## 2 Food-grade Polymers
### Natural Gums  
**Natural gums** are widely used in foods and pharmaceuticals as viscosifying agents at low concentrations. They are hydrophobic compounds obtained mainly from plant or microbial sources. They exhibit high structural heterogeneity, so molecules have significant differences in their properties, including linear chain length, branching characteristics and molecular weight, etc. [67; 68; 69; 70; 71].  
_Gellan gum_ (E418) is a water-soluble polysaccharide synthesized by Sphingomonas elodea; in fact, it belongs to the sphingan family and has been widely used as a stabilizer, thickener, viscosifier, or gelling agent, but its nature allows us to explore other stimulant applications.  
It was originally considered an optimal substrate for the cultivation of thermophilic microorganisms because of its heat resistance. As knowledge about its biological, chemical and physical behavior has progressed, gellan gum has attracted increasing interest in nanomedicine and tissue engineering (e.g., to produce acid- and heat-resistant biomaterials). Chemically, it is a linear anionic polysaccharide identifiable by a typical tetrameric repetitive sequence consisting of [D-Glc(\(\beta\)1\(\rightarrow\)4)D-GlcA(\(\beta\)1\(\rightarrow\)4)D-Glc(\(\beta\)1\(\rightarrow\)4)L-Rha(\(\alpha\)1\(\rightarrow\)3)]n (Figure 8). The natural form of this polymer has an L-glycerol substituent on the third carbon of the 3-linked D-Glc residue and, in some repeated units, an acetyl group on the anomeric carbon of the same residue. By heat treatment with alkali, these residues are removed and the commercial form is obtained [68; 70; 71].  
Bhosale et al. developed a pH-sensitive drug delivery system using gellan gum conjugated to polymethyl methacrylate (PMMA-g-GG) by a radical process. The system loaded with metformin hydrochloride showed pH-sensitive and prolonged drug release over a 12 h period; the release profile followed the Peppas model [72].  
Another interesting example of natural gum used in the production of modified release systems is _almond gum_ (CFR, SS 182.20). This exuded gum is obtained from wild almond trees (Amygdala scaparia Spach.), which grow naturally in Iran and some Middle Eastern countries. Its main fraction is an anionic arabinogalactan polysaccharide (Figure 9). It exhibits different functional and biological properties depending on its molecular weight, protein content and solubility [73; 74; 75; 76].  
Salehi et al. prepared almond gum (AG) and sodium caseinate (CAS) complexes as carriers for propolisis [41]. Different percentages (10 to 50 wt%) of an alcoholic propolisis extract were encapsulated in the two carriers at pH 4.6 (complex coacervates) or pH 7 (soluble complexes). The antioxidant activity and %EE of the complexed coacervates were higher than those of the soluble complexes. Probably, at a pH lower than the isoelectric point of CAS (pH = 4.6), a better interaction between negatively charged AG and positively charged CAS occurs and the complex coacervates are formed. In the complexes at pH = 7, both CAS and AG have a negative charge leading to a slight electrostatic interaction between them.  
Figure 8: Structure of gellan gum.  
The release of total polyphenol content (TPC) of propolis in different food simulants showed, after 1 h, an increasing release rate (%R) of TPC from water (%R = 60%), to 10% ethanol (%R = 68%), to 50% ethanol (%R = 85%). In the first hour of release in all food models, propolis showed a rapid release that can be explained by surface fraction of polyphenols. The maximum amount of release was obtained in 50% ethanol, due to the higher solubility of propolis in this solvent than in water. In simulated GIT environments, propolis showed a sustained release at a constant rate; the cumulative release of propolis in gastric (pH = 1.2, after 2 h) and intestinal (pH 6.8, after 8 h) environments was about 23% and 49%, respectively [77].