Welcoming Remarks

Utilizing inexpensive cellulosic feedstocks or agricultural residues as the sole carbon substrates is one key approach in fermentation cost reduction. Pretreatment and hydrolysis are common techniques employed in sugar recovery from such feedstocks. Nevertheless, some impurities remaining after pretreatment and hydrolysis such as salts, acetic acid, phenolic compounds, and furfurals are claimed to be responsible to lower fermentation efficiency due to their inhibitory effects on microbial metabolism. To eliminate such problems, post-treatment after hydrolysis is required and this undoubtedly results in cost contribution. An acceptable threshold of impurities remaining in the hydrolysate also depends on the robustness of the microbes to be used in fermentation. In this work, the performance of immobilized Rhizopus oryzae in a static bed fermentor was tested against the salt impurities remaining in the cassava pulp hydrolysates for its metabolic responses and lactate production. Similar to that observed in the typical glucose fermentation, after inoculation, spore germination and immobilization on the cotton sheet occurred, generating complete immobilization on the cotton cloth and cell-free fermentation broth. Cell growth and end product formation kinetics were observed during the fermentation using the cassava pulp hydrolysates. The fermentation performance somehow indicated the evidence of stress and impaired metabolic rate as the responses of the remaining impurities in the hydrolysates. The scanning electron micrographs show the salt crystals attached to the growing mycelia resulting in cell impairment. Nevertheless, by cell immobilization in the correct morphological stage, R. oryzae could withstand and subsequently produce lactate and ethanol as the end products under this drastic condition. As a consequence, we can claim the effectiveness of the static bed fermentor configuration.