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The thermotolerant ethanol-producing yeast strain Kluyveromyces marxianus IMB3 was immobilized in calcium alginate and a 1:1 mixture of calcium alginate and the porous volcanic mineral, kissiris. Immobilized preparations were placed in ®xed-bed column bioreactors and continuous ethanol production by systems containing both immobilized preparations was examined at 45°C with a 100 g/l glucose feed. The effect of residence time on product concentration, bioreactor ef®ciency and volumetric productivities have been examined and these were all higher in systems containing the alginate/kissiris mixed immobilization matrix. Maximum ethanol concentrations produced by the continuous system ranged between 46 and 48 g/l representing ef®ciencies of 90±94%. IntroductionThe thermotolerant yeast strain, Kluyveromyces marxianus IMB3, has been shown to produce ethanol during growth on carbohydrate sources such as glucose [1], sucrose [2] and lactose [3] at 45 o C. As a result of its ability to perform this function at relatively high temperatures this yeast is ideally suited for use in simultaneous sacchari®-cation and fermentation (SSF) processes and it has been shown to be capable of ethanol production on substrates such as starch and cellulose [4,5]. In addition to the above advantage associated with the ability of this yeast to function at 45°C or indeed higher, other advantages include a reduced risk of bioreactor contamination and increased ef®ciency with respect to product formation and recovery. In a series of recent studies our group has shown that it is possible to immobilize this yeast in alginate-based matrices and produce ethanol in batch-fed con®gurations from sugars such as glucose, sucrose and lactose [6,7,8]. It has also been demonstrated that these immobilized preparations may be incorporated into simultaneous sacchari®cation and fermentation processes concerned with the conversion of cellulosic materials to ethanol [9]. In many of the above studies the immobilized biocatalyst preparations were much more stable than the free microorganism, particularly over prolonged periods of time. In addition, higher ®nal concentrations of ethanol were produced by the immobilized preparations over longer periods of time in continuous batch-fed systems [8]. Although a variety of con®gurations involving semi-continuous batch-fed systems have been described for ethanol production by K. marxianus IMB3, the major objective of our group has been to apply continuous-¯ow technology to ethanol production by this thermotolerant microorganism.Although alginate is a commonly used immobilization matrix in laboratory studies [10] and provides an ideal and acceptable means of studying a variety of model systems at that level, the economic viability of any ethanol production process would depend heavily on biocatalyst preparation costs. Reducing the quantity of alginate in alginate-based immobilization biocatalyst formulations would be expected to contribute positively to the economics of bioconversion. In recently reported studies it has bee...

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