Researchers looked to winery waste for the treatment of heat-unstable wines and conducted a preliminary study assessing the feasibility of grape seed powder as a novel and sustainable bentonite alternative. Some of the main findings in this study will be reported in this article.
Bentonite is widely used during winemaking to remove haze-forming proteins in white and rosé wines, however, the application has some significant disadvantages. These include:
- Poor settling resulting in significant wine volume losses.
- Potential changes in wine sensory composition.
- Sustainability of the product (non-renewable).
- Environmental impact of bentonite waste.
Research institutions are constantly on the lookout for possible alternatives of which chitosan and proctase have been identified as viable options. The goal is to find a sustainable and economical product that preserves the wine quality, while efficiently removing the haze-forming proteins.
Materials and methods
Grape seed powder (GSP) was added to Australian Sauvignon blanc juice and wine to test the effect of the additive on the heat stability of the wine. The study also included wines from other white grape varieties, however, for these, the effect of the addition of GSP to the juice was not investigated. Further, similar results were obtained for the wines made from different cultivars.
The Sauvignon blanc juice and wine were fined with GSP isolated from unfermented frozen Chardonnay grape marc. The seeds were either processed raw or it was roasted prior to refinement. The refined product was added to the juice and wine at 20°C at various dosages. The GSP was in contact with the medium by stirring for either one or two hours before it was removed by centrifugation and filtration. The juice was then fermented and wine filtered for analyses. A sample was also fined with 0.5 g/L bentonite for comparison.
Heat stability tests were performed on the juice and wine before the addition of GSP and after the removal thereof. The sample was filtered and heated to 80°C for two hours and cooled to room temperature for three hours. The turbidity of the sample was measured using a turbidity meter before heating and after cooling and a difference of less than two nephelometric turbidity units (NTU) was considered stable.
Fining Sauvignon blanc wine with grape seed powder
The wine was successfully fined with GSP and a minimum dose of 25 g/L GSP was needed to obtain a ΔNTU below two. An increase in phenolic content was reported and the wine had a significantly darker colour compared to the control. Compared to the 0.5 g/L bentonite treatment, the GSP lees settled at a faster rate and produced less lees despite bentonite being added at a lower dose.
Interestingly, dosages lower than 20 g/L did not result in improved heat stability results compared to the control (which did not undergo any fining treatment). To the contrary, lower dosages resulted in a significant increase in ΔNTU (between 60 and 120 ΔNTU). The reason for this increase in instability at lower dosages is unclear and a full characterisation of the GSP composition and the extraction of compounds over time will have to be performed to investigate this phenomenon.
In terms of GSP processing and application, neither the GSP contact time (1 h or 2 h), nor the grape seed treatment (roasted or unroasted), affected the efficacy of GSP.
Fining Sauvignon blanc juice with grape seed powder
Fining the Sauvignon blanc juice with GSP produced heat-stable wines at a much lower dose of 5 g/L compared to the dose needed to stabilise the wine (25 g/L). It seems that adding the GSP before fermentation results in better interaction of the grape seed tannins with the proteins with more efficient precipitation during fermentation without affecting fermentation kinetics. Other than a slight increase in phenolic content, the addition of GSP to the juice did not result in any significant wine compositional changes. The wine’s colour also changed marginally at concentrations above 10 g/L (which is higher than the effective dose).
In contrast to when GSP was added to the wine, when added to the juice, there was no increase in haze formation at any of the tested dosages. This would suggest that the presence of ethanol might have an unwanted effect on fining efficiency of GSP especially when present at lower dosages.
This study served as a preliminary study, investigating the possible uses of grape seed powder in winemaking for the removal of heat-unstable proteins. The addition of GSP to the juice was much more effective in removing haze-forming proteins compared to the addition of GSP to the wine. Even though the addition of GSP directly to the wine showed some potential to gain heat stability, the adverse effects on the wine composition might mitigate the potential advantages. Further, the addition to the juice did not have any adverse effects on the wine composition, however, further tests and analyses (including sensory analyses) will have to be done to determine if GSP is a viable option.
The removal of the GSP sediment with the yeast lees after fermentation is an added benefit due to the reduction in the number of racking actions required. GSP also settles at a faster rate and could provide more compact sediment.
Grape seed powder is a sustainable option and is a realistic alternative to bentonite especially when added to juice prior to fermentation. Sources, composition and the preparation of the grape seed powder need further investigation to develop an effective product that imposes minimal changes to the wine composition.
The quest to find an effective bentonite replacement that is sustainable and environmentally friendly has encouraged researchers to start looking at winery waste for possibilities. Sauvignon blanc juice and wine were fined with processed grape seed powder after which the heat stability was tested. Results showed that the addition of the grape seed powder to the juice resulted in a heat-stable wine and was more effective compared to when the powder was added to the wine. Grape seed powder could serve as an effective and sustainable alternative to bentonite, however, further tests are needed to establish the effect on the wine’s sensory and chemical composition.
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- Romanini, E., McRae, J.M., Colangelo, D. & Lambri, M., 2020. First trials to assess the feasibility of grape seed powder (GSP) as a novel and sustainable bentonite alternative. Food Chemistry 305 (March 2019), 125484. https://doi.org/10.1016/j.foodchem.2019.125484.
- McRae, J.M., Barricklow, V., Pocock, K.F. & Smith, P.A., 2018. Predicting protein haze formation in white wines. Australian Journal of Grape and Wine Research 24(4), 504 – 511. https://doi.org/10.1111/ajgw.12354.
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