Pressure bruise is a major defect and source of economic losses in potatoes for all market classes.
In bulk storage, potato tubers are subjected to pile pressure that can damage and deform the potato, which over time could result in flattened or sunken areas, according to Sastry Jayanty and Nora Olsen, Potato Association of America.
In an article published in Spudman magazine, Jayanty and Olsen say that these flattened areas are considered to be a pressure bruise if there is a grey/black discoloration under the depression.
In some cases, there is no discoloration, and these symptoms are called pressure flattening. Discoloration symptoms take time (hours to days) to develop as the damaged area becomes exposed to oxygen. Pressure flattened and bruised tubers can be found throughout the pile depending upon why it occurred, but is often found between the vent pipes or next to the adjacent walls of the storage bin in the bottom 4 to 5 feet of the bulk storage. This is due to a greater force of weight (the pile of potatoes) on less hydrated potatoes.
The development of pressure flattening is a result of dehydration of the tubers. Loss of tuber hydration can happen before and after vine kill and prior to harvest if soils become too dry. Moisture loss from tubers can also occur during harvest and handling due to wounding, skinning, cuts and bruises — increasing exposed surface area for evaporation to occur.
In storage, tubers lose moisture due to a high vapor pressure deficit (function of temperature and humidity), wounded or poorly suberized potatoes, sprouting, improper ventilation and humidity and high respiration.
Measuring tuber texture with a texture analyzer can indicate turgidity of the tuber, cellular arrangement, skin toughness and the tubers’ overall ability to withstand force prior to deforming — potentially causing a pressure bruise.
Texture analysis of a tuber (y-axis) correlates to tuber moisture content and can indicate differences between cultivars. Greater force is needed to deform tuber tissue correlated to a more hydrated tissue. Cultivars vary in the potential for moisture loss and susceptibility to pressure bruise.