Simultaneous Saccharification and Fermentation (SSF)

Ethanol production from lignocellulose involves enzymatic hydrolysis and fermentation. SSF combines both steps. Swedish scientists Olofsson, Bertilsson and Liden found process conditions for this pathway. Below are the process steps.

Pretreatment

Lignocellulose is prepared to increase cellulose hydrolysation rate and minimise inhibitor formation. For example, increase surface area available to enzyme lets the more reaction to occur at the same time. Feedstock characteristics determine the treatment type.
Agricultural residues benefit from:

• Ammonia fibre freeze explosion,
• Lime or calcium hydroxide,
• Alkali or sodium hydroxide, or
• Steam explosion.

Each process has its drawbacks such as equipment costs or by-product and inhibitor formation.

Enzymatic Hydrolysis

Cellulases break β-1-4-glycosidic glucan bonds. Aerobic filamentous fungi such as T. reesei produces cellulose system for industrial use.

Fermenting Microorganisms

The species requirements are:

• High ethanol yield
• High productivity
• Withstand high ethanol concentration

They compared alternative species with bakers yeast to mixed results.

• Xylose fermenting yeasts e.g. Candida shehata,
• Bacteria such as Zymomonas mobilis and E. coli.
• Engineered S. cerevisiae and E. coli.

Bakers Yeast Characteristics	Value
Yield	0.45 g g-1
Specific Growth Rate	1.3 g g-1 cell mass h-1
Ethanol Tolerance	100 g L-1

Process Optimisation

The scientist manipulated the following conditions:

• Substrate loading
• Enzyme loading
• Yeast loading
• Co-fermentation
• Temperature
• Inhibitors

They proposed changes to find better pentose fermenting yeasts, enzyme combinations.

Reference

Olofsson, K., Bertilsson, M & Liden, G. (2008). A short review on SSF – an interesting process option for ethanol production from lignocellulosic feedstocks. Biotechnology for Biofuels, 1(7). DOI:10.1186/1754-6834-1-7