Grape seeds are natural by-products of winemaking and are usually discarded after the separation of the solids from the juice. However, in a quest to find an effective bentonite replacement that is sustainable and environmentally friendly, researchers have turned to winery waste for possible solutions.
Introduction
The use of powdered grape seeds as an alternative to bentonite for the removal of haze-forming proteins have been demonstrated in small-scale preliminary studies.1 Results from these laboratory-scale tests showed that the addition of powdered seeds to Sauvignon blanc in the pre-fermentative phase was effective in reducing the haze-forming proteins resulting in heat-stable wines. However, even though the fining treatment was effective in improving the heat stability, the sensory impact on the resulting wine was not investigated. In a follow-up investigation,2 researchers focussed on the sensory effects of grape seed powder fining to determine the viability of such a treatment for the practical use in the wine industry.
Materials and methods
Sauvignon blanc and Sémillon juices were fined with grape seed powder (GSP) isolated from unfermented frozen Chardonnay grape marc. The seeds were roasted (180°C for 10 minutes) before refinement using a mortar and pestle.1 The refined product was added to the juices (20 L lots done in triplicate) at 20°C at two dosages, 7.5 g/L and 15 g/L. The GSP was in contact with the medium by stirring for one hour before it was removed by racking. The juice was then fermented, and the wine filtered and bottled for analyses.
As a comparison, the same juices were fermented without the addition of GSP, whereafter the finished wines were treated with bentonite at a dosage of 1.1 – 1.2 g/L. The treated wines were filtered and bottled for analyses. A control treatment, where no fining of either GSP or bentonite was applied, was also included in the study.
Heat stability tests3 were performed by filtering the wine and heating it to 80°C for two hours. The sample was then cooled to 22°C 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. A trained sensory panel was used to assess the wines using sensory descriptive analysis by rating the intensity of pre-determined attributes on a 15 cm line scale.
Results
Heat stability
Compared to the control (no fining with either GSP or bentonite), fining the juices with 15 g/L GSP resulted in 80% and 75% decrease in ΔNTU (higher ΔNTU indicating greater instability) for the Sauvignon blanc and Sémillon wines, respectively. The lower dosage of 7.5 g/L showed a lower efficiency, decreasing the ΔNTU by 14% and 22%. No significant changes to the ethanol and titratable acidity content, as well as the pH of the resulting wines, were observed.
Of the treatments tested, the bentonite-treated wines delivered the most heat-stable wines, reducing the ΔNTU with 98% and 99% for Sauvignon blanc and Sémillon, respectively. Even though the GSP treatment in this specific trial was less effective compared to the bentonite treatment, the relatively high dose of bentonite used and the additional disadvantages of the use of bentonite should be considered.
Sensory effects
The sensory assessment of the Sauvignon blanc wines showed only statistically significant differences between the treatments for four of the 17 pre-determined attributes. These four attributes were “yellow colour intensity”, “tropical fruit aroma”, “bitterness” and “pungency” (defined as “intensity of the aroma and effect of alcohol”).
The GSP-treated juice resulted in a wine with increased “yellow colour intensity” when compared to the control and the bentonite-treated wine, with the higher dosage (15 g/L) showing a higher yellow intensity compared to the lower dosage (7.5 g/L). The increased yellow colour is likely due to the increased extraction of phenolics from the seeds. The same trend was also seen for the Sémillon wine.
Compared to the bentonite-treated wines, as well as the control wines, the GSP treatment resulted in a wine with a greater intensity of “tropical fruit aroma”, as well as “bitterness” and “pungency”. The reason for the increased intensity of the generally desired “tropical fruit aroma” attribute is unclear and interactive sensory effects (enhancing/suppressing effects) of seed-derived compounds and wine aroma compounds could contribute to this observation. Increases in proanthocyanidins and low molecular mass polysaccharides from the seeds can possibly be the reason for the increased bitterness reported.4 The intensities of other attributes (such as “citrus aroma”, “fresh grass aroma” and “astringency” among others) were not significantly different between the treatments. However, the judges did score the GSP-treated wines as having a slightly higher viscosity.
Conclusion
Trials have shown that GSP can successfully be used to fine Sauvignon blanc and Sémillon juice to improve the heat stability of the resulting wine. In this study, treating the juice with 15 g/L GSP resulted in up to 80% decrease in ΔNTU. Even though the treatment was not as effective as the high dose of bentonite treatment post-fermentation, the addition of GSP in the juice phase holds several advantages. Treating the juice with GSP could significantly reduce the bentonite requirements post-fermentation, thereby reducing wine volume losses, improving time management, and minimising the environmental impact.
Of the 17 sensory attributes tested, only four of the attributes were reported as being significantly different between the treatments. This, together with the relatively small differences in average intensity rating, would suggest that the sensory differences between the treatments were marginal.
Even though the GSP-treated wines showed an increase in “yellow colour intensity”, “bitterness” and “pungency”, it is unclear if the increase in the intensity of these seemingly undesirable attributes were considered as “unacceptable” and if a consumer would notice and reject the wine based on these changes. Further research is needed to investigate GSP production/processing methods and fine-tune the application to minimise undesirable effects.
The valorisation of the natural grape seeds holds several advantages, the renewability of this resource being key. As a by-product of winemaking, the seeds are readily available and can potentially provide a sustainable and economic product to reduce/eliminate the need for bentonite.
Abstract
Sauvignon blanc and Sémillon juice were fined with grape seed powder and compared to an unfined control and a post-fermentation bentonite treatment. Results showed that grape seed powder was able to improve the heat stability of the resulting wine significantly and had a relatively small sensory impact with both positive and negative influences.
References
- 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.
- Romanini, E., McRae, J.M., Bilogrevic, E., Colangelo, D., Gabrielli, M. & Lambri, M., 2021. Use of grape seeds to reduce haze formation in white wines. Food Chemistry 341 (July 2020): 128250. https://doi.org/10.1016/j.foodchem.2020.128250.
- 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.
- Gombau, J., Nadal, P., Canela, N., Gómez-Alonso, S., García-Romero, E., Smith, P., Hermosín-Gutiérrez, I., Canals, J.M. & Zamora, F., 2019. Measurement of the interaction between mucin and oenological tannins by surface plasmon resonance (SPR); relationship with astringency. Food Chemistry 275: 397 – 406. https://doi.org/10.1016/j.foodchem.2018.09.075.
– For more information, contact Carien Coetzee at carien@basicwine.co.za.
