Effects and mechanism of blanching treatment on peeling of Citrus reticulata Blanco

Abstract: Blanching treatment is often used to reduce the difficulty of peeling Citrus reticulate Blanco before the real peeling process. However, the mechanism of Citrus reticulata blanching is still unclear, particularly for the relevant theoretical support in the actual processing. It is a high demand to clarify the effect of the peeling process in the special way of blanching treatment. This study aims to determine the effect of blanching treatment on the mechanical characteristics, tissue structure, and the peeling performance of Citrus reticulata Blanco peel. A test platform of Citrus reticulata peel separation was used to evaluate the peeling performance of Citrus reticulata under different blanching times. The tensile test was also carried out to evaluate the mechanical characteristics of blanched Citrus reticulata peel. The ultra-depth of field microscope and Electron Microscope (EM) were utilized to characterize the pericarp microstructure, white layer, and flesh structure based on the Ray CT tomography. The size of the exocarp of the Citrus reticulata was observed after blanching treatment. Therefore, the influencing mechanism was then established for the blanching treatment on the peel ability of Citrus reticulata. The experimental results showed that the blanching time posed a significantly vital and crucial effect on the maximum tensile force, the separation displacement, the peel breaking strength, and the porosity between the peel and pulp during Citrus reticulata peel separation (P<0.01). There was a positively correlated relationship between the separation displacement and the breaking strength. A negatively correlated relationship was found between the maximum tensile force and the porosity between peel and pulp. The peeled surface increasingly tended to be wrinkled, rough, and coarse with the increase of blanching time in the experiment. The pectin compounds on the peel cross section were slowly dissolved, whereas, the parenchyma cells and vascular bundle tissues totally collapsed and disappeared forward a single step. There was a gradual increase in the porosity between the peel and pulp. The peel-breaking strength changed accordingly, which first increased and then decreased. As a result, the minimum tension during peeling was reduced, where the separation displacement first increased and then decreased, the same as the peel-breaking strength. Once the blanching time was 3 min, the maximum tensile force reached the minimum of 1.59 N, which was reduced by 42.8%, compared with the control. The separation displacement also reached the maximum of 41.81 mm, which increased by 19.6%, compared with the control. The overall peeling effect was the best at that time. But after the blanching time was more than 3 min, the peel with soft foam to break cannot be easy for the peeling. This study provides a theoretical reference for improving the difficulty of peeling Citrus reticulata reticulata Blanco and enriching the mechanism of blanching peeling.