Accepted Manuscript

3d Printing Technologies Applied for Food Design: Status and Prospects

Fernanda C. Godoi, Sangeeta Prakash, Prof. Bhesh R. Bhandari

PII: S0260-8774(16)30024-3

DOI: 10.1016/j.jfoodeng.2016.01.025

Reference: JFOE 8463

To appear in: Journal of Food Engineering

Received Date: 28 October 2015

Accepted Date: 28 January 2016

Please cite this article as: Godoi, F.C., Prakash, S., Bhandari, B.R, 3d Printing TechnologiesApplied for Food Design: Status and Prospects, Journal of Food Engineering (2016), doi: 10.1016/j.jfoodeng.2016.01.025.

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3D PRINTING TECHNOLOGIES APPLIED FOR FOOD DESIGN: STATUS AND PROSPECTS 1

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Fernanda C. Godoi,a Sangeeta Prakasha and Bhesh R. Bhandaria* 3

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aSchool of Agriculture and Food Sciences, The University of Queensland, Brisbane, 5

Queensland 4072, Australia 6

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*Corresponding author: 18

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Prof. Bhesh Bhandari 20

b.bhandari@uq.edu.au 21

School of Agriculture and Food Sciences 22

Hartley Teakle Building, Room C405 23

The University of Queensland 24

Brisbane QLD 4072 25

Phone: +61 7 33469192; 0401297947 (Mobile) 26

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3D PRINTING TECHNOLOGIES APPLIED FOR FOOD DESIGN: STATUS AND PROSPECTS 28

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Fernanda C. Godoi,a Sangeeta Prakasha and Bhesh R. Bhandaria* 30

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aSchool of Agriculture and Food Sciences, The University of Queensland, Brisbane, 32

Queensland 4072, Australia 33

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Abstract: 36

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The use of 3-Dimensional (3D) printing, also known as additive manufacturing (AM), technology in food 38

sector has a great potential to fabricate 3D constructs with complex geometries, elaborated textures and 39

tailored nutritional contents. For this reason, 3D technology is driving major innovations in food industry. 40

Here, we review the use of 3D printing techniques to design food materials. Our discussions bring a new 41

insight into how essential food material properties behave during application of 3D printing techniques. We 42

suggest that the rational design of 3D food constructs relies on three key factors: (1) printability, (2) 43

applicability and (3) post-processing. Especial emphasis is devoted to how the advantages/limitations of 3D 44

printing techniques affect the end-use properties of the printed food constructs. 45

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Keywords: 3D Printing; Additive Manufacturing; food design; material properties; printability; 3D-food 47

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1 INTRODUCTION 52

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The design of food which meets the unique demand of special consumer categories, such as, elderly, 54

children and athletes, has raised the need for new technologies usable in the processing of additives, flavours 55

and vitamins with tailored chemical and structural characteristics, and longer shelf-life properties. Additive 56

manufacturing (AM), also known as solid freeform fabrication (SFF), is one of these methods that involve 57

techniques applied for building physical parts or structures through the deposition of materials layer by 58

layer. This is also referred as a “3D printing” in a general term. 59

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AM was originally invented to build 3D objects based on materials, such as, metals, ceramics and polymers 61

aiming to perform the fabrication of complexes parts in a single step. In the very first studies, the fabrication 62

of 3D objects from polymers relied on photo-polymerization processes in which ultraviolet curable polymers 63

were used for printing layers upon layers of solid constructs (Hull, 1986; Kodama, 1981). AM technology 64

using photo-sensitive materials is not suitable to design food. However, curable printing inks can be 65

attractive in the field of food packaging where there is a continued need for safer, faster and cheaper inks, 66

functional coating, and overprint varnishes. This technique can also be applied to make films and plastic 67

containers with gas barrier coatings to protect flavour and extend the life of packaged food and beverages. 68

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In the food sector, a relevant application of 3D printing techniques to design food constructs was firstly 70

reported by researchers from Cornell University who introduced the Fab@Home Model 1 as an open source 71

design 3D printer capable of producing forms using a liquid food materials (Malone and Lipson, 2007; 72

Periard et al., 2007). The operation system of the Fab@home printers is based on extrusion processes. In 73

subsequent years many studies were carried out in an effort to adapt AM technology to the design of food 74

constructs (Diaz et al., 2015a; Diaz et al., 2014b; Grood et al., 2013; Hao et al., 2010a; Hao et al., 2010b; 75

Schaal, 2007; Serizawa et al., 2014; Sol et al., 2015). This represents a challenge because AM is not easily 76

applied to the complex food materials with a wide variation in physico-chemical properties. 77

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The purpose of applying AM technology to print food materials does not rely on the concentration of 79

manufacturing processes of product in a single step, but it is associated with the design of food with new 80

textures and potentially enhanced nutritional value. This approach is achieved by the synergetic combination 81

of the essential constituents of food (carbohydrates, proteins and fat), bearing in mind their intrinsic properties 82

and binding mechanisms during deposition of layers. Another trend of the AM in the food sector is the design 83

of complex structures which are not possible to design manually by an artisan, for example. This second 84

strength generally uses sugars and other low nutritional ingredients to produce confectionary items. 85

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In this review, we describe the current 3D printing techniques applied to design food materials. They are 87

classified according to material supply: liquid, powder and culture of cells. The deposition of liquid-based 88

materials can be performed via extrusion and inkjet processes. Powder-based structures are printed by 89

deposition followed by application of a heat source (laser or hot air) or particle binder. A brief description of 90

cell culture deposition (bioprinting) is also described, as this technique was applied to print meat analogue. 91

Our discussions, however, are especially devoted on how the food constituents (not cell cultures) would 92

behave during AM processes. 93

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This review looks into three interactive factors which we consider essentials for the rational choice of 3D 95

printing techniques in the design of food: (1) printability, (2) applicability and (3) post-processing 96

feasibility. We emphasize that the profitable incorporation of AM technology by food industry relies on 97

comprehensive studies of the materials properties and optimization of multicomponent systems containing 98

carbohydrates, proteins and fat. 99

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2 ADDITIVE MANUFACTURING TECHNOLOGIES 100

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Depending on the fabrication principle, number of 3D printing techniques can be introduced in the food field 102

and adapted to meet the demand of food design and materials processing. Table 1 summarizes the 3D 103

printing techniques currently applied for food design. The processes are grouped the type of material used: 104

liquid, powder or cell cultures. Cell culture-based systems have been applied for printing meat. Particular 105

attention was given to the techniques involving the essential constituents of food. 106

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Table 1 108

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2.1 Extrusion processes 110

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The application of extrusion processes into AM was introduced by the Fused Deposition Modelling 112

(FDMTM) method developed by Crump (Crump, 1991; Crump, 1992) and trademarked by Stratasys Inc 113

(Batchelder, 2012). In this method a moving nozzle is used to extrude a hot-melt filament polymer as a 114

continuous melted threat, fusing it to the preceding layer on cooling (Fig. 1a). While FDM is primarily used 115

for prototyping plastics, the technology has been adapted to 3D food printing for few years (Fig. 1b). 116

Depending on the materials used in extrusion processes, the binding mechanisms may happen by the 117

accommodation of layers controlled by the rheological properties of the materials, solidification upon 118

cooling or hydrogel-forming extrusion. 119

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Figure 1 121

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2.1.1 Soft-materials extrusion 123

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In AM, soft-materials extrusion has been applied to print 3D constructs by mixing and depositing self-125

supporting layers of materials such as dough, meat paste and processed cheese. The viscosity of the material 126

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is critical to be both low enough to allow extrusion through a fine nozzle and high enough to support the 127

structure post-deposition. Rheological modifiers, or additives, can be used to achieve the desired rheological 128

properties but must comply with food safety standards. 129

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Periard et al. (2007) applied extrusion at room temperature to print cake frosting and processed cheese using 131

the Fab@home Fabrication system (Periard et al., 2007). Using the same system, Lipton et al. (2010) tested 132

a variety of recipes to print sugar cookies. Variations on the concentration of ingredients such as butter, yolk 133

and sugar played an important role to form natively printable dough and resistant on cooking. The authors 134

have also used transglutaminase and bacon fat as additives to make printable scallop and turkey meat-puree, 135

respectively. The resulted meat-based products kept their shape after cooking (Lipton et al., 2010). 136

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Extrusion-based processes have also been employed by the Netherlands Organisation for Applied Scientific 138

Research (TNO) scientists to print a large variety of foods using essential carbohydrates, proteins, meat 139

purees and other nutrients extracted from alternative sources, such as, algae and insects (Van der Linden, 140

2015). Most recently, TNO and Barilla (Italian pasta company) have presented the preparation of 3D printed 141

pasta using classical pasta recipes (ingredients: durum wheat semolina and water, without additives) (Sol et 142

al., 2015; Van Bommel, 2014; Van der Linden, 2015). Another example, a company called Natural 143

Machines created Foodini Food printer which extrudes fresh food ingredients to design meals. The extruded 144

ingredients are used for surface filling (e.g., pizza or cookie dough and edible burger from meat paste) and 145

graphical decoration (Chang et al., 2014; Kuo et al., 2014). Fig. 2 illustrates some examples of 3D 146

extrusion-based techniques applied to print pasta recipe, pork puree and pizza dough. 147

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Figure 2 149

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2.1.2 Melting extrusion 151

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Melting extrusion has so far been applied to print chocolate 3D objects, denoting a working temperature 153

which ranges from about 28 ºC to 40 ºC (Hao et al., 2010a; Hao et al., 2010b; Schaal, 2007). The 154

formulation of chocolate self-supporting layers is challenging due to the complex crystallization behavior 155

exhibited by cocoa butter, the main structuring material in chocolate and confections. Six different crystal 156

polymorphs have been identified for cocoa butter (Marangoni, 2003). The correct polymorph should be 157

produced in the chocolate for its best melting, textural and shelf-life properties. 158

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The chocolate deposition directly into a 3D object by means of extrusion was introduced by researchers 160

from Cornell University using a Fab@home Fabrication system (Schaal, 2007). Their studies, however, did 161

not look at the materials properties and geometrical accuracy of the extrudate. Hao et al (2010a,b) revealed 162

the factors influencing the geometrical precision of the chocolate deposition: (1) nozzle aperture diameter, 163

(2) optimum nozzle height from the forming bed and (3) the extrusion- axis movement (Hao et al., 2010a; 164

Hao et al., 2010b). The expertise of the research group led by Hao enabled the foundation of ChocEdge Ltd, 165

a spin-off company from the University of Exeter, which pioneered the commercialization of 3D chocolate 166

printers. Fig. 3a shows an example of 3D printed chocolate by ChocEdge. Over the years, many companies 167

applied chocolate extrusion to build 3D objects, for example, Foodini, TNO and recently 3D Systems in 168

partnership with The Hershey Company has introduced the CocoJetTM at the 3D Chocolate Candy printing 169

exhibit (2014) as a breakthrough 3D chocolate printer, enable to build self-supporting layers in a 3D shape, 170

as illustrated by Fig. 3b (3DSystems, 2015). 171

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Figure 3 173

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2.1.3 Hydrogel-forming extrusion 178

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The extrusion of hygrogel-forming materials is critically dependent on the polymer rheological properties 180

and the gel forming mechanism. At first, the polymer solution should present viscoelastic characteristic, and 181

then turn into self-supporting gels prior the consecutive layers are deposited. To prevent premature gelation 182

of the polymer solution inside the printer, temporal control of the gelation mechanisms must be carried out. 183

Generally, the hydrogel-forming mechanisms can be classified in three categories: (1) chemical cross-184

linking, (2) ionotropic cross-linking and (3) complex coacervate formation, as depicted by Fig. 4 185

(Kirchmajer et al., 2015). Chemical cross-linking is unlikely to be applied for food design, as many cross-186

linking reagents are harmful and must be completely removed from the designed structure before they are 187

consumed. Conversely, ionotropic cross-linking has been widely applied by food industry, especially in 188

microencapsulation processes (Bokkhim et al., 2014; Ching et al., 2015). As an example, alginate is a 189

polysaccharide composed of mannuronic and glucaronic acid residues (negatively charged at pH values 190

higher than 2) which are cross-linked by calcium ions, resulting in ionotropic gel. A complex coacervate 191

hydrogel is produced when a polyanion and a polycation are bound with one another. 192

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Figure 4 194

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The use of hydrocolloids in combination with food ingredients was reported by Cohen et al. (2009) as an 196

alternative to create printable food materials composed of starch, protein etc. in a platform of different 197

texture and flavours. Testing solely two hydrocolloids, xanthan and gelatin, they simulated a broad range of 198

mounthfeels. The resultant complex coacervate formed by the mixture between xantham and gelatine has 199

shown granularity, which was not observed when the pristine hydrocolloids were tested (Cohen et al., 2009). 200

This behavior can be explained by the hydrogel-forming mechanism of the combination between a 201

polycation (xanthan) and an amphoteric polymer (gelatine). Cohen et al’s study suggests that further 202

materials developments are required to progress in the field of food design using 3D technology. For 203

example, the combination of alginates of different guluronic/mannuronic acid ratios and pectin of high and 204

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low degree of esterification has potential to reveal a new printable material for food structure design. By 205

mixing alginate and pectin at low pH values a synergistic gel is form in absence of Ca2+ and at high water 206

activity; at this condition, neither of the pristine samples would gel. To promote gel formation, both alginate 207

and pectin chains should be partially positively charged before interacting and methylation is recommended 208

to avoid electrostatic repulsion (Walkenström et al., 2003). 209

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2.2 Inkjet printing (IJP) 211

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Inkjet Printing (IJP) technology relies on the fundamental of accumulation of droplets of material deposited 213

on-demand by ink-jet printing nozzles, as depicted by Fig. 5 (Kruth, 2007). Inkjet printers generally operate 214

using thermal or piezoelectric heads. In a thermal inkjet printer, the print head is electrically heated to 215

generate pulses of pressure that push droplets from the nozzle. Piezoelectric inkjet printers contain a 216

piezoelectric crystal inside the print head which creates an acoustic wave to separate the liquid into droplets 217

at even intervals. Employing a voltage to a piezoelectric material arouses a prompt change in shape, which 218

in succession produces the pressure necessary to eject droplets from the nozzle (Murphy and Atala, 2014). 219

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The technology developed by Grood et al. (2011) for dispensing a liquid onto layers can be classified as drop-221

on-demand deposition (Grood and Grood, 2011; Grood et al., 2013). This technology was commercialized by 222

the name of FoodJet printing and uses an array of pneumatic membrane nozzle-jets which layers tiny drops 223

onto a moving object. The drops together shapes a digital image in the format of a graphical decoration, 224

surface fill or cavity deposition (FoodJet, 2015). Inkjet printers generally handle low viscosity materials; 225

therefore, it does not find application on the construction of complex food structure. Typical deposited 226

materials are: chocolate, liquid dough, sugar icing, meat paste, cheese, jams, gels etc (Fig. 6). 227

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Figure 5 229

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Figure 6 231

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2.3 Powder binding deposition 232

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After extrusion processes, powder binding deposition is the second most popular system in 3D food printing. 234

This category can be divided into three sub-types: (1) Selective Laser Sintering (SLS), (2) Selective hot air 235

sintering and melting (SHASAM) and (3) Liquid binding (LB); which have in common the powder 236

deposition in bed. By SLS and SHASAM the layers of powder are fused together upon application of a heat 237

source, infrared laser and hot air, respectively. In liquid binding, there is no phase change during layer 238

solidification: a liquid binder is overprinted onto layers of powder that are accumulated consecutively, as in 239

directed fusion (Wegrzyn et al., 2012). Liquid-biding method has been patented as 3D printing (3DP). All 240

three techniques require an additional step for removing the unfused material at the end of construction. Fig. 241

7 shows a schematic representation of SLS, LB and SHASAM technologies. 242

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Figure 7 244

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2.3.1 Selective Laser Sintering (SLS) 247

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Selective Laser Sintering (SLS) applies a laser, as a power source, to sinter powder particles. A solid 249

structure is built by directing the laser at points pre-determined by a 3D model. The laser fuses together 250

specific areas of the particulate powder bed by scanning cross-sections. Afterwards, the powder bed is 251

lowered by one layer thickness, a new layer of particles is deposited on top, and the process is repeated until 252

the object is finalised. This method can be applied to generate multiple layers of food matrix each layer 253

containing different food material components (Diaz et al., 2014b). 254

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The interaction between the laser beam and the particles used in SLS is important to define the feasibility 256

and quality of any SLS process (Kruth, 2007). The selection of the laser has a relevant influence on the 257

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fusion of powder for two main reasons: (1) the laser absorptivity of materials relies on the laser wavelength 258

(2) the mechanism for powder densification is affected by the input laser energy density (Gu, 2012). 259

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Although SLS has been extensively used to sinter metals, this technology was also adapted for some food 261

designs. Based on SLS technology, using an infrared laser that heats and sinter the material, TNO has 262

incorporated nutritional value and flavours to powder-based 3D objects. The principle relies on the powder 263

binding as a result of the melting fat and/or sugar of the composition (Diaz et al., 2014a; Diaz et al., 2014b). 264

Fig. 8 illustrates examples of 3D food objects created by TNO using SLS technique. 265

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Figure 8 267

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2.3.2 Liquid binding (LB) 269

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This technology has been originally patented as 3D printing (Bredt and Anderson, 1999). During the 271

fabrication by liquid binding, a liquid binder is ejected by a drop-on-demand print head onto a thin layer of 272

powder following a sliced 2D profile generated by a computer 3D model. The binder plays an important role 273

of joining adjacent particles together creating, therefore, a 3D construct. This can occur due to the dissolution-274

fusion or cross-linking of the particle surfaces (Peltola et al., 2008). 275

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An example of this technology applied for food design is the 3DSystem's ChefJet printer which uses the Z-277

Corp inkjet process to produce a broad range of confectionary recipes including sugar, fondant and sweet 278

and sour candy in a variety of flavours-sculptural, as can be seen by the example illustrated in Fig. 9 (Von 279

Hasseln et al., 2014). Recently, TNO researchers described a liquid binding-based method called Powder 280

Bed Printing (PBP). In this method, edible 3D objects are produced by spatial jetting of food fluid (binder) 281

onto a powder bed containing formulated food powder composed of a water soluble protein and/or a 282

hydrocolloid (Diaz et al., 2015b). 283

Figure 9 284

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2.3.3 Selective hot air sintering and melting (SHASAM) 285

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CandyFab machines, a project by Evil Mad Scientist Laboratories (California, USA), use SHASAM 287

technology to print sugar-based 3D objects. SHASAM technology uses a narrow, directed, low-velocity beam 288

of hot air to selectively fuse together sugar powder, building a two-dimensional (2D) picture out of fused 289

powder. At first, the powder bed is slightly lowered then a thin flat layer of particles is spread to the top of the 290

bed, and selectively fuse the media in the new layer. The freshly printed 2D object is indeed fused to any 291

overlapping connected areas in the previous layer. By performing this step again, a 3D object is gradually built 292

up. Upon finishing the 3D object, the bed is brought to its initial position, disinterring the manufactured model, 293

while the unused powder is kept for use in the construction of the next object (CandyFab, 2006). Fig. 10 294

illustrates an example of 3D structure made of sugar, using SHASAM technology. 295

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Figure 10 297

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2.4 Bio-printing 299

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Bio-printing have been originally applied to build tissues without any biomaterial-based scaffold. This 301

technique relies on the precise layer-by-layer deposition of biological materials and culture of living cells. 302

The most common technologies used for deposition and patterning of biological materials are inkjet, 303

microextrusion and laser-assisted printing (Murphy and Atala, 2014). 304

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Considering meat as a post-mortem tissue, researchers from University of Missouri led by Prof Gabor Forgacs 306

proposed to construct a strip of edible porcine tissue using 3D printing technology. Their technology uses 307

multicellular cylinders as building blocks and thus depends on self-adhering cell types. Droplets of freshly 308

prepared multicellular aggregates (the bio-ink particles) are deposited on-demand via an inkjet nozzle into a 309

biocompatible support structure (in this case, agarose rods). The final construct is transferred to special purpose 310

bioreactor for further maintenance and maturation to make it appropriate for use. During maturation, the 311

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bioreactor promotes pulsatile flow and the maturing graft develops biomechanical properties (Forgacs et al., 312

2014; Marga et al., 2012; Norotte et al., 2009). 313

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Figure 11 315

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According to Marga (2012), the bio-printed meat would find acceptance by the vegetarian community which 317

rejects meat for ethical reasons. Upon affordable price, in the future this technology would benefit the 318

masses with religious restrictions on meat consumption and populations with restrict access to safe meat 319

production. The bio-printing of meat, however, shows many drawbacks to overcome, most of them 320

associated to the spatial resolution of the final construct and long maturation processes (Marga, 2012). 321

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2.5 Advantages/limitations of AM techniques and desirable end-use properties of 3D food structures 324

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Although AM technologies have received a lot of attention in the field of food engineering, the advantages 326

and limitations of 3D printing techniques and their impact on the end-use properties of the materials need to 327

be addressed to exchange, in a profitable manner, the traditional fabrication methods by processes involving 328

AM technology. Ideally, the end-properties related to the mechanical stability of 3D printed food should 329

match with those in conventional manufacturing processes. And, in terms of texture design and nutritional 330

optimization, AM technology would potentially defeat traditional fabrication methods. 331

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Both liquid deposition and powder binding bed techniques are capable to build geometrically complex 333

structures. Liquid-based supplies, however, afford a broad range of materials and mechanisms of binding. 334

Printable mixtures comprised of carbohydrates, proteins and fat can be prepared by tuning the material’s 335

properties, such as, melting and glass transition temperature, gelation and viscosity. The strong interaction 336

between layers is the main factor affecting the stability and self-supporting properties of the final construct. 337

For example, the melting extrusion of chocolate must avoid formation of fissures when the deposited layers 338

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are cooled down. Presence of empty spaces between layers (as result of poor interaction) not only cause 339

fracture of the final build but facilitate the undesirable migration of fat (fat bloom) due to the creation of 340

preferential channels. 341

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Powder binding bed techniques have been used to print sugar structures. Especially when heat is used as power 343

source to bind patterns of powder (SLS and SHASAM), the lack of nutritional value in the final product makes 344

this technique less attractive than liquid-based deposition. For this reason, toughness and uniform surface are 345

relevant end-properties of the construct. The powder patterns formed should, in theory, resist fracture upon 346

presence of a crack. The choice of food grade binder can restrict the application of 3DP technique. In addition, 347

weak interactions between powder and binder may lead to rough surface and unstable buildings. 348

349

Self-supporting layers, palatability and visual appeal are common desirable end-properties of the 3D food 350

constructs designed via liquid-based deposition, powder binding deposition and bio-printing. Particularly, 351

for the resulting material of bio-printing processes where 3D constructs of meat is designed by depositing 352

biological material (cell cultures), visual appeal would rather relevant than liquid and powder-based AM 353

techniques. This is because bio-printing is an unusual technique in the field of food engineering and the 354

consideration of meat as a post-mortem tissue might cause negative effect on the acceptance of the product 355

for consumption. 356

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3 RATIONAL CHOICE OF 3D PRINTING TECHNIQUE BASED ON MATERIALS PROPERTIES 358

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Originally, AM technologies were applied for building 3D objects by means of layering deposition of non-360

food materials, such as, metals, ceramics and synthetic polymers in processes involving the use of organic 361

solvents, extreme temperature conditions or crosslinking agents that do not comply with food safety 362

standards. Therefore, one of the critical challenges in the 3D food printing field has been to align food 363

grade materials with printing processes. Three food materials property related critical factors are 364

suggested here for the rational design of 3D food structures: 365

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366

(1) Printability: This feature relies on how the properties of the material enable handling and deposition 367

by a 3D printer and hold its structure post-deposition. The printability of liquid-based AM 368

technologies, such as, drop-on-demand techniques is influenced by the material viscosity or 369

rheological properties. In addition to rheological properties, 3D printing based on extrusion 370

techniques can be affected by specific gelation mechanisms (crosslinking) and thermal properties 371

(melting point and glass transition temperature). Properties like particle size distribution, bulk 372

density, wettability and flowability can also exert influence on powder-based 3D printing; 373

374

(2) Applicability: AM technologies can be attractive by their capability of building complexes structures 375

and textures. In addition, AM becomes more interesting when nutritional value is incorporated to the 376

unique designed structures. The applicability of AM technology is also ruled by the materials 377

properties. 378

379

(3) Post-processing: Ideally, the 3D construct of food should resist to post-processing, such as, baking in 380

an oven, being cooked by immersing in boiling water or deep frying. In the pursuit of a cooking-381

resistant structures, an accurate selection of materials with appropriate physical-chemical, 382

rheological and mechanical properties are essential. 383

384

We emphasize that printability, applicability and post-processing feasibility can be achieved by controlling 385

the physical-chemical, rheological, structural and mechanical properties of the materials (Fig. 12). 386

Knowledge of the essential constituents of food (carbohydrates, proteins and fat) and how their properties 387

influence AM technology is critical to guarantee quality in the end-use product. 388

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Figure 12 390

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391

3.1 Essential constituents of food and their feasibility for 3D printing 392

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As discussed earlier, the food materials should be flowable (liquid or powder) during deposition and also 394

support its structure during or after the deposition. The flowability is achieved by plasticization and melting. 395

The self-supporting structure can be achieved by the reverse process or by gelation by variation of 396

temperature and/or by an additive. In a multicomponent system, variations in the fraction of proteins, 397

carbohydrates and fat will definitely affect the melting behaviour, glassy state and plasticization of the food-398

materials during liquid-based and powder based 3D printing processes. It has been well reported that the 399

plasticization phenomena by water depresses the glass transition temperature of food polymers such as 400

starch, gluten and gelatin (Bhandari and Howes, 1999; Bhandari and Roos, 2003; Haque and Roos, 2006; 401

Roos, 2010; Slade and Levine, 1994). 402

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3.1.1 Carbohydrates 404

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The Tg of food systems is strongly dependent on the carbohydrates fraction, which may vary from simple 406

sugars (e.g., glucose) or disaccharides (e.g., sucrose and lactose) or may encompass polydisperse 407

carbohydrate oligomers such as maltodextrins. High molecular weight polymeric carbohydrates can be 408

unprintable without modification or addition of water or gelling agent. High molecular weight 409

carbohydrates, such as, maltodextrins have higher Tg (for pure starch, ranging between 100 ºC to 243 ºC) 410

than simple sugars, e.g. fructose glucose and fructose with Tg values at about 31 ºC and 5 ºC, respectively 411

(Adhikari et al., 2000; Bhandari et al., 1997). Slade and Levine reported diverging influences of sugars on 412

the Tg for food systems which deserves consideration for their use in AM techniques. In the presence of 413

water and heat, when the intermolecular bonds of starches are broken down allowing hydrogen bonding sites 414

to engage more water (gelatinization), sugar can act plasticizer in low-moisture systems, by playing a role as 415

a co-solute with starch depressing the Tg. Control of glass transition temperature is important to make the 416

deposited material to be set in order to support its own structure after deposition. 417

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418

The crystallization state of the sugars is important in the 3D printing of chocolate by melting extrusion 419

(section 2.3.1). Upon extrusion, in milk chocolate, sucrose and lactose must be fully crystallized. The 420

advantages of a highly crystalline crumb are the reduced amorphous glassy sugar remained to trap fat. 421

Consequently, less fat is necessary to adjust the final chocolate viscosity. Below the Tg, for sucrose in water, 422

viscosity is increased enabling the growth of many crystal nuclei. By releasing fat from the amorphous 423

sugar, the chocolate manufacture is facilitated because the overall fat required to reach the ideal viscosity is 424

reduced (Gonçalves and da Silva Lannes, 2010). 425

426

Sugars comprise the major component of particulate systems used in powder bed binding processes (section 427

2.3). In this category of AM technology, the ongoing interaction between layers promoted by a heat source 428

(laser or hot air) is based on the melting point range of the material. Parameters, such as, powder density and 429

compressibility also deserves attention, as they have an important effect on the powder flowability inside the 430

vessel which, in turn, contributes for the formation of patterns when the heat source is directed to the powder 431

bed (Berretta et al., 2014; Schmid et al., 2014). Powder flowability of the sugars inside build platform 432

together with wettability of the powders is a critical parameter for liquid-based 3D processing (section 433

2.3.2). The flowability plays an important role when the powder is spread and should allow building up thin 434

layers. Low flowability leads to insufficient recoating and high flowability results in powder bed instability. 435

Wettability of particles is also important as the volume of binder dispersed into the powder bed and also the 436

amount of binder absorbed by the particles governs the resolution and mechanical/structural properties of the 437

constructs. Low wetting of fine particles give rise to powder bed rearrangement and high wettability together 438

with slow powder reaction may reduce the smallest feature size. Particle size distribution is another 439

important parameter which may alter the powder bulk density and pore size distribution within the and 440

consequently affects the drop penetration behavior of a water-based binder. The interactions between 441

powder and binder can be related to adhesive forces or chemical reactions (Shirazi et al., 2015). For food 442

design, water-based binder is probably a better alternative, as polymeric binders may give rise to undesirable 443

non-food safety products. 444

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Hydrocolloids are hydrophilic polymers originated from vegetable, animal, microbial or synthetic sources, 445

which form colloidal dispersions in water. In liquid-based AM techniques, hydrocolloids may combine with 446

food ingredients via gelation mechanism giving rise to a different rheological behavior of the mixture. This 447

approach has been demonstrated by Cohen et al. (2010) as a potential alternative to create a wide range of 448

simulated textures of food using only the combination of xanthan gum and gelatin with flavoring agents 449

(Cohen et al., 2009). In powder-based AM techniques, Diaz et al. (2015) have recently reported that 450

hydrocolloids act as a binder component of an edible powder formulation and enhance the control of the 451

migration and flow of the liquid spray into the powder bed during printing. They suggested that the content 452

of hydrocolloid in the powder composition should ideally lie in the range 0.1-2.0wt.%, based on the total dry 453

weight of the composition (Diaz et al., 2015a; Diaz et al., 2015b). 454

455

3.1.2 Proteins 456

457

Food proteins are composed of a mixture of amino acids containing negative and positively charged 458

functional groups. Proteinaceous polymers can be positively or negatively charged depending on the 459

solution pH and the isoelectric point (pI) of the protein. At pI proteins will show aggregation. This is a very 460

attractive feature for liquid-based AM processes ruled by the particle based-gelation and hydrogel-forming 461

mechanisms described in the section 2.1.3. New textures can also be created by the intercalated deposition 462

of layers of food proteins with polysaccharide materials, such as, gelatin and alginate during AM processes. 463

The application of external stress (temperature or mechanical strength) or compounds (e.g., strong acid or 464

base) can also be incorporated to AM technologies promoting, therefore, aggregation and denaturation of 465

proteins as an alternative to design different textures. 466

467

Food proteins can also have their conformation changed by the addition of enzymes. Lipton et al. (2010) 468

have tested transglutaminase as a food additive to build complex geometries out of meat. By incorporating 469

transglutaminase to the meat puree right before printing, the material retained its rheological properties, 470

however, a new protein matrix has been developed (Lipton et al., 2010). This behavior can be explained by 471

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the fact that enzymes, such as, transglutaminase catalyses the formation of covalent bonds between lysine 472

and glutamine residues in a calcium dependent reaction. Hence, the proteins present in the meat puree were 473

enzymatically crosslinked; giving rise to self-supporting hydrogels (Davis et al., 2010). This approach brings 474

insights to the development of 3D printed meat. 475

476

Gelatin can be one of the potential candidates for 3D printing (Diaz et al., 2015a; Diaz et al., 2015b). 477

Gelatin, a derived protein from the irreversible breakdown of the fibrous structure of the collagen (alkali or 478

acid treatment), is a potential candidate to be used as an ingredient of 3D printer inks. Gelatin gel possesses 479

a unique ‘melt-in-mouth’ texture that provides appreciable mouthfeel and flavour perception. Air-dried 480

gelatin dissolves instantaneously in warm water (approximately 40 °C) when the hydrated particles form 481

flexible single random coils. Upon cooling, junction zones are formed by small segments of polypeptide 482

chains reverting to the collagen triple-helix-like structure, leading to gelation (Burey et al., 2008; Ward and 483

Courts, 1977). Gelatin exhibits Newtonian flow in dilute solution except when extended by charged groups. 484

Hence, the used of flexible protein molecules, such as, gelatin in 3D-extrusion processes must bear in mind 485

that charges on the molecules give rise to relevant effects on viscosity. When both positive and negative 486

charges are present, the molecule is fully contracted at the isoelectric point and a minimum in the viscosity 487

is observed. A change in pH in either direction alters the ionization of the functional groups and increases 488

the preponderance of either positive or negative charges. The mutual repulsion of similar charges extends 489

the molecule and enhances the viscosity. At pH values where the molecule depicts its maximum extension, it 490

may be less flexible and lead to non-Newtonian behavior. The shear rate is another important factor 491

affecting the flow of proteins like gelatin in extrusion processes. Irreversible reduction in viscosity can be 492

observed upon shear application. And, extreme conditions of very high shear rate may lead to a non-493

Newtonian behavior (Kragh, 1961). 494

495

496

497

498

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3.1.3 Fat 499

500

Fat is formed by the reaction between three fatty acid molecules (long, straight chain carboxylic acids) with 501

a glycerol molecule to yield a triglyceride (TAG). The TAG composition and structure can affect the 502

material formulation for AM technology and, consequently, the end-use functional properties of the 503

material, such as melting point range, solid fat index, and crystal structure. As an example, it is well-know 504

that the quality of meat in terms of texture, colour and shelf-life is closely related to the TAG composition. 505

This is because the different melting points of fatty acids have significant effect on firmness or softness of 506

the fat in meat. Colour is affected by the content of fatty acids, as solidified fat with a high melting point 507

appears whiter than liquid fat. Shelf-life of meat (rancidity and colour deterioration) is also influenced by the 508

composition of fatty acids, based on the fact that unsaturated fatty acids, especially those with more than two 509

double bonds, can be easily oxidised (Wood et al., 2004). AM technology has potential to help achieving 510

high quality meat-based products, with texture developed for special consumer categories (e.g., elderly and 511

children), by tuning the content of TAGs throughout the deposition of layers of meat paste. 512

513

Fatty acids with larger number of carbon atoms depicts higher melting point. In an opposite way; larger 514

number of double bonds result in lower melting point. Hence, the TAG composition can help to regulate the 515

melting point of the deposited layers and determine their self-supporting properties prior and post-516

processing. Particularly in melting extrusion-based AM processes, where the mixture is melted upon 517

application of heat and solidified by cooling. Lipton et al. (2010) optimized the content of butter fat in 518

traditional dough recipes to avoid liquefaction of the printed structure when baked (Lipton et al., 2010). The 519

same researchers have used bacon fat as flavour enhancer to print turkey meat puree in combination with 520

transglutaminase additive. 521

522

In the melting extrusion-based AM technique applied to print chocolate 3D structures, it is absolutely 523

essential to understand the mechanisms of fat crystallization and how they contribute for the manufacturing 524

of self-supporting layers. Cocoa butter, the main structuring material in chocolate, can develop six different 525

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polymorphic forms, noted from I to VI in increasing order of melting points (γ: -5 to +5ºC; α: 17-22ºC; 526

β2’and β1’: 20-27ºC; β2and β1: 29-34 ºC) (Marangoni, 2003). The polymorphism of cocoa butter has a 527

relevant impact on the organoleptic and physical characteristics of the final products (self-supporting layers, 528

gloss and blooming during storage). 529

530

531

4 SUMMARY AND FUTURE DIRECTIONS 532

533

As reviewed, the design of 3D food constructs via AM technology is strongly dependent on the material 534

properties and binding mechanisms. In recent years, great efforts have been made aiming to achieve end-535

properties of 3D constructs having end-use properties aligned with or more advantageous than those 536

obtained through traditional manufacturing method. However, there are still many barriers to overcome for 537

AM technology to be incorporated in place of traditional manufacturing fabrication processes. From our 538

point of view, many of the challenges faced by AM technology, in the field of food engineering is attributed 539

to (1) process productivity and (2) product innovation and functionality. 540

541

Process productivity can be achieved by enhancing the cost-effectiveness which, in turn, is closed related to 542

the properties of the materials and speed of deposition of layers. As an example, the resolution of extrusion 543

processes, the most common AM technique, is attained through the deposition of very thin layers. This 544

increases the time and cost of operation; placing AM in an unfavourable circumstance in comparison to 545

traditional manufacturing techniques. In the current scenario of substantial advance in the methods of layer 546

deposition, this engineering barrier faced by the AM technology is likely to determine the commercialization 547

underpinning of 3D printers. 548

549

AM is a powerful technology which shows capability to promote product innovation and functionality. We 550

noted, however, quite a few active groups of researches in the field of AM exploring this feature. Most of 551

the 3D food printers are used to build geometrically complex structures without nutritional value. To 552

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aggregate value to the final product or design novel textures, correlations between materials properties and 553

factors influencing a rational design of food structures need to be addressed. 554

555

We reinforce that materials properties play important role on the achievement of printability, 556

applicability and resistant-constructs upon cooking post-processing. Foreseeing and understanding how 557

the essential constituents of food (carbohydrates, proteins and fat) behave during AM processes bring 558

useful insights into the AM of food structures which will help future research into the optimization of 559

printable multicomponent mixtures. 560

561

ACKNOWLEDGMENT 562

563

The authors are grateful to the financial support provided by Meat and Livestock Australia (MLA) to review 564

the 3D printing technologies and identify potential red meat applications (project number: V.RMH.0034). 565

The authors also thank the infrastructure used in the School of Agriculture and Food Sciences at The 566

University of Queensland during the writing of this article review. 567

568

569

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570

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Table 1: List of 3D printing techniques applied for food design.

Technique Principle Binding Mechanism Application References

Supply: liquid

Soft-materials extrusion Extrusion and deposition

No phase change, accommodation of layers

controlled by rheological properties of the

material

Frosting, processed

cheese, dough, meat puree

(Chang et al., 2014; Kuo et

al., 2014; Lipton et al., 2010;

Periard et al., 2007)

Melting extrusion Extrusion and deposition Solidification upon cooling Chocolate, confection (Hao et al., 2010b; Schaal,

2007)

Hydrogel-forming Extrusion Extrusion and deposition Ionic or enzymic cross-linking Xanthan gum and gelatine (Cohen et al., 2009)

Ink Jet Printing Drop-on-demand deposition

No phase change, accommodation of layers

controlled by rheological properties of the

material

Chocolate, liquid dough,

sugar icing, meat paste,

cheese, jams, gels

(Grood and Grood, 2011)

Supply: powder

Liquid binding Powder binding and binder

drop-on-demand deposition

Adhesive forces or chemical reactions

between powder and binder Chocolate

(3DSystems, 2015; Diaz et

al., 2015a; Diaz et al., 2015b;

Von Hasseln et al., 2014)

Selective Laser Sintering Powder binding and heat source

(laser) Sintering and melting of sugar/fat Sugar, Nesquik

(Diaz et al., 2014a; Diaz et

al., 2014b)

Hot air sintering and melting Powder binding and heat source

(hot air) Sintering and melting Sugar (CandyFab, 2006)

Supply: cell

Bioprinting Drop-on-demand deposition Self-assembly of the cells Meat (Forgacs et al., 2014; Marga,

2012)

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Figure 1: Schematic diagram of 3D food printing extrusion processes.

Figure 2: Examples 3D printing technique based on soft-material extrusion: (a) pasta recipe

(Van der Linden, 2015), (b) pork puree (Van Bommel, 2014) and (c) pizza dough. Image (c)

was reproduced with permission of Natural Machines from data available at

https://www.naturalmachines.com/press-kit/#.

Figure 3: Examples of 3D printing technique based on melting extrusion: chocolate constructs

printed by (a) ChocEdge and (b) 3DSystems (3DSystems, 2015). Image (b) was reproduced

from data available at http://www.chocedge.com/creations.php.

Figure 4: Schematic illustration of hydrogel-forming mechanisms: chemical cross-linking,

ionotropic cross-linking and complex coacervate formation. Adapted of (Kirchmajer et al., 2015).

Figure 5: Schematic diagram of Inkjet printing technology.

Figure 6: Examples of 3D printing technique based on inkjet technology: (a) graphical

decoration, (b) surface filling and (c) cavity deposition. Images (a), (b) and (c) were reproduced

from data available at http://foodjet.com.

Figure 7: Schematic diagram of Powder Binding technologies (SLS and LB).

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and (c) Curry Cube, Paprika Pyramid, Cinnamon Cylinder and Pepernoot Pentagon constructs

printed by TNO (Van Bommel, 2014).

Figure 9: Example of 3D printing technique based on LB technology: 3D chocolate structure.

The image was reproduced from the data available at

http://www.3dsystems.com/culinary/gallery.

Figure 10: Example of 3D printing technique based on SHASAM technology. Image

reproduced with permission from Windell H. Oskay, www.evilmadscientist.com.

Figure 11: Schematic illustration of bio-printing technology applied to construct meat.

Figure 12: Parallel between materials properties and factors to consider for the rational design

of 3D food structures.

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FIGURE CAPTIONS

Figure 1: Schematic diagram of 3D food printing extrusion processes.

Figure 2: Examples 3D printing technique based on soft-material extrusion: (a) pasta recipe

(Van der Linden, 2015), (b) pork puree (Van Bommel, 2014) and (c) pizza dough. Image (c)

was reproduced with permission of Natural Machines from data available at

https://www.naturalmachines.com/press-kit/#.

Figure 3: Examples of 3D printing technique based on melting extrusion: chocolate constructs

printed by (a) ChocEdge and (b) 3DSystems (3DSystems, 2015). Image (b) was reproduced

from data available at http://www.chocedge.com/creations.php.

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Figure 4: Schematic illustration of hydrogel-forming mechanisms: chemical cross-linking,

ionotropic cross-linking and complex coacervate formation. Adapted of (Kirchmajer et al., 2015).

Figure 5: Schematic diagram of Inkjet printing technology.

Figure 6: Examples of 3D printing technique based on inkjet technology: (a) graphical

decoration, (b) surface filling and (c) cavity deposition. Images (a), (b) and (c) were reproduced

from data available at http://foodjet.com.

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Figure 7: Schematic diagram of Powder Binding technologies (SLS and LB).

Figure 8: Examples of 3D printing technique based on SLS technology: (a) Sugar, (b) Nesquik

and (c) Curry Cube, Paprika Pyramid, Cinnamon Cylinder and Pepernoot Pentagon constructs

printed by TNO (Van Bommel, 2014).

Figure 9: Example of 3D printing technique based on LB technology: 3D chocolate structure.

The image was reproduced from the data available at

http://www.3dsystems.com/culinary/gallery.

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Figure 10: Example of 3D printing technique based on SHASAM technology. Image

reproduced with permission from Windell H. Oskay, www.evilmadscientist.com.

Figure 11: Schematic illustration of bio-printing technology applied to construct meat.

Figure 12: Parallel between materials properties and factors to consider for the rational design

of 3D food structures.

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1) 3D food printing technologies overview

2) Design of new textures and tailored nutritional content

3) Design of complex geometries of food structure

4) How 3D printing techniques are influenced by material properties?

5) Relation between advantages/limitations of 3D printing techniques and the end-

use properties of 3D printed food

6) Rational choice of 3D printing technique to design food