## 7 Challenges and Opportunities  
The hydrothermal pre-treatment of lignocellulosic biomass comes with some challenges, for example: production of inhibitory compounds, toxification of the biomass, and production of 5-HMF and furfurals. Various studies show that xylo-oligomers and oligomer sugars can be removed by hydrolysis and fermentation of pentose [111], but other inhibitory compounds like weak acids, furan aldehydes and phenolic compounds are still inhibitory to enzymatic process and fermentation [112]. Over liming, i.e., pH adjustment by using an alkali, vacuum evaporation, sulphite addition, and adsorbent treatment are some of the methods stated by previous studies in order to remove volatile inhibitors [111]. Removal of phenolic compounds and giving higher yield of ethanol can be achieved by various chemical detoxification processes like addition of activated charcoal adsorbents and polymeric resin [113]. At the same time, these detoxification processes may also lead to high manufacturing cost, generation of waste, and fermentable sugar wastage [114; 115]. Bio-abatement methods, using lacase and peroxide as microbial enzymes are also used as detoxication against furan aldehydes, phenolic compounds, and weak acids. _C. ligniaria_ NRRL30616, an ascomyceete, was used as a mitigation bio-abatement against the inhibitors, mainly phenolics, formed during hydrothermal pre-treatment of corn stover slurries. More than 95% acetic acid and \(>\)50% HMF, phenolics, and furfurals were removed from the slurries using this technique and on the contrary, 16% higher yield of cellulose conversion was observed [114].  
One of the methods for detoxification is the evolutionary engineering of fermenting microorganisms to fight the inhibitors. This method is necessary as it requires no extra detoxification treatment technique, and is based on successive cultivation [116]. _S. cerevisiae_, an ethanol-producing microorganism, possesses an inborn tolerance to inhibitors like furans and phenolics, and it also convert them to less harmful compounds [117; 118]. HMF is reduced to 2,5-bis HMF and the furfurrals are reduced to furfural alcohols [119]. Several other studies show that adaptation of yeast to hydrolyses increases the ethanol production and microbial growth [119; 120; 121]. Also, in order to improve microbial performance in the presence of inhibitors, genetically or metabolically engineered of yeast strains can be used. For example, for the detoxification of phenolic compounds from hydrolyses, _S. cerevisiae_ mutants have been developed [122]. Along with this, strains of _S. cerevisiae_, which shows resistance to furan aldehydes, have been constructed by many oxidoreductases, such as alcohol dehydrogenases [119].  
The sources and component of enzyme determine the loss of cellulase activity due to non-productive adsorption of cellulases to lignin. Upon incubation of Cellic CTec2 with lignin, the major part of \(\beta\)-glucosidase was adsorbed to the lignin, while 50-60% of the initial cellobiohydrolase and endoglucanase remained in the enzyme supernatant [95]. The enzymes need to be engineered to become more resistant to adsorption to insoluble lignin. This is one of the solutions for overcoming enzymatic inhibition [64]. [PERSON] et al. [95] stated that adsorption of \(\beta\)-glucosidase to lignin is dependent on the pH and the salt ions concentration of the medium, thus indicating the presence of electrostatic interactions. So, by changing the surface charge, the adsorption of enzyme to lignin can be reduced.