as early as they can. Later-season weather-related sugar ends can sometimes be alleviated by pre-conditioning the crop at slightly higher temperatures than normal, around 16 C.

    Sugar sampling is a good way to monitor crops for sugar ends and other problems. “I think the best option is to properly identify or fingerprint each field or bin of potatoes,” says Forbush. “There is a blend of variety, field choice, agronomy, disease pressure and weather that make up each lot of potatoes for a given growing season. These individual lots need to have their strengths and weakness identified and they should be marketed accordingly. Pre- and post-harvest potato sugar sampling is one tool that we use to fingerprint each lot of potatoes that we work with.”

    The quality of ventilation systems has improved immensely over the past 10 years and, in the hands of savvy storage operators, says Burkholder, these systems can be a huge asset to extend the storability of potatoes, especially in a year when most potatoes were under extreme stress during the growing season.

    Variety choice is likely to become a top consideration as weather-related stresses are affecting more growers across the globe. Seasons like 2012 may help growers sort out which varieties are going to be the best choices under difficult conditions. “The grower’s choice to plant a potato variety that can withstand the stress associated with adverse weather is important,” says Forbush. “As stress-averse varieties are identified during growing seasons like the 2012 season, the choices can be made to adapt these into the variety mix a grower uses on his or her operation.”

    As potatoes go into bins across North America, many growers and storage managers are still not entirely sure what to expect as they approach winter. “It’s going to be an interesting year to store potatoes,” says Burkholder.

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