Research progress of key technology and device for size-grading shell-breaking and shell-kernel separation of walnut

Abstract: Walnut varieties are rich with the wide planting area in China, and the high yield ranked in the world. The walnut kernel is the most important edible part of walnut fruit. The by-products of walnut, such as walnut shell, have also great application potential to serve as cheap raw materials for the extraction of some important medical ingredients and the production of industrial products. However, the by-products are often burned or discarded as wastes in the processing process, leading to environmental pollution. The intensive processing of walnut products is increasing daily demanding with high nutrition and economic added value, indicating the walnut has a broad prospect of comprehensive development and utilization. Walnut size-grading, shell-breaking and kernel-fetching, as well as shell-kernel separating are the most critical the links in the primary processing, which are the prerequisite before the intensive processing of walnut. However, the traditional manual operation mode and the primary processing equipment with simple functions cannot meet the high quality and quantitative requirements of the food industry, which seriously restricts the development of the walnut industry. Therefore, it is imperative to strengthen the research and development of key technologies and devices for walnut primary processing. Based on the size characteristics, four devices were used in walnut size-grading, including the type of taper roller, grid cylinder, sieve, and visual imaging. The status quo and application characteristics of devices were summarized in this review. The combinations of optimized parameters can be obtained from the multi-factor performance tests for different types of devices. The physical characteristics of walnut were important basis for the design of shell-breaking device, such as sphericity, shell thickness, and moisture content, due to they directly determine the mechanical properties of walnut shell and kernel. The specific ranges of parameters were determined to represent different physical characteristics. Three main principles of walnut shell-breaking were then classified, including extrusion, collision, and gas explosion. The characteristics and applications of core shell breaking were reviewed, with emphasis on three innovative shell-breaking devices. The multi-factor performance test can be used to optimize the parameter combination of main influencing factors in each type of device. In shell-kernel separation, the winnowing type was the main application in the relevant devices at present. The morphological characteristics and physical parameters of walnut mixed shell-kernel were the main basis for setting the wind speed and direction. An attempt was made on the separation mechanism of shell-kernel by winnowing type, the optimization parameters of various shell-kernel separation devices and their respective multi-factor performance tests. This review systematically summarized the physical characteristics, working mechanism, device, and performance optimization test, corresponding to the key links of walnut primary processing. The finding can be expected to deepen the understanding of walnut primary processing, and further provide theoretical basis and technical support for improving walnut processing level.