Abstract
A recent study investigated the effect of varying the yeast population at the time of inoculation and how it affects the overall fermentation with a special focus on the formation and release of volatile thiols. Results showed that an increased yeast inoculum could result in wines with elevated concentrations of 3MH and 4MMP.
Introduction
The use of pure yeast cultures (for example, active dried yeast) to perform alcoholic fermentation has become popular in the wine industry mainly due to its ease of use and the fact that it facilitates more reliable, predictable and timely wine fermentations. Pure yeast cultures can also impart specific organoleptic properties to a wine depending on the genetic makeup of the yeast in combination with the fermentation parameters such as fermentation temperature and the concentration and type of nutrition available to the fermenting cells.
One fermentation parameter often overlooked is the yeast inoculum size, as different yeast suppliers usually specify inoculum rates for their products. Following the manufacturers’ recommended rates usually ensures a viable cell population of more than 5 x 106 viable cells/mL, which is, in most cases, needed for timely and successful fermentations. Usually, the proper rehydration of 25 g active dry yeast per hectolitre of juice (25 g/hL) is required to achieve the desired viable cells/mL. This is based on the active dry yeast containing 2 x 1010 cells/g.
In practice, there are rarely intentional deviations from the recommended yeast inoculum rate. However, a recent study1 questioned the effect of varying the yeast population at the time of inoculation and how it affects the overall fermentation performance of a commercial wine yeast with a particular focus on the formation of volatile thiols, which are potent aroma compounds responsible for an array of fruit-driven sensory descriptors such as passionfruit, grapefruit, blackcurrant and guava.
Materials and methods
Small-scale fermentations were carried out in 750 mL bottles using 700 mL of sterilised Sauvignon blanc juice (Marlborough, New Zealand). The juice was inoculated with rehydrated Saccharomyces cerevisiae X5 yeast (Laffort), and fermentation was conducted at 15°C until dryness. The juice was inoculated at four different yeast population rates:
- 1 x 106 yeast cells/mL (control)
- 1 x 108 yeast cells/mL
- 1 x 1010 yeast cells/mL
- 1 x 1012 yeast cells/mL.
Results
Ferments inoculated with an increased number of yeast cells finished the fermentation one to two days earlier than the control. This result is expected as a higher yeast cell density will increase the volumetric productivity of the fermentation process, thus making the fermentation process faster and more robust.
There were no significant differences in the general oenological parameters (alcohol, pH and titratable acidity) of the resulting wines. However, a trend in alcohol reduction was observed as the inoculum size increased. The highest inoculum size (1 x 1012 yeast cells/mL) resulted in a 0.17% v/v reduction in alcohol compared to the control wine.
Interestingly, the volatile thiol concentration in the resulting wines differed significantly depending on the yeast inoculum rate. As the inoculum size increased, the 3-mercaptohexan-1-ol (3MH) concentration also increased. However, this increase was not linear. An increase in the yeast population from 1 x 106 to 1 x 108 cells/mL resulted in a 10% increase in 3MH concentration, while an increase from 1 x 106 to 1 x 1010 cells/mL resulted in a 24% increase. However, further increases in yeast cell density (to 1 x 1012 cells/mL) did not result in any further increases in 3MH.
Similarly, 4-mercapto-4-methylpentan-2-one (4MMP) concentration also increased with yeast cell density; however, the increase was linear in this case. Increasing the yeast population from 1 x 106 to 1 x 108 cells/mL resulted in an 11% increase in 4MMP concentration while increasing the yeast population from 1 x 106 to 1 x 1010 cells/mL resulted in a 69% increase. Further increases in yeast cell density (to 1 x 1012 cells/mL) resulted in a significant change in 4MMP concentration, resulting in a wine with a 153% increase in 4MMP compared to the control wine.
The number of yeast cells present during fermentation had little effect on the concentration of 3-mercaptohexyl acetate (3MHA), and all the wines had relatively similar levels.
Conclusion
Results from the study suggest that the number of inoculated yeast cells could affect not only the fermentation rate, but also the formation and release of potent aroma compounds such as the volatile thiols. Adjusting this fermentation parameter could potentially lead to increases in 3MH and 4MMP, which may result in wines with more intense sensory characteristics, such as passionfruit and blackcurrant. However, further work is needed to ascertain whether this is true for juices of varying composition, volumes and yeast strains. The scientific explanation for the observations in the study also needs to be clarified. The inoculum size is a parameter which can be easily adjusted and does not require specialised equipment or monitoring; however, it will add to production costs.
Abbreviations
4MMP: 4-mercapto-4-methylpentan-2-one; 3MH: 3-mercaptohexan-1-ol; 3MHA: 3-mercaptohexyl acetate.
Reference
- Pinu, F.R., Stuart, L., Topal, T., Albright, A., Martin, D. & Grose, C., 2023. The effect of yeast inoculation methods on the metabolite composition of Sauvignon blanc wines. Fermentation 9(8), 759. https://doi.org/10.3390/fermentation9080759.
For more information, contact Carien Coetzee at carien@basicwine.co.za.
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