of printing technology as 3D printing could be used for modifying the fill density of a product that gives a distinct mouthfeel. This micro‐scale printing also helps to incorporate food bioactive compounds into the 3D printed products. Further 3D printing helps to deliver texture modified foods for people with swallowing disorders. All these potential advantages of food printing would lead to the development of a convenient food product that holds a huge market value. Post‐processing of printed foods will add upon taste and contributes to structural integrity to retain its shape. Thus, it is essential to understand the role of each constituent in printability and the response of 3D structures when subjecting to post‐processing treatments. The structural chemistry of food constituents towards 3D printing is discussed in the subsequent sections.
3.4 Insights on the Printability of Different Food Constituents
3.4.1 Carbohydrates and Starch
Carbohydrates are chemical organic compounds comprised of simple sugars and polysaccharides (starch and non‐starch). The latter group of carbohydrates takes more time to get digested than simple sugars and are referred to as dietary carbohydrates. The concept of well‐being and healthy food provokes the fascination towards fibre‐rich diet possessing a low glycaemic response (Amicucci et al. 2019). The digestible polysaccharide, starch comprises of linear amylose and non‐linear amylopectin fractions. These are homopolymers made of glucosidic bonds forming the backbone of starch molecules. Non‐starch polysaccharides are grouped as cellulosic and non‐cellulosic (pectin and hemicellulose) that forms the major part of the plant cells. The polymeric network of polysaccharides and their physicochemical characteristics makes them more suitable for food printing. Most of the 3D printing works reported in the literature were carried out with starch‐based food materials in assessing its behaviour towards printability. On comparing with the other food constituents, carbohydrate‐based printing material supplies possess more tendency of gelation that tailors the consistency and material flowability and hence the layered deposition of 3D constructs (Figure 3.1).
Figure 3.1 Schematic representation of gelation mechanism of starch granules.
Researchers are quite interested in exploring the printability of common daily foods such as cereals and millets, legumes and pulses, fruits and vegetables, dairy products and meat products. Rice a staple food of South Asia is widely consumed and forms a major part of the regular diet (Ramadoss et al. 2019). Huang et al. (2019) have reported a study on the assessment of printability of brown rice and evaluated its effect on end‐product quality. In this study, ready-to-cook (RTC) brown rice flour was prepared and used for printing trials. Material supply was pre‐gelatinised in order to enhance the chemical integrity of starch molecules. Granule size, amylose, and amylopectin content of flour determined its pasting behaviour (Huang et al. 2019). This could be correlated with the swelling power and gelling behaviour of starch molecules. The formation of hydrogen bonds and corresponding molecular entanglement had a significant impact on textural properties such as the hardness and gumminess of the 3D printed sample.
In another study, mashed potato along with potato starch (0, 1, 2, and 4%, w/w) was used for 3D printing. Raw potatoes were steam cooked and ground to form smooth paste making them suitable for extrusion‐based printing. Application of heat causes the molecules to swell due to which the amorphous nature of the starch molecules was increased with a decrease in their crystallinity. Upon subsequent cooling, the starch mixture would reverse the process to regain its crystalline nature that leads to the formation of a gel. The strong association of hydrogen bonds between water and starch forms a dense network imparting a smooth texture for material supply making it easy for extrudability and hence 3D printing of starch‐based food mixtures (Liu et al. 2018).
Southerland et al. (2011) reported a study on printing a mixture of starch, sugar, and mashed potato as dry fractions using a Z Corporation 3D printer to fabricate teeth like prototypes based on sugar crystals. These type of 3D printers employs a heat source for binding and fusion of materials based on sintering process thereby forming a 3D edible construct. Sintering is a solid‐state process involving the melting of solid particles to their melting point that leads to the formation of grain boundaries between grain‐void interfaces (Figure 3.2). In the case of sugar crystals, the molecular voids open out forming the fusion of particles together with the application of heat and slight pressure. 3D printing of sugars greatly depends on properties like crystallinity, melting point, source and extent of heating, glass transition temperature (T g), compressibility, solubility, and material density. These parameters greatly influence the porosity of fabricated structures and hence the texture of 3D printed candies could be controlled. Similarly, 3D systems had fabricated sugar cubes from milk chocolates, sweet, and sour candies (Sun et al. 2015).
Figure 3.2 Schematic representation of sintering process.
3.4.2 Proteins and Amino Acids
Proteins are a complex group of biopolymers that consists of long chains of linear and branched amino acids linked together by peptide bonds. Chemical break down of proteins could result in a shorter chain amino acid that forms a precursor to nucleic acid, antigen, antibodies, hormones, and co‐enzymes. Proteins play a vital role in imparting structural integrity to cell walls and stiffness to tissues. Other functional role includes regulation of physiological activities, maintenance of normal pH, acts as chemical messenger, and storage pool (Hoffman 2019). Legumes and pulses are a good source of proteins from a plant source. Proteins from the animal source include eggs, red meat, fish, and dairy products such as cheese and whey powders. Based on the amino acid profile, proteins from animal sources are considered as complete proteins while plant proteins are considered incomplete due to lack of one or more essential amino acids. Various factors that affect 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