Thermostable enzymes are gaining wide industrial and biotechnical inter­est due to the fact that they are more stable and thus generally better suited for harsh process conditions. The concept of thermostability is, however, not very clear, and the thermostability is a relative term. The enzymatic activity is known to increase with increasing temperature up to the tem­perature where inactivation starts to occur [25]. Thermostability is usually defined as the retention of activity after heating at a chosen temperature for a prolonged period. The drawback is that it only measures how well an en­zyme tolerates high temperature and does not take into consideration the number of variables affecting this measurement. The most appropriate way to express thermostability is to measure the half-life of enzyme activity at elevated temperatures. Thermostable enzymes are produced both by ther­mophilic and mesophilic organisms. Although thermophilic microorganisms are a potential source for thermostable enzymes, the majority of industrial thermostable enzymes originate from mesophilic organisms. Thermophilic bacteria have, however, received considerable attention as sources of highly active and thermostable enzymes.

Thermostable enzymes in the hydrolysis of lignocellulosic materials have several potential advantages: higher specific activity (decreasing the amount of enzyme needed), higher stability (allowing elongated hydrolysis times) and increased flexibility for the process configurations. The two first character­istics would expectedly improve the overall performance of the enzymatic hydrolysis even at the range of conventional enzymes active at around 50 °C. Thus, carrying out the hydrolysis at higher temperature would ultimately lead to improved performance, i. e. decreased enzyme dosage and reduced hydro­lysis time and, thus, potentially decreased hydrolysis costs. Thermostable enzymes would expectedly also allow hydrolysis at higher consistency due to lower viscosity at elevated temperatures and allow more flexibility in the process configurations. The characteristics of thermostable cellulases are re­viewed in Table 1. The enzymes are categorised as endo — or exoglucanases, based on the information available.

Several hyperthermostable cellulolytic enzymes have been isolated from various thermophilic bacteria including the anaerobic Thermotoga [11,14,21], Anaerocellum thermophilum [82] and Rhodothermus strains [34]. Signifi­cant research efforts have been invested in the thermophilic bacterial cel — lulosome systems of Clostridia (reviewed by [17]). Concepts for the direct conversion of lignocellulose into ethanol using clostridial co-culture pro­cess have been studied [33]. In addition, thermostable ascomycete cellu — lases have been characterised [30,37,57]. Several mesophilic or moderately thermophilic fungal strains are also known to produce thermostable en­zymes. These enzymes are stable and active at temperatures that are essen­tially higher that the optimum temperatures for the growth of the micro­organism [65]. Some filamentous fungi produce cellulases that retain rela­tively high cellulose-degrading activity at elevated temperatures, particularly those from the species Talaromyces emersonii [27,50,78], Thermoascus au — rantiacus [26,59,70], Chaetomium thermophilum [48], Myceliophthora ther-

mophila [63], Thielavia terrestris and Corynascus thermophilus [45]. Ther­mophilic в-glucosidases have been obtained from e. g. Aureobasidium sp. [66], Chaetomium thermophila [79], Talaromyces emersonii [15], Thermoascus au — rantiacus [23,26,59,70] and Thermomyces lanuginosa [40]. The literature data shows that a number of enzymes are stable at temperatures around 70 °C for elongated periods, but the data does not allow comparison of the proper­ties under similar conditions.

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