Using Microwave Technology to Eliminate Spoilage Organisms in Oak Barrels: A Novel Approach

As many, if not all, of you reading this blog already know, the use of oak in winemaking is a very common practice for adding complexity and quality to a wine. Using oak is typically more expensive than using stainless steel tanks, as the cost of the equipment is simply much higher. As a result of this added cost of using oak barrels, winemakers will often use the same barrel for multiple vintages. Of course, using older barrels is also a style choice and not always a financial necessity, as older barrels give a more delicately oaked wine that is well desired among many wine consumers.

All that being said, using a barrel multiple times brings up the issue of having to clean it properly in order to avoid contaminating the next wine that resides inside of it. The porous structure of wood easily allows microbial and other spoilage organisms to “set up camp”, if you will, and is extremely difficult to remove via the current cleaning systems in place at wineries all over the globe. In fact, the washing process will typically only remove the larger particles and

organisms on the surface of the oak, and can easily miss those organisms that are living deeper inside the wood staves. Many organisms, such as the spoilage critters Brettanomyces, can develop into very large colonies while only starting from a very small number of cells. So, even if you think you are cleaning the barrel thoroughly, you could miss just a couple of little buggers and they’ll still propagate and thrive to spoil your next batch of wine by causing off-aromas and unpalatable sensory characteristics.

Since the current method of cleaning and sanitation of wine barrels is relatively sub-par, a method that allows for the removal of all organisms from all crevices of the barrel needs to be found. There is some research on the topic; however, none of the methods so far have been completely effective in reducing spoilage organisms in the oak barrels. The authors of the study presented today introduced the use of microwave technology in order to remove the spoilage organisms from the cracks and crevices of the oak barrel. They claim that this technology could not only potentially solve the spoilage microorganisms problem in oak barrels, but could also reduce the levels of SO2 needed in wine

since it would no longer be needed to prevent the spoilage usually caused by oak barrel contamination. They also mention that using microwave technology to kill the spoilage organisms in oak barrels would result in the lowering of water consumption and energy costs since the usual high-energy blasts of water would no longer be needed to remove the spoilage organisms.

Methods

The equipment used to sanitize the oak was based off a “pulse train generator of high frequency microwaves”. Oak was exposed to the microwaves for 3 minutes at 3000 watts. Over the three minutes, the following treatment was applied: 0-90% for 10 seconds, followed by 90% for 40 seconds, followed by 90-0% for 10 seconds, all repeated 3 times for a total of 3 minutes. The authors noted that the maximum temperature that occurred on the oak was 48oC.

The equipment used in the experiment was only large enough to treat 30cm length oak staves, though ideally the commercial product would be large enough to treat the entire barrel.

The barrels used in this experiment were: a 3 year old American oak barrel, and a 2 year old French oak barrel. Both were confirmed to be highly contaminated with spoilage organisms from previous winemaking and storage. The barrels were first washed with hot water vapor pressure and then were broken down into staves. Half of the staves were randomly assigned to the microwave treatment, while the other half were assigned to the control treatment (no microwave exposure). Treatments were performed in duplicate.

Microbial analysis was performed by collecting wood scrapings from each of the staves to a depth of 8mm.

Results

• The most prominent organism found in both microwave-treated oak staves and control oak staves was Brettanomyces bruxellenxis, representing 99.6% and 98% of the respective microbial populations.
• Even after being washed with hot water vapor pressure, the staves from both American and French staves in the control treatment had a high level of microbial contamination.
o Total cell counts found in control oak staves were: 4.16 log units of total yeasts, 3.27 log units of Brettanomyces, 2.15 log units for lactic acid bacteria, and 2.48 log units for acetic acid bacteria.
• The microwave treatment on the oak staves resulted in significantly fewer microbial population counts than the control treatment.
o Total cell counts found in the microwave-treated oak staves were: 3.24 log units of total yeasts, 2.74 log units of Brettanomyces, 0.20 log units of lactic acid bacteria, and 0 log units of acetic acid bacteria.
• The microwave treatment resulted in a 36-38% reduction in the yeast population, a 35-67% reduction in the Brettanomyces population, a 91-100% reduction in the lactic acid bacteria population, and finally a 100% reduction (i.e. total elimination) of the acetic acid bacteria population.
• The percentage of reduction of microorganisms was larger in American oak barrel staves, which the authors deduce is possibly a result of the greater porosity of French oak barrel staves allowing for greater wine infiltration and thus greater difficulty in cleaning and sanitizing the barrels.
• The microwave treatment did not affect the chemical composition and quality of the wood itself.

Conclusions

The results of this study are fascinating in that microwave treatment of oak barrel staves effectively reduces yeast populations inside the pores of the wood, and nearly eliminates the lactic and acetic acid bacterial populations. While it doesn’t completely eliminate all of the spoilage microorganisms responsible for creating off flavors and aromas in wine (i.e. Brettanomyces yeasts), it does significantly reduce them, and in certain cases remove them all together (i.e. lactic and acetic acid bacteria). According to the authors, this microwave treatment is a significant improvement over any of the current cleaning and

sanitizing methods for removing potential contaminants in oak barrels.

I agree with the authors when they suggested perhaps combining the microwave treatment with another method for microorganism removal, such as laser, ultrasonics, or UV radiation, all of which are currently being studied as possible methods for contaminant removal. Perhaps the combination of one or more of these methods would create a synergy that could remove significantly more of the microbial population than either one on their own. Microwave treatment appears to nearly eliminate all of the lactic acid and acetic acid bacteria in the oak barrel staves, so combining it with a method that effectively reduces the yeasts populations may result in the “perfect storm” of spoilage organism removal techniques.

Determining if a system could be created that would effectively reduce or eliminate the spoilage organism populations in whole oak barrels, and not simply oak barrel staves, is of utmost importance. Real world applications of this technique will occur in the whole barrel, so it is important to determine if this method will work in this scenario, or if the system needs to be altered or tweaked in any way, in order to remain effective in this different scenario.

The authors mentioned that the microwave treatment did not affect the chemical composition of the wood itself, but only affected the microbial population. I would like to see a sensory analysis of wine fermented and/or stored in these microwaved barrels (compared to controls) in order to confirm that, in fact, the chemical composition of the barrels did not change.

If this technique can be applied on a larger scale (i.e. at the intact whole barrel level), it could be a very good investment for the wine industry, particularly if a combination treatment is found to eliminate not only the bacteria populations but also the trouble yeasts. Not only would the winemaker not have to worry about spoilage in their wines (theoretically), but they would not have to use as much SO2 for protecting against these potential contaminants, and they would not be wasting nearly as much water on the hot water vapor pressure washing process. Good for the winemaker, good for the consumer, and good for the environment (theoretically!).

I’d love to hear what you all think! Please feel free to leave your comments!

Source: González-Arenzana, L., Santamaría, P., López, R., Garijo, P., Gutiérrez, A.R., Garde-Cerdán, T., and López-Alfaro, I. 2013. Microwave technology as a new tool to improve microbial control of oak barrels: A preliminary study. Food Control 30: 536-539.