WINETECH Technical Yearbook 2021

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 tests were performed (McRae et al ., 2018) by filtering the wine and heat - ing 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 sig- nificant differences between the treatments for four of the 17 pre-determined attri- butes. These four attributes were “yellow colour intensity”, “tropical fruit aroma”, “bitterness” and “pungency” (defined as “intensity of the aroma and effect of alco - hol”). 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 bentoni te-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 (Gombau et al ., 2019). 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 Tr i a l s 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 rela- tively small differences in average inten - sity 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.

WINETECH TECHNICAL YEARBOOK 2021 | 76