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.

Resistance of Acid-Adapted Salmonellae to Organic Acid Rinses on Beef †.

Four strains of salmonellae, including three bovine isolates and an ATCC strain, were adapted to growth in acidic conditions by sequential transfer in tryptic soy broth with reduced pH values. The cultures were transferred until good growth (approximately log107 CFU/ml) was obtained within 24 h at 37°C at pH 5.0. Lean beef tissue was inoculated by immersion into either the acid-adapted or the homologous parent strain of each bacterium. The inoculated tissue was rinsed for 10s in 1.5% or 3.0% lactic acid solutions at 23°C or 55°C. Reductions in bacterial populations were compared between the parent and acid-adapted strains to determine if the acid-adapted strains were more resistant to the organic acid rinses. Acid-adapted strains had either equal or greater sensitivity to organic acid rinses than their homologous parent strains, indicating that acid adaptation did not result in bacteria which were resistant to organic acid rinses. Acid-adapted strains had significantly lower D55°C- values than their homologous parent strains.