*4.5. Fermentability of the Pretreatment Liquids*

The inhibitory effects on *S. cerevisiae* by the pretreatment liquids were low except for A-205. Additionally, with regard to microbial inhibition, A-190 turned out to be a good initial step for the biorefining of wheat straw. The chemical analyses indicated that the

inhibitory impact on the fermentability of pretreatment liquid from A-205 corresponded with high values for TAC, total phenolics, TCAC, and furfural (Table 4). TAC has recently been found to be a useful indicator of inhibition in studies of hydrothermal pretreatment of sugarcane bagasse and Norway spruce [36]. TAC values are related to the values of total phenolics and furfural, both of which contribute to TAC. The most inhibitory (A-205) and second-most inhibitory (SA-190) pretreatment liquids also exhibited the highest and second-highest values for TAC, total phenolics, and furfural. Regarding TCAC, it is noteworthy that the values were always < 100 mM. Previous experiments with *S. cerevisiae* yeast indicate that inhibitory effects were achieved when the total concentration of aliphatic acids was above 100 mM [48]. Therefore, it is reasonable to assume that it was aromatic and heteroaromatic substances that caused the inhibitory effects rather than carboxylic acids. The absence or very low concentrations of newly discovered inhibitors such as formaldehyde and *p*-benzoquinone can tentatively be attributed to the relatively low lignin content of the raw material.
