Abstract
The objective was to investigate the applicability of UV-C technology to inactivate yeasts and bacteria in must and wine. Experiments were carried out in vintage 2016 with Riesling musts of different quality containing their natural microflora. Yeasts were tested more resistant to UV-C energy than bacteria. Saccharomyces cerevisiae showed higher tolerance against UV-C irradiation than Hanseniaspora uvarum facilitating new opportunities to control spontaneous fermentations. However, inactivation efficacy was strongly dependent on turbidity of musts and the initial degree of contamination suggesting a shadowing effect of individual germs. Compared with thermal pasteurization, UV-C treatment of must with 1 kJ/L showed similar effects in germ-reduction. While thermal pasteurization significantly decreased aroma precursors in musts, UV-C treatment did not change concentrations of glycosidically-bound C6-alcohols, monoterpenes and C13-norisoprenoids as shown by GC-MS analysis. Applying UV-C technology in wines, it was possible to irreversibly stop ongoing alcoholic fermentation indicating that UV-C treatment is capable to replace SO2 addition to produce wines with residual sugar. Besides inactivation power, UV-C is known for its ability to form powerful off-flavours such as methional or methanethiol. Sensory analysis revealed that the application of UV-C at doses < 2 kJ/L in must is uncritical. However, applying UV-C after alcoholic fermentation can result in rising concentrations of mercaptans already at doses < 1 kJ/L. In this context, compounds such as caftaric acid, riboflavin and dissolved oxygen are thought to positively contribute to the UV-induced formation of off-flavours in wine.
Highlights
The objective was to investigate the applicability of UV-C technology to inactivate yeasts and bacteria in must and wine
Four yeasts could be clearly identified in the fresh grape musts, in detail Saccharomyces cerevisiae, Hanseniaspora uvarum, Schizosaccharomyces pombe and Pichia fermentans
While 500 and 900 NTU are unrealistically high turbidity values for fermenting grape musts, these findings indicate that existing UV-C technology can still be improved towards higher efficacy
Summary
Vitis vinifera grapes of the variety Riesling were harvested in 2016 from the vineyards of the DLR Rheinpfalz, Neustadt, Germany. Harvesting by hand was conducted in three consecutive weeks on same sites obtaining grapes of different quality and different microflora. After harvesting, grapes were crushed and pressed obtaining musts which were further processed by means of sedimentation for clarification reasons (< 50 NTU). In order to investigate UV-C efficiency in turbid musts, grape lees were blended into the clarified c The Authors, published by EDP Sciences.
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