## 4 Biosynthesis of Dextran in Optimized Microbial Processes
### Dextran Synthesized by Specific Strains of Leuconostoc Species  
The genus _Leuconostoc_ belongs to a group of lactic acid bacteria usually isolated from fermented vegetables, which includes species involved in the production of exopolysaccharidesrides with commercial potential. High molecular weight dextran produced by _Leuconostoc mesenteroides_ AA1 have been characterized for their potential applications [61]. Dextrans produced by wild and mutant strains of _L. mesenteroides_ were studied for their structural configuration and characterization [73]. Dextran production was studied using LAB strains isolated from the aquamiel of _Agave salimiana_; the product was structurally characterized to analyze the rheological properties of dextran [74].  
Recently, in a study performed by [PERSON] et al. to produce and characterize dextran of different configurations, employing four strains of _L. mesenteroides_, which were isolated from _Agave salimiana_. The extracellular polysaccharides were produced in a 24 h process with sucrose substrate at optimized fermentation conditions set at pH 6 and incubated without mixing or shaking at 30 \({}^{\circ}\)C. The product with 20 g/L yield was precipitated as a gummy material by the addition of ethanol, and in its characterization, it was found to be a long-chain glucose polymer known as dextran. Dextrans produced by these strains have a polymer chain of \(\alpha\) (1 \(\rightarrow\) 6) linkages with branching of \(\alpha\) (1 \(\rightarrow\) 3). The rheological behavior of dextran solutions exhibited typical shear thinning and weak gel properties [74].  
Commercial dextran is biosynthesized by the non-pathogenic organism _L. mesenteroides_ strain NRRL B-512. For industrial production, a method is based on the batch-wise culture of fermentation medium containing sucrose as the main carbon source and supplemented with yeast extract, casein, peptone, malt extract, tryptone, and the salts of calcium and phosphate. During fermentation, the pH drops from 7 to 5 due to the generation of lactic acid; therefore, non-ionic compounds are usually added to maintain the stability of the bacteria and its enzymes [75]. Table 2 presents some fermentation studies performed for the production of dextran using selected strains of _Leuconostoc_ species.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Dextran Product** & **Substrate/s Used in Biosynthesis** & **Strains Used to Conduct** & **Reference** \\
**Specification** & & **Fermentative-Production** & **Reference** \\ \hline
358 MDa & & & \\ with & & & \\ \(\alpha\)-(1\(\rightarrow\)6) & 2\%, \(w/v\) solution of sucrose & _Leuconostoc cartoon_ strain CUPV411 an isolate from apple pomace & [76] \\ \(\alpha\)-(1\(\rightarrow\)3) & & & \\ \hline
46 MDa & & & \\
56\% of \(\alpha\)-(1\(\rightarrow\)6) & 10\%, \(w/v\) solution of sucrose & _L. citreum_ strain SK24.002 & [77] \\
44\% of \(\alpha\)-(1\(\rightarrow\)3) & & & \\ \hline n.a. & 15\%, \(w/v\) solution of sucrose & _Leuconostoc_ sp. strain LS1 & [78] \\  & & & \\ \hline n.a. & 15\%, \(w/v\) solution of sucrose & _Leuconostoc_ sp. strain L11, an isolate from & [78] \\  & & & \\  & & & fermented rice batter \\ \hline
93\% of \(\alpha\)-(1\(\rightarrow\)6) & 10\%, \(w/v\) solution of sucrose & _L. mesenteroides_ strain SD1, an isolate from & \\
07\% of \(\alpha\)-(1\(\rightarrow\)3) & 10\%, \(w/v\) solution of sucrose & green maguey—_Agave salimiana_ & [74] \\ \hline
95\% of \(\alpha\)-(1\(\rightarrow\)6) & 10\%, \(w/v\) solution of sucrose & _L. mesenteroides_ strain SD23, an isolate of & [74] \\  & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Dextrans of diverse MW and configuration synthesized by specific isolate strains of _Leuconostoc_ species.