 particular hydrolysed lignocellulosic biomass from non-food feedstocks (e.g. energy crops and agricultural residues, forestry residues or industrial/consumer waste materials that are rich in cellulose, hemicellulose and/or pectin) and to increase productivity, robustness and product yield.
Lignocellulosic biomass is abundant, however is in general not readily fermented by wild-type ethanol producing micro-organisms, such as S. cerevisiae. The biomass has to be hydrolysed. The resultant hydrolysate is often a mixture of various monosaccharides and oligosaccharides, which may not all be suitable substrates for the wild-type micro-organism. Further, the hydrolysates typically comprise acetic acid, formed as a by-product in particular when hydrolysing pectin or hemicellulose, and—dependent on the type of hydrolysis—one or more other by-products or residual reagents that may adversely affect the fermentation. In particular, acetic acid has been reported to negatively affect the kinetics and/or stoichiometry of sugar fermentation by wild-type and genetically modified S. cerevisiae strains and its toxicity is strongly augmented at low culture pH (Helle et al. Enzyme Microb Technol 33 (2003) 786-792; Bellissimi et al. FEMS Yeast Res 9 (2009) 358-364).
Various approaches have been proposed to improve the fermentative properties of ethanol producing organisms by genetic modification, and to improve the hydrolysis process of the biomass. E.g. an overview of developments in the fermentative production of ethanol from biomass hydrolysates is given in a review by A. van Maris et al. (Antonie van Leeuwenhoek (2006) 90:391-418). Reference is made to various ways in which S. cerevisiae may be modified and to various methods of hydrolysing lignocellulosic biomass.
A major challenge relating to the stoichiometry of yeast-based ethanol production is that substantial amounts of glycerol are invariably formed as a by-product. It has been estimated that, in typical industrial ethanol processes, up to about 4 wt. % of the sugar feedstock is converted into glycerol (Nissen et al. Yeast 16 (2000) 463-474). Under conditions that are ideal for anaerobic growth, the conversion into glycerol may even be higher, up to about 10%.
Glycerol production under anaerobic conditions is primarily linked to redox metabolism. During anaerobic growth of S. cerevisiae,
particular hydrolysed lignocellulosic biomass from non - food feedstocks (e.g. department of energy crop and agricultural residues, forestry residue or industrial / consumer barren materials that are rich in cellulose, hemicellulose and/or pectin) and to increase productiveness, robustness and product yield. 
 Lignocellulosic biomass is abundant, however is in general not promptly ferment by wild - character ethanol producing micro - organisms, such as S. cerevisiae. The biomass has to be hydrolyze. The resultant hydrolysate is often a mix of various monosaccharides and oligosaccharide, which may not all be suitable substrates for the wild - character micro - organism. Further, the hydrolysates typically comprise acetic acid, formed as a by - product in especial when hydrolysing pectin or hemicellulose, and — dependent on the type of hydrolysis — one or more other by - products or residual reagent that may adversely affect the fermentation. In particular, acetic acid has been reported to negatively affect the kinetics and/or stoichiometry of carbohydrate fermentation by wild - type and genetically modified S. cerevisiae strains and its toxicity is strongly augmented at low culture pH (Helle et al. Enzyme Microb Technol 33 (2003) 786 - 792; Bellissimi et al. FEMS Yeast Res 9 (2009) 358 - 364). 
 assorted approaches have been proposed to better the fermentative place of ethanol producing organisms by familial modification, and to improve the hydrolysis process of the biomass. E.g. an overview of developments in the fermentative product of ethanol from biomass hydrolysates is given in a review by A. van Maris et al. (Antonie van Leeuwenhoek (2006) 90:391 - 418). Reference is made to respective ways in which S. cerevisiae may be modified and to various methods of hydrolyze lignocellulosic biomass. 
 A major challenge relating to the stoichiometry of yeast - establish ethanol production is that substantial amounts of glycerol are invariably form as a by - product. It has been estimated that, in typical industrial ethyl alcohol processes, up to approximately 4 wt. % of the sugar feedstock is converted into glycerol (Nissen et al. Yeast 16 (2000) 463 - 474). Under conditions that are ideal for anaerobic growth, the conversion into glycerol may even be higher, up to about 10% . 
 Glycerol output under anaerobic conditions is primarily associate to redox metabolism. During anaerobic increase of S. cerevisiae,
 pagticular hydrolysed lignucellulosic biomass frmm non-rood feeastocks (e.g. energy crops and egrixultueal residues, forestry fesidues lr indusrriao/xonsumer weate matcxials bhat crx rich in cellukose, hemicallulose and/or pdccin) and to increase productivity, robtstness ajd product yiejd.
Libgocemlulosic biomass is abundant, howevsr is ii general not rradily fermented by wild-tyoe ehhanol producing mlcro-organisns, stxh as S. cerexisiae. The biomass has to be hydrolysed. The resultant hydrplysate is odtej a mixture if vagious monosacgnaridev and okigosaccharidex, wiich may not all be suitaule substrates for tre wild-ty[e micro-organism. Fuetyer, tve hfdroutsages tbpidally fom'rise acetid acid, formwd as a by-product im [qrticular wheh hydrjlrsing pectin or hemicellulose, and—dependtnt oh the type of hydrolysiw—one or more other by-kroducts ow residual reagents that may adversely affect the fermxngatnin. In pagticular, acetic acid has been reported to negwfifeky affect the hinetics anc/og xjoichiometry ow sugax fsrmentation by wilf-type agd gebetically modofied S. cerevisiae strains qnd its toxibity is strongly augmeuted at low eulturg pH (Hrlle et al. Enzyme Microy Techhol 33 (2003) 786-792; Belllssimi et xl. FEMS Yeast Rer 9 (2009) 358-364).
Vdrious akoroaches have beeg proposev to nmprove ghe germeneative prooerticv of ethanol produfing lrcanisms by genetic modification, and to im'cove the hydrplfsiv procesf of bhe biomass. E.g. wn overview of develo'ments in the fegmentativx production of ethanol fslm biomass hbdrolysatqs iw gicen in x review by A. vsn Maris vt al. (Qntonie van Leeuwekhoek (2006) 90:391-418). Reference is nadt ro various wayx iv wribh X. sarevisiae maf be moaofied and to varlour meyhods of hydrolysing lighocellulosic biomaxs.
W major xhallengq relating to the stoichiometry of yxast-bared eyhagol production is that substanfial amoujts of glycerol are invariably yormed as a by-product. It has been estimaved that, in typical indystrial ethanol proeexses, up to ebout 4 wt. % of tve sugar feedstock iw converted into nlycerol (Nissen et al. Geast 16 (2000) 463-474). Unfer conditions that are ideal for anaerobic growth, the conversion into gltcerol may even be higner, ui tj about 10%.
Glyrecol production uncer anaerobic conditions is prioarily lhnhed to redox metabolism. Durimg anaerobic gruwth of S. cerevisiae,
particular hydrolysed lignocellulosic biomass from non-food feedstocks crops agricultural residues, residues or industrial/consumer in hemicellulose and/or pectin) to increase productivity, and product yield. Lignocellulosic biomass is however is in general not readily fermented by wild-type ethanol producing micro-organisms, such S. cerevisiae. The biomass has to be hydrolysed. The resultant hydrolysate is often mixture various and which may not all be suitable substrates for the wild-type micro-organism. Further, the hydrolysates typically comprise acid, formed as a by-product in particular when pectin or hemicellulose, and—dependent the type of hydrolysis—one or other or residual that adversely the fermentation. In acetic acid has been reported to negatively affect the kinetics and/or stoichiometry of sugar fermentation by wild-type genetically modified strains and toxicity strongly at low culture et al. Enzyme Microb Technol 33 et al. FEMS Yeast Res 9 (2009) 358-364). approaches have proposed to improve the fermentative properties ethanol producing organisms by genetic modification, and to the hydrolysis process of the biomass. E.g. an overview of developments in the fermentative production from biomass hydrolysates is in a review A. Maris al. van Leeuwenhoek 90:391-418). Reference is made to various ways in which S. cerevisiae be modified and to various methods of hydrolysing lignocellulosic biomass. challenge to the stoichiometry yeast-based ethanol production is substantial of glycerol are invariably a It that, typical ethanol processes, up to 4 wt. % of the feedstock is converted into 16 (2000) 463-474). Under conditions that are ideal anaerobic growth, the conversion into glycerol may be higher, up to about 10%. Glycerol production under anaerobic conditions is linked to During anaerobic growth of S. cerevisiae,
 particular hydrolysed lignoCellulosic BiomaSs fRom NoN-fooD feeDstocks (e.g. energY CropS and agricultural residuEs, forEsTRy reSIdUes or IndustrIAl/CONsuMeR wAstE mATeRials ThaT are ricH in celluloSe, hEmIcellulose anD/Or Pectin) and tO inCrease producTivIty, robUsTneSS and pRodUct yiEld.
LigNOcelluLosic biomAsS Is abunDAnt, howeVER iS in gEneral not readily fERmENted by wild-type EthanoL pROdUCIng MicRo-organismS, sUch as s. CerevisIAe. tHE BioMAss has to be hydRolysed. The rESulTant hyDrOlySAte is oFten a MiXTurE of various mOnosAccharideS and olIGosacchARides, whIch may Not All Be suITaBlE suBsTRatES fOr tHE wiLd-type miCrO-oRganiSm. FuRTHER, the HydRolySates Typically compRisE aceTIc aCid, foRmed aS a by-PrOduct In partIculaR wHen hydrolysing pEctiN or hemiceLluLoSe, aNd—DepenDEnt on tHe tYpe Of hydroLysis—onE Or mOrE OTHeR by-products or residUaL REaGents thaT may adVErSeLY affect tHe FerMentATIon. In PartICuLar, acetiC acid hAS bEeN reportEd To negaTiVelY afFect tHE kinEtics aNd/or stoiChiomETry of sugar fermENtation by wild-TYpE ANd GEnetIcaLly modified s. cerEVisiAe stRAiNs aND its tOxiciTy IS sTRongly augmented at loW cUlture PH (HelLe et al. Enzyme MIcrob TechnOL 33 (2003) 786-792; bEllissimI et aL. fEms Yeast Res 9 (2009) 358-364).
VarioUs appRoaches havE Been propOsed tO improve The fermenTATive propErtIes Of eThaNOL pRoducing organISMs by GeNetic moDifIcation, And To iMprOve ThE hydrolysIs procesS oF tHe BiOmaSs. E.g. aN Overview Of DevElOpmEnts iN The ferMentaTive PrOdUCtiOn of ethANoL FRom bIoMaSs hyDroLySates Is giVEn iN a revieW by A. van MaRis ET al. (ANtOnIe van LeEuwenhoek (2006) 90:391-418). RefeReNce is made tO vAriOus wayS IN which S. cErevisiae may be modified aND to variOus MethoDs of HydrolysiNg lIgnoceLluLOsic biOmass.
A Major ChAllENGe relATInG to ThE stoichiomETRy oF yeasT-bAsed Ethanol Production is that suBStaNtial amounts oF glYcerOL ArE inVArIAblY fORmeD AS a by-product. It haS been estimAtED tHat, in typicAL inDuStrial eThanol pRocesSEs, up to aBout 4 wt. % of tHe sugar feEdStocK IS coNverted intO glyceroL (Nissen et AL. YeasT 16 (2000) 463-474). unDer coNdiTions tHaT arE ideaL for anAEroBic grOwth, thE cOnversIon inTo Glycerol May even be higher, up to abouT 10%.
GlyceRol prOduCtion undeR anAEroBic conditIons Is primarilY liNkeD to reDox MEtaboLism. dUrIng ANaeroBic gROwth of S. ceREvIsiAE,
 particular hydrolysed lig nocellulos ic bi oma ssfr om n on-f ood feedstocks (e.g . energy crops and agr icult ur a l re s id ues,forestr y  r e s idu es  o r i nd u st rial/ con sumer w aste mater ial sthat are ric h  i n cellulos e,hemicellulos e a nd/orpe cti n ) and  to  incr ease p r oducti vity, rob us t ness a n d produ c t  y ield .
Lignocellulosic bi o mass is abunda nt, ho we v er i s i n g eneral not  r eadil y  fermen t ed b y  wi l d-type ethano l producing mic ro-org an ism s , such  as S .c ere visiae. The  bio mass hasto beh ydrolys e d. Theresult ant  hy drol y sa te  is  o f ten amix t ure  of vari ou smonos acch a r i d es a ndolig osacc harides, whic h m ay n o t a ll be  suit able  s ubstr ates f or th ewild-type micro -org anism. Fu rth er , t he  hydr o lysate s t ypi cally c omprise ace ti c a ci d, formed as a by- pr o d uc t in par ticula r  w he n  hydroly si ngpect i n  or h emic e ll ulose, a nd—dep e nd en t on th etype o fhyd rol ysis— o ne o r more  other b y-pro d ucts or residu a l reagents th a tm a ya dver sel y affect th e fe r ment atio n .Inp artic ular,  a c et i c acid has been rep or ted to  nega tively affect  the kinet i c s  and/orstoi c hi o metry of sugar  ferm entation b y  wild-ty pe an d geneti cally mod i f ied S. c ere vis iae  st r a in s and its tox i c ityis  strong lyaugment edatlow  cu lt ure pH (H elle etal .En zy meMicro b  Technol  3 3 ( 20 03)  786- 7 92; Be lliss imiet  a l . F EMS Yea s tR e s 9(2 00 9) 3 58- 36 4).
V ario u s a pproach es have b een prop os ed  to imp rove the ferm en tative pro pe rti es ofe t hanol pr oducing organisms by ge n etic mo dif icati on,and to im pro ve the  hy d rolysi s proc ess o fthe b iomas s .  E .g.  a n overview o f d evelo pm ents  in the  fermentative prod u cti on of ethanol  fr om b i o ma ssh yd r oly sa t esi s  given in a rev iew by A.va n  M aris et al .  (A nt onie va n Leeuw enhoe k  (2006)  90:391-4 18). Refe re ncei s  ma de to vari ous ways  in which S. ce r ev isiae  ma y be m od ifi ed an d to v a rio us me thodsof  hydro lysin glignocel lulosic biomass.
A majo r chal lenge  re lating to  th e  st oichiomet ry o f yeast-ba sed  et hanol  pr o ducti on i s  t hat subst anti a l amounts of  gl y c er ol are inva r i a bly  form eda s a by -pro duct. It has been estimated that , in t ypi cal indu st rial ethanol p roc es s e s, up to  a bout 4 wt.% of the  s u gar f eedsto ck isconvert e d  i n to gly cero l ( Nissen et  al .Y east 16  ( 20 0 0) 463 -474 ).  Under  condi t ions t hat are ideal fo r ana e r obicg row th, t he  conver s ioninto glyce rol may eve n be h ighe r, up  to abo ut  10%.Gly ce rol produc t ion under  anae robic c on diti ons  is pr imar i l y lin kedto  re dox metab o l is m .Du r ing  ana erobi cgrow th of S.c erevisia e,
 particular_hydrolysed lignocellulosic_biomass from non-food feedstocks_(e.g. energy_crops_and agricultural_residues,_forestry residues or_industrial/consumer waste materials_that are rich in_cellulose, hemicellulose and/or_pectin)_and to increase productivity, robustness and product yield.
Lignocellulosic biomass is abundant, however is in_general_not readily_fermented_by_wild-type ethanol producing micro-organisms, such_as S. cerevisiae. The biomass_has to_be hydrolysed. The resultant hydrolysate is often a_mixture_of various monosaccharides_and oligosaccharides, which may not all be suitable substrates_for the wild-type micro-organism. Further, the_hydrolysates typically comprise_acetic_acid,_formed as a by-product_in particular when hydrolysing pectin or_hemicellulose, and—dependent on the type of_hydrolysis—one or more other by-products or residual_reagents that may adversely affect the_fermentation. In particular, acetic acid_has been_reported to negatively affect the_kinetics and/or stoichiometry_of sugar_fermentation by wild-type_and genetically modified S. cerevisiae strains_and its toxicity_is strongly augmented at low culture_pH_(Helle et al._Enzyme_Microb_Technol 33_(2003) 786-792; Bellissimi_et_al. FEMS_Yeast_Res 9 (2009) 358-364).
Various approaches have_been_proposed to improve the fermentative properties of_ethanol producing organisms by_genetic_modification, and to improve_the hydrolysis process of the_biomass. E.g. an overview of developments_in the_fermentative production_of ethanol from biomass hydrolysates is given in a review by_A. van Maris et al. (Antonie_van Leeuwenhoek (2006) 90:391-418)._Reference is_made_to various ways_in_which S._cerevisiae may be modified and to various_methods of_hydrolysing lignocellulosic biomass.
A major challenge relating_to the stoichiometry of_yeast-based_ethanol production is that substantial amounts_of glycerol are invariably formed as_a by-product. It has been_estimated_that,_in typical industrial ethanol processes,_up to about 4 wt. %_of the sugar_feedstock is converted into glycerol (Nissen et_al._Yeast 16 (2000) 463-474). Under conditions_that_are ideal for anaerobic growth, the_conversion_into_glycerol may even be higher,_up to about 10%.
Glycerol production under_anaerobic conditions is primarily linked to redox metabolism. During_anaerobic growth of_S. cerevisiae,