Network structure and functional properties of transparent hydrogel sanxan produced by Sphingomonas sanxanigenens NX02
Introduction
Microbial polysaccharides have been utilized in the food, cosmetic, and medical industries to modify the functional properties or texture of existing products and to create novel gelling, thickening, stabilizing, emulsifying, foaming, and suspending agents (Rehm, 2010; Schmid, Sieber, & Rehm, 2015). Microbial polysaccharides, including fungal polysaccharides, are more advantageous than plant polysaccharide because of their structural diversity, short production cycle, safety, non-toxicity, biodegradability, and lack of limitations regarding of season, location, and pests. Common microbial polysaccharides with excellent rheological characteristics available on the market include xanthan (Becker, Katzen, Pühler, & Lelpi, 1998), cellulose (Ji et al., 2016; Vandamme, De Baets, Vanbaelen, Joris, & De Wulf, 1998), alginate (Hay, Rehman, Moradali, Wang, & Rehm, 2013), succinoglycan (Andhare, Delattre, Pierre, Michaud, & Pathak, 2017), welan (Kaur, Bera, Panesar, Kumar, & Kennedy, 2014), gellan (Fialho et al., 2008), diutan (Schmid, Sperl, & Sieber, 2014), scleroglucan (Castillo, Valdez, & Fariña, 2015), and pullulan (Prajapati, Jani, & Khanda, 2013). For example, gellan gum has been applied in the food industry to confectionery, jams, jellies, fabricated foods, water-based gels, puddings, and various dairy products such as ice cream and yogurt (Sanderson & Clark,1983). Furthermore, in the biomedical field, gellan has been used as a pharmaceutical excipient for nasal, ocular, gastric, and colonic drug delivery applications (Agnihotri, Jawalkar, & Aminabhavi, 2006; Osmalek, Froelich, & Tasarek, 2014; Sultana, Aqil, & Ali, 2006). Xanthan also has been used as a viscosifier, thickener, emulsifier, and stabilizer in food and non-food sectors, and for oil recovery, due to its unique rheological properties. The main applications of xanthan are in the pharmaceutical, cosmetics, and nutritional industries (Garcıa-Ochoa, Santos, Casas, & Gomez, 2000). However, current research has mainly focused on screening and characterizing novel microbial polysaccharides with diversified structures and functions, especially those with the potential to be used in different fields of applied and industrial biotechnology.
Sanxan, a novel extracellular polysaccharide, is produced by Sphingomonas sanxanigenens NX02 (Huang et al., 2009a, Huang et al., 2009b). Based on Fourier transform infrared spectroscopy, periodate oxidation, Smith degradation, composition analysis and nuclear magnetic resonance experiments, the structure of sanxan was identified, as follows (Huang et al., 2016):→4) -D-Man (1 → 4) -D-GlcA (1 → 3) -L-Rha (1 → 3) -D-Glc (1→
Sanxan is very different from sphingans, which are structurally related exopolysaccharides secreted by members of genus Sphingomonas (Fialho et al., 2008), including gellan, welan, and diutan. These sphingans possess similar linear structures with an identical repeating tetrasaccharide unit, (→4) L-Rha (1 → 3) D-Glc (1 → 4) D-GlcA (1 → 4) D-Glc (1 → ), in the backbone (Schmid et al., 2014, Wu et al., 2017). The unique structure of sanxan, largely caused by its scattered and distinctive biosynthetic pathway (Wu et al., 2017), suggests it might possess unusual functional properties. What’s more, a major advantage of sanxan production is that it can be precipitated by acidification of the fermentation broth to about pH 3. Compared with precipitation by alcohol, this method decreases the sanxan production cost significantly (Wu et al., 2016). Sanxan has been used in China for years as a drilling mud and a thickening agent in the recovery of petroleum by water flooding (Huang et al., 2013).
In general, the technological applications of biopolymers are somewhat dependent on their functional properties, in addition to the structure features. In this context, the micro network structures of sanxan in the absence of added cations were observed using atomic force microscope (AFM) to provide a provisional summary of its gelation mechanism. The functional properties of sanxan, or compound systems contained sanxan and gellan, including rheological behavior, emulsion formation, and stabilization, were measured under different chemical and physical conditions to investigate their potential industrial applications.
Section snippets
Sanxan polymer preparation
Sanxan polymer production by Sphingomonas sanxanigenens NX02 was performed at 30 °C for 72 h in liquid fermentation medium composed of: glucose, 40.0; yeast extract, 0.2; K2HPO4, 1.2; NaNO3, 2.0; CaCO3, 1.0; FeSO4, 0.005; NaCl, 0.4; and MgSO4, 0.5 (all g/L); pH 7.5 (Huang et al., 2012). The fermentation broth was diluted with distilled water, heated at 90–100 °C for 10 min, and then centrifuged for 20 min or filtered to separate the supernatant and cell pellet. Transparent supernatant was collected,
Comparison of LT for sanxan and gellan
The molecular weight of sanxan is 4.08 × 105 Da (Huang et al., 2016), and its purity exceeds 89% when using the aforementioned extraction methods (Wu et al., 2016).The light transmittances of sanxan, HG, and LG are compared in Table 1. Hydrogel sanxan was transparent, and its LT value was slightly lower than that of LG, but much higher than that of HG, whether using the 0.2% or 1.0% gelation system. These differences were mainly due to the different extraction processes, structures, and produced
Conclusion
The network structure and functional properties of sanxan, a polysaccharide produced by S. sanxanigenens NX02, were studied to evaluate its potential industrial application. AFM data indicated that side-by-side-type inter-helical aggregation might be predominant in developing macroscopic network structures of sanxan according to the formation of cyclic configurations in the absence of added cations. These properties, namely high light transmittance, gelation character, and emulsion forming and
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 31571790), China Postdoctoral Science Foundation funded project (Grant No. 2016M601251), and the Key Project of Tianjin Science and Technology support program (Grant No. 14ZCZDSY00016).
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2022, Journal of Cereal ScienceCitation Excerpt :The molecular weight and polydispersity (=Mw/Mn) of sanxan are 408 kDa and 1.07, respectively, and its molecular structure is presented in Fig. S1 (Huang et al., 2016). The monosaccharides composition of sanxan are glucose, mannose, rhamnose and glucuronic acid, and the sugar skeleton is similar with that of gellan (Wu et al., 2017). Sanxan at low concentrations exhibits high viscosity, excellent gelling and emulsifying properties (Huang et al., 2016), and is authorized as a food additive by the National Health Commission of China in 2020.
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These authors contributed equally to this work.