Although a plethora of pretreatment methods have been developed in the recent years, very few could be applied in pre-commercial stage. It is therefore difficult to ascertain which method is the best in terms of its efficiency. The term ‘effi­ciency’ includes several criteria, and for a pretreatment method to be an efficient one, it should suffice all or partly. The key criteria for an efficient pretreatment technology and process are as follows (IEA 2008; Kumar et al. 2009):

• must reduce the crystallinity of cellulose and increase porosity of the material;

• increase the yields of both hexoses and pentoses in downstream processing;

• avoid the loss/degradation of sugars;

• recover lignin for further combustion;

• minimize the inhibitors of enzymatic action and fermentation;

• fungibility with different feedstocks;

• avoid expensive capital cost on biomass comminution;

• minimize waste products and use low-cost chemicals; and

• should have low overall capital cost with low energy requirement.

Currently, none of the pretreatment method is suitable for a range of biomass feed­stocks owing to their different degree of action and varying strengths and weak­nesses. Different feedstocks respond to each pretreatment method in a varying way, and it is difficult to find a single method for all feedstocks type. Presently, the dilute and concentrated acid, and steam explosion are very near to commercializa­tion, in spite of their high capital cost. As described in Table 3, each of the methods has its own limitations, which are inherent to the process and difficult to overcome. Therefore, a single pretreatment method cannot have the potential to commerciali­zation unless integrated/combined with other methods. It is imperative to conduct research on combined pretreatment methods to minimize the limitations and overall reduce the capital cost and improve the efficiency of hydrolysis. In fact, all the pre­treatment methods described in Table 3 are in the varying stages of R&D and require extensive trials before any of them reach the commercial viability.