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In this research work, a main biopolymer group of polyhydroxyalkanoates (PHAs) in the form of polyhydroxybutyrate (PHB) was synthesised by a pure bacterial strain of Azotobacter vinelandii via repeated fed-batch fermentation. An agricultural crop, sugar cane, was used as the sole carbon source. Firstly, batch fermentation was investigated considering variations in incubation times (24 h, 48 h, and 96 h). The highest dry cell weight (DCW) and PHAs of 5.15 ± 0.04 g/L and 4.00 ± 0.04 g/L were obtained after 48 h of incubation time. The optimum time obtained was further varied to investigate the effect of the sugar concentrations in the medium. It was found that bacteria could grow very well and produced the highest DCW and PHAs (11.17 ± 0.15 g/L and 8.77 ± 0.06 g/L) when the culture medium with a 100 g/L sugar concentration was added. Later, repeated fed-batch fermentation was carried out to improve productivity. The results obtained revealed that PHA production was increased in the next cycle of the process. Furthermore, the final productivity (0.104 g/L·h) was increased 1.65-fold compared to the first cycle (0.063 g/L·h). Moreover, the culture strategy showed remarkable results, with reductions in both fermentation time and preparation cost. Full article

Yeasts were isolated from four potential sources, sweet sorghum juice, sugar cane juice, grapes and rambutan. The 27 yeast isolates were tested for their ethanol tolerance (15% v/v of ethanol) and ethanol fermentation performance in a synthetic ethanol production medium (200 g/L of total sugar). Only five isolates, SCJ04KKU, SCJ07KKU, SCJ09KKU, SCJ14KKU and SSJ01KKU could tolerate 15% ethanol and produce ethanol at levels higher than 55 g/L. The ethanol production efficiency from sweet sorghum juice under high gravity (HG, 200 and 240 g/L of total sugar) and very high gravity (VHG, 280 g/L of total sugar) conditions of the five isolates was tested. Saccharomyces cerevisiae NP01 and S. cerevisiae ATCC4132 were used as reference strains. The results showed that the SSJ01KKU isolate gave the highest ethanol production efficiency under all conditions. Ethanol concentration (P_E), yield (Y_P/S) and productivity (Q_P) values were 98.89 g/L, 0.50 and 1.18 g/L·h, respectively, with sugar consumption (SC) of 98.96% under the HG condition at 200 g/L of total sugar. Under the HG condition at 240 g/L of total sugar, the P_E, Y_P/S and Q_P values were 118.12 g/L, 0.51 and 1.41 g/L·h, respectively, with the SC of 95.79%. These values were 82.29 g/L, 0.34 and 0.98 g/L·h, respectively, with the SC of 85.59% under the VHG condition. Addition of urea into the sweet sorghum juice under all conditions significantly shortened the fermentation time, resulting in increased Q_P values. Based on molecular taxonomic analysis of the five isolates using sequence analysis of the D1/D2 domain and the ITS1 and ITS2 regions, SSJ01KKU is S. cerevisiae, whereas SCJ04KKU, SCJ07KKU, SCJ09KKU and SCJ14KKU are Pichia caribbica. Full article