**1. Introduction**

Corn stover is a well-studied agricultural residue of the corn kernel production [1]. It is highly available and a significant amount of the produced straw is undervalued and until now not harvested [2]. Reasons, therefore, are the prevention of soil erosion or the maintaining of soil organic carbon [2,3]. Nonetheless, corn stover is one of the most promising feedstock candidates for large-scale lignocellulose biorefineries [4].

Because of the chemical composition of lignocelluloses, containing high amounts of cellulose and hemicelluloses, the enzymatic hydrolysis (EH) with cellulases is a suitable way to obtain monomeric

carbohydrates for fermentation from the mentioned polysaccharides [3]. The hydrolysis of the glucoside linkage with cellulases (mostly from *T. reesei*) is highly specific to the β-(1→4) glycosidic bonds of the cellulose chain, resulting in high yields with nearly no undesired by-products formed in the EH. However, inhibitors are normally formed during pretreatment prior to EH.

The chemical composition of corn stover as a lignocellulosic feedstock involves also disadvantages, like the natural recalcitrance of the complex cellulose-hemicellulose-lignin-structure against microbial degradation and EH [5]. One important way to conquer this recalcitrance is the pretreatment of the raw material [6]. Over the years, different methods and applications have been invented and common classifications distinguish between biological, chemical, physical and (hydro)-thermal processes [6].

One additionally described category is the physicochemical pretreatment [7]. Well-described physicochemical pretreatments are steam pretreatments with and without (an acid) catalyst. They are reported as the most often investigated pretreatment methods and they combine the advantages of a simple process, low energy costs and no necessary recycling of process chemicals [7,8].

An alternative physicochemical pretreatment to the generally used steam explosion technique is steam refining [9–14]. Whereas the defibration of the fibers in steam explosion processes is achieved by pressure relief, some studies revealed that the explosion part of the steam explosion is nearly unnecessary for the enhancement of EH yields [15]. Therefore, the defibration can also be achieved by a refining step at the end of the steaming, then called the steam refining process by, e.g., Schütt et al. [12]. Nonetheless, in contrast to the defibration, the dimension of the used biomass particles has shown a much bigger impact on the results of the subsequent EH [12,15]. For steam-refined poplar wood, EH yields increase up to a severity of around four, which represents temperatures around 200 ◦C and holding times between 10 and 15 min. At severities higher than four the yields decline due to secondary carbohydrate degradation reactions [11]. Unfortunately, degradation of the pentoses to furfural and degradation of the hexoses to 5-hydroxymethylfurfural (5-HMF) occur in processes with increased temperature, like in steam processes. These components have an inhibitory effect on enzymes and it is essential to get knowledge on the best process conditions [16]. However, 5-HMF and furfural are platform chemicals by themselves and can be converted into several value-added products [17,18].

Today, industrial applications of cellulose systems are available at the commercial market for different purposes [19] and the use of nonfood raw material is of high interest. Further, the use of fermentation sugars and the production of platform chemicals from renewable resources are also highlighted in the literature, especially when the applied biorefinery approach is not subject to the food and fuel debate [20–22]. From this point of view, it is favorable to use undervalued agricultural residues, like corn stover, for the renewable production of highly valuable products.

Several authors describe the effect of the lignin content of the used raw material on the performance of the EH. It can be stated that lignin has a negative impact on the yields after EH [23]. Further, the formation of unproductive bindings between lignin and enzymes are described [24]. On the other hand, it was also reported that a nearly complete delignification below 5% lignin content of corn stover with acidified sodium chlorite results in a significant yield loss during the EH, whereas a softer dilute acid pretreatment improves the EH yields [25]. Suggested reasons are the aggregation of the cellulosic microfibrils after the near removal of lignin and xylan and a resulting decreased accessibility. Stücker et al. [13] report further about the utilization of alkaline-extracted poplar lignins in lignin–phenol–formaldehyde resins after a steam refining process. Due to the reported improvement of the enzymatic hydrolysis and the reported possibility of utilization, the influence of an alkaline lignin extraction should be tested for steam-refined corn stover, although alkaline treatment is reported as a preliminary treatment by itself [26].

The aim of the present study was to investigate the effect of different steaming severities on the EH of corn stover. The steamed fibers were subjected to EH with and without alkaline lignin extraction in order to differentiate the effect of lignin on the overall process balance. The degradation products of the carbohydrates were detected to get an overview of inhibitory compounds in the liquid fraction after the process. Furthermore, characterization of the extracted lignins was performed correlating steaming severities and lignin characteristics.
