Enhancement of ethanol production efficiency in repeated-batch fermentation from sweet sorghum stem juice: effect of initial sugar, nitrogen and aeration

BACKGROUND: Ethanol concentration (PE), ethanol productivity (QP) and sugar consumption (SC) are important values in industrial ethanol production. In this study, initial sugar and nitrogen (urea) concentrations in sweet sorghum stem juice (SSJ) were optimized for high PE (≥10%, v/v), QP, (≥2.5 g/L·h) and SC (≥90%) by Saccharomyces cerevisiae SSJKKU01. Then, repeated-batch fermentations under normal gravity (NG) and high gravity (HG) conditions were studied. RESULTS: The initial sugar at 208 g/L and urea at 2.75 g/L were the optimum values to meet the criteria. At the initial yeast cell concentration of ~1 × 108 cells/mL, the PE, QP and SC were 97.06 g/L, 3.24 g/L·h and 95.43%, respectively. Repeated-batch fermentations showed that the ethanol production efficiency of eight successive cycles with and without aeration were not significantly different when the initial sugar of cycles 2 to 8 was under NG conditions (~140 g/L). Positive effects of aeration were observed when the initial sugar from cycle 2 was under HG conditions (180­200 g/L). The PE and QP under no aeration were consecutively lower from cycle 1 to cycle 6. Additionally, aeration affected ergosterol formation in yeast cell membrane at high ethanol concentrations, whereas trehalose content under all conditions was not different. CONCLUSION: Initial sugar, sufficient nitrogen and appropriated aeration are necessary for promoting yeast growth and ethanol fermentation. The SSJ was successfully used as an ethanol production medium for a high level of ethanol production. Aeration was not essential for repeated-batch fermentation under NG conditions, but it was beneficial under HG conditions.

Production of Halophilic α-Amylase by Immobilized Cells of Moderately Halophilic Bacillus sp. Strain TSCVKK.

Aims: To investigate the effect of cell immobilization on amylase production by the moderately halophilic bacterium, Bacillus sp. strain TSCVKK and to compare the properties of the amylase produced under immobilized conditions with the enzyme produced by the free cells. Study Design: Cell immobilization. Place and Duration of Study: Department of Chemistry, Biochemistry Lab, Indian Institute of Technology (IIT Madras), Chennai, Tamil Nadu, between Jan 2009 and March 2009. Methodology: Bacillus sp. strain TSCVKK was immobilized in alginate, agar, polyacrylamide and gelatin. Production of amylase was determined using 3, 5- dinitrosalicylic acid (DNS). Effect of NaCl, pH, temperature on the activity of amylase was determined and compared with the amylase produced by the free cells. Results: Maximum production of 832 mU/ml was achieved with an initial cell load of 1.2% (w/v; wet weight) of 24 h grown cells immobilized in 2% agar of 4 mm3 block size using GSL-2 medium containing 10% NaCl and 1.5% dextrin at pH 8.0 at 30ºC after 36 h of growth. Amylase production was lower when the cells were immobilized in alginate (211 mU/ml) or with the free cells of same biomass concentration as used for immobilization (333 mU/ml). Amylase was not produced when gelatin or polyacrylamide was used as the immobilization matrix. The immobilized cells in 2% agar could be used up to 5 cycles without much reduction in amylase production. Amylase produced through cell immobilization retained all the properties that were shown by amylase produced under submerged fermentation. Conclusion: Agar was the suitable matrix to immobilize Bacillus sp. strain TSCVKK for amylase production. Amylase produced under immobilization conditions retained its temperature, salt and pH requirements. Immobilized cells were used for 5 cycles without much decrease in production.

Study of Sugarcane Pieces as Yeast Supports for Ethanol Production from Sugarcane Juice and Molasses Using Newly Isolated Yeast from Toddy Sap

A repeated batch fermentation system was used to produce ethanol using Saccharomyces cerevisiae strain (NCIM 3640) immobilized on sugarcane (Saccharum officinarum L.) pieces. For comparison free cells were also used to produce ethanol by repeated batch fermentation. Scanning electron microscopy evidently showed that cell immobilization resulted in firm adsorption of the yeast cells within subsurface cavities, capillary flow through the vessels of the vascular bundle structure, and attachment of the yeast to the surface of the sugarcane pieces. Repeated batch fermentations using sugarcane supported biocatalyst were successfully carried out for at least ten times without any significant loss in ethanol production from sugarcane juice and molasses. The number of cells attached to the support increased during the fermentation process, and fewer yeast cells leaked into fermentation broth. Ethanol concentrations (about 72.65~76.28 g/L in an average value) and ethanol productivities (about 2.27~2.36 g/L/hr in an average value) were high and stable, and residual sugar concentrations were low in all fermentations (0.9~3.25 g/L) with conversions ranging from 98.03~99.43%, showing efficiency 91.57~95.43 and operational stability of biocatalyst for ethanol fermentation. The results of the work pertaining to the use of sugarcane as immobilized yeast support could be promising for industrial fermentations.