Reduction of Brettanomyces bruxellensis populations from oak barrel staves using steam

The aim of the study was to determine the impact of oak species, barrels toasting level, location of staves in barrels and barrel age on Brettanomyces recovery, before and after the application of steam.

Experimental layout:

• New 16 L barrels from American oak and French oak with light and heavy toasting levels were used. Each barrel had 24 staves with 20 mm thick midpoints.
• Four three year old 225 L barrels were obtained from a commercial winery: Two American oak (medium-to-heavy toasting) and two French oak (medium-to-light toasting). The barrels were infected with Brett but received hot and cold water wash and SO₂ gas at the winery before transport to the research facility. Each barrel had approximately 30 staves with 25 mm thick midpoints.
• The 16 L barrels were inoculated with commercially prepared Cabernet Sauvignon and then each type of barrel and type of toasting was inoculated with two different strains of Brett. Thus eight treatments in total. There were eight control barrels (different oak and toasting level types) as well that were not inoculated with Brett.
• After 6 to 7 months barrels were drained, rinsed and staves disassembled and numbered. This included the 225 L barrels.
• Yeast penetration was determined in two ways: staves sawn into blocks at midpoints and then further sawed in horizontal cross sections of different depths; and shavings obtained from drilling into the stave midpoints at 2mm depth increments. Cross sections and shavings were incubated in different ways to promote Brett yeast recovery.
• Additional stave blocks were subjected to steam treatment and Brett recovery determined.

Main results for non-steamed staves:

• In the new 16 L barrels Brett growth entered the logarithmic growth phase faster in the heavily toasted French oak than the light toast French oak and American oak barrels.
• For staves from the 16 L barrels Brett cells were recovered only as deep as 4 mm in the American oak, but in the French oak staves they were recovered up to 6mm deep in the light toasted staves and up to 8 mm deep in the heavy toasted staves.
• In the used 225 L barrels Brett could only be recovered in the 0 – 4 mm cross sections from top staves in all four barrels. However in the bottom staves both French and American staves contained Brett cells in the 5 – 9 mm cross sections. Note that these are used barrels so it could explain the difference in results to the new American oak 16 L barrels. The recoveries from shavings corresponded to the recoveries from cross sections. No Brett cells were recovered beyond 8 mm.
• Polymeric pigment and pigmented tannins were generally recovered to the same depths as Brett cells with the furthest penetrations in French oak barrels.
• Bottom and heavy toasted staves had larger Brett populations. Heavy toasting releases more cellobiose that is a sugar that benefits Brett growth.

Main results for steamed staves:

• Heat penetration of staves depended only on depth of staves: 20 mm or 25 mm.
• Steaming American oak stave cross sections from the 16 L barrels for 9 minutes resulted in Brett being undetectable.
• After 9 minutes of steaming French stave cross sections from the 16 L barrels Brett cells were undetectable in the 0-4 mm cross sections, but still detectable in the 5 – 9 mm cross sections. No cells were detected after 12 minutes of steaming.
• The results for the 225 L barrels were the same as for the 16 L barrels.
• In contrast to previous research Brett cells were occasionally detected beyond red wine pigments.

Significance of the study:

This study found that Brett penetration tend to be higher in French oak barrels, used barrels, heavily toasted barrels and in staves at the bottom of the barrel. Twelve minute steam treatments are effective for the treatment of staves if Brett penetration is up to 9 mm. Take note that future research is needed to test steam treatment of highly infected whole barrels.

Reference:

Cartwright, Z.M., Glawe, D.A., and Edwards, C.G. (2018). Reduction of Brettanomyces bruxellensis populations from oak barrel staves using steam. Am. J. Enol. Vitic. 69 (4), 400-409.

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