Analysis of liquid and solid phases from autohydrolysis
In order to understand more easily the effect of autohydrolysis in barley straw, it comes of interest to analyze the transformations that each one of the fractions that constitutes barley straw suffers. Regarding the glucan, it can be found in glucan form in the solid phase and in form of glucooligomers, PCI34051 or hydroxymethylfurfural in the liquid phase. Around the 90% is kept in the solid phase, regardless of the operating conditions, what probably corresponds to cellulose. This assumption is supported by the fact that in the liquid phase there is an almost constant amount of glucooligomers (33 ± 2 g kg−1, corresponding to 9–10% of the initial glucan) which already appears under the lowest severities, what can indicate that they come from any hemicellulosic polymer such as xyloglucan (Barton and Fincher, 2012). These oligomers are broken down with increasing severity, reducing somewhat their quantities and leading to the formation of glucose and hydroxymethylfurfural. The addition of glucooligomers + glucose + hydroxymethylfurfural is constant, with a joint value of 37.2 ± 0.7 g kg−1, (corresponding to 10.3–10.9% of the initial glucan). The main hemicellulosic components are xylan, arabinan and acetyl groups, and they become highly altered because of this treatment: the usual performance is the following: the xylan is solubilized in xylooligomers form, which splits in xylose and this one can dehydrate at the same time producing furfural. For its part, arabinan is solubilized in form of arabinooligomers which becomes separated in order to generate arabinose which also can be dehydrated to furfural form. Acetyl groups can be linked to the hemicelluloses in the solid phase, to the solubilized hemicelluloses (named as acetyl groups linked to oligomers) or in form of acetic acid in the liquid phase. As it was previously mentioned, xylan, arabinan and acetyl groups were solubilized in high percentages (between 8%, arabinan, and 22%, xylan). Achieving the maximum amounts of xylooligomers (more than the 50% of the initial xylan in oligomeric form) and acetyl groups linked to oligomers (more than the 20% of the initial acetyl groups) at severities of about 3.5–4.1, but maximum values of arabinoxylan are found at lower severities of about So = 3.3–3.6. It is also remarkable the relatively low amount of xylose and arabinose. With regards to lignin content, it decreases from a 95% of the initial lignin under the lowest severity down to 86% (experiment carried out at So = 3.61) and then increases up to 96% at the highest severity. This variation can indicate a partial lignin repolymerization, that was also observed by other researchers in treatments of acid hydrolysis ( Cao et al., 2012) or autohydrolysis ( Long et al., 2014), or formation and precipitation of insoluble compounds. Other compounds represent a heterogeneous group containing extracts, proteins and other non determined substances. Their content in the solid decreases continuously, from 185.9 g kg−1 in the raw material to a range between 74.9 g kg−1 (40% of the initial other compounds, at the lowest severity) and 15.6 g kg−1 (8% of the initial other compounds, at the highest severity). In the liquid phase, other compounds present a maximum value of 142.7 g kg−1 in the experiment performed at So = 3.91, and then decreases. The addition of other compounds in the liquid and the solid phase represents an average of a 94% of the ones present in the raw material (with a value of 99.7% in the experiments carried out at So < 4.0).