melting extrusion and soft material extrusion. 3D printing of chocolates and starch is known to be hot‐melt extrusion (HME) that is accompanied by a phase change during printing. On the other hand, cereal‐based dough and meat pastes comes under soft material extrusion that involves direct deposition without any phase change during printing (Godoi et al. 2019). As a multicomponent food system, the binding of the extruded layers is accompanied by the intermolecular interactions of macro and micro constituents. The printing precision and the resolution of the 3D constructs could be achieved through proper optimization of printing variables. The infill percentage and infill pattern are related to the complex internal microstructure and the final appearance of the 3D constructs (Severini et al. 2016). In recent days, attempts are made to determine the printability of the common daily foods includes staple food crops such as rice, millets, and wheat (Severini et al. 2018); fruits and vegetables such as lemon juice (Yang et al. 2019b), spinach (Lee et al. 2019), potato (Dankar et al. 2020), broccoli, peas, and carrot (Kim et al. 2018b; Pant et al. 2021); meat‐based foods such as surimi, beef, chicken, and pork (Dick et al. 2019a; Wang et al. 2018; Wilson et al. 2020). All these studies dealt with the optimization of material concentration, process parameters and post‐processing methods to deliver the customized foods. So far, the basic concept of a single extrusion system has been discussed. Focussing on the development of the novel fascinating 3D constructs, a dual extrusion system is in practice. In contrast to a single extrusion system, the dual extrusion unit consists of an additional extruder unit that works alternatively in a pre‐determined interval that prints a single structure with dual material supplies. Liu et al. (2018b) reported a study on dual extrusion of mashed potato and strawberry juice gel using a commercial 3D printer (Figure 2.5). In this study, dual extrusion was performed by two different methods: the creation of the multi‐part model and assigning each of them to each extruder; and creation of the single part model and assigning different roles to each extruder. The former method aids in varying the inside shapes of the model while the latter method helps in the creation of porous texture by varying the infill levels. In another study, edible constructs with overhanging geometries have been fabricated using a dual‐printing approach (Periard et al. 2007). More recently, a dual extrusion commercial 3D printer was used to fabricate a colourful 4D ready‐to‐eat food using purple sweet potato puree and mashed potato (He et al. 2020). Thus, these studies proved the feasibility of the creation of multi‐material constructs that has a great scope for personalized functional foods.
Figure 2.5 3D printed samples using dual extrusion of different internal structures (a) triangle, (b) square, (c) circle, and (d) hexagon.
Source: From Liu et al. (2018b), Figure 02 [p. 019] / With permission of Elsevier. DOI‐https://doi.org/10.1016/j.lwt.2018.06.014.
Figure 2.6 3D printing of material supply using multi‐head 3D food printer MultiCARK™ (unpublished).
Another variant of the 3D printing system is the applicability of a multi‐head printing unit for the simultaneous printing of food materials (Nachal et al. 2019). The printing process using an in‐house designed multi‐head 3D food printer is presented in Figure 2.6 (MultiCARK™ unpublished). The lower printing speed of the single head system is the major disadvantage that results in lesser production and consumes more time. The incorporation of the multiple heads eventually overcomes the above‐mentioned limitations. However, the printing of multi‐scale ingredients with multi‐head systems adds to the complexity involved in the fabrication of 3D constructs. Hence, appropriate design modifications are required for ensuring the homogeneity of the ingredient mixtures. Conventionally, static‐ and agitating‐mixer units are used for the mixing of food materials (Millen 2012). So, the appropriate integration of the conventional processing units with a multi‐head 3D printing system would result in a homogeneous material supply. Thus, the system design of 3D printers for food applications remains a void research area that must be addressed for exploring the potential benefits of 3D food printing. Also, concerns related to operational safety and system cleanliness are the critical factors that must be considered for the safe delivery of the foods. Some of the available commercial 3D printers based on extrusion mechanism are Choc creator, Foodini, and BeeHex robot pizza printer (Sun et al. 2018a).
2.4.2 Classification of the Extrusion‐Based 3D Printing System
2.4.2.1 Hot‐Melt Extrusion
HME also known as FDM, was first used for the 3D fabrication of polymers and ceramics. Considering food applications, HME is used for those materials considering their melting and solidification behaviour. The process involves the deposition of melted semi‐solid food from a moveable FDM print head through the hot‐end nozzle tip (Mantihal et al. 2019). The deposited layers will solidify immediately after extrusion and bonded together with the previous layers upon cooling. The temperature is precisely controlled and determined based on the melting point of the food materials used. HME is mostly applied for the fabrication of 3D constructs using chocolates, starch, and protein gels (Chen et al. 2019; Hao et al. 2010; Liu et al. 2019a). Understanding the material properties is crucial for the fabrication of 3D constructs in a well‐defined quality. In the case of chocolate printing, the combination of sugar with cocoa fat assists in the easy flow of the material through the extruder. The self‐supported layers of chocolate rely on the thermal properties such as glass transition temperature (T g) and melting point that are critical for the successful solidification of the material (Mantihal et al. 2017). The chocolate ink with pseudoplastic behaviour imparts conducive printability that in turn depends on the temperature used. It is essential to play around the six crystal polymorphs of the cocoa butter for achieving a stable 3D‐oriented product with better texture and glossy appearance (Figure 2.7) (Lanaro et al. 2017). Some of the commercial 3D printers specific to chocolate printing are Choc Creator, ChefJet, and CocoJet. Researchers from the Massachusetts Institute of Technology (MIT) printed the melted chocolate using the direct ink writing (DIW) method using the developed 3D printer ‘Digital Chocolatier’ (Zoran and Coelho 2011). A similar approach has been used by 3D Food‐Inks Printer for the printing of 3D‐colored images on the extruded base (Golding et al. 2011). However, a post‐processing step is required for the fusion of printed layers. More recently, Rando and Ramaioli (2020) studied the effect of heat transfer on the print stability of chocolate. The study investigated the correlation between the rheological and thermal properties for achieving a well‐stable 3D structure. The stability criterion based on the developed yield stress during extrusion explained the stability or deformation of the printed materials.
Starch being an integral macro component of food grains that has a great scope for 3D printing. Recently, research on the fabrication of starch‐based 3D constructs using HME is gaining attention. When the starch suspensions are subjected to heat treatment, the starch granules will swell with the absorption of a large amount of water and results in a thicker gel matrix through the process of gelatinization. This resulted in a starch gel that possesses characteristic viscoelastic, shear‐thinning, and thixotropic behaviour that aids in the smooth continuous flow of material during extrusion and structural stability to printed layers during and after the extrusion process (Maniglia et al. 2020b). The mechanical strength and the extrudability of the starch gels explained its printability. Results showed that corn starch with 20% concentration (w/w) at 70–75
