Yeasts protein extracts: new low impact tool for wine protein stability

Yeast protein extracts (ypes) have flocculating properties, allowing clarification of musts and wines. They are already authorized by OIV for fining purposes with a maximum dosage limit of 60 g/hl for red wines, and 30 g/hl for musts, white and rosè wines. The extraction of ypes from the cytoplasm of yeasts (saccharomyces spp) cells is defined by the resolution OIV OENO 452-2012, that indicate also some specification of the final product. In particular, the total protein content must be greater than 50% of the dry product, and at least 50% of the total proteins must have molecular weights higher than 15 kda. Ypes are very complex mixtures, and their composition and further activities are influenced by the yeast strains and extraction processes. Some authors highlighted that some yeast proteins showed an isoelectric point below common wine ph, which means they show a negative electric charge (Noriega-Dominguez et al., 2010) and, consequently, could be expected to interact with positively charged wine proteins. In view of this specific character of negative electrical charge, several experimental trials were carried out on different unstable aromatic white wines in northern italy and croatia. First of all, the chemical properties of an ype and its effectiveness against wine protein instability were evaluated. In the second step, the effects of different dosages (from 5 to 60 g/hl) and treatment times (from 2 and 10 hours) were investigated. A qualitative analysis of ype was carried out by the determination of potential with electrophoretic light scattering (els) and electrical charge by streaming potential. Instead, the effect of ype addition at different dosages and times was evaluated considering several analytical parameters: turbidity, some protein stability tests, and protein content by hplc analysis. Different experiments were carried out in small laboratory volumes and in real cellar conditions on aromatic white wines. All experiments and analyses were performed in triplicate, and the results are elaborated by a one-way analysis of variance. The potential determinations highlighted a negative electric charge of the chosen ype and its potential to interact with wine pr-proteins. The protein contents and the stability indices of the wines after ype treatments at different dosages and times were well correlated. The ype addition, after 4 hours and at 20 or 30 g/hl, allowed a decrease of pr-protein content ranging between 40 and 50 %. The trials of ype addition at 25 and 50 g/hl on aromatic wines allowed a significant increase in protein wine stability, as measured by the protein charge neutralization test. The almost increase of pr-proteins content at longer treatment times could indicate a reversible interaction with ypes, as pointed out by the stability indices. Treatment time becomes a fundamental factor to be considered to ensure ype effectiveness against pr-proteins and the partial protein stabilization of white wines. The best results were obtained in general at 20-30 g/hl of ype concentration and 4 hours, which induced a significant decrease (40 to 50 %) of initial pr-protein concentration. The experimental trials on unstable wines, in laboratory and cellar conditions, performed with different dosages and treatment times, confirmed the effectiveness of ype, and the results showed potential reversible interaction with haze-related proteins. The ype addition is significant for times above 4-6 hours, and its effects could disappear for longer times. The yeast protein extract increased the protein stability of white wines, but it can’t allow their complete stabilization. The addition of ype could be considered a combined treatment with conventional ones, aimed at decreasing the amount of conventional fining agents (e.g bentonite) and preserving the wine aroma. The ype could represent a new tool for protein stabilization, focused on low-impact and precision enology.