On-farm Bruise Assessment

Data is powerful in minimizing bruise

Published online: Jul 28, 2017 Articles Nora Olsen & Mike Thornton
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This article appears in the August 2017 issue of Potato Grower.

A great tool to identify where bruise may be occurring in your harvest and handling operation is to take tuber samples at various spots, warm them, and evaluate for the level of damage. Creating your own quality assessment protocol allows you to take into account the physiological (nutrient, maturity, specific gravity) and physical (hydration level, temperature, shape, size) aspects of the tuber that contribute to both shatter and blackspot bruise susceptibility—as well as identify the locations of greatest impact. The assessment encompasses all the complex components contributing to bruise formation for your operation and variety. Unfortunately, it is a laborious chore. The game is to hasten the process so changes to equipment or harvest timing can be made as soon as possible.

The development of blackspot bruise takes time after the force of impact damages the cells. The blue-black discoloration is the formation of a pigment called melanin. It typically takes about 24 to 48 hours for the pigment to develop at room temperature. However, the higher the temperature, the quicker the pigment formation will occur, and the sooner the results will be available.

Shatter bruise can be rated without warming tubers, but it is often difficult to see each fissure or opening. We have outlined a few strategies to use in developing an on-farm bruise assessment.



Select various points along the harvest and handling operation to take samples (20 to 25 tubers at each spot). Identify areas or equipment that may have a greater impact on the potatoes. Common points are after the windrower, after the harvester, off of trucks, various drops in conveying, and in the pile. Properly denote each sample bag of potatoes with field and/or variety, which piece of equipment or point of sampling, date and time, and pulp temperature. If you are running more than one windrower or harvester in the same field, know which sample is taken from which piece of equipment. Timing may be off on one windrower but not the other. The idea here is to identify where the cause of the bruise is coming from.

If you are primarily focused on identifying impacts resulting in shatter bruise, after collection, wash tubers and evaluate for damage. See below for an example on how you can evaluate and rate the samples for shatter bruise.

If you need to assess for blackspot bruise, the samples must be warmed to hasten the process. Blackspot color will develop after about 48 hours at 50 degrees Fahrenheit, about 24 hours at 70 degrees, and 6 to 12 hours at 90 degrees. Time needed for color to form will be variety- and field-dependent. There is no need to wash the potatoes first, but keep them in the dark.

Commercial “hot boxes” are available for purchase to warm samples. These boxes are handy, since they are designed to handle higher temperatures, although they are often smaller and can only hold a few samples at a time. Other options are to set samples in a mechanical area where heat is given off by equipment or find or construct a room where a space heater can be placed to increase air temperature. Use a thermometer to get an idea of the temperature and how long samples need to be in the room. Use all space heaters with safety in mind.



Deciding how to evaluate samples is dependent on how you want to use the data to make changes to your operation. You may wish to mimic how the buyer of your potatoes would assess for damage. Here are some ideas for ways to evaluate and rate for shatter and blackspot bruise.

  1. If you just want to determine presence or absence of a bruise, look on the outside for any obvious shatter bruise and peel the potato for any blackspot bruise. Write down how many tubers have a bruise from each location you sample. Divide by the total number of tubers evaluated to give a percent incidence at that location (e.g., 50 percent blackspot and 12 percent shatter bruise from windrower 1).
  2. If you want to know the number of bruises per tuber and whether you are increasing the risk for bruising as the potato is being handled, count the number of shatter bruises, then peel and count the number of blackspot bruises. This will allow you to see if multiple bruises are occurring on the same potato but from different locations (e.g., 50 percent with one blackspot bruise from windrower 1, 15 percent with one blackspot bruise and 65 percent with three-spot blackspot bruise from off of truck).
  3. If you want to know the incidence plus the severity of bruise, there are several options to use. Always keep track of the total number of tubers evaluated in order to calculate an overall percentage.
    1. Count the number of slices from a peeler it takes to no longer see the shatter or blackspot bruise. The more slices removed, the greater the severity of the bruise. You can also denote, for example, if three or more slices equals severe damage. Keep shatter and blackspot bruise numbers in separate columns.
    2. Develop your own subjective rating scale or use one already available. An example for blackspot bruise would be: 0 = no bruise; 1 = slight discoloration; 2 = obvious blackening; 3 = intense black color. Another example is to rate by the size of the bruise: 0 = no bruise; 1 = mild; 2 = less than 0.2 inches in diameter; 3 = 0.2 to 0.5 inches; and 4 = larger than 0.5 inches. For shatter bruise, measure the length of the bruise.
    3. Weigh the total tuber sample, cut away damaged tissue, weigh damaged tissue separately, and divide damaged tissue weight by total weight (e.g., 300 grams damaged/9,600 grams × 100 = 3.1 percent damaged). This is how most potatoes are graded for inspection. Count how many tubers are free from damage to get a percentage of bruise-free tubers.

Regardless of the method you use to evaluate your potatoes for bruise, use the collected data to pinpoint areas of your operation where you can modify equipment operation to minimize damage from impact.