Pretreatment and Characterization of Wheat Straw

Wheat straw, locally harvested in August 2013 and dried on field (Johan Håkansson Lantbruksprodukter, Lunnarp, Sweden), was chopped into pieces up to 50 mm long using a knife mill (Retsch GmbH, Haan, Germany). The dry matter (DM) content of the wheat straw was measured by drying the material in an oven at 105 °C until constant weight was obtained, and was found to be 90 %. The composition was determined using standardized analytical procedures from the National Renewable Energy Laboratory (NREL) [40 ], and is given in Table 1.Table 1

Composition of wheat straw expressed as percentage of dry matter (average and standard deviation of three measurements)

	Ave. content (%)	SD
Glucan	29.3	1.2
Xylan	21.6	1.9
Galactan	0.3	0.0
Arabinan	3.2	0.2
Mannan	0.1	0.1
Lignina	27.5	1.3

^aAcid-soluble and -insoluble lignin and lignin ash are included

The wheat straw was soaked for 1 h in warm tap water with 1 % (by weight) acetic acid solution at room temperature in sealed buckets. The ratio between the wheat straw and the liquid was 1:20 by weight. After 1 h, the soaked material was dewatered to a DM content of 45–55 % (by weight) in a filter press (Tinkturenpressen HP5 M, Fischer Maschinen-fabrik GmbH, Burgkunstadt, Germany), and then stored at room temperature in sealed buckets overnight until pretreated. The impregnated material was steam pretreated in a steam pretreatment unit using a 10-L batch reactor described previously [8 (link)], at 190 °C for 10 min. The steam-pretreated slurry was thoroughly mixed and stored at 4 °C. The structural carbohydrates, lignin, and ash content in the water-insoluble solids and the sugars, by-products (acetic acid), and degradation products [furfural and HMF (5-hydroxymethyl-2-furaldehyde)] in the liquid fraction were determined using standardized NREL analytical procedures [40 , 41 ]. The total DM content was measured by drying the material in an oven at 105 °C until constant weight was obtained. The WIS content of the pretreated material was determined using the method developed by Weiss et al. [42 (link)]. The composition of the pretreated wheat straw is given in Table 2.Table 2

Composition of steam-pretreated wheat straw (average and SD of three measurements)

	Ave. content	SD
DM (%)	16.7	0.3
WIS (%)	11.9	0.4
Content in solid fraction (% WIS)
Glucan	44.7	1.5
Xylan	11.9	0.8
Lignina	32.0	0.9
Content in liquid fraction (g/L)
Glucoseb	6.5 (1.0)	0.0
Xyloseb	36.3 (9.9)	0.1
Acetic acid	4.6	0.0
Furfural	1.5	0.0
HMF	0.2	0.0

^aAcid-soluble and -insoluble lignin and lignin ash are included

^bBoth monomeric and oligomeric forms are included; concentration of monomeric sugars are in parenthesis

Most of the pretreatment slurry was separated into a liquid and a solid fraction using a filter press. The rest of the slurry was used for batch SSCF experiments. The liquid fraction was then filtered using a vacuum filtration unit to remove the particles from the liquid. The particles were mixed with the solid fraction. The WIS content of the solid fraction was 40 %, and both the liquid and solid fractions were stored at 4 °C.