Research in ethanol has been targeted for the devel­opment of second-generation technology, including the strategy of SSF process, which combines in a single unit the cellulose enzymatic hydrolysis and the ethanol fermentation (Santos et al., 2010). In the SSF process, glucose released by cellulase action is directly converted to ethanol by the fermenting microorganisms, which al­leviates problems caused by the end product.

The consumption of glucose and the presence of ethanol in the culture medium would reduce the risk of undesired contamination by glucose-dependent or­ganisms. Recently, consolidated bioprocessing, which combines enzyme production, saccharification and fermentation in a single step, has gained recognition as a potential bioethanol production system, because the costs of capital investment, substance and other raw ma­terials, and utilities associated with enzyme production can be avoided using microorganisms with the capa­bility for efficient cellulose hydrolysis and ethanol pro­duction (Hasunuma and Kondo, 2012).

Recently, there are many reports that SSF is superior to the traditional saccharification and subsequent fermentation in the ethanol production because the SSF process can improve ethanol yields by removing end-product inhibition of saccharification process and decrease the enzyme loading. Moreover, SSF requires a single fermenter for the entire process and eliminates the need for separating reactors for saccharification and fermentation leading to reduce the investment cost (Boonsawang et al., 2012).

Difference between SHF and SSF is in an incipient step of their development. It is possible to note that a significant number of studies reported in Tables 3.2 and 3.3 are mak­ing a comparison between the two techniques, which shows that the SSF researches are trying to develop an effi­cient process to substitute the SHF method. On the other hand the starchy raw materials have a great use in SSF fermentation; this can be explained by the simplicity of this substrate compared to cellulosic (the efficiency of enzymatic hydrolysis is better) and the conditions of operation can be milder, facilitating the adaptation of a fermentation microorganism.

CONCLUDING REMARKS

As can be seen from the tables above, there is a growing interest in ethanol production from agroindus­trial residues of a variety of sources including grains, straws, stalks and husks such as cotton, barley, triticale, wheat, coffee, rice, canola, sugarcane and other fruits and vegetables. In terms of volume, lignocellulosic material is the predominant raw material for second — generation ethanol. However, the production costs associated with the use of lignocellulosic ethanol is high, making it necessary to develop an efficient process for hydrolysis and fermentation, where the use of simul­taneous saccharification and hydrolysis is seen a prom­ising technology, but there is also the necessity to genetically modify a microorganism to grow at high temperatures or obtain an enzyme to carry out the hy­drolysis at normal fermentation temperature. Low-cost biomass residues offer excellent perspective for large — scale application of ethanol.

Acknowledgments

The authors thank CAPES for the scholarships and SCIT-RS and CNPq for the financial support of this work.