SSF combines enzymatic hydrolysis of cellulose and fermentation in one step. As cellulose converts to glucose, a fermenting microorganism is presented in the medium and it immediately consumes the glucose produced. As mentioned, cellobiose and glucose significantly decrease the activity of cellulase. SSF gives higher reported ethanol yields and requires lower amounts of enzyme, because end-product inhibition from cellobiose and glucose formed during enzymatic hydrolysis is relieved by the yeast fermentation. SSF has the following advantages compared to SHF:

■ Fewer vessels are required for SSF, in comparison to SHF, resulting in capital cost savings.

■ Less contamination during enzymatic hydrolysis, since the presence of ethanol reduces the possibility of contamination.

■ Higher yield of ethanol.

■ Lower enzyme-loading requirement.

On the other hand, SSF has the following drawbacks compared to SHF:

■ SSF requires that enzyme and culture conditions be compatible with respect to pH and temperature. In particular, the difference between optimum temperatures of the hydrolyzing enzymes and fermenting microorganisms is usually problematic. Trichoderma reesei cellulases, which constitute the most active preparations, have optimal activity between 450C and 50OC, whereas S. cerevisiae has an optimum tem­perature between 30OC and 350C. The optimal temperature for SSF is around 38oC, which is a compromise between the optimal temper­atures for hydrolysis and fermentation. Hydrolysis is usually the rate-limiting process in SSF [27]. Several thermotolerant yeasts (e. g., C. acidothermophilum and K. marxianus) and bacteria have been used in SSF to raise the temperature close to the optimal hydrolysis temperature.

■ Cellulase is inhibited by ethanol. For instance, at 30 g/L ethanol, the enzyme activity was reduced by 25% [2]. Ethanol inhibition may be a limiting factor in production of high ethanol concentration. However, there has been less attention to ethanol inhibition of cellulase, since practically it is not possible to work with very high substrate con­centration in SSF, because of the problem with mechanical mixing.

■ Another problem arises from the fact that most microorganisms used for converting cellulosic feedstock cannot utilize xylose, a hemicellu- lose hydrolysis product [8].