## 2 Mineral Acid Fractionation
### Conditions and Yields
#### 2.3.1 Low Concentrations of Acid and High Temperatures
It is interesting to anticipate the yield of the following step of sugar fermentation based on this ratio and in this way to compare the efficiency of the fractionation conditions. However, different inhibitors have different inhibition thresholds depending on the fermentation enzymes or microorganisms used; for instance, furans usually decrease their activity at lower concentrations than acetic acid [46; 50]. Besides, the final concentration of monomeric sugars is highly dependent on the S:L ratio used for the dilute acid hydrolysis. Studies also presented the yield of monomeric sugars after dilute acid fractionation and enzymatic saccharification or the yield of ethanol after fermentation of the sugars [51; 52]. Comparison between the different fractionation processes is valid only if the same enzymatic saccharification or fermentation conditions are used, but as there is no standard process, this is rarely the case. Besides, some studies focused on the production of other molecules than ethanol from the enzymatic conversion of monomeric sugars such as hydrogen [49] or xylitol [53]. For these reasons, to compare the different acid fractionation processes, the yields of monomers obtained after the dilute acid hydrolysis (the production monomeric sugars over their total potential regarding the polysaccharides content in the initial biomass) appeared to be of interest. As purification is often required between the saccharification step and the valorization of the monomeric sugars, it would be valuable to look for the highest possible yields for the monomeric sugars after the fractionation step and not consider their purities or concentrations. The results of some studies are displayed in Table 1, with a focus on sugarcane bagasse (SCB) for lignocellulosic biomass and sulfuric acid for the acid used, for an easier comparison between the hydrolysis conditions. SCB is one of the most studied lignocellulosic biomass in the literature. It contains mainly three sugars--glucose, xylose, and arabinose--and its hemicelluloses contains a very low amount of glucose [47], so the quantified amount of glucose in the acid extract can be correlated to the hydrolysis of cellulose. Other biomasses are presented for comparison. When the results of the hydrolysis were expressed in g/L in the literature, they were converted by calculation to yield of monomeric sugar following Equations (2) and (3):  
\[\text{Y}_{\text{glucan}}=\frac{[\text{glucose}]}{C\ 1.11\ \text{r}_{\text{S:L}}} \tag{2}\]  
with [glucose], the concentration of glucose in the acid hydrolysate (g/L), C the cellulose content in the initial biomass (% dry solid(DS)), and r\({}_{\text{S:L}}\) the solid:liquid ratio of the hydrolysis (g/L). C has to be corrected by a factor of 1.11 to represent the initial potential in glucose of the biomass as a molecule of water is added to glucose during the hydrolysis of cellulose. \[\text{Y}_{\text{hemicelluloses}}=\frac{[\text{xylose}]+[\text{ arabinose}]}{\text{H}\ 1.14\ \text{r}_{\text{S:L}}} \tag{3}\]  
with [xylose] and [arabinose] the concentrations of xylose and arabinose in the acid hydrolysate (g/L), H the hemicelluloses (xylan and arabian) content in the initial biomass (%DS), and r\({}_{\text{S:L}}\) the solid:liquidratio of the hydrolysis (g/L). H has to be corrected by a factor of 1.14 to represent the initial potential in xylose and arabinose of the biomass, respectively, as a molecule of water is added to xylose and arabinose during the hydrolysis of hemicelluloses. Sulfuric acid was found to be the most efficient to yield monomeric sugars among all the acids tested (Table 1). The lignocellulosic biomass treated had a significant impact on the yield of monomeric sugars (Table 1). For instance, with the same fractionation conditions, glucose yield reached 33% for Dermudagrass, whereas it was 10% for rye straw [52].