Because of diminishing crude oil resources and increasing fuel
costs in recent years, ethanol has re-emerged as an alternative to
petrochemical-based liquid fuels. Ethanol is a renewable and clean
fuel produced using biomass as the raw material. It is by far the
most widely used biofuel for transportation worldwide [1]. In
Thailand, gasohols E10 and E20 (with mixtures of 10% and 20% ethanol,
respectively, in gasoline) have been widely used in vehicles,
and there are attempts underway to promote the use of E85 in
vehicles in the near future. According to the report on net renewable
energy analysis in Thailand, a positive net energy of 5.95 MJ/L
was obtained from the molasses-based ethanol system [2].
Traditional ethanol industries produce ethanol by batch or fedbatch
processes. Cell immobilization in continuous fermentation
has been proposed as an effective means of improving the ethanol
production rate. Cell immobilization could lead to a higher biomass
concentration and enhanced biological stability. The protection of
immobilized cells against toxicity has been reported previously
[3,4]. Almost all cell immobilization methods are based on adsorption
or entrapment. Gel entrapment methods usually involve the
problems of gel degradation and limitations of oxygen, nutrient
and metabolite mass transfer. In contrast, natural adsorption is generally
a simple and inexpensive procedure for cell immobilization
without internal mass transfer limitations. Moreover, at the end