the functionality and availability of proteins includes heat, pH (acid/ base), enzymatic hydrolysis that leads to denaturation, aggregation, and cross‐linking of proteins (Lassé et al. 2015). These properties in turn affect the printability of native proteins. Proteins being an important macronutrient for bodybuilding, several kinds of research were being conducted focussing on the effect of physicochemical properties on the printability of protein‐based food systems. When subjected to external stress such as heat, acidic/ basic conditions, and mechanical agitation proteins undergo denaturation. The production process of various food products such as cheese, tofu, minced meat, etc., involves simultaneous denaturation and coagulation of protein molecules that enhance the textural properties of the final product (Xiong 2018). Protein denaturation refers to the breakdown of secondary and tertiary structures that has a significant effect on modifying the texture of food products and hence mouth feel.
Modification of textures of food being one of the potential applications of food 3D printing, the printability of the processed cheese was examined by Le Tohic et al. (2018) in which the effect of the printing process on structural properties of hot‐melt cheese was reported (Figure 3.3). Different cheese samples (untreated cheese, melted cheese, and printed cheese) were analysed for their hardness. Results showed that printed cheese was found to be less hard than other samples. The lesser hardness of the printed cheese was attributed to the material’s meltability, and the associated shear stress exhibited during extrudability. The weaker bonds of casein molecules of the printed cheese were responsible for its soft texture and ease of flowability. Combined effects of material shearing during printing and solidification of which during the after‐printing process resulted in the coalescence of fat globules. The resulted food system with disrupted fat globules embedded in the protein matrix imparts the tendency of melt‐in‐mouth texture to the 3D printed cheese. Proteins alone are not conducive for 3D printing, proteins along with other food constituents either with lipids or starch aids in printability as they act as a plasticizing agent. This could be due to the synergistic effects of proteins in combination with other macronutrients that increase crosslinking and hence improve the strength of the gel matrix. These characteristics greatly influence materials viscosity and stability of printed structures.
Figure 3.3 Confocal micrographs of cheese sample showing distribution of fat globules (pigmented spots) in a protein matrix (a) untreated cheese, (b) melted cheese, (c) low speed printed cheese, (d) high speed printed cheese.
Source: From Le Tohic et al. (2018), Figure 05 [p. 61] / With permission of Elsevier. DOI‐https://doi.org/10.1016/j.jfoodeng.2017.02.003.
In another work, Dong et al. (2019) reported a study on the printability of surimi gel with sweet potato starch as a potential structural enhancer for 3D printing. Conventional processing of multi-step surimi preparation deteriorates the gel quality that may not possess enough strength for 3D constructs. Hence, sweet potato starch (0, 2, 6, 8, and 10%, w/w) was used for enhancing the rheological properties of surimi gel. Results showed that surimi gel with 8% sweet potato starch possesses good gel strength with a softer texture. This was due to the cross‐linking of myofibrillar fish proteins with sweet potato starch that forms a uniform aggregate structure making it suitable for 3D printing.
Another protein‐rich formula was made from soy protein isolate (SPI) by utilizing the functional properties of co‐blending of gelatin with sodium alginate that imparts a stable structure to the 3D constructs made from SPI (Chen et al. 2019). The physical interactions of the peptide bonds of gelatin along with sodium salts of alginic acid at its melting temperature could form a mesh‐like network that provides strength to the SPI printing mixture. However, results showed that the addition of these co‐blended systems to SPI does not cause chemical cross‐linking among the protein subunits, but it had a significant effect on improving the textural properties of 3D printed geometries. An increase in the concentration of gelatin had a notable effect on the material’s flowability that in turn resulted in improved hardness and chewiness of the SPI gels. Thus, SPI with 2, 6, and 10% (w/v) of gelatin was found to be printable with better resolution (Chen et al. 2019). This study reveals the potential utilization of hydrogels in protein‐based food systems for 3D printing applications.
3.4.3 Lipids and Fatty Acids
Another major food constituent that regulates and assist in printability is lipids. Lipids are organic compounds either in form of fat or oil that are essential for storing energy in the human body. It consists of fatty acids, triglycerides (an ester derived glycerol with three fatty acids), and phospholipids. Based on the presence of double bonds in the carbon chain, fatty acids are classified as saturated and unsaturated fatty acids. Dietary fat from plant‐based sources includes oil seeds, nuts, and fruits (olive, palm, and avocado). While animal‐based sources such as meat, fish (salmon and mackerel), eggs, and dairy products like butter and margarine also possess a considerable amount of fat. Various factors that affect the functionality of dietary fat include solidification/ meltability, crystal structure/ polymorphism, globule size, esterification, level of hydrogenation, fatty acid composition, and its distribution within triacylglycerol (Devi and Khatkar 2016). Any changes in these properties have a significant effect on the printability of material supply.
An illustration for characterizing the printability of lipid‐based material is chocolates which consist of a mixture of cocoa solids, milk fat possessing several triglycerides. Based on thermostability, chocolates are categorized into six different forms. The highly polymorphic nature of the fat in the chocolate was responsible for its varied physiochemical properties. Among all the six forms of triglycerides, the V th form seems to be stable possessing β2 crystal with a melting point of around 35 °C. These forms are commonly used in food industries and are referred to as tempered chocolates (Lanaro et al. 2019). They possess high‐quality colour and gloss with desired hardness and melting point suitable for conching and tempering. The crystallinity of fat plays an important role in flowability and stability which can be correlated with its printability. In a study conducted by Mantihal et al. (2017), the extrudability of the chocolate was linked with the melting temperature of the fat crystals (29–32 °C). For the printing of chocolates, magnesium stearate was added as an emulsifier in order to maintain stable nucleation of β crystals as 3D printing eliminates tempering which is an essential step to achieve the desired rheology of chocolates. Stearic acid, a saturated fat act as a lubricant that imparts anti‐sticking characteristics and aids in regulating the flow of chocolate mixture through the printing nozzle. Results showed that the addition of magnesium stearate has no impact on the thermal properties but in turn, it delays the crystallization thereby enhancing post‐deposition of the chocolates after printing, which is an essential criterion in determining the end quality (solidification, mechanical strength, and appearance) of printed 3D constructs from chocolates (Mantihal et al. 2017).
Researchers were working on tailoring the nutritional profile of foods using 3D food printing. In context with this, a study was conducted in analysing the effect of fat from varied animal sources and analyzed the integration of which for the printing of fibrous beef (Dick et al. 2019). Natively presented undesirable saturated fat in beef was removed and a desired fatty acid profile composed of polyunsaturated fatty acid from the pig (lard) possessing 39% of saturated non‐hydrogenated fat was incorporated as a separate layer into the beef using 3D printing. This study evaluates the effect and amount of lard fat as interlayers on the printability of beef (Figure 3.4). Results showed that lipids were responsible for the smooth flow of the fibrous meat paste through the printing nozzle. However, the fatty layers sandwiched
