Effect of rheological properties of potato, rice and corn starches on their hot-extrusion 3D printing behaviors
Introduction
Emphasis has been placed on the diversification and personalization of food to meet the special demands of particular groups of consumers such as the elderly, children and athletes. Given this, 3D printing technologies have been introduced and adapted to meet the demand of food design and related food materials processing. Food 3D printing, also known as food layered manufacturing (Wegrzyn et al., 2012; Yang et al., 2017), is capable of eliminating the requirement of particularly shaped molds and potentially offers a much wider design space beyond unusual shaping (Kokkinis et al., 2015). Moreover, 3D printing technology can also revolutionize food manufacturing by the ability to fabricate 3D constructs with complex geometries, elaborated textures, and tailored nutritional contents (Sun et al., 2015). Among all food 3D printing technologies, extrusion 3D printing, especially hot-extrusion 3D printing (HE-3DP), has drawn much attention due to its ability to deposit ingredients to solid geometries (Long et al., 2017). HE-3DP involves extruding a molten or semi-solid material through a small-diameter nozzle moving along the X- and Y-directions, and the printing platform moves down in the Z-direction for the deposition of the next layer (Jafari et al., 2000).
A series of preferable properties of printing media for HE-3DP includes the ease of loading into the printer syringe and extruding from its fine nozzle, the sufficient mechanical integrity of printed threads to support stacked layers without printing defects such as buckling and sagging, and the high stability of threads after their deposits to ensure a good resolution of the printed object. All of these properties can be well reflected by the rheological behaviors of printing media. Specifically, the printing media should be shear-thinning and with suitable flow stress to be easily extruded from the fine nozzle (Duoss et al., 2014; Le Tohic et al., 2018). Furthermore, the printing media should be not only viscoelastic but also elasticity-dominant (tan δ < 1), and have high yield stress to avoid the inconsistent printing from broken threads. More importantly, the media should present a rapid and reversible modulus response to shear stress to ensure a good resolution of printed objects (Zhang et al., 2015). Given this, the rheological properties of printing media are critical for their HE-3DP (Hong et al., 2015; Liu et al., 2018).
Starch, as one of the most important carbohydrates in human diets, has been extensively used in food applications to improve the process convenience and the quality of final products (Zheng et al., 2018). In food systems, starch often undergoes gelatinization during cooking. During this process, starch granules swell extensively with the resultant disrupted crystalline structure. Meanwhile, amylose molecules diffuse out from the swollen granules (Wang et al., 2018). As a result, starch pastes can be regarded as a continuous matrix of entangled amylose molecules reinforced by embedded swollen granules (Ring, 1985). This particular structural feature endows the gelatinized starch paste with viscoelasticity, which shows a shear-thinning behavior and instant responses to the applied shear strains (Evans and Haisman, 1980). Regarding this, starch shows high potential for HE-3DP.
Despite the huge advantages of HE-3DP technology, research in food printing has just been started. Various food materials have been used to print a complex structure, such as chocolates (Lanaro et al., 2017), confections (Hao et al., 2010), proteins, meat purees, and other nutrients (Cohen et al., 2009; Lipton et al., 2010; Serizawa et al., 2014). These printable food materials either are based on its own thermal characteristics (typically, melting upon heating and solidification on cooling) or need further modification to acquire printability. There is limited data about the 3D printing of grain-based food, which highly hampers the application of 3D printing in the production of next-generation daily dietary food since the major ingredients of most snack foods are grain-based. Only a few studies have concerned 3D printed grain-based products based on, for instance, mashed potato products with different contents of potato starch (Liu et al., 2018). Also, potato starch was reported to adjust the rheological properties of lemon juice gels in order to develop new 3D printed food constructs in lemon juice gel systems (Yang et al., 2018). Still, this field is in its infancy and the improvement of the currently developed systems is urgently needed. Therefore, motivated by the excellent rheological properties of starch, this study focuses on the rheological behaviors of rice starch (RS), potato starch (PS), and corn starch (CS) under the conditions mimicking the HE-3DP process, and their actual printing behaviors. The aim of this work is to illuminate the underlying relationship between starch rheological properties and printability, and provide insights into the 3D printing of starch-based staple food.
Section snippets
Materials
RS was supplied by National Starch Pty Ltd. (Lane Cove, NSW, 2066; Australia). CS was obtained from Huanglong Food Industry Co., Ltd. (P. R. China). PS was provided by Sanjiang Group Co., Ltd. (Xining, China). Anhydrous ethanol was supplied by Nanjing Chemical Reagents Co., Ltd. (Nanjing, China). Sodium hydroxide was obtained from Tianjin Baishi Chemical Co., Ltd. (Tianjin, China). Iodine and Potassium iodine were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Acetic acid
Steady shear rheological study
An ideal printing medium for extrusion through a small-diameter nozzle in HE-3DP is a shear-thinning material to ensure smooth extrusion (Zhang et al., 2015). To understand the viscoelasticity of starch suspensions as HE-3DP printing media, steady shear rheological measurements were carried out and the results are shown in Fig. 2. A linear relationship between viscosity and shear rate can be seen on the double logarithmic plot for CS at 75 °C, RS at 80 °C and PS at 70 °C of various
Conclusion
This study focused on the HE-3DP of starch and we have established the relationship between rheological properties and printability. The results indicated that concentrated starches present shear-thinning and strain-responsiveness, which were printable as HE-3DP materials. Moreover, the τy and G′ parameters of all the samples, which are crucial for supporting subsequently deposited layers and maintaining printed shapes, increased with the increased starch concentration. Nevertheless, the
Acknowledgments
This article has been financially supported by the National Key R&D Program of China (2016YFD04012021), the Key Project of Guangzhou Science and Technology Program (No.201804020036) and YangFan Innovative and Entrepreneurial Research Team Project (2014YT02S029). F. Xie acknowledges the European Union's Marie Skłodowska-Curie Actions (MSCA) and the Institute of Advanced Study (IAS), University of Warwick for the Warwick Interdisciplinary Research Leadership Programme (WIRL-COFUND).
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