Three‐dimensional printing and its application to legume proteins: A review

Additive manufacturing, or three‐dimensional printing (3DP) technology, has been recently explored in every field of science and technology for its unique features and ease of operation. 3DP technology has a wide range of applications in the food industry. The technology has enormous potential for the development of functional foods, for food and drug delivery, and for people with special needs, including dysphagia patients. This review envisages the recent interest in plant proteins and the development of food products using additive manufacturing (3DP), which is an exciting opportunity for the food industry and legume producers. The development of the formulation prior to printing requires a thorough rheological characterization for the smooth operation of the printer and fidelity. The steady flow properties and viscoelasticity of legume proteins, as well as their role in 3DP, are elucidated. Finally, this review provides insights into the current state of legume protein‐based 3DP and food product development.

printers has increased tremendously because of the easy access to software that has led to a reduction in manufacturing costs (Provaggi & Kalaskar, 2017). So, it makes 3DP technology an emerging technology that helps to improve the design of products used for various applications.
Since 1986, with the development and patenting of the first 3D printer by Charles Hull, the technology has flourished so fast that all can consult elsewhere industrial sectors have welcomed the technology into their work. Following other industries, 3DP technology has emerged as a subject of research in food engineering and nutrition.
Because of its potential advantages including the design of the desired shape, flavor, texture, and, predominantly, nutritional profile (Chen et al., 2019;Shi et al., 2022), multiple food applications have been explored. Furthermore, 3DP provides an opportunity to produce customized food with uniquely designed food structures to fulfill the needs of population groups with special meal requirements (Chen, 2016;Godoi et al., 2016;Kim et al., 2018). This emerging technology is appropriate for people with exclusive nutritional requirements, for example, dysphagia patients, who have difficulties in eating or swallowing food (Pérez et al., 2019). Similarly, celiac patients require gluten-free (GF) food products, which can be produced using 3DP (Agarwal et al., 2022). The ability of 3D printers to personalize and customize food can transform traditional foods into digital ones. However, more time and research are required for the adaptation of 3DP technology to industrial applications in the food sector, in particular, legumes, because of the wide variability in food ingredients with different compositions and compatibility. The rheological and structural properties of the food 3DP ink should be understood well to ensure the printability and structural stability of the printed products.
In recent times, global consumption and processing of legumes have increased manyfold because of the rising consumer demand for healthy plant protein products as a substitute for animal proteins.
Additionally, the popularity of meat analogs has sparked an interest in using legume proteins as a major protein ingredient in meat analog products. To fulfill the demands of consumers with a desired product shape and texture, 3DP could be a potential choice for food manufacturers using various legume protein isolates as printing ink. Therefore, it is important to understand legume ingredients for their functionality, rheological behavior, and printability so that those ingredients can be used for AM with their health benefits.
The focus of this review is to present a brief outline of the AM techniques and their applications in legume products. The role of rheology in 3DP has also been discussed.

| TYPES OF 3D PRINTERS
According to the American Society for Testing and Materials (ASTM), 3DP technologies can be divided into seven major categories: vat polymerization, direct energy deposition, binder jetting, sheet lamination, material jetting, material extrusion, and powder bed fusion (ASTM International, 2013). Among these, a few are suitable for food printing, including fused deposition modeling (FDM) or extrusion-based printing, selective laser sintering (SLS), stereolithography (SLA), binder jetting, and inkjet printing. Extrusion-based printing, also known as FDM, is the most frequently studied method for 3DP of food Pham & Gault, 1998). These techniques are discussed briefly here so that readers can easily understand the subject. For more details, readers can consult books on AM and review papers (Liu et al., 2017).

| FDM
FDM is one of the preferred extrusion-based AM technologies used for a range of products. The most frequent use of FDM lies in its simplicity, low cost, and flexibility in processing (Badouard et al., 2019). In the FDM technique, the thermoplastic type of filament (in the form of a filament on a spool) heated above its melting temperature acts as a piston for forcing the melt out of the nozzle, and then, the molten filament is deposited onto a platform in a raster pattern so that it can form each layer of the model, and the final 3D object is manufactured by building up continuous layers (Jia et al., 2017;Zhong et al., 2001).
A typical schematic diagram of the FDM technique is illustrated in Figure 1 (Jin et al., 2015). The process consists of a build platform, printing bed, liquefier head, and building material spool. The thermoplastic polymer-based filaments, including polylactides (PLAs), polycarbonate (PC), and acrylonitrile butadiene styrene (ABS), are materials F I G U R E 1 Schematic diagram of three-dimensional (3D) printing using the fused deposition modeling (FDM) technique (Reference: Jin et al., 2015) commonly used as filaments because of their optimum thermal and rheological properties, which are suitable for FDM. The filaments are rounded to a roll of 1.75 and 3 mm in diameter. The heating element transforms the filament into a semiliquid phase, which is then extruded through the nozzle to the printing area for printing the actual component. The main important task in the process is to fuse the subsequent layers before they get solidified, as solidification before getting fused can have a greater impact on the other properties of the building part.
Several printing parameters play important roles in the built-in part characteristics and the production efficiencies of FDM. The major factors considered in 3DP are thickness of the layer, density of the infill, speed of printing, temperature of extrusion, raster angle and width, and nozzle diameter (Dey & Yodo, 2019). Because FDM is a layer-by-layer deposition, the deposition path and processing parameters must be optimally controlled so as to ensure sufficient internal stress relaxation and good bonding quality between the adjacent polymer beads (Tambrallimath et al., 2019).
The FDM is basically designed for prototyping polymers.
However, the technology has been successfully employed for food printing. In fact, the plastic filament of FDM is replaced by dough or a blend of food ingredients. Godoi et al. (2016) demonstrated 3D food extrusion through an illustration shown in Figure 2. The food ingredients are treated as the formulated ink and loaded into a cylinder (extruder) for the extrusion. The layer-to-layer deposition is performed by setting the cylinder at points set by a 3D model. Depending on the materials used in extrusion processes, the surface adhesion of the extruded materials is controlled by the rheological properties of the materials.
Instead of using ink as a medium, many consumer-level 3D printers use melted plastic that solidifies almost immediately after it is released from the printing nozzle. Other printing media are available, however, including a relatively new one-powdered or liquid food material. Sugar, liquid chocolate, and puréed food have all been used to create new food items with interesting and complex shapes and designs. In some cases, using a 3D printer to produce a food product is easier than manual production.
Previously, FDM was used to print chocolate by melting the material and printing it layer by layer into the desired shape (Sun et al., 2015). However, the process could not achieve its print precision; only a rough image of the object was obtained. Additionally, the material it can use is limited, and it needs additives to print chocolate, which destroys the ideal taste condition.

| Binder jetting printing
Binder jetting is one of the attractive AM techniques in which an industrial print head deposits a liquid binder onto thin layers of powder based on object profiles that have been created by software (Kullmann et al., 2012). Such printing has many applications in the electronic industries (Cao et al., 2013;Voss, 2000), ceramics (Huang et al., 2020), and personalized medicine (Öblom et al., 2020). Two types of drop-on-demand (DOD) heads, namely, piezoelectric and thermal heads, are used in this system. In thermal head systems, an electrical pulse is applied at the head so that a high current can pass through a thin-film resistor and vaporize the fluid in contact with it, forming a vapor bubble over the resistor. The vapor bubble inflates in the fluid reservoir, and the rise in the pressure generates a droplet that ejects through the nozzle (Kumar et al., 2004). In the piezoelectric head system, a change in the reservoir fluid volume is carried out by the application of a voltage pulse to a piezoelectric material element, which is coupled, directly or indirectly, to the fluid. Such a volumetric change triggers pressure/velocity transients to happen within the fluid, and these are directed to produce a drop that issues from the nozzle (Noguera et al., 2005).
For food printing, individually controlled nozzle jets are employed.
A commercial printer (FoodJet, The Netherlands) contains a depositor head with a straight row of nozzles, which can "jet" droplets of liquid foodstuff at a high frequency. Each "nozzle jet" can be regulated exclusively and produces droplets with variable volumes onto the surface of a detected moving object. Printable liquids may contain solid particles, but the particles should be smaller than half of the used nozzle diameter. The inkjet technique enables us to produce personalized food products with unique information-rich patterns, for example, quick F I G U R E 2 Schematic diagram of three-dimensional (3D) printing using the extrusion process (Reference: Godoi et al., 2016) response (QR) codes (Öblom et al., 2019;Trenfield et al., 2019). This is an innovative approach utilizing inkjet printing of QR-encoded dosage forms containing both a personalized dose of the food and information relevant to the end user in a QR pattern, readable by a standard smartphone (Edinger et al., 2018).

| SLS 3DP
SLS is the third most common 3DP method, which is mostly used in printing metal and plastic parts, implants, and tissue scaffolds. The technology has several advantages over other methods of 3DP. The printing is solvent-free; there is no need for filament as a material; and the printing is relatively fast compared with other methods.
Additionally, printlets do not need post-processing and are readily available after printing for dispensing and consumption. SLS printing technology has recently received attention from the food industry for printing foods (Jonkers, van Dijk, et al., 2022;Jonkers, van Dommelen, et al., 2022;Shahbazi et al., 2022). Printlets containing multiple bioactive compounds with controlled release and desired properties can be designed by manipulating process and material attributes.
SLS uses a laser beam to construct solid objects by heating powder particles close to the melting point and fusing them together at their surfaces (Fina et al., 2018). Upon completion of the first layer, a roller dispenses another layer of powder on top of the earlier one.
The object is assembled layer by layer, which is retrieved from the bottom of the powder bed later on. The SLS printer consists of six parts, namely, (i) a building platform that assembles the 3D object; (ii) a laser that is used for the sintering process; (iii) Galvano mirrors that guide and direct the laser beam to the correct printing positions; (iv) a powder reservoir platform or hopper to hold and dispense fresh powder onto the building platform; (v) a mechanical roller, which spreads and flattens fresh powder on the building platform; and (vi) a material vat that recovers unsintered powder material ( Figure 4) (Akande et al., 2016;Tiwari et al., 2015). Mostly, SLS printers use carbon dioxide (CO 2 ) lasers, which provide higher power at lower cost, permitting the use of a wide array of powdered thermoplastic materials (Awad et al., 2020).

| RHEOLOGY AND 3DP
3.1 | Basics of rheology related to 3DP Various flow models have been reported to characterize the 3DP inks. Nonetheless, the power law model (Equations 1 and 2) is the most appropriate for non-Newtonian fluids. It is applicable to a wide range of inks and can describe Newtonian, shear-thinning, and shear-thickening behaviors based on the flow behavior index, n. For a Newtonian fluid, n = 1, and the equation reduces to the Newtonian model. If n is less than 1, the fluid is shear thinning; if it is greater than 1, the fluid is shear thickening. Mostly, shear-thinning behavior of inks is desired for the 3DP because it allows small extrusion pressure at the same speed as compared with other types of materials. where τ and _ γ are the shear stress and shear rate, respectively; K and n are the consistency index and the flow behavior index, respectively; and η is the apparent viscosity.
On the other hand, many non-Newtonian fluids exhibit viscoelastic behavior, which is a combination of elastic (solid-like) and viscous (fluidlike) behaviors. Viscoelasticity is measured by applying a small oscillatory stress and measuring the resulting strain. In smallamplitude oscillatory measurements, the viscoelasticity of the inks can be described in terms of the elastic modulus (G 0 ), viscous modulus (G 00 ), phase angle (δ), or complex viscosity (η*). Readers can consult elsewhere for more details about basics of rheology (Ahmed, 2021).

| Importance of rheology in 3DP and ink formulation
In addition to processing parameters, the rheological properties of materials (e.g., ink) play a pivotal role in the material's behavior at different stages of printing and print variability (e.g., shear-thinning behavior and shape fidelity) . printer. An illustration of the rheology-controlled shape fidelity of the printing product is illustrated in Figure 7. As can be seen, ceramic inks with different rheological properties were employed to elucidate the shape fidelity, self-support capacity, and shape retention ability by printing a similar structure on a 3D printer (del-Mazo-Barbara & Ginebra, 2021). Figure 7a shows adequate shape fidelity with shape retention and self-supporting capacity, whereas Figure 7b shows partial shape distortion and Figure 7c shows complete shape distortion.
These differences in shape fidelity and distortion of the structures of the ceramic inks have been attributed to their poor rheological properties. Therefore, it can be concluded that the rheological properties of inks have strongly influenced the structures of 3D-printed materials.
The flow of the viscous ink and the transitions of the viscosity during an extrusion-based printing are illustrated in Figure 8. At the beginning, there is no flow inside the cartridge. However, when the force is applied, the ink deforms and starts to flow in a high-shear zone through the nozzle (Schwab et al., 2020;Seoane-Viano et al., 2021). Finally, the extruded material forms a desirable shape.
It suggests that the flow of the ink requires a minimum force prior to flow, which is termed as the "yield stress" in rheological terms. The basic requirements for inks in the 3DP system are that they flow smoothly through the nozzle, should not cause clogging, form a continuous filament, retain the shape of the nozzle, support layer stacking, and reproduce the printing path. In order to fulfill these requirements, the printing ink should behave similarly to fluid and viscoelastic materials during the extrusion process and at rest or under any external stress.
F I G U R E 7 Illustrative image to the effect of viscosity on shape fidelity, shape retention, and self-supporting capacity. The rheology of the ink must be tailored meticulously to maintain the desired shape and span gaps of extruded objects. Therefore, the use of ink solutions with shear-thinning behavior is ideal for hot-melt extrusion flows where the viscosity decreases with increasing shear rate.
Temperature has a significant impact on rheology. The food materials transform from viscoelastic to non-Newtonian shear-thinning fluid at a high temperature, so a time lapse is required for the structuring of the printed materials, in particular in layer-by-layer structuring.
It is another area of research, which is known as four-dimensional printing (4DP), and more information is available elsewhere (Ahmed, 2019). Rando et al. (2020) Wang et al., 2021). In order to avoid these issues, the filament should be used to balance the surface tension, gravitational force, and applied stress perfectly. This rheological characteristic is reflected in shape fidelity. Higher viscosity results in high shape fidelity. Under shape fidelity, the rheological parameters include damping factor, elastic behavior, and yield stress, which represent the elastic features and stiffness of the material at rest. The damping factor is directly used to classify the materials at rest on the basis of stiffness, either elastic or viscous. A few materials, such as cellulose nanofibrils (CNFs), are popularly used as rheological modifiers to promote printability, extrusion, and shape fidelity of the ink materials. The details of the relationship between the rheological parameters and printing parameters and their significance are presented in Table 1. lupin or chickpea flour in combination with pea protein isolates (PPIs) (Agarwal et al., 2022). All the rheological data fitted well into the WoDS, considering two object initial heights (12 and 36 mm). All formulations show deformation below 5% of WoDS at two tested heights. This implies that these GF food-based inks could be expected to have good dimensional stability and post-printing dimensional stability. Δh

| Application of 3DP in legume-based products
Legumes play an important role in human nutrition and have acted as a primary food source in low-income countries (Maphosa & Jideani, 2017). There has been a recent interest in developed countries for utilizing legumes in mainstream food products. Legumes show favorable physicochemical and functional properties that make them a suitable ingredient for edible printing ink (Pasqualone et al., 2020;. In recent years, various studies have evaluated multiple legume-based ingredients for their suitability to create 3D-printed structures. Extrusion-based 3DP performance can be affected by various characteristics of the ink. These include (i) smooth nozzle flow, (ii) recovery to printed shape after deposition; and (iii) ability to self-support the printed structure Rowat et al., 2021). To achieve those characteristics, the printed ink should have to exhibit shear-thinning behavior with thermo-reversibility . Several legume ingredients exhibit shear-thinning behavior. However, it can be significantly improved by incorporating suitable functional ingredients such as hydrocolloids and biosurfactants to modify the rheological and structural characteristics of the legume-based formulation. The following section explores the studies that have used legume-based ingredients for extrusion-based 3DP and evaluates the opportunities and challenges.
Faba bean is one of the inexpensive protein sources in developing countries. Faba bean proteins have shown good emulsifying, foaming, and solubility characteristics, enabling them to be suitable ingredients for multiple food applications (Boye et al., 2010;Dong et al., 2022). Johansson et al. (2022) investigated the printability of faba beanbased edible inks by varying the protein, starch, and fiber contents in the formulation. The formulations are shaped into 3D-printed cubes at room temperature with two different infill patterns, namely, honeycomb and grid and freeze-dried ( Figure 10). The ink rich in the fiber-rich fraction exhibited better printability and shape stability as compared with the protein and starch-rich fractions. The protein and starch-rich inks had the highest printing failure due to unstable flow and low shape stability. The fiber-and starch-rich printed products were crispier in sensory perception as compared with the protein-rich products, which were perceived to have a harder texture. Principal component analysis (PCA) indicated that the fiber content of the faba bean ink was related to higher elastic modulus and higher shape F I G U R E 9 Variables relevant in characterizing the deformation of a cylinder of food ink under its own weight: G 0 and G 00 are the elastic and viscous moduli; τ y is the yield stress; ρ is the density of the food ink; H, Δh, and A are the initial height, deformation, and crosssectional area of the cylinder, respectively; and g is the gravitational constant (Reference: Nijdam et al., 2021).
T A B L E 1 Relationship between the rheological parameters and printing parameters and their significance (References: Bom et al., 2022;del-Mazo-Barbara & Ginebra, 2021)   that the 3D-printed snack-bites were harder and had an intense crumbly texture than the snack-bites manufactured conventionally. In conclusion, the 3DP technology allowed for the alignment of structural filaments and air gaps layer by layer in 3D-printed objects, resulting in a modified structure that affected texture and taste, depending on the formulation and moisture content.
When mixed with potato starch, pea protein at concentrations below 2% enhanced the cross-linking between the starch granules.
The presence of pea protein contributes to the cohesiveness and adhesiveness of the potato starch paste and thereby improves its printability (Chuanxing et al., 2018). Although 3DP of banana paste was explored, it was found that the paste is not a good candidate for 3DP and it had longtail effects, like negatively affecting printed objects and having a low resolution. However, the addition of 15% PPI to banana paste improved the printability with optimal shape retention by overcoming those limitations (Kim et al., 2021). When F I G U R E 1 0 Faba bean-based three-dimensional (3D)-printed cubes (14 Â 14 Â 14 mm) after freeze-drying (Reference: Johansson et al., 2022) the PPI concentration is increased to 20%, the printing performance is reduced with discontinuous extrusion and the lower resolution of the printed products ( Figure 12). The decrease in printing performance has been attributed to the increased particle volume fractions caused by the protein aggregation that led to challenges in extrudability and flexibility. This study identifies the challenges with dynamic oscillatory tests to evaluate printability at higher particle fraction and suggests alternate experimental techniques such as tack tests and dashed line printing tests to determine the printing resolution (Kim et al., 2021).  (Phuhongsung, Zhang, & Bhandari, 2020). The printed product was microwaved and showed improved the stability and flavor of the 3DP gel. Chen et al. (2021) evaluated the possibility of the development of 3D-printed soybean protein-based steak-like products using six different hydrocolloids on the printability of textured soy protein (TSP) and drawing soy protein (DSP). A steak-like model of food was printed, followed by freezing and frying in hot oil (170 C) for 5 min ( Figure 13). The presence of hydrocolloid improved printability, and F I G U R E 1 1 Transverse-sectional two-dimensional (2D) X-ray micro-tomography images of conventional (left) and three-dimensional (3D)printed (right) chickpea flour (CPF, top) and lupin flour (LPF, bottom) gluten-free snack-bite baked for 30 min. The images represent the "gray" region associated with protein to starch networks, whereas "black gaps" inside the gray region represents air gaps in the microstructure (Reference: Agarwal et al., 2022). the addition of xanthan gum resulted in the best product. The TSP inks had better printability and shape stability than the DSP inks although same hydrocolloids were used for the preparation. The addition of xanthan gum to TSP improved the shear-thinning behavior, enabling a smooth extrusion flow from the printer nozzle. The corresponding dynamic moduli were lowered with the xanthan gum incorporation. The printed samples also maintained their shape after being fried in hot oil. The study, furthermore, evaluated the effect of the infill pattern and infill ratio on the texture of the sample after frying.
Among four different infill patterns, namely, grid, triangular, wiggle, and honeycomb, it was observed that triangular infill with an infill of 60% had textural characteristics similar to that of a fried chicken nugget.
Blending of SPI with other food materials has been tested by many researchers. While working with the printing performance of SPI and strawberry ink systems, Fan et al. (2020) examined the effects of the addition of salt and microwave pre-treatment to blended powder mixes, and it was found that both salt and microwave pre-treatment enhanced the printability and shape stability of the ink system. The influence of pH on the SPI, pumpkin, and beetroot powder mixed ink showed that the pH had a significant impact on the color, aroma, and flavor of the printed products (Phuhongsung, Zhang, & Devahastin, 2020a) ( Figure 14). The best sensorial results were obtained with samples having a pH of 6 to 10.
The effects of xanthan gum and sodium chloride at selected concentrations were evaluated for the printability of SPI gels (Phuhongsung, Zhang, & Devahastin, 2020b). The addition of xanthan gum at 0.5 g/30 g and 1 g/100 ml NaCl to SPI gel resulted in the best printability. The addition of NaCl lowered gel viscosity, enabling smooth extrusion. In one study,   biopolymers for the syringe-based 3D food printer and gear-based 3D food printer, respectively. The first ink formulation includes CBPE, sodium alginate, and gelatin, whereas the second formulation consists of agar, xanthan, and CBPE. For the syringe-based and gear-based 3D food printers, superfine grinding significantly reduced the particle size of CBPE and resulted in a reduction in the printability by increasing the adhesiveness and the swell powder of food-ink systems. Both the food-ink formulations exhibited shear-thinning behavior, and the viscosity was low at a high shear rate that made for an easier extrusion of the materials from the nozzle. The syringe-based 3D food printer retained the α-amylase activity in printed CBPE; however, the enzyme activity was lost after the gear-based extrusion process.
These findings could provide ideas for the potential application of white common bean protein in 3DP technology.
Legume-based food ingredients such as faba bean protein, soy protein, common bean, and pea protein have been examined for their 3DP capability, performance, and fidelity in multiple studies. Those studies demonstrated that legume-based proteins have the potential to form suitable edible inks because of their shear-thinning behavior, optimum yield stress, and thermo-reversible characteristics. At optimum concentrations, inks containing such legume-based ingredients could print self-supportive 3D-printed structures. The addition of hydrocolloids such as xanthan gum and biosurfactants enhanced the printability and shape stability of the legume-based ingredients.
In certain cases, the addition of legumes to starch-and fruit-based ingredients improved the printing performance of the composite inks.

| CONCLUSIONS
3DP is an innovative tool to design food products following consumer demand within a short time with a personal touch. Among available printers, extrusion-based printers are mostly preferred for the printing of legume-based products. Legume protein isolates have excellent functional and rheological properties. Rheologically, the food inks mostly exhibit shear-thinning behavior. Therefore, protein isolates, either individually or in a blend, can be successfully printed with personal needs. The effect of viscosity on shape fidelity, shape retention, and self-supporting capacity is also discussed in this review. The layered construction of food using 3DP technology has a significant effect on food microstructure and sensory parameters. Legume proteins are excellent candidates for AM. Understanding the interactions between the proteins, hydrocolloids, and additives will be key to optimizing the material properties for new applications. The potential use of legume starch or starch-protein blend needs to be investigated as a potential ink for food printing, so that a large amount of starch pooled through the extraction of proteins should be used appropriately.

DATA AVAILABILITY STATEMENT
Data sharing not applicable-no new data were generated, or the article describes entirely theoretical research.