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 °C printing temperature and rice starch with 15–20% concentration (w/w) at 75 to 80 °C was suitable for HME (Zeng et al. 2020). A similar study on comparative analysis of printability of potato, corn, and rice starch gels has been reported (Chen et al. 2019). Nowadays, potato starch is widely used as a thickening and gelling agent in processed foods. In addition, the characteristic high degree of polymerization imparts viscosity and expansion behaviour to potato starch gels making it a suitable ingredient for the fabrication of personalized 3D printed food using HME. It was reported that potato starch of 15–25% concentration at 70 °C exhibits optimal printability. The hot‐melt extruded layers of potato starch result in a uniform extrusion with a compact gel network and possess greater structural integrity (Liu et al. 2020). Likewise, detailed studies on the gelation and gelatinization of millets and pseudo‐cereals would lead to the delivery of nutritious and personalized 3D printed foods that reduces the emergence of lifestyle disorders such as diabetes and obesity.
Figure 2.7 Hot‐melt extrusion of complex 3D geometry bunny using chocolate.
Source: From Lanaro et al. (2017), [p.30] / With permission of Elsevier. DOI‐https://doi.org/10.1016/j.jfoodeng.2017.06.029.
2.4.2.2 Cold Extrusion
The cold extrusion also known as room temperature extrusion (RTE) refers to the process of extrusion and deposition of materials without phase change. Most of the natively printable materials and pre‐processed non‐natively printable, as well as alternative ingredients, can be printed using RTE (Nachal et al. 2019). This type of extrusion is applied for the fabrication of 3D printed products that are difficult to produce by conventional food processing methods. Adoption of 3D printing would result in the production of foods with higher repeatability and smooth finish making them suitable for mass production and customization. Ready to cook (RTC) pasta products with novel 3D designs can be printed from a mixture of wheat semolina and water without the influence of temperature using RTE. Similarly, surface filling and graphical decorations of confectioneries are done using RTE (Van der Linden 2015). The semi‐solid paste‐like food materials are more appropriate for RTE; hence it is also referred to as soft material extrusion. The process involves the continuous extrusion of material from the moving print head that results in layered deposition of materials adhered to the preceding layers upon cooling without phase transition. The printing material supplies not only possess the adequate yield stress and elastic modulus but also shear‐thinning tendency to withstand the desired shape after printing (Huang 2018). Although the temperature is less significant during RTE, the temperature must be fine‐tuned that has a direct influence over the material consistency (K) and flow behaviour (n) (Hamilton et al. 2018). The printability of vegemite and marmite has been determined based on the K and n values in achieving a proper extrusion rate. Apart from the rheological properties, the particle size, crystallinity, and material composition would greatly affect the printability and quality of the 3D constructs.
Cereal and millet‐based doughs, cheese, creamy peanut butter, cake frostings, jam, jelly, hummus, and Nutella are some of the common food materials that are suitable for RTE (Cohen et al. 2009; Millen 2012; Periard et al. 2007). Among the food printing technologies, extrusion‐based 3D printing is widely used for a diverse range of food materials. More recently, the non‐printable surimi (Scomberomorus niphonius) paste was printed with the addition of microbial transglutaminase as an additive (Dong et al. 2020). Results showed that the addition of transglutaminase in the range of 0.2–0.3% (w/w) enhanced the printability of surimi paste. Further, the textural properties such as hardness, cohesiveness, and resilience of the surimi gel were gradually increased with an increase in the concentration of transglutaminase up to 1.4%. The entire printing process was performed using an extrusion‐based 3D printer at room temperature (20 °C). In a similar approach, the printability of the microalgae was assessed for the development of nutritious 3D printed snacks from pastry wheat flour. In this study, a dual printhead system was employed for the coaxial extrusion of microalgae‐enriched snacks. Results showed that the material supplies with 3 and 4% Chlorella resulted in desired 3D construct with accurate precision (Uribe‐Wandurraga et al. 2020). Other than pre‐processing of the materials to enhance printability, post‐processing is another significant step that ensures the safety and edibility of the 3D printed foods. Few of the commercial 3D food printers integrate the post‐processing cooking step along with the printing process. However, most of the lab‐scale 3D printers reported in the literature are at their initial stage that assists in the printing process alone. Hence, additional post‐processing must be followed immediately after the printing process (Yang et al. 2019a). It is crucial to ensure the shape stability and structural integrity of the 3D printed layers during post‐processing without any deformations. Post‐processing methods such as drying, frying, baking, and freezing are common food processing methods used for cooking 3D printed samples (Krishnaraj et al. 2019). A detailed discussion on the various post‐processing methods and its feasibility are presented in subsequent chapter of the present book.
2.4.2.3 Hydrogel‐Forming Extrusion
Hydrogel‐forming extrusion (HFE) is the process of the extrusion of hydrocolloid solutions/ dispersions into a polymeric/ hardening/ gel setting bath using a syringe‐based extrusion mechanism through a moving printing nozzle (Kuo et al. 2021). Here, the solution temperature is a key criterion that determines the stability of droplets. The gel droplet’s diameter ranges about 0.2–5 mm that forms a smooth distinct layer on deposition (Sun et al. 2018a). The rheological property and the gelation characteristics of the polymeric solution have significant implications on the successful printing of hydrogels. During the printing process, the polymer solution in the liquid state gets transformed into a stable gel state upon deposition. Research works on the fabrication of edible hydrogels are quite increased due to the advantage of the development of soft foods for aged people with swallowing disorders (Serizawa et al. 2014). Commercially available hydrogel printers are equipped with advanced dispensing units for the precise deposition of material. A 3D fruit printer developed by a UK firm, Dovetailed combined strawberry fruit flavour with the sodium gel for 3D printing of little spheres into a cold solution of calcium chloride bath to resemble a raspberry fruit (Molitch‐Hou 2014).