After asking Elmus how the mead production is going, he explains he requires some juniper berries to give it more flavor:
How is the mead?"We lost most of the batches we had brewing when... before those things were here. One is just ready to take its turn, though, and it's about time to add some of the flavorings. You can't find them on Solstheim, but if you find yourself back in Skyrim, I wouldn't mind you bringing some juniper berries. They add a different kind of sweetness. Tart. Makes the flavor more round."
Simply bring Elmus one sample of juniper berries. They can be found all over the Reach or sometimes sold by alchemists.
Upon receiving them, he will excitedly say "Oh, perfect. Just the right ripeness, too. Good work, friend." and rewards the Dragonborn with a leveled sum of .