Impact of particle size reduction on high gravity enzymatic hydrolysis of steam-exploded wheat straw

Quantitative correlation analysis between particle liquefaction and saccharification through dynamic changes of slurry rheological behavior and particle characteristics during high-solid enzymatic hydrolysis of sugarcane bagasse

This study identified the intrinsic relationships among slurry rheology, particle characteristics, and lignocellulosic liquefaction/saccharification based on correlation analysis and principal component analysis during the hydrolysis of sugarcane bagasse pretreated by deep eutectic solvents (DES) and mechanical milling (MM). The DES-MM pretreated lignocellulosic slurry (20% solids) exhibited high apparent viscosity of 1.4 × 104 Pa·s and shear stress of 929.0 Pa under steady state. Glucose production had a negative linear correlation with slurry viscosity (R2, 0.69-0.97), whereas its correlation with yield stress (R2, 0.85-0.98) depended on the particle liquefaction rate. The availability of free water provided a major contribution to improving slurry rheology. However, the size reduction of submillimeter particles and the changes in particle hydrophilicity during liquefaction were not significantly correlated with rheological changes. Various interrelated particle characteristics and rheological changes were integrated into two simple principal variables to predict glucose production with a high R2 of 0.96.

Investigation of stress tolerance of Pichia kudriavzevii for high gravity bioethanol production from steam-exploded wheat straw hydrolysate

This study investigated a newly isolated thermotolerant strain of Pichia kudriavzevii with respect to its stress tolerance and fermentation performance. Response surface methodology was applied to evaluate the combined effects of furfural, osmotic and thermal stress on ethanol yield. The proposed model shows that P. kudriavzevii has a natural resistance against multiple stress factors. Further evolutionary adaptation of the isolated strain in lignocellulosic hydrolysates improved the ethanol yield by ≥ 24 %. The adapted strain HYPK213_ELA was able to produce ethanol from wheat straw hydrolysates at a high solid loading of 37 %ww-1 at 40 °C and anaerobic conditions. The highest ethanol concentration of 56.8 ± 1.0 gL-1 was reached at 40°C with an inoculum size of 2.5 × 106cellsmL-1. The results show that Pichia kudriavzevii has the potential to enable high gravity bioethanol production under conditions where most yeast strains are unable to grow.

Mechanical pretreatment of lignocellulosic biomass toward enzymatic/fermentative valorization

Lignocellulosic biomass (LCB) has the potential to replace fossil fuels, thanks to the concept of biorefinery. This material is formed mainly by cellulose, lignin, and hemicellulose. To maximize the valorization potential of this material, LCB needs to be pretreated. Milling is always performed before any other treatments. It does not produce chemical change and improves the efficiency of the upcoming processes.

Additionally, it makes LCB easier to handle and increases bulk density and transfer phenomena of the next pretreatment step. However, this treatment is energy consuming, so it needs to be optimized. Several mills can be used, and the equipment selection depends on the characteristics of the material, the final size required, and the operational regime: continuous or batch. Among them, ball, knife, and hammer mills are the most used at the laboratory scale, especially before enzymatic or fermentative treatments. The continuous operational regime (knife and hammer mill) allows us to work with high volumes of raw material and can continuously reduce particle size, unlike the batch operating regime (ball mill).

This review recollects the information about the application of these machines, the effect on particle size, and subsequent treatments. On the one hand, ball milling reduced particle size the most; on the other hand, hammer and knife milling consumed less energy. Furthermore, the latter reached a small final particle size (units of millimeters) suitable for valorization.