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Another diverse class of polysaccharides that are widely employed in the food industry are the hydrocolloids which form a group of long‐chain hydrophilic polymers (Li and Nie 2016). The presence of one or more hydroxyl groups rendered them water‐loving thereby forming a colloidal suspension when added into the food mixture. Hydrocolloids are derived either from plants/ seaweeds (xanthan gum, carrageenan, gum arabic, locust bean gum, guar gum, gellan gum, glucomannan, and pectin) or animal sources (gelatin, chitin, and chitosan) (Mehta et al. 2019) (Table 3.1). They exhibit a varied range of functional property as they can act as thickeners, stabilizers, foaming agents, emulsifiers, and gelling agents that help in tailoring the rheology of food systems. Concerning 3D printing, hydrocolloids are commonly used as food additives for making the material supply attain a conducive consistency suitable for 3D printing by altering its flow and viscosity. Further many studies were reported on food hydrocolloids as reference material for 3D food printing. Kim et al. (2018) conducted a study on a comparative assessment of various hydrocolloids such as guar gum, gellan gum, xanthan gum, locust bean gum, hydroxypropyl methylcellulose (HPMC), methylcellulose (MC), and gelatin for their applicability in food printing. The study was performed for framing a classification system based on printability assessment. Among all the hydrocolloids considered for the study, MC was found to be capable of simulating the handling properties and deformation behaviour of foods and possess good mechanical stability which was comparable to most of the food systems with diverse viscosity. Thus, a classification system was made based on dimensional and storage stability as grade A to D with grade A more suitable for 3D printing that requires less extrudable force as with cheese. On the other hand, the latter category of grade D includes materials like cookie dough which requires more force to extrude out of the printing nozzle (Kim et al. 2018).

Table 3.1 Classification of food hydrocolloids.

Origin	Examples of hydrocolloids
Plant	Gum arabic, basil seed gum, gum karaya, konjac, locust bean gum, flaxseed gum, guar gum, and starches
Animal	Chitin, chitosan, and gelatin
Seaweeds	Agar, alginate, xylan, carrageenan, and ulvan
Microbial	Gellan gum, tara gun, xanthan gum, dextran, curdlan, and pullulan
Synthetic	Carboxymethyl cellulose, methylcellulose, hydroxypropyl methylcellulose, and other cellulose derivatives

In another study, Vancauwenberghe et al. (2017) developed a pectin‐based food stimulant for 3D printing applications. The study correlates the effect of the degree of methoxylation of pectin with a concentration of Ca²⁺ ions as a cross‐linking agent that aids in printability. Results showed that the formulation of food ink was greatly influenced by the density of the polymeric network and its degree of cross‐linking with Ca²⁺ ions. The addition of sugar to the food stimulant affects the viscoelastic behaviour and hence could be positively correlated with printability. As the incorporation of sugar dehydrates the polymeric matrix thereby enhancing the gelation mechanism with the formation of hydrogen bonds that would result in higher young’s modulus with firmer pectin‐based gel (Vancauwenberghe et al. 2017). Further, the study demonstrates the production of porous aerated 3D structure with the use of bovine serum albumin (BSA) and remains to be a base for future works for the development of food stimulants with added flavours and textures for food 3D printing.

Basic mechanisms responsible for the gelation of hydrogels are ionotropic cross‐linking, chemical cross‐linking and coacervate formation. Among which ionotropic cross‐linking is the most commonly employed technique for hydrogel formation. The physiochemical characteristics of the hydrocolloids were greatly affected by several factors such as temperature, melting and solidification, ionic strength, gelation behaviour, and cross‐linking (Godoi et al. 2016). Hydrocolloids in the form of hydrogels are gaining attention in recent days as it possesses the centric value as that of dietary fibres. A study was reported on the satiety effects of hydrocolloids, as it could result in slowing down of enzymatic actions or could delay gastric emptying (Li and Nie 2016). As an important application, hydrocolloids are utilized for increasing the fibre content of the diet. Along with other major food constituents, hydrocolloids play a significant role in modifying the sensorial aspects of food and hence found to possess a key role in regulating the dietary aspects of food for providing a healthy 3D printed customized diet.