OPTIMIZATION OF THE ALCOHOLIC FERMENTATION BY CORRELATING THE INITIAL SUGAR CONCENTRATION WITH THE INOCULUM SIZE OF YEASTS AND ASSIMILABLE NITROGEN REQUIREMENTS

It is known from the literature that the yeasts require assimilable nitrogen (YAN) in a certain dosage in order to ferment sugar and for the exact calculation of the nitrogen can be done by applying a certain equation derived from the Bisson and Butzke tables. This equation however does not take into account the yeast strain and its requirements for assimilable nitrogen, nor the possibilities of yeasts to ferment all the available sugar. Taking into account the high concentration of sugar which is more and more found in our grape musts in the later years the selection of yeast and the inoculum size is also important. We have used a 'Feteasca regala' must with various amounts of initial sugar and we have corrected the YAN in accordance with the Bisson and Butzke table for each sample. In order to optimize the fermentation, we tested 3 different yeast strains, used in quantities proportional to the initial sugar concentration. In this way the yeast quantity used is optimized in accordance to sugar fermentation requirements. We have observed that not all the yeasts are able to totally ferment the sugar in samples with 26-28° Brix, leaving in the same time unconsumed YAN. Therefore, the correction of nitrogen should not by applied only in accordance to the calculations, but should be also adapted and limited in the case of high sugar concentration musts. The moment of nitrogen correction, the yeast strain and the inoculum size are evaluated and discussed.