room temp orange juice and grape tannin

I've got a book, Making Wild Meads and Wines and it looks like every single recipe calls for 1 1/2 C room temp orange juice. This is beyond the tablespoon of acid blend it calls for. They also call for 1/4t grape tannin in each recipe. I'm hoping to get a little information on WHY? What do OJ and grape tannin do to the profile of mead?

Cheers

I've also seen recipe books and other references routinely call for citrus and tannin additions...one recipe book had lemon juice and a small amt. of black tea in most recipes.

This had similarly led me to believe all meads needed some acid and tannin, but after reading Ken Schramm's book (which has a nice discussion on how to use acid and tannin additions to fine tune taste and mouthfeel), I really think that you need to wait to the bitter end to see if you really need them.

I have no idea why your source would recommend both acid blend and a citrus routinely