Suitability of Two Instruments to Determine Skinning Resistance in Sweetpotato

Skinning or surface abrasion in sweetpotato (Ipomoea batatas) occurs during harvest and causes substantial losses in marketable products.  Skinning occurs when abrasion forces the periderm to break across the phellem (tensile fracture) and along the phellogen (shear fracture) resulting in the separation of the phellem from the phelloderm.  In this study, we evaluated two instruments to measure skinning resistance in sweetpotato and how differences in washing and handling to remove the soil, which may cause partial loss of the phellem, influence the subsequent measurements. Beauregard (B-14) plants grown in the field at the Pontotoc Experiment Station (Pontotoc, MS) were devined or treated with ethephon (1.5 kg·ha^-1) before harvest. Storage roots were taken at 3 and 7 d after treatment (DAT) and divided into two groups for measurements.  Roots from one group were gently washed to remove the soil, while the other roots were washed by moderately rubbing the root surface.  Skinning resistance was measured by both a digital force gauge (model DS2-11 3100; Imada, Northbrook, IL) that measures the force required to peel the skin (shear fracture) and a torque meter (model TQS050FUA; Snap-on. Kenosha, WI) that measures the torque to twist and snap off the skin (tensile and shear fracture).  Skin phenolics and lignin/suberin contents were also measured.  Both instruments were able to detect differences in skinning resistance among treatments, but handling methods had an effect on the results from the force gauge.  In general, ethephon treatment increased skinning resistance in comparison to devining and untreated control.  In contrast, ethephon reduced skin phenolics content at 3 DAT, but this difference was lost at 7 DAT.  Handling, however, resulted in consistent differences in skin phenolics content.  Finally, skin lignin/suberin content was not different among treatments as well as between handling methods.