PRELIMINARY STUDIES OF THE COMBINED EFFECT OF ULTRASOUND AND ASPERGILLOPEPSINS I ON THE PROTEIN INSTABILITY INDICES OF GEWÜRZTRAMINER WINE

The formation of protein haze is one the main organoleptic faults that can occur in white wines after bottling. Normally, to remove the excess of unstable proteins, winemakers resort to specific fining agents, of which bentonite is the most common. However, this cation exchanger can negatively affect the mouthfeel and the volatile composition of wines. Recent studies have reported the effect of ultrasound (US) on the protein stability of wines, suggesting that sonication promotes conformational changes in wine proteins that lead to more stable wines. Depending on the frequency and the intensity, US have been also reported to increase enzyme activity, improving mass transfer and substrate availability. In this contest, the work aimed to acquire preliminary results related of the combined effect of sonication and Aspergillopepsins I (AP-I), acid endo-pectinases commonly known as proteases, capable of degrading proteins at wine pH and recently approved by OIV. Sonication treatments at 20 kHz and 100 % amplitude were performed on a protein-unstable Gewürztraminer wine at batch conditions, in the presence or absence of AP-I (100 mg/L). Two different sonication times (20 and 60 s) were adopted, leading wine to different temperatures (40 and 70°C, respectively) as a consequence of the batch treatment. The wines were evaluated for the concentration of PR proteins and several indices related to protein instability (i.e.: turbidity, protein charge neutralisation, zeta potential and mean particle size). Results showed that every parameter was affected by treatments (Tukey HSD test; n=3; p<0.05). Respect to control wines, US lowered the concentration of the total PR proteins at both treatment conditions: - ~10% and - ~18% with 20 s and 60 s treatment respectively. Besides, sonication increased turbidity and protein charge neutralisation (PCN), and diminished particle size and zeta potential. The presence of AP-I during sonication further reduced the concentration of PR proteins at the more powerful conditions (~24%), and increased the zeta potential, even if maintaining negative values. The 20 s treatment supplemented with AP-I increased PCN and reduced turbidity respect to the corresponding untreated treatment and, vice versa, reduced PCN and increased turbidity at the most energetic conditions. The presence of proteases during sonication did not statistically modify the zeta potential and the particle size. Results confirmed the effect of US on the reduction of unstable proteins of the Gewürztraminer wine, at a different extent depending on the treatment conditions. The presence of proteases during sonication further allowed the reduction of these compounds, directly related to the protein instability of wines. Further research is needed to understand the mechanisms of interaction and the effects on the multifactorial phenomenon of protein instability, as an alternative to the heating of wine foresaw by the OIV regulation linked to the proteases utilisation, thus leading to a more sustainable oenology.