Any substance at a temperature and pressure above its thermodynamic critical point will become supercritical fluid, which can diffuse through solids like a gas and dissolve materials like a liquid (Hawthorne 1990). Additionally, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties to be adjusted. Supercritical fluids may be suitable as a substitute for organic solvents in a range of industrial and laboratory processes.

Persson et al. (2002b) performed countercurrent flow supercritical CO2 (200 bar and 40oC) extraction of hydrolysates. A reduction in the concentration of a variety of inhibi­tors, such as furan derivatives, aliphatic acid, and phenolic compounds, were observed. The effect of the SFE treatment was examined with respect to alcoholic fermentation by Saccharomyces cerevisiae. The ethanol yield increased from 0.30 to 0.43 g/g glucose, and its productivity from 0.14 to 0.46g/Lh. SFE has several advantages such as clean­ness, biocompatibility, and high concentration factor. But the capital cost for SFE is usually high.

Encapsulation

With cell encapsulation, fermenting yeasts are protected by an artificial membrane, and successful fermentation with toxic hydrolysates has been reported (Talebnia et al. 2005) . Talebnia et al. (2005) used encapsulated S. cerevisiae CBS 8066 to ferment two different types of dilute-acid hydrolysates in the presence of furfural (0.39 and 0.31 g/L) and hydroxy — methylfurfural (HMF; 0.74 g/L and 1.58g/L). While the free cells were not able to ferment the hydrolysates in 24 hours, the encapsulated yeast successfully converted glucose and mannose in the hydrolysates in less than 10 hours with minimal lag phase. Talebnia and Taherzadeh (2006) further demonstrated that encapsulation is a promising method to keep the cells viable in a toxic environment and help the process to run continuously at high dilution rates and high productivities. The physiological and morphological characteristics of the encapsulated S. cerevisiae CBS 8066 were studied by Talebnia and Taherzadeh et al. (2007). After 20 consecutive batch cultivations in a defined synthetic medium, the ethanol yield increased from 0.43 to 0.46 g/g, while the biomass and glycerol yields decreased by 58% and 23%, respectively. The growth rate of the encapsulated cells in the first batch was 0.13/hour, but decreased gradually to 0.01/hour. After long-term application, most of the encapsulated yeast existed in the form of single and non-budding cells. Total RNA content of these yeast cells decreased by 39%, while the total protein content decreased by 24%. On the other hand, the stored carbohydrates (glycogen and trehalose) content increased. Because of the higher biomass concentrations inside capsules, the glucose dif­fusion rate through the membrane drastically decreased to 1/5 of that seen in cell-free capsules.

Molecular Sieve

Molecular sieves are used as adsorbents for gases and liquids. Molecular sieves have tiny pores of a precise and uniform size. Molecules that are small enough to pass through these pores will be adsorbed, while larger molecules are not. For instance, Tran and Chambers (1986) treated unfermentable red oak hydrolysates with a molecular sieve. The treatment with the molecular sieve decreased the concentration of acetic acid by 40% and furfural, by 82%. Treatment of hydrolysates with molecular sieve, however, resulted in a 10% loss in xylose concentration.