Research progress on technology and equipment for epidermis removal in agricultural products

Efficient and low-damage separation of the epidermis (skin/rind/hull) is often required in the deep processing of agricultural products, directly impacting further efficiency and product quality. Factory processing has ever rapidly expanding in recent years, as the large-scale and intensified processing industry accelerates the agricultural products. However, manual operation is heavily confined to the epidermis separation, due to the low efficiency, consistency, and high cost. The automatic equipment can be expected to fully meet the demands of modern processing. Particularly, some characteristics can be commonly found in the agricultural products: irregular shape, individual variation, and high mechanical fragility of the flesh. Existing machinery can be subjected to complex shapes and size variations, leading to low separation precision, damage to flesh tissue, and even low stability during operation. The separation effectiveness can also severely hinder the quality of subsequent processing or direct consumption. Therefore, this study aims to systematically analyze the relationship between the morphological characteristics of different materials (e.g., shape, size, epidermal adhesion properties, and flesh mechanical properties) and the adaptability to epidermis separation, starting from the fundamental scientific question of skin-flesh interfacial bonding mechanisms. A summary was also given on the working mechanisms, typical mechanical implementations, and representative examples of applicable agricultural products for mainstream techniques of epidermis separation. Some treatments included the compressive shelling, tearing-based peeling, frictional comb-brushing, cutting-based peeling, and chemical/high-temperature softening. A systematic review was presented on the mechanical research, in order to improve the high production efficiency and low-damage separation. Furthermore, there were the dynamic mechanical responses of materials during separation, the required separation forces (e.g., peeling force, shear force), and the effective detachment force between flesh and epidermis. Furthermore, the current research and development status of the technologies and equipment was also obtained for the epidermis separation, including the mechanical mechanisms, equipment innovations, and applications. The key technological and equipment advances were also reviewed for the pre-separation processing (e.g., grading pretreatment) and post-separation processing (e.g., sorting, and epidermis recycling). Specifically, the development trends and application prospects of intelligent technologies were explored in the field of epidermis separation. Intelligent equipment was outlined to integrate the machine vision (MV), deep learning (DL), multi-sensor information fusion, as well as the environmental perception and decision-making, in order to enhance the equipment's adaptability and intelligence levels. Finally, some recommendations should be focused on: the highly adaptable and intelligent epidermis separation equipment; the standardized process systems; and the collaborative innovation across the entire industry chain to integrate the raw materials and equipment with the process optimization. This finding can provide a systematic theoretical reference and practical guidance to select the epidermis separation of agricultural products, as well as the key technologies and equipment. It holds significant importance to promote the technological upgrading and high-quality development of product processing in the agricultural industry.