WINETECH Technical Yearbook 2019

cerevisiae to metabolise glucose, leading to residual fructose at the end of fermentation. By reducing the initial fructose concentration at the beginning of fermentation, the risk of residual fructose at the end is reduced. The ethanol production in the mixed inoculation treatment was slightly lower compared to the pure inoculation. This difference only amounted to 0.1 to 0.2 % v/v, however, similar studies (using the same yeast strains and inoculation delay) reported a larger decrease in ethanol content (up to 0.5 % v/v) (Englezos et al ., 2016B), which could hold significant advantages for the production of wines from high sugar musts. The glycerol content was significantly higher for the mixed inoculations (9.3 g/L to 10.3 g/L) compared to the pure inoculations (7.8 g/L to 8.4 g/L). The higher concentration of glycerol in the mixed fermentations could contribute to the viscosity and palate weight of the wine (Nieuwoudt et al ., 2002), a property that is widely sought after. The volatile composition showed variable results between the different varieties. The higher alcohol 2-phenyl ethanol (“rose” and “floral”), as well as 1-hexanol (“green” and “cut grass”), were higher in the mixed treatment for all varieties, however, the differences were not always statistically significant. The compound, isobutanol (“herbaceous”), was also higher for the mixed treatment, but only in the Sauvignon blanc

and Riesling fermentations. For Chardonnay and Muscat, the pure inoculation resulted in a higher concentration of isobutanol, as well as isoamyl alcohol. Esters are a very important aroma group responsible for the overall fruity aroma of wine. The ethyl esters (ethyl octanoate, ethyl decanoate and ethyl dodecanoate) were all present in lower concentrations in the mixed fermentation for all cultivars with the exception of Sauvignon blanc. For Sauvignon blanc, these esters were present at higher concentrations in the mixed treatment compared to the pure inoculation. A higher concentration of these compounds could contribute to a stronger intensity of attributes such as “pineapple”, “pear”, “fruity” and “floral”. The acetate esters (hexyl acetate, octyl acetate, 2-phenyl ethyl acetate and 3-methyl-1-butanol acetate) were all much lower for the mixed fermentations for all the cultivars. This could lead to decreased intensities of attributes such as “floral”, “honey”, “banana” and “fruity”. Ethy l acetate can be a t roub l esome compound when present in elevated concentrations with an aroma reminiscent of nail polish, however, at low concentration, ethyl acetate can contribute to the overall fruity aroma of a wine. Ethyl acetate was significantly lower in the mixed inoculation for Chardonnay and Muscat, while no significant differences were reported for Riesling and Sauvignon blanc. The

concentration of ethyl acetate was low for all the varieties and would not contribute negatively to the aromatic composition of the wines. Wine produced from mixed inoculations contained significantly lower fatty acids (octanoi c and decanoi c ac i d) . These compounds are generally undesirable at h i gher concent rat i ons , however, the concentrations reported in this study was not at a level where it would contribute negatively to the wine aromatic composition. For Muscat, the mixed fermentation led to a decrease in terpenes and norisoprenoids, compounds responsible for contributing to the typical Muscat characters in wine. These results indicate that sequential inoculation using these specific strains could be detrimental to the composition and quality of a Muscat driven wine. However, the Riesling, a cultivar whose varietal aroma also heavily relies on the presence of terpenes and norisoprenoids, showed no significant difference between the treatments. Volatile thiols are very powerful aromatic compounds that contribute attributes such as passion fruit, grapefruit, tomato leaf and guava. In the present study, the mixed inoculation led to a marginal (but statistically significant) increase in the volatile thiol concentration of the wines. The effect of sequential inoculation on these compounds needs further investigation.

EXPERIMENTAL LAYOUT The varietals used in this study, were Sauvignon blanc, Riesling, Muscat and Chardonnay. There were two treatments for each cultivar: 1. Inoculation with Starmerella bacillaris w i t h s e q u e n t i a l i n o c u l a t i o n o f Saccharomyces cerevisiae 48 hours after the first inoculation (mixed); 2. I no c u l a t i on w i t h Sa c c ha r omy c e s cerevisiae only (pure). The fermentations were conducted in 1 L glass vessels and the growth dynamics of the two yeasts during fermentation were monitored by plate count. Volatile compounds were analysed using GC/MS. MAIN RESULTS Analyses of non-volatile components showed that Starmerella bacillaris preferred to metabolise fructose to glucose. During the slower initial stages of the mixed fermentation, the Starmerella bacillaris consumed mainly fructose at 9 g/L after 48 hours. Saccharomyces cerevisiae , on the other hand, consumed an average of 81 g/L (glucose and fructose) in the first 48 hours after inoculation. The fructophilic behaviour of Starmerella bacillaris holds significant advantages for the wine industry. Stuck fermentations are often a result of unfavourable fructose to glucose ratios, due to the preference of Saccharomyces