There is no single pretreatment method that can be called as "Best Method" for all type of feedstocks. It totally depends on from what prospective you want to call it best. Some methods are good in removing hemicellulose and lignin, and results in high sugar yield during hydrolysis but are no economically competent. Other methods that use less chemicals and energy, results in relatively low sugar yields. Another thing is type of biomass, e.g. methods using alkali solutions are in general good for biomass having high lignin content. Dilute acid pretreatment and AFEX processes are among the most thoroughly investigated pretreatments.

Two step acid hydrolysis can result in almost complete hydrolysis of cellulose but process require very high amount of energy (because of severe operating conditions) and require corrosion resistant equipment. Because of these reasons, enzymatic hydrolysis is preferred over acid hydrolysis.

You can refer some papers published by our group:

- Kumar, D. and Murthy, G.S. (2011) Impact of pretreatment and downstream processing technologies on economics and energy use in cellulosic ethanol production. Biotechnology for Biofuels 4:27

- Kumar, D. and Murthy, G.S. (2012) Life cycle assessment of energy and GHG emissions during ethanol production from grass straws using various pretreatment processes. The International Journal of Life Cycle Assessment, 17(4): 388-401

- Kumar, D. and Murthy, G.S. (2011) Pretreatment and enzymatic hydrolysis of grass straws for ethanol production in Pacific Northwest U.S. Biological Engineering, 3(2): 97-110

in my opinion for the pretreatment/ hydrolysis the most suitable method is two step acid hydrolysis. in which first step is to solublize the carbohydrates present in lignocellulose by using concentrated acid at room temperature and second step is to hydrolyze the sugars at lower acid conc on elevated temperature. the acid can also be recovered very efficiently with the advanced separation technologies like SMB, and can be reused.

There is no single pretreatment method that can be called as "Best Method" for all type of feedstocks. It totally depends on from what prospective you want to call it best. Some methods are good in removing hemicellulose and lignin, and results in high sugar yield during hydrolysis but are no economically competent. Other methods that use less chemicals and energy, results in relatively low sugar yields. Another thing is type of biomass, e.g. methods using alkali solutions are in general good for biomass having high lignin content. Dilute acid pretreatment and AFEX processes are among the most thoroughly investigated pretreatments.

Two step acid hydrolysis can result in almost complete hydrolysis of cellulose but process require very high amount of energy (because of severe operating conditions) and require corrosion resistant equipment. Because of these reasons, enzymatic hydrolysis is preferred over acid hydrolysis.

You can refer some papers published by our group:

- Kumar, D. and Murthy, G.S. (2011) Impact of pretreatment and downstream processing technologies on economics and energy use in cellulosic ethanol production. Biotechnology for Biofuels 4:27

- Kumar, D. and Murthy, G.S. (2012) Life cycle assessment of energy and GHG emissions during ethanol production from grass straws using various pretreatment processes. The International Journal of Life Cycle Assessment, 17(4): 388-401

- Kumar, D. and Murthy, G.S. (2011) Pretreatment and enzymatic hydrolysis of grass straws for ethanol production in Pacific Northwest U.S. Biological Engineering, 3(2): 97-110

Perhaps you would like to read some of this articles we have published:

1. Téllez-Luis, S.J., Ramírez, J.A. and Vázquez, M. 2002. Modelling of the hydrolysis of sorghum straw at atmospheric pressure. Journal of Science of Food and Agriculture. 82(5): 505-512.

2. Téllez-Luis, S.J., Ramírez, J.A. and Vázquez, M. 2002. Mathematical modelling of hemicellulosic sugar production from sorghum straw. Journal of Food Engineering. 52(3): 285-291.

3. Aguilar, R, Ramírez, J.A., Garrote, G and Vázquez, M. 2002. Kinetic study of the acid hydrolysis of sugar cane baggase. Journal of Food Engineering. 55(4):309-318.

4. Herrera, A., Téllez-Luis, S.J., Ramírez, J. A., Vázquez, M. 2003. Production of xylose from sorghum straw using hydrochloric acid. Journal of Cereal Science. 37(3): 267-274.

5. Bustos, G., Ramírez, J.A., Garrote, G., & Vázquez, M. 2003. Modeling of the hydrolysis of sugar cane bagasse with hydrochloric acid. Applied Biochemistry and Biotechnology. 104(1): 51-68.

6. Chong, A., Ramírez, J.A., Garrote, G. and Vázquez, M. 2004. Hydrolisis of sugar cane bagasse using nitric acid, a kinetic assessment. Journal of Food Engineering. 61(1): 143-152.

7. Herrera, A., González-Cabriales, J.J., Ramírez, J.A. and Vázquez, M. 2004. Effect of the hydrochloric acid concentration on the hydrolysis of sorghum straw at atmospheric pressure. Journal of Food Engineering. 63(1): 103-109.

8. Gámez, S., Ramírez, J.A., Garrote, Gil. and Vázquez, M. 2004. Manufacture for fermentable sugar solutions from sugar cane baggase hydrolysed with phosphoric acid at atmospheric pressure. Journal of Agricultural and Food Chemistry. 52(13): 4172-4177.

9. Uresti, R. M., Velazquez, G., Ramírez, J. A., Torres, J. A., 2004. Effect of high pressure treatments on mechanical and functional properties of restructured products from arrowtooth flounder (Atheresthes stomias). Journal of the Science of Food and Agriculture. 84 (13):1741-1749.

10. Gámez, S., González-Cabriales, J. J., Ramírez, J. A., Garrote, G. & Vázquez, M. 2006. Study of the hydrolysis of sugar cane bagasse using phosphoric acid. Journal of Food Engineering 74 (2006) 78–88.

Most of researches prefer sulphuric acid because is easy to remove in lab conditions, but I have observed an increasing interest in phosphoric acid because residual phosphate might be used by microorganisms.