Although extrusion‐based 3D printing has a great scope and applicability for printing foods, the printing of delicate complex shapes is very challenging. The extrusion‐based 3D constructs are more prone to structural collapse and deformation that requires additional support for stable product geometry which must be removed after 3D printing process (Liu and Zhang 2019). In case of polymer printing, the removal of additional support structures is easy, however, for food applications it greatly affects the final resolution of the printed 3D construct. Hence, more studies on material properties and post‐stability behaviour are required for deeper understanding of the food printing technologies.
Figure 2.4 Different types of extrusion mechanism (a) piston‐based extrusion system, (b) syringe‐based extrusion system, and (c) screw‐based extrusion system.
Material characteristics like moisture content, rheological, and thermal properties are crucial for the successful printing of 3D constructs based on extrusion printing. It was reported that the viscosity must be low enough for easy flow of the material and high enough for stable deposition and proper adhesion of the printed layers (Wilson et al. 2020). Based on the temperature used for the deposition of material, extrusion‐based printing can be categorized as 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