Bruise Testing

Catechol and Tetrazolium Tests

Other pages about bruise defects:

There are three ways of evaluating internal bruises, especially (internal) black (brown) spot: abrasive peeling, homogenization and impact-bruising methods (Dean et al., 1993). The first two give the most consistent results. The most common way growers and processors use is the catechol test in conjunction with peeling. The catechol test is used primarily to detect and evaluate external bruises: skinning, shatter and depth of cuts. Catechol is a polyphenol that reacts with wound enzymes turning purple to dark red. The following procedure is adapted from Gould, W.A., 1995 and Kleinschmidt, G. and Thornton, M. (Eds.), 1991.

  • Take a 10 tuber sample, three to five pounds, and wash off soil etc.
  • Immerse tubers for one to five minutes in 1.5% catechol or 2 oz catechol per gallon with a bit of soap, detergent or nonionic surfactant to break the water tension.
  • Drain and let stand for one to 10 minutes.

If there are purple or dark red areas than there are bruises. The severity of the bruises is estimated by the number of strokes with a potato peeler in takes to get below the coloration. One stroke indicates a surface bruise probably skinning or slight shatter. Two strokes indicate a shallow bruise, shatter, deep abrasion or slight cut. If more than two strokes are needed to remove the catechol stain than the bruise is deep and serious.

Tetrazolium testA specific test for (internal) blackspot (or brownspot) or IBS is called the tetrazolium test. The method is:

  • Collect a tuber sample and wash them.
  • Peel the tubers slightly then place them into a solution of 1% 2,3,5-tetrazolium chloride for about 40 minutes.

Bruises will appear as dark pink areas in a matter of minutes (Picture). The staining is most pronounced when this test is done under sunlight and when the tetrazolium was dissolved in water warmed to room temperature (70 F). Note tetrazolium is toxic to animals including us; so the potatoes and left over solution needs to be disposed of safely.


Dean, B.B., Jackowiak, N., Nagle, M., Pavek, J., and Corsini, D. 1993. Blackspot pigment development of resistant and susceptible Solanum tuberosum L. genotypes at harvest and during storage measured by three methods of evaluation. Amer Potato J 70:201-217.

Gould, W.A. 1995. Methods of detecting bruise damage at harvest. Valley Potato Grower. August. pp. 11-12.

Kleinschmidt, G. and Thornton, M. (Eds.) 1991. Bruise-Free Potatoes: Our Goal. Univ Idaho Coop. Ext. Bull. # 725.