Natural sweet wine production by repeated use of yeast cells immobilized on Penicillium chrysogenum

The production of sweet wines from sun-dried grapes is difficult because yeasts are affected by a hyperosmotic stress due to the high sugar concentration, leading to altered metabolism and growth. The aim of this study is to improve and simplify this process by means of a repeated batch fermentation system using Saccharomyces cerevisiae immobilized on spheres of Penicillium chrysogenum. Successive reuse of the immobilized yeasts revealed its gradual adaptation to the fermentation conditions and an increasingly uniform behaviour in terms of fermentation kinetics and production of metabolites. Immobilized yeasts produced higher concentration of carbonyl compounds, esters and polyols than free yeast, and the opposite was true for major alcohols. Nitrogen compounds were depending on the state of cells (free or immobilized) and also on the number of time the yeasts were used. Using this immobilization system might provide some advantages as to obtain the desired ethanol level by easier removal of yeast cells from the medium or to reduce the production costs in the inocula preparation. The operational stability of the immobilization system proposed might enable its use at the commercial scale for production of sweet wine.