**1. Introduction**

The Walnut (*Juglans regia* L.) is one of the oldest cultivated fruit species in the world [1]. The kernels have excellent nutritional and therapeutic value due to their high content of unsaturated fatty acids [2] and abundant amino acids and minerals [3,4]. In postharvest and processing, compared to other operations (e.g., cleaning, drying, storing, etc.), walnut-shell cracking to extract the kernel from the internal nut is not only the most important operation but also the fundamental goal, which can be attributed to the fact that the usable part of tree nuts is not the walnut itself but the kernel, especially the whole kernel because consumers prefer whole kernels [5]. However, during the process of walnut cracking, unexpected phenomena (e.g., incomplete nutshell cracking and broken nut kernels) occur with existing cracking devices, which are the key factors that affect the quality of the final kernel and limit the development of the initial processing [6]. The cracking performance is strongly related to the intrinsic properties of the walnut (e.g., shell thickness and moisture content), cracking device configurations (e.g., roll and hammer),

**Citation:** Zhang, H.; Liu, H.; Zeng, Y.; Tang, Y.; Zhang, Z.; Che, J. Design and Performance Evaluation of a Multi-Point Extrusion Walnut Cracking Device. *Agriculture* **2022**, *12*, 1494. https://doi.org/10.3390/ agriculture12091494

Academic Editor: Bengang Wu

Received: 11 August 2022 Accepted: 13 September 2022 Published: 18 September 2022

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and operational conditions (e.g., shaft rotation speed). Hence, several researchers have proposed a series of effective methods to improve cracking performance in terms of the three aspects mentioned above [7–9], particularly the latter two factors.

Li [10] studied the effects of the gap between the two rotating cracking rollers and roller speed on walnut cracking. It was found that increasing the rotational speed and decreasing the pitch increased the fracture force, thereby resulting in an increase in the specific deformation of the walnut shell [11]. Shi et al. [12] evaluated a cam rocker bidirectional extrusion walnut cracking device using squeeze clearance, camshaft speed, and walnut circumference as the test factors. Bernik et al. [5] investigated the cracking quality of three types of walnuts at different speeds under a modified centrifugal cracking machine. By analyzing the above-mentioned research, it was found that the contact type between the walnut and the cracking device was mostly a single point or line load. However, it was also found that the contact types mentioned above would result in uneven forces on the shell, poor crack extension, and easy damage to the inner kernel [13]. Additionally, studies have also suggested that different contact types have a significant effect on walnut cracking. For example, Zhang et al. [14] reported that spherical compression was the best process for walnut cracking. In addition, Shen [15] discovered that adding spikes to the surface of the V-indenter for shell cracking was substantially more successful. According to the above analyses, increasing the stress concentration areas contributes to the generation and expansion of cracks while reducing the inner shell force and deformation value of walnut kernels, thus further minimizing mechanical damage to the kernels.

Unfortunately, there are few practical applications for walnut cracking devices based on multiple load contact. For instance, Cao et al. [13] observed superior walnut-cracking outcomes using a hammer head with seven grooves. Furthermore, He et al. [16] improved cracking device performance by adding rectangular or trapezoidal grooves to the surfaces of the cracking rollers. Nonetheless, the quality of cracking still needs to be improved. Therefore, by integrating the results of previous research, a multi-point extrusion walnut cracking device was designed and its operating parameters were optimized using the RSM in this work. The influence of each factor on the evaluation index and the interaction between the factors were analyzed, respectively. The best combinations of parameters were determined using the regression analysis method with the help of MATLAB software, which was used to verify the practical suitability of several walnut varieties.

#### **2. Materials and Methods**
