This article appears in the December 2017 issue of Potato Grower.
During long-term bulk storage, potatoes often develop the depressed or sunken areas referred to as pressure flattening. Pressure flattening occurs as potatoes are exposed to pressure over time from the weight of other potatoes in the bulk pile. The height of the pile in which the potatoes are stored and the duration they are stored generate the accumulated pressure to which the tubers are exposed.
The development of pressure flattening is also a result of dehydration of the tubers that occurs during harvest and storage. This is because less-hydrated potatoes are less resistant to pressure. During shipping, pressure flattened areas may be large enough for potatoes to fail to meet USDA grade standards. Later in the storage season, pressure flattening often causes a shipment to be downgraded from U.S. No. 1 to U.S. No. 2 or lower, causing large economic losses to the grower. In some cultivars, pressure flattening can result in discoloration beneath the affected area. This discolored and depressed area is referred to as a pressure bruise and is a major concern for chipping and processing as well as for fresh market users.
Due to environmental conditions, agronomic management practices and cultivar differences, some fields and/or cultivars are more likely to develop pressure flattening earlier in the storage season than others. Our objective has been to develop a predictive test that can be done at-harvest or early in the storage season that could identify the tubers that may be more prone to pressure flattening. With that information, growers and shippers could determine which potato bins to ship first to reduce pressure flattening losses.
To conduct our testing we used both a 10-kilogram and a 25-kilogram load capacity instrumented penetrometer to measure the maximum amount of force required to deform the surface of several dozen tuber samples each from many fields and varieties. This “peak load” value in grams was the value we used to compare the different fields, treatments and varieties. The higher the peak load, the more resistant the samples were to deformation under pressure. The use of a penetrometer or texture analyzer to predict relative pressure flattening of stored potatoes was considered a novel concept. What our data has shown is that penetrometer testing at-harvest or early in the storage season can identify differences in moisture loss and possibly variety susceptibility to early pressure flattening.
We tested tubers subjected to moisture loss with the penetrometer to evaluate the peak load as tubers lost weight at 0.5 percent intervals. The testing was able to identify fairly small amounts of moisture loss from tubers. Potatoes prone to moisture loss are considered to be more likely to pressure flatten during storage. It was also interesting that the peak loads consistently separated the varieties as firmer or softer.
The at-harvest peak loads of tubers samples were compared to the pressure flattened area per tuber after three months of storage. The results indicated a correlation between at-harvest peak load required for deformation and pressure flattening development after three months of storage.
After a few years of testing across a lot of different fields and varieties, we wanted to know how much pressure flattening occurred after storage from the highest peak load fields versus the lowest peak load fields. In other words, although the predictions were not always correct for every single field, how much difference in pressure flattening was found when the top half of the fields were compared to the bottom half? Overall, would a grower come out ahead by shipping one group of fields first and another later? So we took all of our peak load values from many fields and varieties and separated them from highest to lowest and looked at the results. On average, there was about twice as much pressure flattening on samples from the lowest peak load fields compared to samples from the highest peak load fields.
The height of the bulk stored potato pile is another important factor in the development of pressure flattening. Our results show that the peak load results were predictive of differences in pressure flattening even when samples were stored at different pile heights.
At-harvest penetrometer testing was able to show differences among varieties as well as individual fields within each variety. In a situation where test results are similar to those shown in Figure 5, our research suggests that pressure flattening losses will be reduced if Gold Variety 3 is shipped first, followed by Gold Variety 1 and maybe some of the 
Gold Variety 2 fields such as 12 and 1. Fields 2, 3, 4, 10 and 14 from Gold Variety 2 should be among the last to be shipped unless disease pressure or skinning are worse in those fields.
Based on the differences in the resulting pressure flattening between the groups of fields and cultivars arranged based on peak load values, it appears that use of penetrometer testing at harvest will identify the majority of potatoes that are likely to pressure flatten earlier in the storage season. Growers may also be able to use at-harvest penetrometer testing to adjust pile height for a new cultivar or a cultivar for which a grower is not aware of its pressure flattening susceptibility.
Putting the penetrometer data together with other at-harvest observations on the disease, skinning and bruising factors will make it easier to determine the most profitable order to ship potatoes out of bulk storage.