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Extrusion-based 3D food printing – Materials and machines

that patients recognize as conventional food and then, non-food applications such as bioprinting, UV light is
hopefully, increase the food intake of patients with commonly used to cure gels. In the example of Section
dysphagia. Other advantages that 3D printing can confer 2.7, alginate and carrageenan gelling was promoted by
to healthcare providers is the customization of specific the spraying of calcium ions. In other cases, such as in
nutritional content by carefully tailoring the food ink Section 2.5, the respective gels were cured by heat.
formulations. For example, Azam et al. was able to Understanding these two requirements of printable
successfully 3D-print an orange concentrate gel enriched food formulations, it is therefore beneficial to investigate,
[9]
with vitamin D into various shapes . and if possible, create shear-thinning food inks. This
In this review article, Section 2 describes some recent takes advantage of the high shear experienced at the
research works that have been done for formulation printing nozzle to allow smooth flow through the nozzle
of food inks using a variety of texture modifiers. and onto the print bed. Once printed, no more shear is
These include additives like starch, pectin, gelatin, experienced and the food’s 3D shape may be retained.
nanocellulose, alginate, carrageenan etc. It should be In a food system, shear thinning behavior is caused by
noted that due to the commercial sensitivity of such macromolecular disentanglement in solution (e.g. long-
studies, there are few researchers who are willing to chain food fibers) or layered alignment of concentrated
share their optimized formulations on public platforms. dispersions or emulsions. This is illustrated in Figure 1.
Thus, the depth of the reviews in this aspect is lacking. In the former system, the disentanglement caused by
Nevertheless, this review attempts to discuss them in external shear will reduce intermolecular forces such
detail where possible. Section 3 lists and evaluates 3D as Van der Waals forces, electrostatic attractions and
food printers that have been developed for academic hydrogen bonding. In the latter system, the layered
and/or commercial purposes. alignment reduces particle collision. Both phenomena

2. 3D Food Printing Materials reduce the viscosity of the solution and thus impart
shear-thinning properties.
In practical application, the use of a pre-determined
2.1 Favourable material properties for printing hydrocolloid mixture in optimized amounts will allow
There are many different approaches to make food us to fine-tune the desired extent of shear thinning in any
purees printable. However, the underlying mechanism specific food system we want to adapt for 3D printing.
to obtain a successful 3D printout are very similar, The hydrocolloids chosen will form a macromolecular
regardless of the additive used. gel network that disentangles under shear and reforms
In the pre-extrusion phase, the food formulation must when at rest. Also, by strictly controlling food particle
remain fluid. This is commonly achieved by ensuring size and concentration, it is possible to achieve the loose
that the food material used has a small particle size. particle cluster arrangement that also imparts shear
In Sections 2.3 and 2.4, we see that the fibrous food thinning properties. In the following sections, we discuss
materials used were first mechanically degraded by use some examples of printability-enhancing additives.
of high shear equipment such as a kitchen blender. In 2.2 Potato starch in lemon juice [10]
Sections 2.5 and 2.6, milk powders, starch powders,
sugar crystals, and protein powders were used. In In this research, Yang et al. attempted to make lemon
Section 2.1, we see that even liquids such as lemon juice juice printable by gelling the juice using various potato
could be used. starch concentrations (10%, 12.5%, 15%, 17.5%,
In the post-extrusion phase, the printed food has to 20% w/w). It was found that 17.5%–20% starch
be able to resist structural deformation after deposition. concentrations resulted in discontinuous deposition due
This can be done by “curing” the printed the food. In to high viscosity. The opposite (i.e. over-extrusion) was

Figure 1. Shear-thinning mechanism in food systems. (Left) Disentanglement of long macromolecular chains. (Right) Breakdown of loose
particle clusters to form layered alignment.

2 International Journal of Bioprinting (2018)–Volume 4, Issue 2