Grape must fermentation by the combination of immobilized Candida stellata cells and Saccharomyces cerevisiae was carried out in order to enhance the analytical profiles of wine. Batch and continuous pre-treatment of must with immobilized C. stellata cells, followed by an inoculum of S. Cerevisiae, enhanced the analytical profiles of fermentates. The metabolic interactions between the two yeast species showed a positive influence on reducing sugars, acetaldehyde and acetoin metabolism. Sequential fermentation was the best combination for improving the analytical profiles of wine but caused a loss of viability and metabolic activity of beads by limiting their successive use. Continuous pre-treatment of must on the beads of C. stellata could be a more interesting modality to improve the quality of wines. This biotechnological process could be profitably used to produce specific and special wines.

Screening tests carried out for 10 strains of Candida stellata confirmed high levels of glycerol production, although a low fermentation rate and reduced ethanol content were observed. To overcome the poor competition with Saccharomyces cerevisiae, fermentation tests with immobilized C. stellata cells, alone or in combination with S. cerevisiae, have been carried out. The immobilization of C. stellata cells consistently reduced the fermentation length when compared with that obtained with free cells, immobilized cells exhibiting about a 30and a 2-fold improvement in fermentation rate compared with rates for C. stellata and S. cerevisiae free cells, respectively. Moreover, immobilized C. stellata cells produced a twofold increase in ethanol content and a strong reduction in acetaldehyde and acetoin production in comparison with levels for free cells. The evaluation of different combinations of C. stellata immobilized cells and S. cerevisiae showed interesting results with regard to analytical profiles for practical application in wine making. In fact, analytical profiles of combinations showed, apart from a high glycerol content, a reduction in the amounts of acetic acid and higher alcohols and a consistent increase in succinic acid content in comparison with values for the S. cerevisiae control strain. Sequential fermentation first with immobilized C. stellata cells and then after 3 days with an added inoculum of S. cerevisiae free cells was the best combination, producing 15.10 g of glycerol per liter, i.e., 136% more than the S. cerevisiae control strain produced. Fermentation with immobilized C. stellata cells could be an interesting process by which to enhance glycerol content in wine.

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:Since the large occurrence of Brettanomyces yeasts in strict anaerobiosis environments (sparkling wines) has been found without an increase in acetic acid content, we evaluated the inÑuence of the oxygen concentration on acetic acid production. Results showed that the oxygen concentration exerted a strong inÑuence on both growth and acetic acid production by Brettanomyces yeasts in winemaking. Full aerobiosis lead to a large production of acetic acid causing a block of metabolic activity. Semi-aerobiosis resulted in the best condition for alcoholic fermentation (Custers e †ect) combined with acetic acid production. In anaerobic condition Brettanomyces yeasts did not result in high acetic acid production and a pure, even if slow, alcoholic fermentation occurred. The absence of an increase in acetic acid in wines, does not exclude the active presence of Brettanomyces yeast since the characteristic "high acetic producerÏ in Brettanomyces yeast is linked to the presence of oxygen.

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