Yeasts as Biocatalysts

When looking at a biological reaction (a reaction due to a microorganism) it appears clearly that it can take place in two different ways: – A system where the microorganisms are suspended in the medium (here considered as homogeneous): that is the case of the fermentations in winemaking or in brewing, for example. – A system where the microorganisms are not free in the medium but where they are attached to a support, giving a two-phase system. Such a system is illustrated in nature by biofilms, for example. In the controlled use of microorganisms in an industrial frame, like alcoholic fermentation, free cells of microorganisms were used first, surely because of tradition, which existed prior to knowledge of the biological nature of the reaction. In oenology, and also in the major part of industrial fermentation processes, this concept of free cells is largely pre-eminent. In some others fields of applications, in contrast, as early as 1960s the purpose was to attach the microorganisms and some industrial applications quickly followed the conceptual developments: electrodes with attached enzymes, bacterial beds in water treatment, adsorbed bacteria for vinegar production, etc. Thus it appears quite logical to ask: – Why should the microorganisms be attached? – How should it be done? – What interest and what applications are there for the immobilized yeasts in the food industry?