title: Impact of osmotic stress and ethanol inhibition in yeast cells on process oscillation associated with continuous very-high-gravity ethanol fermentation.
abstract: BACKGROUND
  VHG fermentation is a promising process engineering strategy aiming at improving ethanol titer, and thus saving energy consumption for ethanol distillation and distillage treatment. However, sustained process oscillation was observed during continuous VHG ethanol fermentation, which significantly affected ethanol fermentation performance of the system.
  
  
  RESULTS
  Sustained process oscillation was investigated in continuous VHG ethanol fermentation, and stresses exerted on yeast cells by osmotic pressure from unfermented sugars and ethanol inhibition developed within the fermentation system were postulated to be major factors triggering this phenomenon. In this article, steady state was established for continuous ethanol fermentation with LG medium containing 120 g/L glucose, and then 160 g/L non-fermentable xylose was supplemented into the LG medium to simulate the osmotic stress on yeast cells under the VHG fermentation condition, but the fermentation process was still at steady state, indicating that the impact of osmotic stress on yeast cells was not the main reason for the process oscillation. However, when 30 g/L ethanol was supplemented into the LG medium to simulate the ethanol inhibition in yeast cells under the VHG fermentation condition, process oscillation was triggered, which was augmented with extended oscillation period and exaggerated oscillation amplitude as ethanol supplementation was increased to 50 g/L, but the process oscillation was gradually attenuated when the ethanol supplementations were stopped, and the steady state was restored. Furthermore, gas stripping was incorporated into the continuous VHG fermentation system to in situ remove ethanol produced by Saccharomyces cerevisiae, and the process oscillation was also attenuated, but restored after the gas stripping was interrupted.
  
  
  CONCLUSIONS
  Experimental results indicated that ethanol inhibition rather than osmotic stress on yeast cells is one of the main factors triggering the process oscillation under the VHG fermentation condition, and in the meantime gas stripping was validated to be an effective strategy for attenuating the process oscillation.
authors.affiliation: School of Life Sciences and Biotechnology, Dalian University of Technology, 2 Linggong Rd,, Dalian 116023, China. xue.1@dlut.edu.cn.
authors.forename: Liang
authors.initials: L
authors.lastname: Wang
authors.forename: Xin-Qing
authors.initials: XQ
authors.lastname: Zhao
authors.forename: Chuang
authors.initials: C
authors.lastname: Xue
authors.forename: Feng-Wu
authors.initials: FW
authors.lastname: Bai
references.citation: Proc Natl Acad Sci U S A. 1964 Jun;51:1244-51
references.pmid: 14215650
references.citation: Biochem Biophys Res Commun. 1964 Jun 1;16(2):174-81
references.pmid: 4224521
references.citation: J Appl Microbiol. 2010 Jul;109(1):13-24
references.pmid: 20070446
references.citation: Eur J Biochem. 1996 Jan 15;235(1-2):238-41
references.pmid: 8631335
references.citation: Bioresour Technol. 2008 Sep;99(13):5270-95
references.pmid: 18158236
references.citation: Biophys J. 2001 Feb;80(2):626-34
references.pmid: 11159431
references.citation: FEMS Microbiol Lett. 1992 Nov 1;77(1-3):261-7
references.pmid: 1334018
references.citation: Biotechnol Adv. 2001 Jun;19(3):201-23
references.pmid: 14538083
references.citation: Proc Natl Acad Sci U S A. 1964 Jun;51:989-94
references.pmid: 14215656
references.citation: FEMS Yeast Res. 2009 May;9(3):365-71
references.pmid: 19416102
references.citation: Microbiol Rev. 1989 Mar;53(1):121-47
references.pmid: 2651863
references.citation: Biotechnol Adv. 2008 Jan-Feb;26(1):89-105
references.pmid: 17964107
references.citation: Exp Cell Res. 2003 Jul 1;287(1):10-5
references.pmid: 12799177
references.citation: Biotechnol Bioeng. 2005 Jun 20;90(6):703-14
references.pmid: 15812801
references.citation: Biotechnol Bioeng. 2009 Jan 1;102(1):113-21
references.pmid: 18949752
references.citation: Biotechnol Bioeng. 2004 Dec 5;88(5):558-66
references.pmid: 15470717
references.citation: Biotechnol Bioeng. 1997 Oct 5;56(1):99-105
references.pmid: 18636614
references.citation: Biotechnol Bioeng. 1990 Jun 5;36(1):28-38
references.pmid: 18592606
references.citation: Biochem Biophys Res Commun. 2009 Oct 2;387(4):778-83
references.pmid: 19635452
references.citation: J Biotechnol. 2004 Jun 10;110(3):287-93
references.pmid: 15163519
references.citation: J Ind Microbiol Biotechnol. 2001 Aug;27(2):87-93
references.pmid: 11641766
references.citation: Biotechnol Adv. 2003 May;21(3):183-92
references.pmid: 14499128
references.citation: Biotechnol Bioeng. 1990 Jun 5;36(1):19-27
references.pmid: 18592605
references.citation: Biophys J. 2000 Mar;78(3):1145-53
references.pmid: 10692304
references.citation: J Biotechnol. 1996 Oct 18;51(1):57-72
references.pmid: 8987629