A survey of earlier literature suggests that various natural polymers such as starch , chitosan , and tannic acid  have been reported as reducing agents for the synthesis of silver and gold nanoparticles. It has been demonstrated that the plant-based exudate gums such as gum Acacia
 and gum kondagogu  can be utilized as reducing and stabilizing agents for the silver nanoparticle biosynthesis. Gum gellan, a microbial heteropolysaccharide, was employed for similar purpose in the case of gold nanoparticles . Gum ghatti is a naturally occurring water soluble, complex polysaccharide derived as an exudate from the bark of Anogeissus latifolia (Combretaceae family), a native tree of the Indian sub-continent. The name gum ghatti has originated from its transportation through mountain passes or ghats. This native Indian gum is collected from the forests by the tribals and marketed through government organizations such as Girijan Co-operative Corporation Ltd., Visakhapatnam, India. The world production of gum ghatti is about 1,000–1,500 MT/year [7, 8]. This biopolymer is an arabinogalactan type of natural gum and its morphological, structural, physico-chemical, compositional, solution, thermal, rheological, and emulsifying properties have been well documented and studied [9–17]. This biopolymer is a high-arabinose, protein rich, acidic heteropolysaccharide, occurring in nature as mixed calcium, magnesium, potassium, and sodium salt [12–14, 16]. The primary structure of this gum is composed of sugars such as, l-arabinose, d-galactose, d-mannose, d-xylose, and d-glucuronic acid in a molar ratio of 48:29:10:5:10 and < 1% of rhamnose, which is present as non-reducing end-groups. The gum contains alternating 4-O- substituted and 2-O-substituted α d-mannopyranose units and chains of 1 → 6 linked β d-galactopyranose units with side chains of l-arabinofuranose residues. Six percent of rhamnose in the polysaccharide is linked to the galactose backbone as α-Rhap-(1 → 4) β-galactopyranose side chain. It has a molecular weight of 8.94 × 107 g/mol [12, 13, 15, 16].
The gum ghatti with a CAS number 9000-28-6 is recognized as “generally recognized as safe” (GRAS) and approved as a food ingredient (Code 184.1333) by the Food and Drug Administration, USA, under the function of emulsifier and emulsifier salt. Its use in food is also approved in Japan, China, South Korea, Singapore, Russia, Australia, South Africa, Iran, Saudi Arabia, Latin America, and other countries. But, it is not approved as a food additive in European Union and not been accorded a European food safety E number. It is considered as a food grade additives of food by the Bureau of Indian Standards, India under Indian Standard IS 7239:1974 [13, 15, 16]. In India, the application of this hydrocolloid in traditional medicine and food preparations is well known for centuries. The gum is fed to the lactating mothers in the form of laddu to enhance the nutrients in milk as well as to prevent the post-delivery backache . The gum laddu is also eaten as a heating agent during winter season [18, 19]. The gum ghatti is comprised of around 80% soluble dietary fiber and acts a prebiotic by supplying the matrix required to sustain the bacterial flora of the human colon. This hydrocolloid is resistant to gastrointestinal enzymes and known to be degraded enzymatically only by the specific microflora of the colon such as Bifidobacterium longum, thereby aiding in bifidus fermentation [20–22]. This gum is also given for the treatment of diarrhea and diabetes . Earlier studies on gum ghatti fed white leghorn cockerels and albino rats have established the hypolipidemic activity of gum ghatti [24, 25]. Recent studies have established that gum ghatti has a potential application as a release modifier for controlled drug delivery . Gum ghatti has long been used in non-food applications, such as, calico printing, explosives, varnishes, car polishes, ceramics, cosmetics; and in pharmaceutical, textile, paper, petroleum, and mining industries. Also, this biopolymer aids in various photoelectric determinations [7, 8, 13, 16, 23].
The attractive features of gum ghatti prompted us to use this biopolymer for the synthesis and stabilization of silver nanoparticles due to its (i) edible nature and GRAS ; (ii) natural availability and low cost ; (iii) intermediate viscosity between gum arabic and gum karaya [14, 15]; (iv) greater stability to pH acidification, electrolyte addition, and high-pressure treatment [15, 17]; (v) higher emulsification ability and superior emulsion storage stability at lower concentrations , and (vi) exceptional interfacial characteristics with faster kinetics . The green synthesis of inherently safer silver nanoparticles depends on the adoption of the basic requirements of green chemistry; the solvent medium, the benign reducing agent, and the non-hazardous stabilizing agent [1, 27]. In this context, we have explored and developed a facile and green synthetic route for the production of silver nanoparticles using a proteinaceous, edible, renewable natural plant polymer, gum ghatti as both the reducing and stabilizing agents. Being a natural polymer, gum ghatti is amenable for biodegradation. The synthesis was carried out in aqueous medium by autoclaving, without the addition of any external chemical reducing agent. In this study, autoclaving was adopted as a synthetic route to produce sterile silver nanoparticles that are completely free from bacteria, viruses, and spores, which would suit biological applications. The focus of this study was on (i) the synthesis, (ii) characterization, and (iii) capping and stabilization of silver nanoparticles. In addition, we have also demonstrated the antibacterial activity of the prepared nanoparticles on Gram-positive and Gram-negative bacteria for finding out the potential of the generated nanoparticles for various environmental and biomedical applications.