*3.2. Fractions after Steam Refining and Characterization*

## 3.2.1. Fiber and Extract Yields

Steam refining was conducted in a temperature range from 160 ◦C to 210 ◦C with severities from 2.77 to 4.54 (Table 1). To check the statistical scattering, a triplicate at the same severity was tested before. From this previous experiment is known that the standard deviation for the fiber and extract yields is below ± 1%. After steaming, the solid fiber fraction and the liquid extract fraction were separated, and the yields were calculated after measuring the moisture content of the fiber fraction and the solid content of the extract fraction (Figure 2).

**Figure 2.** Fiber, extract and total yields after steam refining at different severities.

As illustrated in Figure 2 there is a clear tendency for the fiber, extract and total yield with increasing severity of the steam refining. While the fiber fraction yield decreases steadily with enhanced severity, the extract fraction yield shows a maximum at a severity around four. After that point, the extract yield decreases due to degradation reactions of the carbohydrates at severities higher than four. The total yield decreases continuously and at a severity of 4.54, it falls below 75% based on raw material input. The reduced recovery with increasing severity can be attributed to the formation of volatile components, which were not accounted for in the mass balance.

As stated for the fiber yield, extract yields and total yield, there is a decreasing tendency with increasing severity. For mild severity corn stover steam treatments for 10 min at 140 ◦C (log *<sup>R</sup>*<sup>0</sup> <sup>=</sup> 2.18) and 160 ◦C (log *<sup>R</sup>*<sup>0</sup> <sup>=</sup> 2.77), Takada et al. [38] report about slight decreasing yields, as shown in Figure 2. In the present study, only the extracts show a maximum yield at severities around 3.6. Schütt et al. [11] made similar findings for steam refining of poplar wood. In comparison, the fiber, extract and total yields are very similar; however, the described extract maximum occurs at higher severity.
