title: Ensiling and hydrothermal pretreatment of grass: consequences for enzymatic biomass conversion and total monosaccharide yields.
abstract: BACKGROUND
  Ensiling may act as a pretreatment of fresh grass biomass and increase the enzymatic conversion of structural carbohydrates to fermentable sugars. However, ensiling does not provide sufficient severity to be a standalone pretreatment method. Here, ensiling of grass is combined with hydrothermal treatment (HTT) with the aim of improving the enzymatic biomass convertibility and decrease the required temperature of the HTT.
  
  
  RESULTS
  Grass silage (Festulolium Hykor) was hydrothermally treated at temperatures of 170, 180, and 190°C for 10 minutes. Relative to HTT treated dry grass, ensiling increased the solubilization of dry matter (DM) during HTT and gave increased glucan content, but lower lignin in the insoluble fiber fraction. Ensiling improved glucose yields in the enzymatic hydrolysis of the washed solid fiber fraction at the lower HTT temperatures. At 170°C glucose yield improved from 17 to 24 (w/w)% (45 to 57% cellulose convertibility), and at 180°C glucose yield improved from 22 to 29 (w/w)% (54 to 69% cellulose convertibility). Direct HTT of grass at 190°C gave the same high glucose yield as for grass silage (35 (w/w)% (77% cellulose convertibility)) and improved xylan yields (27% xylan convertibility). The effect of ensiling of grass prior to HTT improved the enzymatic conversion of cellulose for HTT at 170 and 180°C, but the increased glucose release did not make up for the loss of water soluble carbohydrates (WSC) during ensiling. Overall, sugar yields (C6 + C5) were similar for HTT of grass and grass silage at both 170 and 180°C, but at 190°C the overall sugar yield was better for HTT of dry grass.
  
  
  CONCLUSIONS
  This study unequivocally establishes that ensiling of grass as a biomass pretreatment method comes with a loss of WSC. The loss of WSC by ensiling is not necessarily compensated for by providing a lower temperature requirement for HTT for high enzymatic monosaccharide release. However, ensiling can be an advantageous storage method prior to grass processing.
authors.affiliation: Center for BioProcess Engineering, Department of Chemical and Biochemical Engineering, Technical University of Denmark, DTU, DK-2800 Kgs Lyngby, Denmark.
authors.forename: Morten
authors.initials: M
authors.lastname: Ambye-Jensen
authors.affiliation: Novozymes A/S, Krogshøjvej 36, DK-2880 Bagsværd, Denmark.
authors.forename: Katja S
authors.initials: KS
authors.lastname: Johansen
authors.affiliation: Danish Plant Breeding Research Division, DLF TRIFOLIUM A/S, Højerupvej 31, DK-4660 Store Heddinge, Denmark.
authors.forename: Thomas
authors.initials: T
authors.lastname: Didion
authors.affiliation: Center for BioProcess Engineering, Department of Chemical and Biochemical Engineering, Technical University of Denmark, DTU, DK-2800 Kgs Lyngby, Denmark.
authors.forename: Zsófia
authors.initials: Z
authors.lastname: Kádár
authors.affiliation: Center for BioProcess Engineering, Department of Chemical and Biochemical Engineering, Technical University of Denmark, DTU, DK-2800 Kgs Lyngby, Denmark.
authors.forename: Anne S
authors.initials: AS
authors.lastname: Meyer
keywords: Combined pretreatment; Ensiling; Enzymatic hydrolysis; Grass; Hydrothermal treatment; Silage