The aim of the study was to investigate the efficacy of UV-C treatment of grape musts for inactivation of wine-related microorganisms. In addition the effect on the physicochemical, chemical and sensory characteristics of wine was investigated, as well as the mutagenic potential of UV-C treated grape must.
- Riesling, Müller-Thurgau, Pinot blanc and Pinot noir were harvested during 2014, 2015 and 2016 in the Rheinpfalz.
- Musts were clarified to below 50 NTU and pasteurized.
- A portion of Riesling was not pasteurized and further subdivided into clarified (below 50 NTU) and a turbid fraction.
- Musts were inoculated with Saccharomyces cerevisiae, Metschnikowia pulcherrima, Pichia fermentans, Candida sp., Hanseniaspora uvarum and Schizosaccharomyces pombe at 106 CFU/mL.
- Grape musts were subjected to different amounts of UV-C treatment at 10°C.
- Physicochemical determination, oxygen measurement and mutagenicity measurements were done.
- Wines were made from the freshly pressed Riesling to assess the impact of UV-C treatment on chemical and sensory properties. The must was fermented with Oenoferm Riesling at 16°C.
- Various analytical methods were used to evaluate chemical parameters.
- Sensory analysis was performed by Descriptive Analysis with 21 panellists experienced in wine tasting.
- Metschnikowia pulcherrima had the highest resistance to UV-C treatment followed by P. fermentans and Candida sp. S. cerevisiae and H. uvarum were the most vulnerable. The researchers hypothesize that these differences can be due to differential DNA repair and protection mechanisms between yeasts, and/or cell wall structure differences that affect UV-C transmission depths.
- Initial yeast cell count had an effect on the efficacy of UV-C treatments with treatments being less effective with higher initial yeast cell counts. The researchers hypothesize that a shadowing effect might explain this effect.
- UV-C efficacy was also influenced by must turbidity and optical density.
- UV-C induced colour bleaching (doses higher than 3 kJ/L) in clarified must and browning in turbid must. Since the turbid must was not pasteurized polyphenol oxidase enzyme was suggested to be responsible for the browning effect.
- UV-C treatments did not affect optical density (254 nm), turbidity, pH, viscosity, total soluble solids, total acidity or relative density of Riesling, Pinot blanc and Pinot noir musts.
- Chemical analysis revealed significant differences in various flavour active compounds. However these differences were not perceived by the panellists at UV-C treatment levels relevant for microbial inactivation.
- There was no mutagenic effect of UV-C treatments utilized in this experiment.
Significance of the study:
This study indicates the possibility of microbially stabilising grape must with UV-C treatment that will neither affect the sensory properties of the resulting wine or consumer safety.