pharmaceutics Article Drying Behavior and Kinetics of Drying Process of Plant-Based Enteric Hard Capsules Chuqi He 1,2 , Haodong Wang 1,2 , Yucheng Yang 1,2 , Yayan Huang 1,2 , Xueqin Zhang 1,2 , Moses Arowo 3 , Jing Ye 1,2 , Na Zhang 1,2, * and Meitian Xiao 1,2, *   Citation: He, C.; Wang, H.; Yang, Y.; Huang, Y.; Zhang, X.; Arowo, M.; Ye, J.; Zhang, N.; Xiao, M. Drying Behavior and Kinetics of Drying Process of Plant-Based Enteric Hard Capsules. Pharmaceutics 2021, 13, 335. https:// doi.org/10.3390/pharmaceutics 13030335 Received: 3 February 2021 Accepted: 26 February 2021 Published: 5 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 College of Chemical Engineering, Huaqiao University, Xiamen 361021, China; he-sylvi@stu.hqu.edu.cn (C.H.); ishdwang@126.com(H.W.); yangyc@hqu.edu.cn (Y.Y.); yyhuang@hqu.edu.cn (Y.H.); xqzhang2009@hqu.edu.cn (X.Z.); yejenny@hqu.edu.cn (J.Y.) 2 Xiamen Engineering and Technological Research Center for Comprehensive Utilization of Marine Biological Resources, Xiamen 361021, China 3 Department of Chemical & Process Engineering, Moi University, 3900-30100 Eldoret, Kenya; musarowo@yahoo.com * Correspondence: zhangna@hqu.edu.cn (N.Z.); mtxiao@hqu.edu.cn (M.X.) Abstract: The drying process is a significant step in the manufacturing process of enteric hard capsules, which affects the physical and chemical properties of the capsules. Thus, the drying characteristics of plant-based enteric hard capsules were investigated at a constant air velocity of 2 m/s in a bench scale hot-air dryer under a temperature range of 25 to 45 ◦ C and relative humidity of 40 to 80%. Results indicate that the drying process of the capsules mainly occur in a falling-rate period, implying that moisture transfer in the capsules is governed by internal moisture diffusion rate. High temperature and low relative humidity reduce drying time but increase the drying rate of the capsules. Investigation results of the mechanical properties and storage stability of the capsules, however, reveal that a fast drying rate leads to plant-based enteric hard capsules of low quality. Scanning electron microscopy further demonstrates that more layered cracks appear in capsules produced under a faster drying rate. The Page model yielded the best fit for describing thin-layer drying of the capsules based on the coefficient of determination and reduced chi-square. Moreover, it was established that the effective moisture diffusivity of the capsules increases with an increase in drying temperature or reduction in relative humidity. Keywords: plant-based enteric hard capsules; hot-air drying; drying characteristics; modeling 1. Introduction The word capsule originates from the Latin word “capsula” and its original meaning is “small box.” The development of capsules has nearly 173 years of history since James Mur- dock patented the two-piece hard capsules in 1847 [1]. Enteric hard capsules have attracted attention because of their role in targeted drug release [2], where they are designed to remain intact in the stomach and then release the active substance in the upper intestine [3]. Nonetheless, gelatin capsules have deficient physicochemical properties and safety issues such as risks of animal disease [4], cross-linking with aldehyde materials and ability to get soft and brittle under extreme conditions [5–7]. Moreover, the process of coating enteric material on the gelatin capsule’s surface is complicated so that the distribution of enteric coated materials on the surface of gelatin capsules is poor [8]. Meanwhile enteric coated materials are also relatively expensive [9]. Therefore, much attention has been paid to the development of plant-based enteric hard capsules [10]. The enteric coated materials that have been developed for plant-based capsules are usually anionic polymethacrylate, cellulose acetate phthalate or polyvinyl acetate phthalate. Huyghebaert et al. had used each of the three materials to make enteric-coated hydroxypropyl methylcellulose (HPMC) capsules [11]. But calcified coating is one of the emerging simple methods of preparing Pharmaceutics 2021, 13, 335. https://doi.org/10.3390/pharmaceutics13030335 https://www.mdpi.com/journal/pharmaceutics