3.3.2. Analysis of the Lignins

The effect of steaming severity on the molar mass of the precipitated lignins is illustrated in Figure 7. It becomes evident that the molar mass of the lignins increased gradually with intensifying the steam refining conditions. Furthermore, the main peak of the molar mass distribution is visible between 6800 and 7700 g/mol (Figure 7).

At a severity of 3.65 (190 ◦C/10 min), components with low molar masses (100–250 g/mol) are visible in the distribution curves. They are significantly reduced at severities above four. The increasing shoulder at high molar masses is a clear indication for condensation reactions at higher severities. Thus, the SEC results confirm the assumption that lignin condensation occurs to a higher extent under harsh steaming conditions.

For considerations on the utilization potential of the extracted lignins, knowledge of the lignin characteristics and purity is essential. Therefore, their carbohydrate contents were determined after acid hydrolysis (Figure 8). With increasing severity, the carbohydrate impurities of lignin decrease. Nevertheless, after pretreatment at lower severities than 3.95, more carbohydrate impurities can occur in the alkaline extract. At severities above four, the carbohydrate content of lignins is negligible. For this finding, Schütt et al. [11] and Schütt [39] also report decreasing carbohydrate impurities with increasing severity for extracted lignins of steam-refined poplar wood.

**Figure 7.** Molar mass distribution, molecular weight (MW) and dispersity (D) of alkaline-extracted lignin.

Several authors discussed the alkaline extraction of steam-treated fibers and following the influence of lignin removal on EH. For the process of lignin extraction, Schütt et al. [11] and Schütt [39] reported higher molar masses and a higher dispersity of the extracted lignin with increasing severity. The influence of one or two steam explosion steps on the extraction behavior of the lignin and the influence of SO<sup>2</sup> as a catalyst were also investigated by Li et al. [51]. However, the reported findings are in good accordance with the illustrated results in the present study (Figures 7 and 8).

**Figure 8.** Effect of steaming severity on the carbohydrate content of alkaline-extracted lignins.
