IMMOBILIZED YEASTS: ACTUAL OENOLOGICAL UTILIZATIONS

The biological reactions observation (realized by a micro-organism) puts in evidence that these reactions can occur in two different systems: -A system in which the micro-organisms are in suspension in the liquid phase (which seems to be homogeneous): it is the example of the fermentations in oenologywine-making or in bread making. -A system in which the micro-organisms are not free, but appear fixed to a support thereby creating a system of two distinct phases: the depuration procedure with bacterial beds may be considered an example. In oenology, traditionally, the concept of free micro-organism largely dominates. On the other hand, in other areas we have since the 60’s, tried to fix the “catalyzers” of the reaction and industrial applications of this process soon have been developed: adsorbed bacteria’s involved in vinegar production, bacterial discs to depuration processes… Before developing some oenological applications with immobilized micro-organisms, we will remember which the interest of such a process is and define which different existing technical possibilities are. Why should we immobilize the micro-organisms? The immobilization (or confinement) of micro-organisms have several advantages: – The micro-organisms can be reused after a working/functioning period (see the application in the desacidification of the musts). – The micro-organisms are not dispersed in a medium turning easier the recuperation at the end of culture (see the application in the foaming process) It is possible to activate an important microbial population without the growing phase, which allows us increasing the reaction speeds (see in the application in the treatment of stuck fermentations) How can we immobilize micro-organisms? The immobilization or confining of micro-organisms can be done through different methods, which had been mostly proposed to the enzymes by Chibata (1979). -The inclusion: in this method the microbial cells are embodied in a rigid polymer matrix; the most used is the calcium alginate (Margaritis et Merchant, 1984). This procedure (adapted to the realization of the double layer dry spheres) will be later developed. -The adsorption: in this method the fixation of micro-organisms to the support is related to a weak connection between the microbial cell walls and the support (wood, pozzolana…) One of the major inconveniences of this technique is the disadsorption risk, for example, during the death of the micro-organisms. -The retention of micro-organisms without support. These terminologies rejoin: – Systems that are a result of the spontaneous agglomeration (or provoked) of microbial cells. The capacity to flocculation of certain strain yeasts was, for example, rehearsed in the second fermentation of sparkling wines. – The procedures that aim in confining the micro-organisms in a part or the reactor through a physical barrier, generally a micro filtration membrane. In oenology, the interesting realization was the cartage “Milispark” developed by Milipore to the second fermentation of sparkling wine. (Lemonnier et Duteurtre,1989). In this work, we will only present results with importance at the winery level. They originate from works done since more than ten years ago, both by the (UMR CNRS 5503) laboratory and by Proenol, Lda (Oporto). These realizations refer to the “prise de mousse” both in the traditional and ancestral methods, the desacidification of the musts or wines by the Schizosaccharomyces pombe, the treatment of stuck fermentations and the control of sweet fortified wine fermentation.