Enzymatic extruded starch as a carbon source for the production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) by Haloferax mediterranei

The extremely halophilic Archaeon Haloferax mediterranei can accumulate polyhydroxyalkanoate (PHA) on a medium containing high salt concentration. The available carbon source for the cell growth and PHA production was the native cornstarch being treated by an enzymatic reactive extrusion process. The extrusion process was processed at a 25 mm single-screw extruder with α-amylase quantities at 1–5 g/100 g wet weight of cornstarch, screw speed at 39–78 rpm and temperatures controlled at 55 °C for zone 1, 65 °C for zone 2, 85 °C for zone 3 and 90 °C for the die. By using an extruded starch/yeast extract ratio (ES/YE ratio) of 1/1.7 g/g in the feeding medium to maintain starch and organic nitrogen concentrations during the pH-stat fed-batch fermentation, the PHA content could reach 50.8% (w/w) of dry cell. From the nuclear magnetic resonance (1H NMR) spectrum, the PHA was confirmed to be a poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) copolymer with a composition of 10.4 mol% of HV unit. The glass transition temperature of this copolymer was −1.2 °C as measured by a differential scanning calorimeter. On the same thermogram, two melting peaks were found at 129.1 and 144.0 °C, respectively; which were much lower than those of the polyhydroxybutyrate (PHB) homopolymer.