The two main objectives of storing potatoes are to maintain quality and minimize losses. We manage the crop in the field, at harvest and in storage to lessen the risk of losses. Sometimes the season or weather creates challenges that elevate the risk and make managing the stored crop more complex. Losses can come in multiple forms, e.g., direct loss due to diseases, evaporation (water loss), respiration (carbon loss) and indirect losses from quality issues such as unacceptable fry color or pressure flattening/bruise. Many of the losses are interrelated, and when one form of loss occurs, other types will follow. The snowball effect can result in even greater losses. An example of this is the link between evaporative loss and pressure bruise: Once potatoes lose enough water, the potential for pressure flattening/bruise is elevated.
There are a few questions to ask when looking at losses in storage. First and foremost, how many potatoes were originally in the storage? Once you know this answer, the overall storage loss can be estimated based on the weight taken at delivery. There are a few ways to know the original weight of potatoes in storage. Each truck can be weighed before unloading, which will also include dirt and other materials taken out during handling, but it provides a good idea of how many potatoes are going into storage.
Another method is to use newer technology that incorporates scales or sensors on conveyors to measure the incoming weight. An alternative method, without directly weighing the crop, is to calculate the volume of potatoes in the storage in cubic feet (length X width X height) and multiply by the bulk density of potatoes (40 to 43 pounds per cubic foot) to estimate the weight of potatoes (in pounds) in the storage. The volume calculation may require more complex math depending on the shape of the storage: i.e., slant wall or Quonset-style building.
In addition, 40 pounds per cubic foot is used as the typical bulk density in the calculation. Three things primarily affect bulk density: size, shape and specific gravity of the potatoes. Tuber size will influence the porosity or air space in the pile and, in general, is assumed to be about 40%. The smaller the potatoes, the less air space and the more weight within a given volume. If storing a smaller size profile crop, it may be more appropriate to use 42-43 pounds per cubic foot. Research out of India in 2018 reported the bulk density of potatoes sized 2 to 3 ounces may be closer to 45 pounds per cubic foot. If storing potatoes with a 1.068 specific gravity versus 1.09, the bulk density could shift downward by 2% (e.g., 39.2 versus 40 pounds per cubic foot).
Overall, if using the bulk density formula, keep in mind that size, shape and specific gravity of the stored crop can contribute to the mathematical estimation of the weight in storage. Once you know how many pounds of potatoes are in storage, you can dive into the question of where losses may occur.
The greatest loss in storage can come in the form of disease or frost. Fortunately, disease and frost are not regular issues that appear in each season or storage. The amount of loss from disease can vary due to the type, level and severity of the disease. Wet rot due to pythium leak or pink rot can have acute consequences on losses, whereas fusarium dry rot may promote a slower loss with time in storage. Ultimately, secondary infection by soft rot can rapidly create severe losses.
Identifying the disease causing the breakdown issues is powerful – it allows you to effectively react to the biology of the pathogen, assess the risk of storing the crop and estimate the potential crop loss. This knowledge will help make current and future management decisions in the field, at harvest and in storage.
In storages with no or low disease pressure, the next greatest source of loss is from evaporation. This means water is being lost from the tuber surface. Humidity, temperature and airflow will affect the vapor pressure difference (VPD) at the surface of the potato, which in turn influences the level of evaporation. Higher temperatures and lower humidity will increase evaporative losses. Evaporation is even greater if the potato is immature and lacks a good skin set or if the skin is damaged due to a shatter bruise, thumbnail crack or a wound. Minimizing wounds at harvest will help lower losses due to evaporation in storage. Loss due to evaporation is highly dependent upon how the storage is managed.
Lastly, carbon dioxide (CO2) loss due to respiration, which is dependent upon cultivar, temperature and time in storage, is somewhat fixed but will be highest after loading the storage and if sprouting occurs. Although respiration loss is much lower than the loss due to dehydration (5%-10%), dry weight loss can be significant at the beginning of the storage period as potatoes are kept warm. The respiration rate is usually higher at the beginning of the storage and decreases during storage as tubers are cooled down to hold temperatures.
The theoretical equation for weight loss per week (according to ASAE EP475.3) is (A = 0.1S)D, where A is 0.7 for the first two weeks and 0.2 for the remainder of the storage season. S is the percentage (%) of sprouts by weight. D is the vapor pressure difference (VPD), which is dependent upon the relative humidity and the saturation pressure of water at a given temperature. This complex equation provides great insight into where the greatest potential for loss will come from – greater in the first two weeks in storage, due to higher respiration and temperatures, and if the storage experiences lower relative humidities. High temperatures also play a factor in the calculation. In the absence of disease or wounding, this calculation can provide insight into the potential loss in storage due to how the environment is managed.
Losses in storage equate to losses of the crop and money. Having a working knowledge of how much you have to start with and what is causing losses in storage will allow management to be fine-tuned to minimize loss and maintain quality. Loss in storage is inevitable but can be minimized with proper management.
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