Shrink of potatoes in storage is directly related to pressure bruise. Dehydration makes potatoes soft, allowing for indentation from the weight of potatoes in a pile. Shrink is weight loss; pressure bruise is a symptom and the result of that shrink. Shrink loss can be heavy—as much as 15 to 20 percent of the potato crop—and significantly reduce the bottom line. Any amount of shrink can reduce profit proportionally to a grower, and in too many cases, can even represent a grower’s entire annual profit.
Shrink begins in the field during the growing season, making it essential for proper and efficient irrigation practices. However, it manifests itself most in storage. If potatoes are placed in storage in a flabby, dehydrated condition, they have already lost weight and are subject to pressure bruise immediately upon being placed in the pile. Reducing the height of a pile or using basket or box containers has proven to reduce pressure bruise by reducing the weight of potatoes stored high in the pile on those stored below. However, pressure bruise can also be avoided by keeping potatoes hydrated before placing them into storage. Growers have learned that irrigating the field prior to digging helps keep the potatoes firm when harvested.
The leakage of poorly constructed storage buildings can have a negative and costly affect on the preservation of potatoes. If you enter a storage when it is empty and can see light through cracks and holes in the walls and doors, these should be repaired or at least covered prior to filling with potatoes. Studies done in the UK have pointed out that “it’s difficult to spot energy leaking from a store through badly fitting doors, old louvers and the like, but it can cause no end of problems, including high energy use, hot and cold spots where condensation can form on potatoes—leading to the development of disease—and reduced effectiveness of chlorpropham (CIPC) applications.”
Respiration that occurs in the first six weeks of storage, commonly known as the sweat period, can cause additional dehydration. Test results produced by North Dakota State University have shown that “respiration accounted for about one-tenth to one-half of the total weight loss, except when tubers were stored in 100 percent relative humidity. Under this condition, there was an actual net gain, apparently from the tubers absorbing moisture from the surrounding atmosphere.”
Air distribution of conditioned air is severely reduced or lost if the conditioned air is leaked or intentionally ventilated excessively from the storage throughout the storage season. Excessive ventilation is applied for cooling and as a precaution against the assumed accumulation of carbon dioxide (CO2) produced by stored potatoes. CO2 should be monitored and controlled as needed by systems specific for the purpose such as the system provided by Isaacs Hydropermutation Technologies (IHT) for its Humigation process and atmosphere control. Ventilation activated by CO2 reading instead of by temperature is a better method for controlling shrink and disease than applying excessive ventilation at random.
IHT’s Humigation process rehydrates potatoes immediately after they are placed in storage without producing excess water in the plenum or ventilation tubes. However, since pressure bruise occurs before the rehydration process is implemented, the flat spots caused by pressure will prevail and cannot be removed, even though the potatoes are firm and solid and there is no weight loss. The value of potatoes with flat spots is reduced because of appearance and the perception of buyers and consumers who have seen deterioration and brown coloring related to pressure bruise.
Refrigeration immediately after harvesting lowers the temperature of the storage air. Cold air does not hold much moisture even when relative humidity is high. Cold, dry air shrinks potatoes and hinders rehydration, which also contributes to pressure bruise. Preserving potatoes at higher temperatures increases profits of growers, packers and processors because shrink and pressure bruise can be avoided and less starch is converted to sugars, making the cooking process and appearance better. Refrigeration has become habit in an attempt to reduce the spread and multiplication of pathogens of diseases such as soft rot, silver scurf, black dot and Fusarium. Reducing the population of these costly, airborne disease agents by means other than chemicals or refrigeration enhances the profits of potatoes in storage. All of these agents are airborne and prevalent in storage air even though they are invisible. Scrubbing storage air with processes like Humigation and removing disease-causing agents prevents the spread of disease, thereby reducing the need for aggressive refrigeration. The value of storing potatoes above 50 degrees Fahrenheit is immeasurable but is certainly an advantage.
Conscientious and advanced storage measures for the reduction of shrink and pressure bruise are sound management practices that protect the bottom line. A little effort and a small investment toward improvement to storage facilities and ventilation practices can be very profitable for potato growers, packers and processors.
This article appears in the September 2016 issue of Potato Grower.