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3D Printing of Food Foams
Table 1. Formulation for each foam ink of 1 cm in radius. The filter paper was left to stand for
Sample Ingredients pH 30 min to allow the liquid to spread before the filter
Ink 1 • Egg white 87.5 wt% 9.19 paper was photographed. The area covered by the fluid
• Honeydew extract 12.5 wt% was measured using a Python program that detects the
Ink 2 • Egg white 85.5 wt% 8.91 edge of the foam inks and the edge of the spread fluid. An
1 × 1 cm red square was included in the photograph as
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• Honeydew extract 12.5 wt% a reference for image analysis. Each sample was carried
• Xanthan gum 2 wt%
Ink 3 • Methocel F50 2 wt% 8.33 out in replicates of three.
• Water 2.4. Rheological characterization of the foams
• Honeydew extract 12.5 wt%
Ink 4 • Foam Magic 2 wt% 8.16 The rheological properties of the foam inks were tested
• Water using an oscillatory rheometer (Discovery Hybrid
• Honeydew extract 12.5 wt% Rheometer DHR-3, TA Instruments, Delaware, USA). The
Ink 5 • Methocel F50 2 wt% 8.20 measurements were conducted using stainless steel parallel
• Foam Magic 2 wt% plates with a diameter of 40 mm and a truncation gap of
• Water 500 µm. Viscosity shear-thinning tests were performed on
• Honeydew extract 12.5 wt% the foam inks by applying a stepwise shear ramp rate from
0.001 s to 100 s . Stress sweep measurements were
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carried out with a logarithmically increasing shear stress at
2.2. Foaming characteristics a constant frequency of 1 Hz from 0.1 to 2000 Pa to study
(1) Foaming ability the viscoelastic properties of the inks. A recovery test
was done to mimic the three phases the ink experienced
The foams were prepared at room temperature by – before extrusion, during extrusion, and after extrusion
whipping 50 mL of respective suspensions in a 500-mL from the nozzle. A multiple stepped flow ramp (peak hold)
beaker using a multi-quick hand blender (Kenwood, test was conducted wherein the initial shear rate of 0.1 s
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800W) at a constant turbo speed for 5 min. The foaming – 0.2 s was applied for 20 s, followed by a rapid increase
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ability, which is also known as the overrun, is the in shear rate from 0.2 s to 200.0 s in 1 s. The shear rate
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percentage of volume increase after the whipping process. was held at 200.0 s for 5 s before reducing the shear rate
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Three replicates were used for each type of foam, and the from 200.0 s to 0.1 s in 1 s and held for 20 s.
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averages from the results were reported.
2.5. 3D printing of foams
(2) Foam stability
Wiiboox Sweetin 3D Food Printer (Wiiboox, China), an
Foam stability was characterized by the half-life of the extrusion-based 3D food printer, was used to print the 3D
foam, which was measured as the time required to lose samples of different foam inks. The nozzle size used for
50% of the volume of the foam. Three replicates of 30 mL these experiments was 1.5 mm. The foams were printed
of foams were placed in a 50-mL Falcon tube. The volume into a six-pointed star with a length and width of 3 cm
of the foam was recorded over time to determine the time and a height of 0.5 cm to assess its printability. The star
required for the volume of the foam to reduce to half. Only was used to assess if the ink was able to handle sharp
the volume of the foam, not including the volume of the curves. Three replicates were printed with each foam ink.
liquid formed as foams burst, was measured in this study. Photographs of the printed samples were taken after the
printing and assessed visually. The first three prints of the
(3) Foam density
inks were rated visually from 1 (poorest) to 5 (best) on
The foams were prepared as mentioned in part (1) of two categories, self-supporting structure and shape, with
section 2.2. Then, 50 mL of each foam ink was placed a maximum total score of 10. The smallest observation
in a 50-mL Falcon tube and weighed using an electronic for each category was given a score of 1.
balance. Each ink was measured in three replicates. The The foams were printed at a print speed of 25 mm/s
average density was calculated by dividing the average with an initial layer thickness of 0.8 mm and subsequent
mass from the volume of 50 mL. layer thickness of 1 mm. The fill density was set to 80%
with a flow of 80%.
(4) Syneresis of foam inks
Syneresis was analyzed by placing 0.2 g of the foam 2.6. Scanning electron microscopy (SEM)
ink at the center of a piece of Whatman Grade 4 filter The micromorphological structure of the printed parts
paper. The foams were spread evenly to cover a circle was observed by using SEM. All samples were gold-

158 International Journal of Bioprinting (2021)–Volume 7, Issue 4