Many types of bacteria and yeasts are relied upon in the winemaking process. However, sometimes these organisms can cause the resulting wine to spoil. Often, this spoilage results in undesirable textures as well as off odors or flavors in the finished wine, including bitterness, too much buttery character, and excessive volatile acidity.
|Acetobacter pasteurianus http://bacmap.wishartlab.com/system/images/
Winemakers often combat these issues conventionally by adding antimicrobials such as sulfur dioxide or dimethyl dicarbonate, pasteurization, or by physically removing the organisms by filtration methods. These methods may not always be completely effective, and sometimes results in other off-flavors in the finished wine.
In other aspects of the food industry, ultrasound is often used for the treatment of microbial spoilage. It is because of this, the authors of the study presented today suggested that perhaps the technology could also be used in treating microbial spoilage in wine.
How this ultrasound technique works is that high power ultrasound waves pass through a liquid (potentially wine) causing small bubbles to form and pop (a.k.a. cavitation). This cavitation process creates localized areas of high temperatures and pressures, as well as the formation of hydroxyl radicals. These conditions of high temperature and pressure can cause breakdowns in microbial cell walls, rendering the organisms more susceptible to the hydroxyl radicals, which ultimately destroy them.
Perhaps by applying these ultrasound waves to wine at some point during the winemaking process, spoilage-causing microbes could be destroyed without causing ill effects on the wine. The study presented today set out to research just that very idea.
The microbes chosen for this study were representative strains of bacteria and yeasts that are commonly associated with spoilage in wine.
The grape juice used in the study was a Semillon from the 2008 vintage. The finished red wine used in the study was produced in 2007 at the Hickinbotham Roseworthy Wine Science Laboratory at the University of Adelaide. Yeast and bacteria cells were cultured and then inoculated into saline solution (control), sterile grape juice, or sterile red wine. If you’d like to know more details about the culture methodology, just ask.
High power ultrasound was performed using a titanium alloy radial probe. The different media (grape juice or red wine) was kept in an ice bath to maintain a temperature of 23oC during the high power ultrasound irradiation. During ultrasound exposure, cultures were exposed for 2 minutes followed by a 2 minute rest. This cycle was repeated for up to 40 minutes, for a total ultrasound exposure time of 20 minutes.
Viability of bacteria and yeasts were calculated for each media. In a nutshell, 0% viability indicates that all cells are dead, whereas 100% viability means that all cells are healthy and alive.
Ultrasound-treated wines were tested for sensory impact by 31 participants (unclear who they were) who were asked to pick the wine with the same flavor and aroma as a reference that did not undergo ultrasound treatment.
- Decreases in viability following ultrasound treatments occurred for all yeast strains, though some yeasts were more susceptible than others.
- Saccharomycescerevisiae viability decreased to 20% in the saline control, and decreased only to 75% in the grape juice.
- Schizosaccharomyces pombe viability decreased to 20% in the saline control, and decreased only to 75% in the grape juice.
- Z. bailii viability initially increased after ultrasound exposure, but after 20 minutes decreased by 15% in wine, and 50% in grape juice.
o This initial viability increase may be due to the fact that Z. bailii grows in clusters, creating the illusion of increased viability when the individual cells were shaken away by the ultrasound waves from the cluster group.
- Yeasts least affected by the ultrasound treatment were Hanseniaspora uvarum (47% viability loss) and Dekkera bruxellensis (43% viability loss).
- The control for yeasts (no ultrasound treatment) did not show any increase or decrease in viability over the entire 40 minute incubation period.
- The control for bacteria (no ultrasound treatment did not show any increase or decrease in viability over the entire 40 minute incubation period.
- Viability of A. aceti in grape juice was 82%, but dropped to 40% in the saline control medium. A. aceti did not grow at all in the wine medium, so viability could not be tested.
- Acetobacter pasteurianus viability decreased to 12.8% in saline, 24.4% in red wine, and remained around 74% in juice.
- Oenococcus oeni viability decreased by around 25-35% in all media.
- Pediococcus sp. showed 45% viability in wine, 75% viability in grape juice, and 100% viability in the saline control.
- Lactobacillus plantarum viability remained between 75% and 100% for all media.
- There were significant sensory differences between wines treated with high power ultrasound and untreated control wines.
It appears from the results of this study that some yeasts and bacteria were more affected than others during the high power ultrasound treatments. The most greatly affected yeast was S. cerevisiae, while the greatest affected bacteria was A. pasteuranus. According to the authors, the variability in the effectiveness of ultrasound on yeast and bacteria viability could be a result of differing configurations of cell walls between the different species. For bacteria, some species have a thicker cell wall (gram-positive), thereby possibly making them more resistant to the physical damage caused by ultrasound treatment compared to bacteria with thinner cell walls (gram-negative).
The medium used also greatly affected the ability of the ultrasound to decrease viability of the yeasts and bacteria. According to the authors, this could be a result of the viscosity, vapor pressure, and surface tension of the different media, as all of these things alter cavitation efficiency in liquid media.
In regards to sensory changes, it is clear from this study that ultrasound treatment significantly altered the flavor/aroma of the wine. The authors speculated that this may be due to the increase in free radicals as a result of the cavitation process of ultrasound treatments. Since they do not know for sure what exactly caused this different in sensory characteristics, more research needs to be done. The authors bring up the concept of “bottle shock”, in that perhaps this change in sensory characteristics is only temporary, and needs to undergo a rest period before consumption. Again, this idea would need testing to confirm or refute it.
High power ultrasound technology may be a way to control different types of wine spoilage microorganisms, however, a lot more work needs to be done to determine exactly when in the winemaking process is ideal for treatment, as well as exactly which microorganisms the treatment would be used to treat against. This study touches briefly on these questions, but much more research needs to be done for a better understanding of the mechanisms involved.
Also, since the ultrasound treatment significantly altered the sensory characteristics of the wine, a lot more research needs to be done to try and optimize the procedure so that spoilage is maximally decreased while sensory characteristics remain unchanged (if that’s even possible). This study did not go into exactly how the flavor and aroma of the wine was just, just that it was different. More detailed sensory analysis should be conducted, in order to get a greater understanding of how this technology affects these characteristics.
Overall, I think the moral of the story is that ultrasound technologies could potentially be a treatment for preventing wine spoilage; however, there is a significant amount of research that needs to be done in order to optimize the system without resulting in any negative impacts on wine sensory characteristics.
What do you think about using ultrasound technology for combating wine spoilage? Do you think these initial results warrant further research? Or should they just toss the idea in the trash? Please leave your comments below! (no html tags, please).
Source: Luo, H., Schmid, F., Grbin, P.R., and Jiranek, V. 2012. Viability of common wine spoilage organisms after exposure to high power ultrasonics. Ultrasonics Sonochemistry 19: 415-420.
I am not a health professional, nor do I pretend to be. Please consult your doctor before altering your alcohol consumption habits. Do not consume alcohol if you are under the age of 21. Do not drink and drive. Enjoy responsibly!