Funky Buddha has become quite close and cozy with grape must.
Must is freshly-pressed grape juice, with all of the skins, seeds, twigs, and other stuff still there. Those solids (called pumice) are kept in the juice as a flavoring, then filtered out and pasteurized to make grape juice, or fermented to become wine.
A few months ago, I tried Undefeated Saison which featured, among other ingredients, Concord grape must. Quite delicious, and provided a great sweetness to the beer.
Of course I was excited to see that grape must was an important ingredient to their new bottle release, Iron and Barleywine (English Barleywine, 10.5% ABV, 30 IBU).