Micromechanical Behavior of Potato Tissue by Scanning Electron Microscopy: Effect of Storage Time, Impact Energy and Curvature Radius of Impact Location

Abstract

Introduction The mechanical impacts occur mainly during harvesting and post-harvesting operations, lead to the breaking of cell membranes in cellular structure that dependS on impact intensity. Furthermore, turgor pressure of potato tissue is influenced by the micromechanical and the physiological changes in the storage duration. Micromechanical changes of potato tissue due to the mechanical impact need to be monitored by microscopic images during storage. Scanning electron microscopy (SEM) is a high-resolution technique used to investigate the micromechanical behavior of potato tissue. Materials and Methods Potato samples (‘Sante’ cultivar), were stored at 5 ± 0.5°C and 85% relative humidity for 16 weeks. By 2-week intervals, potatoes were removed from the storage and then the impact test was done. Experimental factors were impacted energy at three levels of control (no impact was done), impact energy 1 (0.031 ± 0.002 J) and impact energy 2 (0.320± 0.020 J) and the radius of curvature at two levels of (35 and 45 mm). Water content was measured by drying thin slab samples in an oven at 70°C to a constant weight. The cell turgor pressure of potato tissue at 2-week intervals was estimated from the linear regression between turgor values of each mannitol solution (0–0.6 M) and relative volume change. The microstructural changes of impact location on the potato tubers were analyzed by SEM images at 2-week intervals during storage period. The surface and depth sections cutting from the impact location were immediately immersed in 2.5% glutaraldehyde in 0.1 M sodium phosphate buffer (2 h) at 4± 0.5°C. The specimens were then rinsed 3 × 10 min in 0.1 M sodium phosphate buffer (pH 7.2), and dehydrated through an ethanol series, 25, 50, 70, 90 and 100% dry, 15 min each step, 2 × 100%. In this study the HMDS as a high-quality chemical drying was investigated. The sample preparation for SEM observation then followed by chemical drying via HMDS, under a laboratory hood overnight. Analysis of variance test based on completely randomized design (CRD) was considered for all of the data using SPSS 23. Results and Discussion Superior preservation of potato microstructure was obtained by hexamethyldisilazane (HMDS) drying during sample preparation for SEM observation. The MIP software was used for quantitative analysis of SEM images and the microstructural features of potato tissue at the impact location were determined. So that each cell outline was manually separated by drawing the lines along the visible contours of cell walls. Measurement of the impact damage dimensions was done by MIP software for the surface section (major and minor width, w1 and w2) and the depth section (depth, d and major width, w1). The results indicated the significant differences between water content, cell turgor pressure, cell area and cell perimeter over 16 weeks storage. Generally, by increasing impact intensity the water content, cell turgor pressure, cell area and cell perimeter significantly decreased. Also interaction effect of storage time, impact level and radius of curvature for impact damage of potato tissue was significant. Conclusions The cell turgor pressure at the impact location on the potato tubers had the similar trend with the water content. SEM investigation showed that potato parenchyma, which was high preserved by HMDS drying, had consisted of the pentagon and hexagonal thin-wall cells with the average cell area of 23.14 × 103 ± 0.178 μm2, the average cell perimeter of 564.98 ± 2.008 μm at week 0. The higher impact damage was at week 16 of storage, impact level 2 and the radius of curvature of 35 mm compared to the other treatment.

Keywords

• cell turgor pressure
• impact damage
• micromechanical behavior
• scanning electron microscopy (sem)
• storage time