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Lee, et al.
coated with 10 mA current using JEOL JFC-1600 Auto massive extent of over 700% of its original volume, it
Fine Coater machine. The images were viewed and has an overrun that is even lower than Ink 4. While inks
taken under 10 – 13 kV condition under a JEOL JSM- with XG have reduced overrun, the overall increase in
5600LV SEM. Methocel F50 and Foam Magic mass, contributing to the
increase in density that made it difficult to hold the air
2.7. Texture profile analysis bubbles without bursting too. The EW foam ink generally
Texture Pro CT V1.3 Build 15 (Brookfield Engineering has a lower overrun compared to the eggless foams.
Labs, Inc) was used for double-cycle compression tests (2) Foam stability of the inks
to obtain force-time curves. The foams were prepared and
filled into a 20-mm deep tray of aluminum foil for testing. The foam inks’ stability is a critical factor as the food inks
Both as-printed and baked foam inks were tested. The have to retain the foams after being printed. The viscosity
baked foam samples were printed and baked at 70°C for of the continuous fluid phase is one of the main factors
[36]
2 h before being tested at the height of 20 mm. The test affecting foam stability . The higher viscosity of the
parameters were as follows: Block probe with a trigger continuous fluid phase delays the movement of the liquid
load of 5 g, pre-test, test and return speed at 2.0 mm/s, through the network of films that enclose the air bubble,
and the compressive strain was set to 45% for two cycles. hence slowing the formation of larger bubbles and the
Each ink was tested in triplicates. Hardness, chewiness, liquid drainage.
adhesiveness, gumminess, stringiness, and springiness As shown in Figure 1B, the foam inks without
were measured. The averages of the three replicates were XG have much poorer foam stability. The significant
represented in a radar graph. They were also normalized difference in the half-life of foams shows that XG plays
to the highest value obtained among the food inks and an important role in foam stability for both groups (EW
represented in another radar graph for ease of comparison. and HPMC). Ink 3 with only HPMC is unable to retain
the foam for more than 15 min. The poor foam stability is
2.8. Data analysis due to the lack of foam stabilizer in the mixture. HPMC

Data were plotted by using the OriginLab software. The generally stabilizes the hydration layer when it is heated
results were analyzed using unpaired student’s t-test with and sets into a gel at a specific temperature. However,
this ink was prepared at room temperature. Therefore,
n = 3. The significance of the results is denoted on top
of the columns where * represents P ≤ 0.1, **P ≤ 0.01, while it was able to foam, it was unable to retain the foam
***P ≤ 0.001, and ****P ≤ 0.0001. The results were and stabilize the foam at room temperature. As for the
compared within the groups of EW and non-EW inks on EW-based Ink 1, it has the foam stability of over 80 min.
whether XG affects the properties of food foams. The first While its stability is not as high as inks that contain XG,
group (EW-based) was compared between Inks 1 and it is stable enough to be printed within a certain duration
immediately after preparation. EW contains globulins that
2, and the second group (HPMC-based) was compared
between Ink 3 and Inks 4,5. facilitate foam formation, and also contains ovomucin
that stabilizes the foam . During the whipping, the
[37]
3. Results and discussion EW proteins adsorb at the interface of air bubbles and
the liquid through the hydrophobic areas. The partially
3.1. Foam properties unfolded proteins (denatured through whipping) stabilize
the protein films formed. The foam collapses when large
(1) Foaming ability of the inks
gas bubbles grow at the expense of tiny bubbles. These
Both EW and HPMC are known to have excellent films counteract the growth of the large bubbles, thus
[23]
[35]
foaming abilities. The overrun quantitatively measures stabilizing the foam. However, the EW foams, without
the foaming ability of each foam ink. Figure 1A shows additives, tend to destabilize too .
[28]
that all the food ink formulations can foam more than Inks with the inclusion of XG have a significant
300% of their original volume. improvement in foam stability of more than 1000% in
Ink 1 is based on EW, and Ink 3 is based on terms of half-life. With the use of Foam Magic which has
methylcellulose without the addition of hydrocolloids. a proprietary mix of HPMC and XG, Inks 4 and 5 have a
They were able to foam more than their counterparts with foam stability of 158 min and 104 min, respectively. An
added XG. The inclusion of XG makes the foam denser inclusion of 2 wt% of XG in the EW-based foam allows
and more sticky, thus less able to hold more air bubbles. Ink 2 to have long half-life of 1200 min. XG is widely
The reduced foaming ability may be attributed to the used as a food thickener. It thickens the inks and retains
increase in density, as seen from the difference between the liquid phase in foam structures against gravitational
Inks 3 and 4. While Ink 5 contains Methocel F50, a force . The fixation of the liquid phase due to the
[29]
component present in Ink 3 which is able to foam to a presence of XG makes it harder for small bubbles to grow
International Journal of Bioprinting (2021)–Volume 7, Issue 4 159