An Electron Microscopy Study on the Texture of Fresh, Blanched and Sterilized Green Bean Pods (Phaseolus vulgarisL.)

Abstract

The influence of blanching and sterilization on the tissues of green-bean pods (Phaseolus vulgarisL.) was studied using scanning (SEM) and transmission electron microscopy (TEM). Fracture planes were examined by SEM, showing that the outer and inner parenchyma tissues had a different contribution to texture after heating. After heating at 90 °C the outer parenchyma tissue fractured along the middle lamellae, whereas the inner parenchyma tissue fractured through the cells. After sterilization, all cells could be easily separated along the middle lamellae, but the adhesion between adjacent cells was more retained in the inner parenchyma. By using TEM, the middle lamellae were shown to be more pronounced in the outer parenchyma tissue. A proposed causal relationship between the difference in middle lamella ultrastructure and the different fracturing behaviour of the two tissues is discussed. Blanching and sterilization in general caused partial degradation of the cell wall and middle lamella as indicated by swelling and loss of contrast. However, differences between the middle lamella of the inner and outer parenchyma tissues remained evident after these heat treatments. Cellulose microfibrils were visualized after extraction of the matrix materials from the cell walls and seemed to be unaffected by the heat treatment.