Detection of freeze damage in Virginia peanuts (Arachis hypogaea L.) by conductivity, tetrazolium, dipicrylamine, and visual tests

There has been controversy over the accuracy of grading peanuts for freeze damage by United States Department of Agriculture (USDA) guidelines. Several other tests for estimating freeze damage in peanuts were investigated. The conductivity test, which measures electrolyte leakage from cells, and the tetrazolium test, a test currently used for estimating viability of seed peanuts, were adapted for assessing freeze damage. The dipicrylamine test, a qualitative spot test for potassium, was developed and used to detect potassium leakage from cells onto the surface of the cotyledon. A visual test was developed based on differences in glossiness of the flat surface of the peanut cotyledon.

A freezing apparatus was designed and operated so that peanuts could be cooled at a slow, even rate to a series of low temperatures. These peanuts and peanuts exposed to a natural freeze were tested for freeze damage.

The USDA visual test underestimated freeze damage compared to all of the tests used in this study. The VPI visual test differentiated damaged from undamaged peanuts using subtle differences in glossiness that are not considered by USDA visual test guidelines. Results of the VPI visual, tetrazolium, and dipicrylamine tests for peanuts exposed to the laboratory freeze treatments were not significantly different. The dipicrylamine test is a rapid test, but interpretation of color differences proved to be difficult. The tetrazolium test revealed the greatest differences between damaged and undamaged peanuts and these differences were easily interpreted. The tetrazolium test is, therefore, favored over the other tests for grading freeze damage in peanuts.