The two most promising processes for industrial production of ethanol from cellulosic materials are two-stage dilute-acid hydrolysis (a chemi­cal process) and SSF (an enzymatic process). Advantages and disad­vantages of dilute-acid and enzymatic hydrolyses are summarized in Table 3.3. Enzymatic hydrolysis is carried out under mild conditions, whereas high temperature and low pH result in corrosive conditions for acid hydrolysis. While it is possible to obtain a cellulose hydrolysis of close to 100% by enzymatic hydrolysis after a pretreatment, it is diffi­cult to achieve such a high yield with acid hydrolysis. The yield of con­version of cellulose to sugar with dilute-acid hydrolysis is usually less than 60%. Furthermore, the previously mentioned inhibitory compounds are formed during acid hydrolysis, whereas this problem is not so severe for enzymatic hydrolysis. Acid hydrolysis conditions may destroy nutri­ents sensitive to acid and high temperature such as vitamins, which may introduce the process together with the lignocellulosic materials.

On the other hand, enzymatic hydrolysis has its own problems in comparison to dilute-acid hydrolysis. Hydrolysis for several days is nec­essary for enzymatic hydrolysis, whereas a few minutes are enough for acid hydrolysis. The prices of the enzymes are still very high, although a new development has claimed a 30-fold decrease in the price of cellulase.