3D Printing of Foods. C. Anandharamakrishnan

Figure 2.11 Schematic diagram of inkjet printing.

      The most significant material properties that affect the print fidelity of the inkjet printing are the rheological and thermal behaviour of the food ink. In general, the inkjet printing system uses a low viscosity material for easy ejection of liquid through tiny orifice channels of the printhead. Material viscosity is a crucial factor that ensures the flowability of material. So, the desired material viscosity ranges between 2.8 and 6 mPas. While the viscosity above 10 mPas causes cavitation inside the printhead during the printing process and the viscosity below 2 mPas is not stable enough to form a droplet (Liu and Zhang 2019). The surface filling and decoration involve the jetting of food inks dispensing through the micro‐sized channels in the range from 20 to 50 μm. About 1‐pl (picolitre) of ink is dispensed during ejection of each droplets that typically range about 13 μm across (Xaar 2018). The principle of dispersion of ink involves the breaking up of a stream of droplets of the same volume with reduced surface area. The underlying process of ink‐jetting technology is based on the Rayleigh–Plateau instability phenomenon (Godoi et al. 2019). Another 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.