Chef Naomi Pomeroy recommends BLiS #9 barrel aged sherry vinegar since it begins to concentrate with time, taking on the flavor of a fortified wine like Madeira. 
"Lately I’ve been cooking from Taste and Technique, the intense/great new cookbook by Naomi Pomeroy. The Portland, OR, chef recommends aged sherry vinegar since it begins to concentrate with time, taking on the flavor of a fortified wine like Madeira. She uses a sherry vinegar from BLiS that’s been aged eight years in bourbon barrels. I’m not quite there yet, but something tells me I will be soon."
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