Extrusion-based 3D printing

As 3D food printing offers numerous benefits, various types of printers differing in their operating principles have been developed over the last few decades. A detailed description of each type is provided in the following section. Fig. 1 shows the schematic diagram of a FoodBot food printer. Fig. 2 gives the workflow of a 3D food printing machine. The four major techniques of food printing are extrusion-based printing, binder jet printing, inkjet printing and selective laser sintering. Among these technologies, the most commonly employed technique is extrusion printing (9). In this technique, food designs are formed through extrusion using a nozzle under static pressure. Both solid materials and low-viscosity pastes can be printed using this method. Food materials to be printed are loaded into the machine and then forced through the nozzle to form layers of food material, one over the other. Foods commonly printed by this method include dough, meat, paste and cheese (10). Fish surimi gel has been printed using an extrusion-type printer, and it was reported that the diameter and velocity of the nozzle, along with the extrusion rate, influence the quality of 3D printing (11). Different types of sugar cookies have been printed with an extrusion printer (12). Fig. 3a shows the schematic representation of an extrusion-oriented 3D printer. The major advantages of this technique are the low cost of basic models and ease of customization to accommodate a variety of raw materials. However, its disadvantages include low level of printing precision and a very long build time.

Fig. 1

Schematic diagram of a FoodBot 3D food printer (scanned image from the manufacturer's manual)

Fig. 2

Schematic diagram of basic workflow of a 3D food printer

Fig. 3

Schematic diagram of: a) extrusion-type 3D food printer, b) binder jet printer, c) inkjet printer, and d) selective laser sintering 3D printer (Corel Draw graphics editor)

Binder jet 3D printing

In the binder jet technique, layers of food-grade powders are spread uniformly over the fabrication platform (13). These layers are often sprayed with a stream of water to make them stable, minimizing the disturbances caused by binder jetting. Small binder droplets are dropped onto the surface of the powder bed from a printer head, binding the adjacent powder layers. The powder surface is then heated using a hot lamp to provide mechanical strength to withstand the printing process. These steps are performed sequentially for all layers (13). The properties of both the powder and the binder play a crucial role in efficient printing. This process has some benefits like faster fabrication, the ability to create complex structures, and reduced cost of ingredients. With this technology, Sugar Lab fabricated sculptural cakes with sugar and different flavour binders. Fig. 3b shows the schematic diagram of the binder jet printer. Binder jet 3D printing is a versatile technique and its production speed is greater than that of other techniques. Another advantage is the possibility of including support structures in the fabrication of layers. The disadvantage of this technique is the rough or grainy appearance of the end product. Producing fine and soft-textured products is very difficult with this technique. This type of printing requires post-processing step to reduce moisture content and improve the strength of the printed foods.

Inkjet 3D printing

In the inkjet printing, food materials are dispensed from a heated head with multiple channels to the selected regions of the food surface (14). The droplets are dispensed in a drop-on-demand manner. They fall under gravity and are dried through solvent evaporation. These droplets can create two-and-a-half-dimensional images (14). This technique is commonly used for surface filling or decorating purposes. Baked items like cookies, cakes, pastries and pizza are decorated using this method (15). Food materials with complex structures are rarely printed by this method. Fig. 3c shows a schematic diagram of the inkjet printer. Inkjet printing offers advantages like high resolution of the prints and accurate printing. With this technique, a variety of raw materials can be used for 3D printing. The main disadvantage is the damage that can occur to the food after the processing due to its delicate structure.

Selective laser sintering 3D printing

Selective laser sintering technology can be applied to powders, where they are sintered selectively to create any desired shapes (16). A sintering source like hot air or a laser is passed over a bed of powdered food spread evenly. The source moves across both x and y axes of the powder bed to bind the particles together. The sintering process is repeated continuously, so that the fused material attains a 3D geometry through simultaneous layering and binding of the particles (16). The food jetting printer from TNO (17) has used lasers for fusing sugars and Nesquik powders into 3D designs. The unsintered material is retained in place as a support for the whole structure. Candy Fab (18) used a stream of hot air at minimal speed to sinter a layer of sugar. Application of heat to the bed at a temperature lower than the melting point of the powdered material prevents thermal destruction of the product and enhances the binding of particles. Both hot air and laser sintering methods enable rapid fabrication of foods without requiring any post-printing operations. However, a limitation of this technique is that it can only sinter sugar and fat-containing particles with low melting points (19). Fig. 3d shows a schematic diagram of a selective laser sintering type 3D printer. The advantages and disadvantages of different printing techniques are summarised in Table 1 (10, 20-23).

Materials for 3D food printing

The materials used for printing are often classified into three main categories. Natively printable materials are those that can be extruded easily using syringes (24). Cake frosting, chocolate and cheese are some examples. These materials can be fully personalized regarding the nutritional, textural and sensory attributes. Some materials retain their shape for a long time after deposition, so post-processing is not necessary. Non-printable materials include food products that are consumed daily, such as meat, fish, rice, fruit and vegetables. They can be made printable only by incorporating compounds like hydrocolloids. Xanthan gum and gelatine are usually used for this purpose (25). The addition of suitable additives to these materials modifies their printing properties, resulting in a new formulation (12). Alternative ingredients include extracts from organisms such as algae, fungi, seaweed and lupins, along with insects as the newer sources of protein and fibre. According to the Insects Au Gratin Project, formulations containing insect powders, low-viscosity icings and cheese have been used as raw materials to fabricate foods and create tastier recipes (26). The leftovers from the current agriculture and food sector can be transformed into metabolites, enzymes and food-grade flavours, which are known for their biological activity. The advantages and disadvantages of the formulations used in printing are given in Table 2 (11, 27-30).

The key benefit of 3D food printing is the rapid fabrication of customized food products with complex geometric patterns and novel textures (25). As 3D foods are categorized as soft foods due to their high digestibility, they form an important part of geriatric nutrition, especially for the dysphagic community (31). Apart from designing foods with customized patterns, 3D printing develops foods with tailored nutritional content through the incorporation of multiple ingredients. The ability of food printing to provide personalized nutrition makes this technology an excellent solution for addressing nutritional disorders caused by deficiencies in different nutrients. This review aims to highlight the role of functional foods with bioactive ingredients in addressing nutrient deficiency disorders, which mainly prevail in developing nations like India, Nepal and various parts of Africa. The evolution of 3D food printers, numerous printing techniques employed and novel 3D-printed functional foods that could enhance and transform the realm of food, both aesthetically and functionally, are also thoroughly reviewed in this article.

Plant-based functional foods

The bioavailability of iron has increased significantly in beverages fortified with more than one mineral. Additionally, enrichment with chickpeas, which are rich in iron and zinc, increased their iron and zinc content fivefold (61). Fortification with more than one nutrient increases the bioavailability of the added nutrients (62). An infant formula with sweet potato as the base was developed in Africa. This formula contained nutrients like vitamin A, iron and zinc, was of low cost and met CODEX standards for all micronutrients. However, it lacked calcium, which the author recommended to incorporate by adding dried powdered soft fish bones. The amount of iron in the formula was in the range of 8–10 mg/100 g, which was much higher than that in traditional weaning foods (51).

Meat analogues with nutritional content comparable to that of meat have been developed from plant sources for vegetarians and vegan consumers. To achieve this, microalgal powder and soy protein concentrate were effectively combined using high moisture extrusion technology. The resulting product had an appealing colour and a good nutritional profile, with high levels of nutrients like vitamins A and B. The fibrous texture preferred by consumers was achieved by adding 30 % microalgal powder (63).

Noodles were prepared by incorporating mushrooms, which can provide various health benefits, as they are known to be rich in protein and vitamin. They are also excellent sources of micronutrients like iron calcium, copper and zinc (64). They are low-calorie foods and are rich in dietary fibre. When mushrooms were incorporated at 5 %, sensory tests yielded good results. Additionally, the levels of protein, fibre, iron, calcium and potassium in the final product were reported to be higher than those in the noodle brands available on the market (64).

There are several naturally occurring plant-based polysaccharides like pectin, galactomannan, inulin, xanthan and gums, etc. (65). These polysaccharides have several applications in the food industry. It can be used as thickeners (66), stabilizers (67) and gelling agents (68). Konjac glucomannan was selenized by adding the trace element selenium. Through fortification, a new glucomannan hydrolase enzyme was formed that can convert konjac glucomannan to its corresponding oligosaccharides. Selenization was found to be beneficial for providing synergistic biological activity and for developing a functional ingredient. The newly developed konjac glucomannan oligosaccharides have antitumour activity (69).

The base material for most baked products is refined wheat flour. Although wheat flour provides energy and protein, it is deficient in minerals like calcium, iron and zinc (70). The popularity of bakery products like biscuits and cakes could be utilised to develop healthier variants. To enhance the bioaccessibility of minerals like calcium, iron and zinc and to produce a mineral-rich bread, wheat flour was replaced with sesame, cumin and moringa leaves in one combination, sesame and finger millet in another and sesame seeds in a third combination. All the bread samples developed had higher amounts of these minerals and the enrichment altered both the total and bioaccessible quantities of the minerals (71).

Wastewater from oil mills and olive paste, discarded from the olive oil industry, is rich in bioactive ingredients like polyphenols (72). These were added to enrich cereal foods like bread and pasta (73). After enrichment, it was observed that the products containing water from the oil mill had mildly improved chemical properties, while those enriched with olive paste showed a significant change in chemical properties. Additionally, products enriched with olive paste showed higher phenolic and antioxidant activity. Based on the overall quality index values of the products, it was reported that olive paste was more suitable for fortification than wastewater from oil mills (73).

Dairy-based functional foods

Milk and milk products have been fortified with spices and herbs (74). Apart from providing flavour, herbs and spices offer protection against oxidative, inflammatory, hypertensive, diabetic and microbial infections, but create favourable conditions for the growth of beneficial microorganisms (74). The addition of cinnamon to yoghurt resulted in enhanced growth of lactic acid bacteria. The yoghurt also showed increased total phenolic content and radical scavenging activity (74). Milk from cow, buffalo and goat fortified with ginger and beetroot extracts showed improved antioxidant activity (75). New cottage cheese was prepared with the addition of spices like pepper, parsley, garlic, dill and rosemary (76). This cheese showed improved sensory properties, increased shelf life and higher biological value (76). Fortification of butter with rosemary herb reduced the rate of lipolysis in butter (77).

Egg and meat-based functional foods

Table eggs were fortified with lipids from both microalgae and fish (78). The oil extracted from fish was rich in docosahexaenoic acid (DHA) and polyunsaturated fatty acids (PUFA). To retain the sensory properties of eggs, fish oil should be limited to 1.5 % (78). The sensory properties, acceptability and flavour of eggs supplemented with both oils were compared and it was observed that DHA supplemented with microalgal oil yielded better results in terms of sensory characteristics, overall acceptability and flavour of eggs (79).

Vegetable oils like sunflower oil in broiler chicken meat were replaced with PUFA (80). Soybean oil, mustard oil, fish oil and linseed oil were used as sources of PUFA. The meat was reported to be fortified with polyunsaturated fatty acids, with minimal variations in the sensory characteristics of the meat. The fortified meat showed enhanced levels of DHA and eicosapentaenoic acid (EPA) and lower levels of saturated fatty acids (80). Dietary recommendations for EPA and DHA based on cardiovascular risk considerations for European adults are between 250 and 500 mg/day (81). Supplemental intakes of DHA alone at 1 g/day do not raise safety concerns for the general population (81).

Broiler chicken meat was enriched by feeding chickens with high oleic peanuts, instead of grains, canola seeds and soybean meal (82). The protein content of peanuts is twice as high as that of grains. They are also rich in oleic acid (80 %). After enrichment, the meat showed increase amounts of polyunsaturated fatty acids and a low level of trans-elaidic acid, which when present in large amounts can lead to cardiovascular diseases. The protein and amino acid composition remained unaltered after enrichment (82).

When the feed of slow-growing chickens was enriched with linseed oil and tuna oil at 6 %, the chicken meat showed enhanced levels of PUFA with minimal alteration in the cholesterol content of the meat (83). The DHA content was observed to be 250 mg/100 g of meat, both in breast and thigh portions, which was adequate to meet the daily requirements of DHA, as per the European Food Safety Authority (EFSA) (81). According to EFSA, DHA is responsible for the proper functioning of the brain as well as vision (81).

Mono- and multilayered microcapsules were prepared from fish oil emulsions and added to meat products like sausages after cooking and curing (84). They served as vehicles for essential fatty acids, which are known for their beneficial effects on health. The capsules were developed from two combinations, one from lecithin and maltodextrin, and the other from lecithin and chitosan-maltodextrin. Through the addition of microcapsules, meat was fortified with EPA and DHA, and the products were analyzed to meet the labelling requirements for omega-3 fatty acid content according to the European Union legislation. The lipid profile of the product developed, as well as the percentage of fat loss, remained after fortification. The combination of lecithin and maltodextrin yielded better results than the other combinations in terms of bioavailability of EPA and DHA (54, 84). Despite all the health benefits of fortified foods, they were seldom accepted by consumers wholeheartedly, primarily due to their unfamiliar taste and inconvenient processing methods (85). With the development of a sophisticated processing technology like 3D food printing, both the acceptability and accessibility of functional foods are expected to be enhanced to a greater extent.

3D food printing with food grade inks

Customized, highly porous, aqueous, 3D food simulants were developed with the food-grade pectin gel ink with low methylation (92). The incorporation of pectin resulted in a firmer product after printing. Products with different properties were printed by altering the composition of the food inks while keeping the printing parameters unchanged. Calcium chloride was added in amounts required to form cross-links of pectin, contributing to the 3D structure. Adding sugar resulted in a more viscous product with better printability and qualitative attributes. For printing formulations with a high pectin content, the amount of added calcium was reduced to achieve products with desired flow properties. Through this method, customized food inks with numerous applications could be developed to meet consumer preferences. That study may be considered an initial step in the fabrication of 3D-printed pectin-incorporated porous products (92).

A FujiFilm Dimatix inkjet printer using xanthan gum as the printing ink was employed with powdered cellulose (93). Xanthan gum was dispensed over the powdered cellulose for binding. properties and other parameters were optimized to improve the effectiveness of binding. Cellulose was selected as the binding agent due to its low cost and the ability to recrystallize after application. The newly formulated food-grade inks formed a firm structure with cellulose powder, which was used effectively for 2D inkjet printing. They could also be used successfully for 3D printing after recrystallization of the powdered cellulose (93). The performance of a 3D extruder-type printer at ambient temperature was assessed with model components such as modified starch and xanthan gum (94). The food inks used include carrot puree, xanthan gum pastes and modified starch. Rheological tests were performed and the results were analysed to evaluate whether the developed inks were suitable for printing. Carrot puree yielded better results for use as the food ink (94).

3D-printed foods with functional ingredients and antioxidants

A study was conducted to assess the relationship between the printability of formulations and their rheological properties using tomato paste as the model (95). It was observed that the pressure for extruding the material increased with higher flow stress. The dispensing behaviour of the material also increased proportionally with the flow stress, except for fat-containing products, which vary in their dispensing characteristics (95).

A 3D-printed lemon juice gel was developed by incorporating potato starch at different concentrations, and the influence of potato starch addition at different levels on various characteristics of the gel was studied (96). Lemon gel, which was otherwise less transparent and chewy, became printable with the incorporation of potato starch gel, which exhibited anti-ageing properties, transparency and the ability to retain moisture. The impact of printing parameters (nozzle height, nozzle diameter, extrusion rate and nozzle movement speed) as well as material properties on the quality of the final products was also investigated.

When potato starch was incorporated at 15 %, the flow and mechanical properties of the gel were observed to be optimum. The printing parameters were optimal when the nozzle diameter was around 1 mm, the extrusion rate was 24 mm/s and the nozzle movement speed was 30 mm/s. This work provided insight into the printing of gels in combination with starch (96). The impact of material composition on the quality of 3D-printed food was investigated using wheat flour, freeze-dried mango powder, olive oil and water (90). The optimal formulation with the best printing quality had flour and water in the ratio of 5:3 with no added freeze-dried mango powder and olive oil. For the formulation containing 2 % olive oil, the printing quality was optimal with the flour/water/olive oil ratio of 55:2.75:30. When 2.5 g mango powder was incorporated to the formulation, optimal printing quality was observed for the formulation with flour/water/olive oil/lyophilized mango powder ratio of 57.5:30:3:2.5. Additionally, a compressive pressure of 600 kPa, needle velocity of 6 mm/s, needle diameter of 0.58 mm and internal filling ratio of 60 % were found to be optimum. Foods printed with these printing parameters had numerous benefits like an organized packing structure, better interior texture profile and reduced deformation (90).

Mashed potatoes were printed three-dimensionally with different internal structures (97). The dimensional, structural and textural parameters were studied as a function of different infill patterns, infill percentages and shell perimeters. The results showed that the 3D-printed structures closely resembled the 3D designs greatly. Textural properties like hardness and gumminess were highly related to the infill levels. Microstructural analysis of the printed and cast materials revealed that the cast sample had a uniform internal structure, compared to the layered and porous structure of the printed material in longitudinal and cross- section, respectively. Textural analysis showed that the hardness of the printed sample was lower than that of the cast sample. That study demonstrated that 3D printing technology could alter the characteristics of foods, providing a novel approach for customizing the textural properties through the fabrication of internal structures (97).

A 3D-printed smoothie was developed from a mixture of fruit and vegetables of different colours (27). The ingredients used in the formulation were carrots, kiwifruit, avocado, pears and broccoli leaves. That study served as a stepping stone towards the printability of fruit- and vegetable-based formulations. The smoothie was printed in a pyramid shape and the 3D-printed version had a better visual appeal than its non-printed counterpart. The sensory properties, antioxidant and phenolic content were unaltered after printing. Two defects noted during printing were the discrepancy between the actual and estimated deposition positions of food and the inadequate sanitation of the printer areas in direct contact with food. The study explored the effect of printing variables like printing speed and flow on the printability of food containing fruit (27).

A snack food was formulated from fresh bananas, dried mushrooms, canned white beans, dried non-fat milk and lemon juice after 3D printing (98). The printed samples were similar to the 3D design. Material flow had a significant impact on the microstructural properties. When the material flow was reduced, the interior structure of the snacks was irregular with large pores, but when the flow was higher, the internal structure became thicker, with a decrease in porosity. The nutritional analysis of the printed snack revealed that 100 g of the snack was sufficient to meet the calorie requirements of children aged 3-10. The printed snack was reported to be rich in calcium, iron and vitamin D (98).

Another study explored the possibility of printing strawberry tree fruits, which have an exceptional nutritional profile, namely vitamins, antioxidants, minerals, sugars and bioactive components (99). Hydrocolloids like wheat flour and corn flour were added to the fruit blend at various amounts to make the formulation printable. The developed formulation was rich in polyphenols and had anti-microbial activity. The type and quantity of incorporated hydrocolloids, as well as the choice of printing program, played a significant role in the bioactive and functional properties of the formulation (99).

A Pickering emulsion rich in curcumin, initially stabilized by pea protein isolate and κ-carrageenan complex, was 3D printed (100). The printing results showed that the emulsion containing more than 0.3 % κ-carrageenan exhibited good extrusion, stability and shape retention. The bioaccessibility and stability of curcumin was greatly improved after in vitro digestion (100).

3D-printed biscuits were fabricated from processed flour and multi-grain flour, which contained cowpea sourdough and quinoa malt (101). Traditional biscuits were also developed and these two types of biscuits were compared in terms of colour profile, hardness and image parameters to examine the structural differences between the whole grain and composite flour (101). The 3D-printed biscuits were reported to be better due to their multi-flour formulation, low values for redness, browning index and hardness. Traditional biscuits were found to be harder than 3D-printed biscuits. The 3D-printed biscuits had similar quality characteristics to their non-printed counterparts. A slight reduction in antioxidant activity was observed after 3D printing, but the amount of minerals like iron and phosphorus was enhanced in 3D-printed biscuits (101).

3D-printed foods with cereals as the base material

3D-printed cereal foods containing probiotic microorganisms were produced from wheat dough by varying the amounts of water and calcium caseinate (30). The dough was printed in two designs: a honeycomb structure and a concentric circle. It was observed that the printability of the formulation depended on factors like water content, type of flour used and the quantity of additives like calcium caseinate. After printing, strains of probiotic bacteria were incorporated and the prints were baked. The results of the study revealed that the viability of probiotic bacteria increased with an increase in surface-to-volume ratio. Between the two structures, the honeycomb structure yielded higher number of viable bacteria than the other structure (30). The gel formation properties and the physical characteristics of the dough changed with any alteration in its composition. The study investigated the effect of these changes on 3D printing of the dough (102). The results showed that a pseudoplastic gel with high extrudability, gel strength, elasticity and low ductility was necessary to produce extruded samples of optimal shape. The formulation with water (29 g), sucrose (6.6 g), butter (6.0 g), flour (48 g) and egg (10.4 g) per 100 g yielded optimum results in terms of shape, gel formation and physical properties. Apart from optimizing the formulation for a good dough, the physical properties of all the dough formulations were studied. It was observed that sucrose, butter and flour were vital for the desired modelling results (102). A 3D pizza was developed from a gluten-free flour blend and its physical characteristics were compared with those of commercially available pizza (103). Although the fermentation time required for the gluten-free pizza formulation was double that required for commercial pizza dough, the textural properties and colour of the 3D-printed gluten-free pizza was comparable to those of the commercial pizza. The healthier version of pizza developed had the potential to replace commercially available pizza from wheat flour (103).

3D-printed foods for geriatric and dysphagic population

Around 50 % of the elderly population suffers from dysphagia, which is primarily characterized by difficulty in swallowing. To enable people with dysphagia to enjoy their meals, a 3D printer was developed to fabricate softer foods (104). A syringe pump was used to extrude the gel and a dispenser was employed to create 3D objects. Agar solution was used as the ink for printing. As agar gelation is influenced by temperature, the temperature of the gel should be regulated for optimal printing. Four types of soft foods were made using agar and gelatine with the developed printer. Compression tests and hardness measurements of the printed foods were carried out, and the results indicated that all the printed agar gels had a soft texture resembling jelly, which is appropriate for people with dysphagia (104). To meet the highly challenging requirements of dysphagic diet, novel edible formulations were developed from black fungus with the addition of gums like xanthan gum, gum Arabic and κ-carrageenan gum (91). It was reported that the formulation containing gum Arabic lacked self-supporting stability, and the results obtained for the International Dysphagia Diet Standardization Initiative (IDDSI) tests were poor (91). Although the formulation incorporated with carrageenan gum showed good self-supporting stability, the formulation failed the IDDSI tests due to its sticky nature, but the printed structure was self-stable. The formulation with xanthan gum closely resembled the level 5 minced and moist dysphagia food in the IDDSI tests (91). A dysphagia diet, suitable for the elderly population, was 3D printed (105). The diet was composed of cellulose nanocrystal-based (CNC) protein/polysaccharide edible inks, with protein sources including gelatine extracted from bovine skin and whey protein isolate, along with xanthan gum, an essential ingredient in dysphagia food. The formulation containing 2 % gelatine, 8 % whey protein isolate and 8 % xanthan gum was considered optimal due to its good printability and ability to retain shape. The formulation with bovine gelatine, xanthan gum and 1.5 % CNC met the criteria for level 5 minced and moist dysphagia diet according to the IDDSI guidelines (105).

The possibility of using flaxseed gum as the component with the ability to modify texture to develop dysphagia diets using 3D printing technology was studied (106). The ink formulation also contained mung bean protein and rose powder. The formulation containing 0.9 % flaxseed gum yielded optimal results in terms of printing characteristics and was classified a level 4 pureed/extremely thick dysphagia food according to the IDDSI guidelines. That study fabricated an inexpensive dysphagia diet containing phenolics exclusively from plant sources (106).

The impact of the incorporation of xanthan gum and guar gum on different properties of pork paste after cooking and 3D printing was investigated (107). The addition of hydrocolloids imparted a shear-thinning effect to the paste, improving the extrudability of the formulation. The incorporation of gums after the printing operations significantly altered the texture, making it suitable for people suffering from dysphagia. The final products could be classified as transitional foods after the IDDSI tests (107).

The 3D printer developed by Liu et al. (108) had a peristaltic pump and a pressurized tank instead of the syringe and piston commonly employed in other printers. This modification facilitated the printing of meat, which is otherwise complicated due to its fibrous structure. The benefits of the modified meat printer were improved storage capacity and material properties. Operational efficiency and productivity were increased. The chewing experience was enhanced after printing, facilitating easier intake by elderly people. This printing method could transfer ingredients from the interior of the meat to the surface (108).

Protein- and fibre-enriched 3D-printed snack foods

3D-printed snack foods were fabricated from different ingredients like oat protein concentrate, faba bean protein, skimmed milk powder and starch (109). The study investigated whether extrusion-based 3D food printing could be used for pastes rich in starch, protein and fibre. The printability of the pastes was evaluated based on factors like the ease of extrusion, printing precision and print stability. The paste formulated with semi-skimmed milk powder achieved optimal printing precision and shape stability. The results showed that the applicability of the process depended entirely on the material and its binding properties. That study also demonstrated that the optimization of different formulations is a starting point for developing snack foods in future (109).

A 3D-printed snack food was developed from wheat flour fortified with ground larvae of yellow mealworms as a protein source (110). When insects were added at a rate of 20 %, a softer dough texture was obtained. The structural quality and nutritional characteristics were assessed and they remained unchanged after baking. To obtain a desirable product, baking was optimized at 22 min and 200 °C. The incorporation of insects improved the nutritional value in terms of protein quality and essential amino acid score (110).

A fibre-rich snack food was developed from button mushrooms using 3D printing technology (88). Lyophilized mushroom powder, which is not printable in nature, was made printable by incorporating different amounts of wheat flour. The formulation containing 20 % mushroom powder produced the best results in terms of nutritional value and printability. Sensory evaluation of the savoury products gave better results than the sweeter version of the snacks (88). To produce valuable by-products from industrial waste, functional cookies were developed using 3D printing technology from grape pomace and broken wheat, which would otherwise be discarded (89). An increase in grape content enhanced the viscosity of the formulation, which resulted in reduced printing speed. The developed cookie had a good sensory profile, structural integrity and was rich in protein and fibre (89).

Healthier 3D-printed snacks were made using composite flour from barnyard millet, green gram, fried gram and ajwain seeds (111). The printed products were subjected to different post-printing treatments. The printed snack had an attractive visual appeal and good overall acceptability. They also had protein and fibre in amounts needed to meet their daily requirements. Although the snacks developed using different post-processing methods received good sensory scores, microwave-dried snacks closely matched the untreated samples and showed better results in terms of nutritional and textural profile (111).

3D-printed foods from milk, eggs, meat and fish

A milk protein-based 3D food simulant was fabricated by incorporating whey protein isolate into milk protein concentrate (29). The optimal formulation had milk protein concentrate and whey protein isolate at a ratio of 5:2, demonstrating the applicability of a simulative high-protein system using extrusion technology. The prints based on this formulation closely resembled the digital model. The viscosity and mechanical strength of this formulation were adequate to facilitate deposition and adhesion, which are essential criteria in extrusion-based printing. That study would be useful to future works involving 3D printing of protein-rich foods (29).

The application of 3D printing to process commercially available cheese was investigated in another study (112). After printing, both printed and non-printed cheese were compared in terms of hardness and meltability. The printed cheese exhibited decreased hardness and increased meltability than the untreated cheese samples. The cheese samples after printing were found to be highly flexible in terms of geometry and texture. That study emphasized the role of 3D printing in altering the material properties, thereby constructing tailored products (112).

The study by Anukiruthika et al. (28) investigated the printability of both the white and yolk fractions of eggs. As eggs are highly nutritious and have numerous functional properties, 3D printing of egg-based formulations is highly relevant in the functional food industry. To make the egg fractions printable, rice flour was added as a filler at different proportions. Printing parameters including nozzle height, diameter, printing speed, extrusion motor speed and the rate of extrusion were optimized in the study. The incorporation of rice flour enhanced the strength and printing stability of egg whites and yolk. The printability of egg yolk was reported to be better than that of the egg white, with very little deformation. The optimum egg yolk formulation contained egg yolk and rice flour in a ratio of 1:2 (28).

Chicken meat was printed by incorporating composite millet as a source of dietary fibre (113). The composite millet-based flour comprised barnyard millet, fried gram, green gram and ajwain seeds. The optimal formulation contained chicken meat and flour in a ratio of 2:1. This ratio was selected based on the sensory results, as the incorporation of composite flour could adversely affect the sensory characteristics of the chicken meat. The printed product contained good amounts of both protein and dietary fibre, so that the product could be consumed on a daily basis by people of all the age groups. That study initiated the development of meat products enriched with dietary fibre, thus improving their health benefits (113).

Lean meat-lard composite layers were 3D printed and then cooked (114). It was reported that the structure of the products remained unaltered after cooking. The study investigated the effect of various fat content and infill densities on the physical and textural properties of meat. High fat content caused greater cooking loss, shrinkage, cohesiveness and lower fat retention, moisture retention, hardness and chewiness. With increased infill density, high moisture retention, and reduced shrinkage and cohesiveness, an increase in hardness and chewiness was observed (114). A 3D-printed fish surimi gel was developed and the effect of the addition of sodium chloride on the physiochemical properties of the gel, including water-holding capacity, gel strength and network structure was investigated (11). An increase in the addition of sodium chloride improved these properties and the gel containing 1.5 % sodium chloride yielded the best results in terms of rheological parameters. In that study, functional characteristics were the main criteria for printing surimi gel additively (11). Developing novel surimi products using fat fortification and 3D printing technology was also investigated. The work aimed to produce PUFA-rich high internal phase emulsions stabilized with microbial transglutaminase (MTGase) cross-linked fish scale gelatine (FSG) particles. Edible inks were developed by incorporating stabilized emulsion into surimi using 3D food printing and the effect of this addition on different characteristics of surimi gel was studied. When the oil phase volume was 80 %, the stability, flow properties and structural characteristics of the emulsion were affected by FSG. The mechanical properties of the emulsion-incorporated surimi were reported to be enhanced. The incorporation also had a significant effect on the printing accuracy. Incorporation of emulsions using 3D printing technology produces affordable, healthy, PUFA-enriched surimi products (11). Three-dimensional printing is now used to print different types of food components, through proper selection of raw materials, pre-printing steps to make them extrudable, optimization of formulations, and nozzle diameter and height. The ability to print multiple components, together with innovations in both software and hardware, enables this technology to fabricate foods for the general public, regardless of age group, occupation or lifestyle (115).