Designing 3D printable food based on fruit and vegetable products—opportunities and challenges

Abstract

3D printing is an innovative technology for food industry, which provides tremendous opportunities for the designing of customized and personalized nutrition for fruit and vegetable based food products. Various researchers have worked on the development of printable ink, stability of printed products and quality parameters. The aim of this review is to cover the most recent advancements on 3D food printing for fruits and vegetables and explore the prospect. Extrusion-based 3D printing is the most extensively used technique due to their several advantages. The review examines the three groups of extrusion printing such as room temperature extrusion, fused deposition manufacturing and gel forming extrusion. The development in last few years in the area of 3D food products of fruits and vegetables powder has been assembled in this review article. Based on these results, it can conclude that fruit and vegetable has been successfully used to fabricate 3D and even 4D food products. Future studies are required for improvement in pre and post-processing technique. Functional food can also be developed by using 3D printing but more research is required in this area.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-022-05386-4.

Introduction

An emerging technology which has tremendous applications in manufacturing of physical prototypes, practical evaluation of product designs, wellness products, and living biological systems, is three-dimensional printing (3D Printing). 3D printing has been adopted by various fields of industry such as automobiles, architecture, textiles, medical, military and also food. 3D printing is also known as additive manufacturing (AM) technique which was invented in 1980s (Guo et al. 2019; Pitayachaval et al. 2018). Additive manufacturing is a rapid proto typing and rapid manufacturing technique which converts a digital model of a 3D computer-aided design (CAD) tool to exactly into similar 3D physical object by inserting the layer-by-layer addition of materials. And thus, the process can also be named as food-layered manufacture when applied to the food sector (Lipson and Kurman 2013; Liu et al. 2017). 3D printing in food technology was introduced in 2007 at Cornell University where researchers invented the Fab@home™ model. This developed model is an open-source kit that allows the user to structure their personal solid freeform construction system in which layer by layer substance is deposited from a pre-designed file (Liu et al. 2017; Periard et al. 2007). The complicated and incredible food design which is not easy to be printed manually or by traditional molding can be printed by common man with the help of 3D printer (Sun et al. 2015a, b). Hence, 3D printing provides several benefits which include personalized geometry, decreasing the manufacturing cost, widening the usage of existing food ingredients, minimizing the manufacturing cycle, enhancing the competitiveness (Feng et al. 2019). Along with this, 3D printing is environmental friendly process as it uses minute raw material and less energy. 3D printing is very economical technique as that can combine numerous steps while processing into one step and food will be prepared only when it is ordered. Hence, it reduces the quantity of food waste and energy consumption (Dankar et al. 2018a).

The universal plan and main idea of bringing 3D printing is to provide sustainable nutrition and food security (Pitayachaval et al. 2018). Various techniques are employed to construct 3D food printing such as extrusion-based printing, selective sintering printing (SLS), binder jetting, and inkjet printing (Liu et al. 2017). Food inks are the final printable form of an edible substance prepared from one or more raw materials, often in the form of a paste or gel that is ready to be extruded from a printer nozzle at ambient temperature. Edible ink should be able to make layer-by-layer addition of materials and "self-supporting" strength to maintain the deposited formation. The physico-chemical, rheological, structural and mechanical properties of edible ink play a significant role in retaining this "self-supporting" strength (Feng et al. 2019; Godoi et al. 2016). Edible inks are divided into 2 categories (1) Natively extrudable and (2) Non natively extrudable. Natively printable materials include cake frosting, hummus, cream, chocolate which can be smoothly extruded through a 3D printing nozzle. The characteristics of these materials play an important role in efficient 3D food printing (Yang et al. 2018). Non-natively extrudable category includes food such as plants and meat, which can also be extruded but only after processing (Feng et al. 2019).

Fruits and vegetables are an essential part of the human diet as they are useful tools to supply carbohydrate, vitamins, sugars, minerals, and bioactive compounds. So the application of fruits and vegetables to 3D printing plays a significant role on the design of personalized nutritional foods. Application of fruits and vegetables (Non natively extrudable) in 3D printing becomes a barrier due to its high moisture content which reduces viscosity and the fibre content can block the nozzle. There is also a probability of the segregation between solid and liquid phase (Feng et al. 2019; Ricciet al. 2019; Voon et al. 2019). The addition of hydrocolloids to the non-printable food items like rice, meat, fruit and vegetables has proved to elevate their capability of extrusion (Sun et al. 2015a, b). Rheological properties of food products are also essential for predicting the analysis of process design and flow conditions in 3D printing. Water is the most abundant constituent of fruits and vegetables, and it may vary from one plant to another. Moisture content affect the constituent for the implementation of 3D printing in the fruit and vegetable food due to its great effect on the viscosity of pastes (Ricci et al. 2019). Various efforts have been tried by researchers to develop soft and nutritious food for the elderly people and those have difficulty with swallowing and mastication. Children are also enthusiastic to eat healthy and nutritious snacks if it is given in creative and fun way. 3D food printing satisfy this need by designing food which is safe, appealing and nutritious by utilizing fruits and vegetables (Dankar et al. 2018a). This paper aims to review the detailed application of extrusion-based 3D printing which supports the extensive utilization for fruits and vegetables by retaining the post printed deposition.

3D printing process

3D food printing is a sequential process that includes major steps such as 3D model building, material printing and Post-treatment process. 3D model building starts with designing a 3D model by CAD (Computer-Aided Design) followed by its conversion into STL files. Some commonly used CAD software models are AutoCAD, 3ds Max, Rhino 3D, Tinker cad, 3D Slash, Autodesk 123D (Guo et al. 2019). With the help of appropriate slicing software, the model is sliced into independent layers to achieve an outline of each section. During the slicing process, G-codes and M-codes are created for each slice layer which is transported to the printer for printing a preferred material. G-codes lead motors related to the printing region, printing speed, and printing axis whereas M codes helps machine functioning. The next step is the deposition of materials layer by layer with a predetermined thickness to achieve three-dimensional objects from different materials used as food inks. The food inks are derived from the food materials itself and chocolate, fat, gelatin, dough, mashed potatoes, cream, sugar, cheese are commonly used food inks (Feng et al. 2019). The third step is post-treatment process in which printed objects are particularly trimmed, cooked, baked, fried as per the requirement of a final product (Lille et al. 2018).

Printing techniques such as extrusion-based printing, binder jetting, inkjet printing, and selective laser sintering are categorized depending on the fabrication principle (Liu et al. 2017). The extrusion-based printing technique builds the food model by forcing food ink through a nozzle with a constant application of pressure which is previously loaded in the extruder (Liu et al. 2017). Extrusion based printing is different from food extrusion cooking as the printing method is a digitally controlled extrusion process to construct 3D food products layer by layer. Among all the printing technologies, the extrusion-based printing method is mostly preferred (He et al. 2019). Inkjet printing depends on the principle of dispensing droplets of food material from a thermal or piezoelectric head of ink-jet printing nozzles. Thermal inkjet printer operates by electrically heating print head to produce pulses of pressure that forces droplets from the nozzle whereas piezoelectric inkjet printer machine contains piezoelectric crystal inside the print head which generates an acoustic wave to separate the liquid into droplets at even intervals (Pitayachaval et al. 2018). In binder jetting, the powdered materials are distributed evenly layer by layer across the fabrication platform, and the liquid binder is ejected upon each consecutive powder layer at particular regions depending on the data file for the object being built (Liu et al. 2017). Selective laser sintering is a technology that applies a power laser as a sintering source to fuse powder particles layer by layer to form a solid 3D structure. After the laser scanning the surface of the layer cross sectionally, the powder bed is dropped and a fresh layer of powder is covered on top. This process of dropping and layering is repeated until the desired structure is achieved. The unbound powder is removed and recycled for further use. The sintering process constructs complex food materials in a short time without post-curing process (Liu et al. 2017; Sun et al. 2015a, b).

Printing parameters

The 3D food printing is considered successful only when all the printing parameters are optimized specifically and accurately (Dankar et al. 2018a). The nature of every food ingredient will differ during 3D food printing based on the type of process, the composition of ingredients and the printing parameters. In 3D food printing, two types of parameters should be controlled during the process. First are parameters related to the printer machine such as print speed, nozzle diameter, nozzle height and the others related to food itself. The precisely control of both two types of parameters will result in the best and desired 3D printed object (Dankar et al. 2018a). Printing parameters play a major role in the effective realization of 3D printed edible food objects. Print speed (mm/s), layer height (mm), nozzle diameter (mm), flow (%), infill (%) are different printing parameters, potentially affecting the quality of 3D printed food structure. The print heads in 3D food printers are mostly extruder-type with single or dual nozzle models (Dick et al. 2019). The nozzle diameter significantly influences the printing accuracy and perfection of the sample surface. The reduced nozzle diameter increases the degree of refinement and quality of the printed material. But the reduced nozzle diameter also raises the printing time and feed pressure (Feng et al. 2019). During printing perishable foods like meat or seafoods, the temperature of the feeding system, hopper, nozzle, and platform should be regulated below 4 °C throughout the printing process to prevent microbial contamination (Dick et al. 2019).

3D printing methodologies suitable for fruit and vegetable products

Among the various 3D printing technologies, extrusion-based 3D printing is the most extensively used technique due to their several benefits. Cost is restricted to the digital design of the product, equipment cost, ingredients amount and consumers can build their own complicated structure (Azam et al. 2017). This extrusion food printing technique is further divided on the basis of printing material states, into three groups: room temperature extrusion, fused deposition manufacturing and gel forming extrusion (Chang et al. 2020). And, three extrusion mechanisms are employed to extrude liquid or semiliquid material: syringe-based extrusion, air pressure driven extrusion and screw-based extrusion (Sun et al. 2018). Table 1 shows the summary of various fruits and vegetables used in extrusion-based printing.

Table 1.

Brief description of various fruit and vegetables used in extrusion-based printing

Vegetable/Fruit	General formulations	Printing material states	Extrusion mechanisms	Important material properties/processing parameters	Outcome	Reference
Potato	Puree of dehydrated potato powder and whole milk	–	Syringe-based extrusion	Printing temperature, composition of the potato puree	Composition has more influence on retaining the structure of printed material than temperature	Martínez-Monz et al. (2018)
	Mashed potatoes prepared by gelatinized potato flakes and gums	Gel forming extrusion	–	Rheological properties	The printed product with mashed potatoes, k-carrageenan and xanthan gum provides excellent printability and mechanical strength	Zhenbin et al. (2018)
	Mashed potatoes prepared with potato flakes and boiling water	Room temperature extrusion	Syringe-based extrusion	Shear thinning behaviour, LF-NMR	Principal Component Regression and Partial Least Squares were effective to predict rheological propertied	Zhenbin et al. (2020)
	Potato flakes, milk and gums	–	Two nozzle printer	Printing parameters, survival rate of probiotics	Developed appealing 3D printed food fortified with probiotics microorganisms	Liu et al. (2019)
	Potato puree of potato powder and whole milk	–	Syringe-based extrusion	Mechanical and Microstructural Properties	Alginate and agar achieved the best printing product	Dankar et al. (2018a, b)
Yam	Paste of freeze-dried yam and potato processing by-product	Room temperature extrusion	Dual-nozzle 3D printer	Rheological properties, Dimensional properties	3D printed materials were close to the model structure	Feng et al. (2020)
Mushroom	Dough of freeze-dried mushroom powder and wheat flour	Room temperature extrusion	Screw-driven mechanism	Printing speed, nozzle diameter	Fiber-enriched snacks were developed	Keerthana et al. (2020)
Tomato	Tomato puree was centrifuged to produce tomato paste	Room temperature extrusion	Syringe-based extrusion	Rheological properties, printing stability	The extrusion pressure used to extrude tomato paste expand linearly with increasing flow stress	Zhua et al. (2019)
Spinach	Mixture of freeze-dried spinach powder and xanthan gum	–	Syringe-based extrusion	Dynamic viscoelastic properties, particle size	Increase in the particle size of the spinach powder, increases the mechanical strength	Lee et al. (2019)
Broccoli, spinach, carrot	Freeze-dried vegetable powders added to the hydrocolloid mixtures	Gel forming extrusion	Syringe-based extrusion	Printability and rheological properties	Xanthan gum is the optimum hydrocolloid that gives good extrudability and resolution	Kim et al. (2018)
Carrots, pears, kiwi, broccoli raab leaves and avocado	Paste of fruit and vegetable with fish-collagen	–	Stainless piston chamber	Print speed, flow, morphological and microstructural properties	A printed smoothie of selected fruit and vegetables was developed	Severini et al. (2017)
Mango	Mango juice gel developed by concentrated mango juice and potato starch	Gel forming extrusion	Syringe-based extrusion	Dielectric properties of products	Post treatment with microwave vacuum drying enhances the shape accuracy	Yang et al. (2018)
	Dough of freeze-dried mango powder, wheat flour and olive oil	Room temperature extrusion	Air pressure driven extrusion	Material composition	The optimized parameters produced excellent geometry with lesser total deformation	Liu et al. (2019)
Orange	Blend of orange concentrate, wheat starch and gums was prepared by cooking	Fused deposition modeling	Syringe type extrusion	Rheological properties	Vitamin D enriched 3D printed food was prepared	Azam et al. (2018)
	Blend of orange concentrate and wheat starch	–	Screw-based extrusion	Rheological properties, nozzle diameter	Optimized printed materials stayed consistent and obtained best resolution	Azam et al. (2017)
Lemon	Gel is developed by lemon juice and potato starch	Gel forming extrusion	Screw-based extrusion	Rheological properties and mechanical properties	An equation is suggested to describe the relationship between extrusion rate, nozzle diameter and nozzle movement speed	Yang et.al (2017)
	Gel is prepared by lemon juice and various starches (potato starch, sweet potato starch, wheat starch and corn starch)	Gel forming extrusion	Syringe-based extrusion	Rheological measurements, POLYFLOW software numerical analysis	Filling rate of 90% suitable because of the swelling effect	Yang et al. (2019a, b)
Strawberry	Gel prepared by adding potato starch to strawberry juice concentrate	Gel forming extrusion	Dual (2-nozzle) extruder	Rheological properties	Dual extrusion produces highly appealing and complex geometry product	Liu et al. (2018a, b, c)
Banana, white beans, mushrooms	Formulation prepared with selected fresh produce and pectin	–	Stainless piston chamber	Print speed and flow level	Fabricated fruit-based snack for children	Derossi et al. (2017)

Various steps are included in the processing of plant materials to obtain the final printable food inks (Ricci et al. 2019). Major steps involved in the fabrication of printed object are shown in Fig. 1. In pre-processing step, raw plant material should be selected considering its rheological properties, nutritional properties, and also sensory characteristics. The mashing of fruits and vegetables is necessary to achieve a homogenous paste. Viscosity and rheology of pastes should be adjusted by adding hydrocolloids such as agar, alginates, carrageenan, cellulose derivatives, gellan gum, xanthan gum (Imeson 1992). Whereas, particle size distribution, bulk density, wettability and flowability should be considered to prepare bio-ink in powder form. Severini et al. (2018) suggested sanitizing each printing object coming in contact with the food material. This was recommended due to his finding of 4.28 Log CFU/g bacteria in a smoothie obtained from fruits and vegetables. Post-processing is required to enhance the appetizing of printed food. On the commercial level, 3D printed fruit and vegetables lies in the category of ready-to-eat foods which should have fresh taste and appearance and prolonged shelf life.

Fig. 1.

Major steps involved in 3D food printing

Room temperature extrusion

Room temperature extrusion (RTE) applies to smooth extrusion of supporting layers of natively printable materials such as dough, cheese, frosting, creamy peanut butter at room temperature to print 3D material by mixing and depositing process (Godoi et al. 2016). RTE is performed without temperature control and phase conversion. In this type of printing technique, the viscoelasticity of the material plays a critical role to extrude through a fine nozzle and to obtain a successful printing (Chang et al. 2020). Recently, Zhua et al. (2019) studied correlations between printing stability of the material and rheological properties using tomato paste as a model system which showed linear interaction among material’s flow stress, zero shear viscosity, and printing stability. In this study, flow stress was observed to be a good sign for printing stability. Flow stress experiences the stress in which the formation of viscoelastic material changes from solids state to some liquid state. This may improve the stability of the material after printing. A study was conducted by Feng et al. (2020) to examine the 3D printing properties of yam powder by varying the mass ratios of 10:0, 9:1, 8:2 and 7:3 with high-fiber potato by-product. The mass ratio of yam powder and potato by-product in 7:3 achieved more G’. Less infill level, produces printed sample with less weight and more porosity and vice versa. The 3D printed samples were stored at − 20 °C for 24 h and then post processed for air frying. Visible display of all product prior and after air frying are shown in Fig. 2. The size of printed sample was marginally bigger than the printed model because of the swelling phenomenon in extrusion. The printing material was subjected to tensile deformation over a shrinkage part of the nozzle diameter to retain some elastic potential energy and was discharged at nozzle. Material viscosity and inlet volume flow had influence on the swelling ratio. The greater the viscosity of the material, the lesser the swelling ratio. In similar type of experiment, the geometric characteristics, quality and textural properties of printed mashed potato were studied as a function of 10%, 40% and 70% infill percentage, infill parameters and shell circumference (Liu et al. 2018a). Internal structure of mashed potato construct by 3D printing and its textural properties were studied. Comparison was performed between 3D printed sample and cast samples for the microstructure analysis for various infill percentages as shown in Fig. 3. Microstructure characteristics showed homogeneous internal structure for cast sample from both cross-sectional and longitudinal sectional views. However, apparently layered structure was seen in longitudinal sectional views which is represented by red lines and porous structure in cross-sectional of 3D printed product for both the infill percentage. As compared to 70% infill level, relative larger pore size distribution and thicker wall was found for 100% infill mashed potato. This may be due to the less hard texture of 3D printed product when a low infill level was used. Zhenbin et al. (2020) developed principal component regression and partial least squares model to predict rheological properties of mashed potatoes. These researchers also suggested that while extrusion, viscosity of food ink should be less enough to facilitate easy flow from a thin nozzle tip and more enough to stick between different layers. This is rapid approach to predict 3D printing behaviour of mashed potatoes without performing printing trials.

Fig. 2.

Visual appearance of printed product with 20%, 50% and 80% infill levels (and infill structures before and after air-frying).Yam-PP1, Yam-PP2 and Yam-PP3 in mass ratios of 10:0, 9:1, 8:2 and 7:3, respectively. PP:potato processing by-product. (

Source: Reproduced with the permission from Feng et al. 2020. Copyright © 2020, Elsevier)

Fig. 3.

Microstructure features of cast sample and 3D printed product at 100% and 70% infill level observed from cross-sectional and longitudinal-sectional views. Cross-section: magnification 300 × ;Longitudinal section: magnification 160 × . Red lines represent the layered structure. (

Source: Reproduced with the permission from Liu et al. 2018a, b, c. Copyright © 2018, Elsevier)

The influence of ingredient composition on the quality parameters of 3D printed product was studied by Liu et al. (2019). Wheat flour, freeze-dried mango powder, olive oil and water were the ingredients selected in this research to prepare the dough for 3D printing. Wheat flour to water ratio of 5:3 presented the best printing quality. Wheat flour: water: olive oil ratio of 55:2.75:30 provided the best printing quality. However, when freeze dried mango powder was used, the optimal ratio was wheat flour: water: olive oil: freeze-dried mango powder with 57.5:30:3:2.5, respectively. The extrudability of the dough and printing disrupt if the ingredient composition crosses threshold value. The recent study by Keerthana et al. (2020) on fibre enriched snacks, formulated from 20% mushroom powder in combination with wheat flour at 800 mm/min printing speed using a 1.28 mm diameter nozzle, 300 rpm extrusion motor speed at 4 bar pressure at room temperature showed 78% printing precision and extrusion rate of 0.383 g/min. Inclusion of wheat flour enhanced the printability of material, however, the freeze -dried mushroom powder was not able to be print. Hence, composition of material plays an important role in structure fabrication.

Hot melt extrusion

Hot melt extrusion also known as fused deposition modelling has gained lot of attention because it deposits material to solid structures after printing (Long et al. 2017). This method involves extruding a molten or semi-solid material through a small-diameter nozzle moving along the X- and Y-directions, and the printing platform moves down in the Z-direction for the deposition of the next layer. After extrusion, the layer solidifies instantly. The food materials for fused deposition modelling should have adequate rheological properties that can extrude easily and maintain stability. This method is extensively used to construct personalized 3D chocolate products (Chang et al. 2020; Lee et al. 2019; Sun et al. 2015a, b). Printing through hot melt extrusion is still challenging because this technique is constructed for thermoplastic polymers which has invariable properties instead of extremely changeable rheological properties both after and before a 3D printing processing (Lee et al. 2019). Very few studies are published on the application of fused deposition modelling for fruits and vegetables. Starch can change the rheological characteristics of aqueous solution by absorbing the free water. Addition of starch to orange concentrate with the application of heat alters the physico-chemical, rheological structural and mechanical characteristics. This has been proved by Azam et al. (2018) for 3D printed vitamin D fortified orange concentrate and wheat starch blend (OC-WS). This research showed that the gums except gum arabic enhance the apparent viscosity and storage modulus of the blend, whereas, gum arabic reduces the apparent viscosity and storage modulus. 3D printed objects from mixture of OC-WS and gums are shown in Fig. 4. Among the entire 3D printed objects, blend of vitamin D fortified OC-WS- k-carrageenan gum presented the smoothest visible surface of the 3D printed product, which may be due to the regular extrusion without any fragmented extrudate threads. A study by Chen et al. (2019) showed that the starch suspensions with concentrations of 15–25% (w/w) heated to 70–85 °C were optimal for good extrusion precision and mechanical stability. This type of research could help in choosing the correct starch-based food materials.

Fig. 4.

3D printed samples of OC-WS and OC-WS-Gum blends; a Hollow triangle of OC-WSmixture; b Cylinder of OC-WS- guar gum mixture, c Hollow cylinder of OC-WS-gum arabicmixture; d Hollowsquare of OC-WS mixture, e Hollow triangle by OC-WS-xanthan gummixture, and f Hollow square byOC-WS-k-carrageenan gum mixture. (

Source: Reproduced with the permission from Azam et al. (2018). Copyright © Springer Science + Business Media2018)

Gel extrusion molding

Gel extrusion molding is a type of technique extruding hydrocolloid solution or dispersion into a polymer/hardening/gel setting bath using a syringe pipette, jet cutter or vibrating nozzle. The temperature control and the rheological properties of the polymer is a critical factor in the mechanism of gel formation (Sun et al. 2018). The gel extrusion molding has been extensively used in various fruits and vegetables based applications such as concentrated mango juice (Yang et al. 2019b), mashed potatoes (Liu et al. 2018a, 2019; Zhenbin et al. 2018) strawberry juice (Liu et al. 2018a), lemon juice (Yang et al. 2018, 2019a), potato puree (Dankar et al. 2018b) broccoli, spinach, or carrot (Kim et al. 2018). Potato flakes are frequently used as an ingredient in 3D printed food products. The reconstituted potato flakes have nice aroma and more consumer acceptability (Liu et al. 2019). The addition of hydrocolloids increases the viscosity and this was proven in the experiment where k-carrageenan- xanthan gum- mashed potato blend possessed a smooth surface structure of printed objects. This was possible due to xanthan gum providing creamy effect to the printed sample while k-carrageenan provided mashed potatoes with sufficient mechanical strength to hold the material (Zhenbin et al. 2018). In similar type of experiment, Liu et al. (2018a) addressed rheological properties of the mashed potatoes with inclusion of various proportion of potato starch and its influence on 3D printing. Mashed potato with 2% potato starch showed excellent extrudability, printability and printed geometry retention. In another study on mashed potato, Liu et al. (2020) successfully incorporated probiotics (Bifidobacterium animalis subsp. Lactis BB-12) in 3D printed object. The influence of various nozzle diameters (0.6, 1.0 and 1.4 mm) and printing temperatures (25, 35, 45 and 55 °C) was studied to investigate the viability of BB-12. To achieve the health benefit, the minimum count of probiotic bacteria while consuming the food should be more than 6 log CFU/g. As shown in Fig. 5, the viable counts of BB-12 were 9.93 log CFU/g for control, 9.89 log CFU/g for object printed with 1.4 mm nozzle and 9.85 log CFU/g for 1.0 mm nozzle. No significant variation was found among control and 1.0- and 1.4-mm nozzle printed mashed potato. Low nozzle tip diameter (0.6 mm) decreases the viability of BB-12 from 9.93 log CFU/g to 9.74 log CFU/g. This might be because the maximum shear force experienced by probiotic bacteria while passing through a 0.6 mm nozzle which reduces the viability. Higher decrease in probiotic count was observed when the mashed potato was held in heating nozzle chamber at 55 °C for 45 min. However, the count of probiotic was not reduced significantly during 12-day storage period at 5 °C. This research has given a new aspect to fabricate 3D printed functional food enriched with probiotics bacteria. In another study on potato puree, the influence of four different additives (agar, alginate, lecithin, and glycerol) on the internal stability, mechanical characteristics, microstructure and color of 3D printed object was evaluated (Dankar et al. 2018a). The best printing and stability of printed object was obtained with a nozzle size of 4 mm at a nozzle height of 0.5 cm by using potato puree with alginate or agar. Scanning electron microscopy showed that greater convolutions were generated in the potato puree by adding agar or alginate which increase the stability. However, the color of the printed object was not altered by the process. The majority of food materials require post processing such as cooking, baking, steaming, or frying after the buildup of food prints. The process involves different levels of heat penetration inside the food matrix and leads to a non-homogenous structure (Sun et al. 2018). The influence of microwave vacuum drying on 3D printed mango juice gel was studied by Yang et al. (2019b). Mango juice gel has been formulated by mixing of potato starch and concentrated mango juice in proportion of 13.04:86.96 by weight. Pressure was given on the mango juice gel within the extrusion system through a syringe. The main aim of this study was to evaluate the influence of microwave vacuum drying on the quality of 3D printed sample. Microwave vacuum drying was conducted under a microwave power setting of 150 W and an upper air temperature in the chamber of the dryer at 35 °C for 4 min which was found to be suitable for 3D printed mango juice gel. These authors concluded that the microwave vacuum drying post treatment is a promising way to enhance the quality of 3D printed food as the process gave the best shape retention and accuracy to the mango juice gel.

Fig. 5.

Effect of various nozzle diameter on the viability of probiotic microorganisms in 3Dprinted mashed potatoes. (

Source: Reproduced with the permission from Liu et al. 2019. Copyright © 2019, Elsevier)

The addition of hydrocolloids in fruits and vegetables powder improves the printability of the object. The rheological properties are highly influenced by powder volume portion, which is linked to the particle swelling and hydrocolloids has the ability to prevent the swelling. An attempt was made by Kim et al. (2018) to print three different vegetable (broccoli, spinach, or carrot) powders using hydrocolloids as an additive to provide printability. This study proved that the xanthan gum was the suitable hydrocolloid to prevent the swelling of the particles, thus decreases the rise in the rheological values at more portion of the vegetable powder. The addition of 30% vegetable powder with xanthan gum, proved best extrudability and high resolution in the printed samples. Liu et al. (2018a) fabricated highly attractive structure with dual extrusion 3D printer. These scientists added 17.5 g of potato starch into 100 gm of strawberry juice concentrate to develop gel. Dual extrusion provided attractive structures with drop-on-demand way. In similar way lemon juice gel has been prepared with potato starch and used in 3D printing (Yang et al. 2018). The influence of different concentration of potato starch has been studied on rheological and mechanical properties of lemon juice gels. As shown in Fig. 6c, 15% potato starch deposited smooth surface, less defects and no compressed deformation. Figure 6a and b, contains 10 and 12.5% of potato starch respectively, has low viscosity which shows higher quantity of extrusion than the set parameters and not able to retain the desired shape of printed object. However, 17.5 and 20% potato starch have high viscosity which was difficult to extrude and showed broken deposited (Fig. 6d and e). One more study was performed on 3D printing of lemon juice gel adding different starches such as potato starch, sweet potato starch, wheat starch and corn starch (Yang et al. 2019a). This study showed that the viscosity and mechanical properties of lemon juice gel differ with the addition of various types of starches.

Fig. 6.

Various structural design of 3D printed lemon juice gel product at varying potato starch percentage (A: 10%, B: 12.5%, C: 15%, D: 17.5, E: 20%). (

Source: Reproduced with the permission from Yang et al. 2017. Copyright © 2017, Elsevier)

4D printing

Fourth dimension or 4D printing is similar to 3D printing but it needs extra stimulus and stimulus responsive materials. These stimuli may include temperature, pressure, wind, pH, light. In 4D printing process, the shape, properties or functionality of 3D printed object alters as a function of time under stimuli (Choi et al. 2015; Khoo et al. 2015; Le-Bail et al. 2020). Mathematical modelling for 4D printing provides benefits in terms 1) to predict shape change as a function of time 2) to hinder destruction among ingredients of the design during self-assembly operations and 3) to decrease the number of preliminary experiments (Momeni et al. 2017). 4D printing depends on smart materials. It is in fact, a fusion of 3D printing with smart material. The 3D printed food is exposed to predetermined stimuli from the natural environment or through people intervention, which causes changes in physical and chemical parameter as a function of time. The new self-transformation of 3D printed object is known as 4D printing (Chang et al. 2020; Pei 2014). 4D printing has many applications in various fields such as science, manufacturing industry, bioengineering, and medical fields (Chang et al. 2020). In one experiment on anthocyanin-rich purple sweet potato and mashed potato, a dual extrusion was used to design 4D printed product in terms of impulsive color change induced by pH (He et al. 2020). The influence of pH (2.5, 6.5 and 7.8) and potato flake content (15%, 19%, 23% and 27%) on 3D printed object was studied. 1% citric acid (CA) and 1% sodium bicarbonate (SB) was added to the mashed potato. In this research, mashed potato was printed above the purple sweet potato puree (Fig. 7). With the rise in potato flake amount from 23 to 27%, color change of mashed potato was corresponding to pH value and the potato flake amount. Anthocyanin is a water-soluble pigment which produces variation in color with the pH values. When both the parts are in contact with each other, the anthocyanins gently move from purple sweet potato puree into the mashed potato due to the concentration pressure difference. Further, according to the acidity and alkalinity, the color of mashed potato varies. The color of mashed potato deepened with the increase of storage time. Anthocyanin presents red color in acidic environment, purple in neutral and green in alkaline pH.

Fig. 7.

Color changes of printed mashed potatoes with various formulations over time. (

Source: Reproduced with the permission from He et al. 2020. Copyright © 2019, Elsevier)

Challenges and future perspectives

In the last few years, 3D printing has been extensively used for various food operations. This technology provides several benefits in terms of customized food, personalized nutrition and utilization of different types of food ingredients. Despite the extensive benefits of 3D printing, the research in this field is still in its infancy and the limitations of this technology cannot be ignored.

• Fruits and vegetables provide nutrients to food but their utilization for printing is difficult. More studies are required to print these materials and even underutilized or byproducts of fruit and vegetable can be processed with 3D printing.

• Demand of consumers for products with high nutritional and functional values is growing rapidly. Very few studies are available on the 3D printed functional food. Fruit concentrate or fruit powder can be combined with different bioactive compounds such as vitamins, polyphenols, probiotics, minerals etc. to design food for consumers with special requirements. Moreover, the successful adaptation of probiotic microorganisms in fruit products is strongly strain dependent. Different strain of probiotic, individually or in combination can be used to fabricate probiotic fruit based 3D food. Studies on the viability of probiotic during storage period can also be the future research area.

• Fruits and vegetables cannot be directly extruded and thus require suitable pretreatment such as comminution, gelation, preparation of dough, etc. These pretreatments are commonly used but these are not efficient. Hence, novel and effective treatments such as microwave and ultrasonic can be used for future research (Chang et al. 2020). With this more research should be focused on the preservation of nutrients which can get lost during pre-treatment process for fruits and vegetables. Since extremely few studies are available on post treatment, further studies should also focus on post-processing of 3D printed object by using novel drying techniques, to enhance the structure stability and for the customer compliances.

• 4D food printing has not yet been adopted for the research purpose. The influence of different stimuli for example temperature, water, pH, moisture can be used to fabricate smart material with colorful or multimaterial printed structures.

• Most of the 3D food printing techniques rely on the rheological and mechanical properties of food materials. The printing parameters also play a critical role in the extrusion of food materials which varies with each type of food ink. So, there is a massive opportunity to work on food materials considering the human nutrition and food safety. Maximum optimization of trials needs to be done on printed food before its launch in the market. The food construction process should be enhanced to develop individuals’ creativity and also should be quantified to obtain consistent production results.

• The next biggest challenge of this digitalized technology is the piracy of the formulated recipes. This 3D digitalized technology needs new policies considering intellectual property right and piracy to prevent any misuse of set standards (Brown et al. 2014). The limited printing speed is also a major concern for commercialization of 3D printing which limits the production in the market.

• Shelf life of printed food is also a concern due to rigorous food operation on printer. Hence, food safety is a major challenge in this technology. The research should also focus on the microbial load and safety of the food during and after printing. 4D printing has emerged as a supplement 3D printing, developed as an altered form of the existing printing parameters.

Conclusion

3D printing has put forth the capability of fabricating personalized food products by using various fruits and vegetables. Printing material characteristics, processing parameters and post-processing are the factors which influence the accuracy and fidelity of the printed object. For printing fruits and vegetables, extrusion-based printing of hydrogels has been widely used. Extrusion 3D printing provides benefits as various ingredients can be printed and easy to customize. However, some more important obstacles should be overcome in terms of printing precision and accuracy, printing speed and printed products quality. Very few studies have emphasized on the development of functional food which can be a key interest for the researchers. Further, it is essential to determine the food areas where 3D printed products are focused, such as elderly food, soldiers and space food, snack food etc. Through this inspection, it will be easy to determine the specific material to be printed, the technology to be used and other critical parameters can be optimized to print the specific food. Extensive applications will be the future for this emerging 3D printing once the challenges are overcome.

Supplementary Information

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Authors' contribution

RW: Conceptualization, Project administration, Writing—review and editing. DS: Writing —review and editing. SK: Writing—review and editing.

Funding

This work did not receive any grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declarations

Conflict of interest

Author declares no any conflicts of interest.

Ethical approval

Not applicable.