Oak barrels, used widely in winemaking, are a major concern as a source of potential microbial contamination. They require labor-intensive care and maintenance, and the porous wood is notoriously difficult to thoroughly clean and sanitize. Confirming that sanitization procedures have been successful is also an issue for barrels, unlike with non-porous metal surfaces that can be tested by swabbing or other methods.
In a recent paper published in the American Journal of Enology and Viticulture (doi: 10.5344/ajev.2020.20047), researchers described their search for a better way to reliably predict how much time is required for steam to effectively sterilize oak wine barrels.
The researchers used a 5-log (99.999%) reduction in the number of living microbial cells as their criterion for effective sterilization, and they calculated the temperatures required to reach various levels of microbial reduction for three organisms, including the yeasts Saccharomyces cerevisiae and Brettanomyces bruxellensis. Using the physical properties of the steam and the oak barrel, such as density and thermal conductivity, the researchers also calculated the temperature change at various depths within the oak barrel staves during steaming.
With these calculations, they were able to mathematically predict the relationship between the length of steaming time, the depth within the oak staves, the temperature at that depth, and the amount of reduction in living cell numbers.
Based on their mathematical model, the researchers concluded that 9 minutes of steaming was capable of a 5-log reduction in Brettanomyces bruxellensis at a depth of 0.8 cm within the staves, and 12 minutes of steaming was enough to achieve a 5-log reduction of Saccharomyces cerevisiae at a depth of 0.8 cm. However, the exact physical properties of a barrel may depend on characteristics such as grain tightness. The authors also point out that different microbes have different rates of thermal inactivation (heat death). For these reasons, winemakers should consider these to be the minimum guidelines for effective barrel steaming.
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