Background

As one of the main agricultural wastes, corncob is produced in large quantities. It is considered to be an important raw material for the production of high-valued biochemicals and energy fuels. Like the common lignocellulosic biomass, corncob is mainly composed of three major components, namely, cellulose, hemicellulose, and lignin [1, 2]. Hemicellulose of biomass is more sensitive and easily to be hydrolyzed; however, it is often neglected, because it is difficult to obtain high value-added products from it [3]. Hemicellulose is a miscellaneous polysaccharides containing branched polymers. It contains three types, and 65–85% of hemicellulose is made up of xylan backbones [4].The hydrolysis products of xylan include low xylan fragments (DP > 6), xylo-oligosaccharides (XOSs, DP < 6), and xylose, and the health benefits of XOSs have been reporte, including lowering blood cholesterol, increasing calcium absorption, antioxidant effects, maintaining gastrointestinal health, and reducing the risk of colon cancer. Also, they have toxic effects on human leukemia cells, and are benefits for patients suffering from type 2 diabetes [5]. Currently, there are different ways to obtain XOSs from biomass. For example, acquiring XOSs from poplar [6,7,

Materials and methods

Materials

Compound cellulase was purchased from Novozymes (China) and used as received. Corncob acquired from Guangdong, China was ground to a particle size of 65 mesh by a grinder. Then, the ground raw corncob was air-dried (water content was 6.0 wt%) before use. Choline chloride (ChCl, 98%) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Macklin, Shanghai, China). Glycolic acid (98%), lactic acid (85%), guanidine hydrochloride (99%), ferrous chloride tetrahydrate (98%), aluminum chloride hexahydrate (99.99%), magnesium chloride hexahydrate (99.9%), glucose (99%), xylose (99%), and furfural (99%) were purchased from Aladdin Biochemical Technology Co., Ltd. (Aladdin, Shanghai, China). Xylobiose (98%) and Xylotriose (98%) were purchased from Shanghai Yuanye Bio-Technology Co., Ltd. (Yuanye, Shanghai, China). Xylotetraose (99%) and Xylopentaose (99%) were purchased from Shanghai ZZBIO Co., Ltd. (ZZBIO, Shanghai, China). Other chemicals were of the highest purity commercially available.

DES preparation

Glycolic acid (G), choline chloride (C), lactic acid (L), guanidine hydrochloride (G), ferrous chloride tetrahydrate, aluminum chloride hexahydrate, and magnesium chloride hexahydrate were dried under vacuum at 80 °C for 5 h before use. The DES were prepared by mixing glycolic acid with choline chloride (GC), lactic acid (GL), and guanidine hydrochloride (GG) in a molar ratio of 1:1or 3:1. For the metal inorganic salts contained system, 5% (w/w) ferrous chloride tetrahydrate and aluminum chloride hexahydrate were added to GC (3:1), GL (3:1), and GG (3:1), respectively. Magnesium chloride hexahydrate could only dissolve in GL (3:1). The mixture was heated and stirred at a certain temperature in a closed flask until a homogenous colorless solution was formed. The prepared DES were then stored in a vacuum drying oven before use.

Treatment of corncob with DES

Briefly, corncob samples were mixed with DES with a biomass loading of 5 wt%, and then the mixtures were stirred and kept at 120 °C for a specific time (10 min, 20 min, 40 min, 1 h, 2 h, 3 h, or 4 h). Once the treatment was completed, the residues were thoroughly washed with anhydrous ethanol and then water, and placed in a − 20 °C refrigerator for freeze-drying. After lyophilization, the samples were placed in a sealed bag and stored in a drying oven for the subsequent use.

Compositional analysis of the corncob samples

The cellulose, xylan, lignin, and ash contents of the corncob samples were determined according to the standard NREL analytical procedure, including acid hydrolysis, the subsequent HPLC, and gravimetric analysis [38], and these experiments were conducted in duplicate. The sugars were monitored using HPLC (Agilent 1260) equipped with a Bio-Rad Aminex HPX-87H column and a refractive index detector (Agilent 1260). The mobile phase was a 5 mM sulfuric acid aqueous solution, the flow rate was 0.5 mL/min, and the column and detector temperatures were 65 and 50 °C, respectively. The retention times for glucose and xylose were 11.4 and 12.2 min respectively.

Enzymatic hydrolysis of the corncob samples

Enzymatic hydrolysis was implemented by mixing 20 mg of biomass and 8.3 U mL−1 cellulase in 7 mL of a citrate buffer (50 mmol L−1, pH 4.8) with stirring (120 rpm) at 50 °C. Aliquot samples (300 μL) were extraction at specified time spaces and boiled for 5 min to quench the enzymatic reaction. After being filtrated through a 0.22 μm membrane, the glucose and xylose concentrations were detected using the HPLC, as described above. All reactions were performed in duplicate, and the related data showed as mean values with standard derivations. The polysaccharide digestibility were calculated as follows:

Polysaccharide digestibility (%) = (released sugar amount)/Theoretic sugar amount in the sample used for enzymatic hydrolysis × 100.

Fourier transform infrared spectroscopy (FT-IR) analysis

FT-IR patterns of corncob and DES to treat solids were tested from an FT-IR (Nexus Thermo Nicolet, USA). The samples for test were combined with KBr (1/10 mass ratio), evenly grinding both and to press into flakes with 7 MPa and 30 s. 32 background and scans were taken from 400 to 4000 cm−1.

Analysis of the total xylo-oligomers and XOSs (DP 2–5)

To collect the total xylo-oligomers, deionized water of a certain volume was added after the reaction completed, and the mixture was centrifuged at 10,000 g for 5 min. The supernatant was collected. The xylo-oligomers measurement was conducted according to the previous report [18]. Briefly, the supernatant was obtained by centrifugation. Then, the supernatant was hydrolyzed with 4% H2SO4 at 121 °C for 1 h. The total xylo-oligomers yield was calculated based on the discrepancy between the content of xylose before and after hydrolysis. The equation was showed as follows:

$${\text{Total xylo}} - {\text{oligomers yield }}\left( \% \right) \, = \, \left( {{\text{xylose amount after acidolysis }} - {\text{ xylose amount before acidolysis}}} \right) \, \times 0.{88 }/{\text{Xylan amount in the raw corncob}} \times {1}00.$$

XOSs (DP 2–5) were analyzed based on the previous report [39]. The XOSs (DP 2–5) were monitored using HPLC (Waters 1525) equipped with an Agilent Hi-Plex Na column and a refractive index detector. The mobile phase was water, the flow rate was 0.3 mL/min, and the column and detector temperatures were 80 °C and 50 °C, respectively. The equation was expressed as follows:

$${\text{XOSs }}\left( {{\text{DP 2}} - {5}} \right){\text{ yield }}\% \, = \, \left( {{\text{X2}} + {\text{X3}} + {\text{X4}} + {\text{X5 in liquor}}} \right) \, /{\text{Xylan amount in the raw corncob}} \times {1}00.$$

Mass balance of xylan

Furfural were analyzed based on our previous report [24]. The furfural was monitored using HPLC (Waters 2695) equipped with an Agilent C-18 column and a DAD detector at 280 nm. The mobile phase was acetonitrile/water (15/85, v/v), and the flow rate was 1 mL/min−1. The equation was expressed as follows:

$${\text{Conversion of Furfural }}\left( \% \right) \, = \, \left( {{\text{furfural amount in liquid}} \times {1}.{375}} \right) \, /{\text{Total xylan and arabinan amount in raw corncob}} \times {1}00$$
$${\text{Residual xylan }}\left( \% \right) \, = \, \left( {\text{xylan amount in corncob residues}} \right)/{\text{ Xylan amount in the raw corncob}} \times {1}00$$
$${\text{Xylan loss }}\left( \% \right) \, = { 1} - {\text{ Residual xylan }} - {\text{ Conversion }}\left( {{\text{xylo}} - {\text{oligomers}}} \right) \, - {\text{ Conversion }}\left( {{\text{xylose}}} \right) - {\text{ Conversion }}\left( {{\text{furfural}}} \right).$$

Kinetic study of xylan hydrolysis and xylo-oligomers accumulation

Kinetic models used in this study were referred as the previous report [16]. Briefly, DES of six kinds, GC (3:1), GL (3:1), GG (3:1), GL (3:1)/FeCl2·4H2O, GL (3:1)/AlCl3·6H2O, and GL (3:1) /MgCl2·6H2O were kept at 120℃ for 10 min, 20 min, 40 min, 1 h, 2 h, 3 h, and 4 h to treat corncob, and the content percentage of xylan in the residue and the concentration of xylo-oligomers in supernatant at different times were determined according to the above-mentioned methods. Origin 95 was used to fit the models. Xylan solubility and xylo-oligomers concentration were fitted according to the formula listed as follows:

$${\text{Xs}}\,{ = }\,{\text{H}}_{{\text{d}}} \,{ - }\,{\text{exp}}\left( {-{\text{k}}_{{1}} {\text{t}}} \right)$$
(1)
$${\text{C}}_{{{\text{XOS}}}} \,{ = }\,\frac{{{1}{\text{.136C}}_{{0}} {\text{k}}_{{1}} {\text{H}}_{{\text{d}}} }}{{{\text{k}}_{{2}} \,{ - }\,{\text{k}}_{{1}} }}\left[ {{\text{exp}}\left( {-{\text{k}}_{{1}} {\text{t}}} \right)-{\text{exp}}\left( {-{\text{k}}_{{2}} {\text{t}}} \right)} \right],$$
(2)

where Hd is the ‘potential degree of hydrolysis’ of xylan, and 0 ≤ Hd ≤ 1; C0 is the initial concentration of xylan in the pseudo-homogeneous system (mg/ml); Cxos is the concentration of the total xylo-oligomers in the treatment liquor (mg/mL); 1.136 is the conversion coefficient of xylan into xylose.