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Sonication of yeast cells accelerates aging on lees

Aging on dead yeast cells, the “lees”, is a technique to improve the wine’s aromas and mouthfeel. However, aging on lees takes a very long time and is therefore expensive (and requires patience). Sonication of yeast cells may accelerate this process.

Dead yeast cells are slowly broken down during the aging process. This automatic process, the autolysis of yeast cells, results in the release of proteins, amino acids, lipids and polysaccharides from the yeast cells into the wine. These substances make the wine more complex, fuller and more resistant to oxidation. Aging on lees is used for all wine types, but is mainly used for making sparkling wines such as cava and champagne. This wine is aged “sur lattes” for months, or even years, on the yeast in the bottle. This requires long-term storage and management of the bottles, which makes the production expensive and delays revenue. Sonication – the treatment of the yeast cells with ultrasound waves – can potentially speed up this process.

Ultrasound waves

Sonication apparatus
Sonication apparatus. Fifty mL of pure yeast biomass (in the small flask) is treated with ultrasound waves.
Adapted from Del Fresno, 2019 via CC BY 4.0

Ultrasound has a higher frequency than a human can hear (> 20 kHz). With sonication, the ultrasound waves are passed through a solution. Due to the sound waves, bubbles form and disappear very quickly in the liquid which accelerates chemical reactions. The acceleration of these reactions causes a faster aging of the wine. Sonication of a maturing red wine (while aging on lees), increases the amount of polysaccharides released from the yeast, but also reduces the color intensity of the wine, and has an adverse effect on the amount of volatile compounds. Therefore, despite the additional polysaccharides, sonication of the entire wine has more disadvantages than advantages.

Spanish researchers have investigated whether sonication of only the yeast cells, separately from the wine, prevents these adverse effects of sonication. Sonication causes the cell membrane of the yeast cells to break down, resulting in a faster disintegration of the yeast cells. A sonication treatment of twenty minutes results in yeast cells with a partially degraded cell membrane. These treated yeast cells look similar to yeast cells that have undergone autolysis.

Sonication of yeast cells
Treatment of yeast with ultrasound, so-called “sonication”. Yeast that has not been treated (a) and yeast that has been treated with ultrasound for 20 minutes (b).
Adapted from Del Fresno, 2019 via CC BY 4.0

Effect of sonication of yeast cells

The sonication of yeast cells thus visually appears to have the desired effect, but does it also improve the composition of the wine? After adding the ultrasound-treated yeasts to the wine, the composition of the wine (or wine-like solution) was monitored for sixty days. In comparison with aging on normal yeast, aging on sonicated yeast cells results in 20% more polysaccharides. In addition, the sonicated yeast also appears to have an increased antioxidant effect on the wine. This means that sonication may also help to (further) reduce the addition of SO2 to the wine.

More polysaccharides due to sonication
More polysaccharides due to sonication of yeast. The effect of normal yeast (bottom line) and sonicated yeast (top line) on the amount of polysaccharides in an alcohol solution during a 60-day period.
Adapted from Del Fresno, 2019 via CC BY 4.0

In general, aging on lees reduces the total anthocyanin content of the wine. This is probably caused by the adsorbent effect of the cell walls of the yeast cells. However, sonicated yeasts, with less intact cell walls, have a protective effect on the total anthocyanin content in the wine, possibly due to a reduced adsorption capacity. The sonicated yeast cells have no significant measurable effect on the total amount of polyphenols or the amount of fermentative volatile compounds in the wine. The wines treated with sonicated lees did however show reduced color intensity in the absorbance tests.

Test panel

The wines were tasted and scored blind by nine trained wine tasters. For ultimately, all measurements aside, the smell, mouthfeel and taste of the wine counts. The wines with the sonicated yeast had better and more intense aromas, more body, and were better appreciated than wine aged on non-sonicated yeast. Remarkably, the reduced color intensity as observed in the absorbance tests in the laboratory was not observed by the test panel. The sonicated yeast cells have no adverse effects on the smell and taste of the wine, and give the wine the same expected properties as when aged on lees for a longer period of time.

How can the winemaker benefit from this research?

This study provides a “proof-of-concept” and shows that sonication can be used to accelerate the aging on lees of wines. Unfortunately this study only considers aging for two months. This is in stark contrast to the maturing times that are used in the production of sparkling wines according to the traditional method. It would be very interesting to see how wines treated with sonicated yeasts develop over a longer period of time. Moreover, it would also be interesting to determine when sonicated yeasts have a similar effect on the wine as non-sonicated yeasts and what the actual time gain is. Nevertheless, the effects that have already been demonstrated within the two-month period are promising. If it is possible to sonicate the lees separately from the wine on a large scale – not in small flasks, but with tens of liters at a time – then it may become an interesting alternative technique for long-term storage and aging on lees.

Reference:
Del Fresno JM, Morata A, Escott C, Loira I, Cuerda R, Suárez-Lepe JA. Sonication of Yeast Biomasses to Improve the Ageing on Lees Technique in Red Wines. Molecules. 2019 Feb 12;24(3). pii: E635. https://doi.org/10.3390/molecules24030635.

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