factor that affects droplet deposition is temperature. Variations in the temperature modify the rheological properties and influence the surface energy of the food inks. Application of lower temperature reduces the surface energy and spreading tendency of food inks (Willcocks et al. 2011). The ingredient composition is another criterion that must be optimized for achieving the desired flowability for fabricating 3D designs on food substrate. Only a limited range of food materials has been analyzed for inkjet printing, in which chocolate ink is the most commonly used ingredient (Lanaro et al. 2019). However, other food materials in form of emulsion, slurry, and suspensions have a great scope for 2D and 3D inkjet printing. Future studies on the analysis of different material supplies suitable for inkjet printing are required to broaden the food applications.
2.6.2 Classification of Inkjet Printing
2.6.2.1 Drop‐On‐Demand Inkjet Printing
In a DoD inkjet printing method, the food ink material supply is deposited as required as discontinuous droplets in DoD at appropriate intervals. Mainly the ink is dispensed from the printhead under pressure exerted by a valve (Nachal et al. 2019). Irrespective of the flowability method, the compatibility between the ink and substrate surface is essential in order to impart the adhesiveness and binding of the printed layers. A proper flowability with appropriate viscosity must be required for seamless printing. The viscosity of the food ink is influenced by process temperature that in turn depends on the material composition which has to be optimized for achieving good printing precision and resolution. About 100–1000 heads can be used for the multi‐head operation of the DoD system that works based on either using thermal or piezoelectric heads (Godoi et al. 2019). Inkjet printing is well suited for low viscosity materials that can be used for image filling and decoration. The DoD printer was used to dispense edible inks onto the food surface for the creation of appealing images and graphics (De Grood and De Grood 2013). The commercial food printer ‘FoodJet’ employs pneumatic membrane nozzle jets for dispersing of food ink droplets over the moving food substrate for customized designing of pizza base and biscuit filling (FoodJet 2020).
2.6.2.2 Continuous Inkjet Printing
In continuous inkjet printing, the edible ink is continuously dispensed without any pause through a piezoelectric vibrating crystal at a constant frequency. The use of a high‐pressure pump creates a continuous flow that directs the liquid food ink through an orifice of smaller diameter ranges between 50 and 80 μm (Godoi et al. 2019). In some cases, the material flowability can be ensured by incorporating conductive agents into food ink to impart charges. The use of electrically charged conducting agents for improving the performance efficiency limits its application in food customization. The droplet generation rate of continuous inkjet printing is comparatively higher than the DoD inkjet printing. However, the resolution and precision of the produced images from DoD inkjet printing seem to be superior with neat dispensing than continuous inkjet printing. Willcocks et al. (2011) reported that the maximum resolution of inkjet printed images using a single head continuous ink‐jetting system of about 70–90 dots per square inch (dpi).
2.7 Binder Jetting
Binder jetting is also referred to as the liquid binding where the powdered material gets deposited in a layer‐by‐layer manner with the selective dispensing of liquid binder based on a 3D data file. The process involves the accumulation of the powdered layers through direct fusion with the ejection of binder solution from the printhead (Le‐Bail et al. 2020). Here, the binder fuses the cross‐section of one layer with the succeeding layer thereby forming a 3D object (Figure 2.12). The working principle of the binder jetting technique is similar to that of inkjet printing apart from spraying the liquid binder droplets instead of food ink as in inkjet printing. Binder jetting allows us to fabricate complex 3D porous structures with precise control over the micro and macro structures of the material supply (Farzadi et al. 2014). The unfused powder materials act as a support during the fabrication of a 3D structure that can be easily removed after the completion of printing and can be recycled for further use. One of the attractive features of binder jetting technology is the production of colourful 3D structures. Variation in the liquid binder composition helps in the creation of complex, delicate, and attractive coloured 3D objects (Sun et al. 2015). Due to the spray of liquid binder over the food substrate, the fabricated 3D construct would require an additional drying process for removing the residual moisture. Further, the drying treatment improves the structural stability and the mechanical integrity of the 3D construct (Vithani et al. 2019). Considering the food applications, binder jetting is limited with its applicability only to the powdered materials and the use of edible binder considering food safety.
Figure 2.12 Schematic diagram of binder jetting.
2.7.1 Working Principle, System Components, and Process Variables
The process of binder jetting follows an even distribution of the powder layer across the printing platform and a liquid binder is sprayed onto the powdered bed to bind the two consecutive powder layers. In general, 3D printing using binder jetting technology starts by spraying a mist of water in order to stabilize the powdered material and to reduce the effects of distortion caused by spraying the liquid binder (Sun et al. 2018a). The system components of binder jetting are the same as that of a 3D ink‐jetting system instead of a material reservoir, a liquid binder reservoir is used. So, the components are inkjet printhead, levelling and spreading roller, built envelop, binder feeders, powder bed, build piston, and powder feed piston (Trenfield et al. 2018). The first layer is formed by spreading powdered material of appropriate thickness onto the build plate and the subsequent ejection of the liquid binder. It is followed by lowering the powder feed piston in order to ensure the adequate gap for the formation of the next layer that gets fused with the previous layer (Ziaee and Crane 2019). The process is repeated until the pre‐determined 3D object is produced based on a 3D data file. The customized 3D shapes were printed out of sugars and starch mixtures using the Z Corporation powder binder 3D printer in the edible 3D printing project (Southerland et al. 2011). Similar to inkjet printing, binder jetting offers fast fabrication at reduced operational cost. However, the 3D printing machine costs higher, and also the end quality of the fabricated 3D structure possess a rough surface. The commercial firm, 3D Systems utilizes colour jet printing technology for the fabrication of sugar‐based complex 3D constructs with different flavoured binders (3D Systems 2013). This approach is well‐known for the fabrication of complex sculptural cakes for special occasions and events. The developed system demonstrated a safe printing process that meets all the food safety standards.
Common process variables that influence the printing precision and accuracy are head type, droplet size and shape, droplet spacing, printing speed, velocity, and frequency of droplets (Shirazi et al. 2015). All these process variables are interrelated with the material properties. The compatibility of the liquid binder with powder bed substrate is adequate for achieving good finishing quality and resolution of the printed image. The surface tension of liquid binder and substrate material determines the interaction and fusion of printed layers onto substrate base (Liu et al. 2017). Sometimes, the compatibility of the substrate surface is improved by coating the surface with a binder or compatibility‐enhancing film before the start of the printing process. Considering this aspect, binder shellac (poly 1‐vinyl‐2‐pyrrolidone) was added into edible food ink for increasing surface compatibility (Willcocks et al. 2016). It was also reported that water‐based glazing along with gums or other surfactants such as polysorbates and polyglycerol oleates were used for modifying the chocolate ink for achieving high‐resolution images (Willcocks et al. 2011). Thus, the application of multilayered surfactants over the substrate surface had proved to significantly improve the compatibility yields a good precision and higher resolution of the printed
