Cellular viability and kinetics of osmotic stress associated metabolites of Saccharomyces cerevisiae during traditional batch and fed-batch alcoholic fermentations at constant sugar concentrations

Effect of citrate buffer on hydrogen production by photosynthetic bacteria

The effect of citrate buffer on biohydrogen production using photosynthetic bacteria was studied. The study was performed in two steps. First, specific concentrations of citrate and sodium citrate as buffers were mixed into batch cultures, and the effects of these buffers on fermentation broth characteristics and biohydrogen production were analyzed. The maximum overall biohydrogen yield of 411.4 mL, which was 42% higher to the control group, was obtained with 0.05 mol/L citrate buffer. Then, the effect of 0.05 mol/L citrate buffer on biohydrogen yield at different pH values (5.5-7.5) were explored. The maximum biohydrogen yield of 429.82 mL was obtained at pH 6, and the final pH values were effectively controlled. The findings indicated that citrate buffer seriously affected the pH of the reaction liquid. The results provide technical support to stabilize the pH of photo-fermentation broth and improve biohydrogen production performance.

Modified semi-continuous fermentation for resuscitating nongrowing cells during high-temperature gluconic acid production by Acetobacter senegalensis

AIMS

The formation of metabolically inactive and nongrowing cells is an inevitable by-product of intensive fermentation. This study investigated whether co-feeding can be used to resuscitate nongrowing Acetobacter senegalensis cells to enable them to produce gluconic acid in successive fermentation runs at 38°C.

METHODS AND RESULTS

In the first fermentation cycle, 75 g l^-1 of glucose were converted to gluconic acid. Subsequently, however, stationary-phase cells were unable to initiate a new fermentation cycle. The majority of stationary-phase cells (97%) were nonculturable on glucose at 38°C. In addition, 54 and 41% of cells contained non-active cellular dehydrogenases and a compromised cell envelope respectively. Co-feeding stationary-phase cells with a mixture of ethanol, glucose and acetic acid for 7 h enabled these cells to grow on 75 g l^-1 of glucose and produce gluconic acid. Additionally, 74% of cells contained active forms of cellular dehydrogenases after 7 h of co-feeding. However, co-feeding did not improve cell envelope integrity. Quantification of cellular NAD content showed that stationary-phase cells contained moderately reduced levels of total NAD (NADt) as compared with exponential-phase cells. Interestingly, the analysis of stationary-phase cells showed that co-feeding resulted in higher levels of NADt and NADH, suggesting that the regeneration of NADH is one of the limiting factors of glucose consumption. Expression of catalase and superoxide dismutase was increased in stationary-phase cells, but analysis of protein carbonylation and lipid peroxidation did not confirm an extensive oxidative stress.

CONCLUSIONS

Co-feeding with favourable nutrients may enable resuscitation of cells and utilization of less-favourable carbon sources in successive cycles.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study proposed a unique method for resuscitation of nongrowing cells during high-temperature fermentation. By applying this method, cells can be used for consecutive fermentation cycles.