Reduction of Brettanomyces bruxellensis Populations from Oak Barrel Staves Using Steam
Zachary Cartwright,* Charles Edwards, and Dean Glawe
*Washington State University, School of Food Science, PO Box 646376, Pullman, WA 99164 (email@example.com)
Brettanomyces bruxellensis is a spoilage yeast associated with oak barrels used for aging of wines. Infected barrels made from French (Quercus petraea) or American (Q. alba) oak were used to determine the impact of different toasting levels (low or high), size (16 or 225 L), or age (one or three years) on yeast populations and penetration into staves. In addition, time/temperature combinations using steam were explored as means to reduce, if not eliminate, resident populations. Penetration of the yeast into states was evaluated by collecting shavings prepared by a Forstner drill bit which were then incubated in a Brettanomyces recovery medium. Additional staves were cut into 3 × 10 cm blocks and randomly assigned to steam treatments lasting 0 to 12 min. Following heating, blocks were sawn into 4 mm thick layers and assessed for yeast viability by placement in a nutrient-rich red wine for >60 days. Yeast penetration studies revealed populations of 103 cfu/mm3 in the layer closest to the wine (<2 mm) with lower values (≤102 cfu/mm3) at 2 to 4 mm deep. French oak staves consistently contained higher populations than American oak and penetrated farther (up to 8 mm) regardless of barrel size or age (p < 0.05). If located at a depth of ≤4 mm, exposure to steam for a minimum of 9 min was required to no longer recover viable yeasts. A minimum heating of 12 min was necessary to achieve the same extent of reduction for yeast penetration of 5 to 9 mm. Different strains of B. bruxellensis yielded similar temperature/time reductions (p ≥ 0.05). When present within 9 mm of the inner surface, a steam-ing time at least 12 min is recommended to effectively reduce B. bruxellensis in infected oak barrels.
Funding Support: Washington Wine Advisory Committee, Ivory Tower Scholarship Fund