Continuous production of ethanol from fructose by immobilized growing cells of Zymomonas mobilis

Zymomonas mobilis as a model system for production of biofuels and biochemicals

Zymomonas mobilis is a natural ethanologen with many desirable industrial biocatalyst characteristics. In this review, we will discuss work to develop Z. mobilis as a model system for biofuel production from the perspectives of substrate utilization, development for industrial robustness, potential product spectrum, strain evaluation and fermentation strategies. This review also encompasses perspectives related to classical genetic tools and emerging technologies in this context.

Continuous ethanol production by Zymomonas mobilis and Saccharomyces cerevisiae in biofilm reactors

Continuous ethanol fermentations were performed in duplicate for 60 days with Zymomonas mobilis ATCC 331821 or Saccharomyces cerevisiae ATCC 24859 in packed-bed reactors with polypropylene or plastic composite-supports. The plastic composite-supports used contained polypropylene (75%) with ground soybean-hulls (20%) and zein (5%) for Z. mobilis, or with ground soybean-hulls (20%) and soybean flour (5%) for S. cerevisiae. Maximum ethanol productivities of 536 g L-1 h-1 (39% yield) and 499 g L-1 h-1 (37% yield) were obtained with Z. mobilis on polypropylene and plastic composite-supports of soybean hull-zein, respectively. For Z. mobilis, an optimal yield of 50% was observed at a 1.92 h-1 dilution rate for soybean hull-zein plastic composite-supports with a productivity of 96 g L-1 h-1, whereas with polypropylene-supports the yield was 32% and the productivity was 60 g L-1 h-1. With a S. cerevisiae fermentation, the ethanol production was less, with a maximum productivity of 76 g L-1 h-1 on the plastic composite-support at a 2.88 h-1 dilution rate with a 45% yield. Polypropylene-support bioreactors were discontinued due to reactor plugging by the cell mass accumulation. Support shape (3-mm chips) was responsible for bioreactor plugging due to extensive biofilm development on the plastic composite-supports. With suspension-culture continuous fermentations in continuously-stirred benchtop fermentors, maximum productivities of 5 g L-1 h-1 were obtained with a yield of 24 and 26% with S. cerevisiae and Z. mobilis, respectively. Cell washout in suspension-culture continuous fermentations was observed at a 1.0 h-1 dilution rate. Therefore, for continuous ethanol fermentations, biofilm reactors out-performed suspension-culture reactors, with 15 to 100-fold higher productivities (g L-1 h-1) and with higher percentage yields for S. cerevisiae and Z. mobilis, respectively. Further research is needed with these novel supports to evaluate different support shapes and medium compositions that will permit medium flow, stimulate biofilm formation, reduce fermentation costs, and produce maximum yields and productivities.

Zymomonas mobilis--science and industrial application

Zymomonas mobilis is undoubtedly one of the most unique bacterium within the microbial world. Known since 1912 under the names Termobacterium mobilis, Pseudomonas linderi, and Zymomonas mobilis, reviews on its uniqueness have been published in 1977 and 1988. The bacterium Zymomonas mobilis not only exhibits an extraordinarily uniqueness in its biochemistry, but also in its growth behavior, energy production, and response to culture conditions, as well as cultivation techniques used. This uniqueness caused great interest in the scientific, biotechnological, and industrial worlds. Its ability to couple and uncouple energy production in favor of product formation, to respond to physical and chemical environment manipulation, as well as its restricted product formation, makes it an ideal microorganism for microbial process development. This review explores the advances made since 1987, together with new developments in the pure scientific and applied commercial areas.

New alcohol resistant strains of Saccharomyces cerevisiae species for potable alcohol production using molasse

Two alcohol resistant strains of Saccharomyces cerevisiae species were isolated from a Greek vineyard plantation. The strain AXAZ-1 gave a concentration of 17.6% v/v alcohol and AXAZ-2 16.5%, when musts from raisin and sultana grapes, respectively, were employed in alcoholic fermentations. They were found to be more alcohol tolerant and fermentative in the fermentation of molasse than the traditional baker's yeast. Specifically, using an initial [symbol: see text] Be density of 16 [symbol: see text] Be at the repeated batch fermentation process, in the first as well as fourth batch, the better AXAZ-1 gave final [symbol: see text] Be densities of 6.0 and 10.5 respectively, and the baker's yeast 11.6 and 14.5.

Continuous potable alcohol production by immobilized Saccharomyces cerevisiae on mineral kissiris

A biocatalyst prepared by the immobilization of Saccharomyces cerevisiae on the surface of the mineral kissiris was used in the present study for continuous potable-alcohol production. An ethanol productivity (calculated on the basis of liquid volume) of 10.5 g/L/h was obtained at a 0.7/h dilution rate, 121 g/L sucrose content, and 29.6% conversion employing molasse as feed material. Glucose, raisin extracts, and molasse were successively used as feed materials without stopping the operation of the reactor for 6 mo. The ethanol productivity and yield remained constant during the operational-stability study of the reactor, carried out for 44 d. Biomass productivity, yield, and free-cell concentration in glucose, raisin extracts, and molasse were examined. Finally, a system with two continuous reactors joined successively was also studied in the present investigation.