Application of ultrasonic and refractometric measurements in enological samples and related model solution.

AIM: The refractive index is a basic optical property of materials and a key tool for the determination of major components in musts,such as sugars. Ultrasonic based technology is emerging as a well-suited tool for monitoring food processes and quality, and inparticular it has been applied in predicting the alcohol content and measuring the ultrasonic velocity in enological products. The aimof this study is to combine the applications of these two technologies to quantify alcohol and residual sugar in real and modelenological samples. The eff ects of major wine and must components on the combined application of these analytical tools areinvestigated. METHODS: Refractometric and sonic Brix values were determined using a LP10 sensor (Maselli Misure, Parma, Italy)working at controlled temperature. The selection of the real enological samples for the study has been made during the 2020 harvest inSouth Tyrol (Italy). The model solutions were prepared at diff erent combinations of diff erent levels of ethanol in water, glucose,tartaric acid, malic acid and gallic acid as major wine components. For these model solutions, fi rstly the levels of each componentwere chosen upon a likelihood criterium, then to reduce the number of analyses to run. A refi ned randomized selection of thesecombinations was chosen according to a D-Optimal experimental design, calculated on the R-based application CAT, with a largeenough number of selected experiments to ensure representativeness. The statistical analysis of the results for the real and modelsamples has been performed on CAT and XLStat (Addinsoft, France). RESULTS: A statistical analysis of the obtained results allowed toinvestigate the eff ects of the major constituents on the Brix and alcohol determinations by combined refractometric and acousticanalyses, in real and model enological samples. The calculated models showed that the eff ects of interactions betweenvariables/chemical components and non-linear contributions cannot be a priori ruled out in order to accurately explain theinstrumental responses to the variations of these major components. As an alternative, the evaluation of separate models for morelimited experimental domains (e.g. only musts, only dry wines or musts undergoing fermentation) is also discussed. The eff ects of theresidual sugar and alcohol content on the determination of refractive and sonic Brix are discussed in detail. CONCLUSIONS: Insamples collected during fermentations or in wines, the application of this combined technique for residual sugar and alcoholdeterminations could be feasible, provided that the full dependence of these refractometric and acoustic parameters on the majorsample components and their interactions is well understood, and the related mathematical models validated over the widestpossible range of enological samples. Some mathematical models have been proposed and discussed along with validation.