Managing wine extraction, retention, clarity and stability for defined styles and efficient production
The project will investigate:
- the role of macromolecules such as tannins, polysaccharides, proteins and their aggregate colloids, and their impact on stability, clarity, filtration and fouling;
- the impact of other wine matrix components on macromolecule extraction, retention and function;
- the source of these molecules or their precursors in grapes and yeast, and the impact of winemaking processes such as clarification, flotation, vinification and filtration on their retention and/or transformation;
- the impact of fouling of surfaces by macromolecules leading to production inefficiencies;
- alternative strategies for achieving protein stability and cold stability, for example, through use of novel additives and/or processing techniques;
- practical methods for wineries to determine likely extractability of macromolecules during winemaking and the factors that affect extraction and retention (e.g. enzymes, water additions and heat treatments).
Tools to manage wine protein stability faster and better
Protein removal is the primary means used by winemakers to prevent formation of unsightly protein hazes in finished wines. An accurate test to predict the bentonite dose required to prevent instability is a critical step. A highlight of the previous annual report was the development of an updated heat test which could consistently and reliably be completed within a five-hour turnaround time (two hours’ heating at 80°C and three hours’ cooling at 20°C). This year, guidelines for the new heat test were publicised via a peer-reviewed publication (McRae et al. 2018), an article in AWRI Technical Review and an AWRI webinar. Projects investigating pasteurisation and functionalised magnetic nanoparticles as alternatives to bentonite have also continued.
Pasteurisation of grape juice has been previously shown to reduce the concentration of haze-forming proteins in white wines without adverse effects on sensory profiles when conducted for one minute at 75°C. The effects of longer juice heating times on protein concentrations, heat stability and sensory profiles of wines have now been assessed. Laboratory-scale trials demonstrated that juice pasteurisation for up to two minutes effectively heat stabilised wines containing high concentrations of proteins. These trials were followed by industry-scale trials using a prototype pasteurisation unit. Semillon and Sauvignon Blanc juices were heated at 75°C for one or two minutes before undergoing fermentation. The two-minute pasteurisation removed most of the protein in juice. These results demonstrate that pasteurisation may be a viable alternative to bentonite for protein stabilisation of wines.
In recent years, magnetic separation technology has allowed for selective removal of pathogenesis-related proteins using carefully-tuned surface functionalities (Mierczynska-Vasilev et al. 2017). The process uses magnetic nano-beads which encapsulate haze proteins and are subsequently removed by applying an external magnetic force. New experiments examined the influence of different surface coatings, enabling different mechanisms by which proteins are selectively bound to be compared (Figure 10). Plasma coatings rich in amine, carboxyl and oxazoline functional groups were prepared and their efficacy in removing haze-forming proteins was examined using unfined Semillon and Sauvignon Blanc wines. This work will help guide design of new technology for the selective removal of haze proteins from white wines and has contributed to the broad understanding of protein interactions with surfaces.
When considering magnetic nanoparticles for this type of application, another important consideration is their reusability to reduce costs. Since protein adsorption onto magnetic nanoparticle surfaces is a reversible process, their regeneration and reuse is possible. The effectiveness of three different protein cleaning solvents (water, 10% sodium dodecyl sulfate (SDS)/water and acetone/water) were evaluated on bare, pre-washed and acrylic acid plasma coated magnetic nanoparticles. After each wash cycle, the nanoparticles were re-tested in wine, with monitoring of protein, phenolics, organic acids and elemental composition. The acrylic acid plasma coated magnetic nanoparticles exhibited a high stability and good reusability within six successive adsorption–desorption processes (Figure 11). The results suggest the strong potential for the application and reuse of magnetic nanoparticles as an alternative approach to bentonite for protein removal from wines, reducing waste and potentially enabling recovery of useful materials.
Improved understanding of red wine colour stability
Tannins react with anthocyanins to form polymeric pigments during fermentation and ageing, and are thought to contribute to the bulk of stable colour in aged wines. A study aimed to explore the role of tannin composition and size on the formation and stability of polymeric pigments. Tannins of a defined size range were prepared from white Cabernet grape seed (1,250 – 6,000 g/mol) and skins (2,000 – 13,000 g/mol) and were combined with anthocyanin in three different media: wine made from chemically defined must (CDM), wine made from CDM with added acetaldehyde (CDMA) and model wine made up of acidified 15% ethanol (MW). After an ageing period, it was found that the formation of polymeric pigments (and hence wine colour) was quite different among the three media. In model wine (which contained neither natural nor added acetaldehyde) large molecular weight tannins formed greater quantities of polymeric pigment with anthocyanin, and had a more intense colour. In CDM and CDMA wines the reverse trend was observed – losses of polymeric pigment and colour increased as tannin molecular mass increased, and the effect was greater for seed than skin tannins. The loss in colour was found to be primarily due to precipitation of large molecular mass tannins after reaction with anthocyanin. To explain this phenomenon, tannin particle size was compared using nanoparticle tracking (Figure 12). In the model wine, average particle sizes of the tannins ranged from 80 to 140 nm but far larger particle sizes were found in CDM and CDMA wines. This suggested that the principal mechanism for tannin and colour precipitation was the polymer size increase mediated by acetaldehyde, and this might partially explain the lack of large tannins observed in red wine. Acetaldehyde is generally considered to be beneficial to wine colour due to its enhancement of tannin-anthocyanin linkages; however, this study has shown that under certain circumstances it may lead to colour loss.
Developing guidelines on the use of tannin and polysaccharide additives in winemaking
Oenotannin and mannoprotein additives can be used to improve wine protein, cold or colour stability and may also be used to modify wine texture. Fourteen grape-based oenotannins and eight mannoproteins from the Australian market were screened, and their composition and molecular size distribution were determined (Li et al. 2018a,b). It was interesting to find that the analysis of the commercial oenotannins did not always agree with the labelled origin of material (i.e. grape skin or seed tannin). It was also found that for certain manufacturers, products marketed for different oenological purposes were found to have similar compositions.
Commercial mannoproteins also had diverse composition, notably in terms of their relative protein content (10 to 50%) and the presence of gum arabic. The protein content of mannoprotein additives may significantly affect wine composition, since these yeast-derived proteins can precipitate tannins, and hence influence texture and colour. To further investigate this possibility, the stability of commercial mannoprotein and gum arabic products in the presence of tannins was investigated. Studies were conducted in model wine solutions at different ethanol concentrations and characterised using NTA. Results showed aggregation of polysaccharide additives and tannin. Mannoprotein formed large, highly light-scattering aggregates, while gum arabic exhibited only weak interactions with tannin. A 3% reduction in alcohol concentration from 15% v/v to 12% v/v was found to increase aggregate size for mannoprotein, but had no impact on gum arabic. Under the conditions of the study, the polysaccharide additives were found to be stable and did not precipitate or cause tannin loss. However, the results showed that there are differences in colloidal behaviour between polysaccharide products, and further work will aim to determine the alcohol or tannin concentration at which instability, identified by precipitation or turbidity, might occur. Together, the results suggest a range of effects could potentially be achieved by applying different products, and this warrants ongoing study.
Li, S., Wilkinson, K.L., Bindon, K.A. 2018a. Compositional variability in commercial tannin and mannoprotein products. Am. J. Enol. Vitic. 69(2): 176-181.
Li, S., Wilkinson, K., Bindon, K., Bastian, S., Ristic, R. 2018b. Winemaking supplements: what’s inside? Wine Vitic. J. 33: 30-33.
McRae, J.M., Barricklow, V., Pocock, K., Smith, P.A. 2018. Predicting protein haze formation in white wines. Aust. J. Grape Wine Res. 24(4): 504-511.
Mierczynska-Vasilev, A.; Boyer, P.; Vasilev, K.; Smith, P. A. 2017. A novel technology for the rapid, selective, magnetic removal of pathogenesis-related proteins from wines. Food Chem. 232: 508-514.