A cluster or two now and then should be no problem. Or do you have a huge amount of grapes. If so have lots of dry cardboard paper type items on hand to add dry. Have several plans in place to prevent and deal with fruit flys. The worms will love the grapes. The stems are persistant.

The problem with citrus has nothing to do with acid. Toss the grapes in your bin.

I have given my wigglers slice oranges on occasion, and they LOVE them !! I cut an orange in half and buried the halves on the very ends of my worm bin. In just a day, the worms had migrated to them and that orange was filled with little worms just gorging themselves on orange. Gluttons !! LOL

I wouldn't put too much in though.

From a grocer I got about 2 shopping bags full of rotten grapes. I removed them from the stem and put the grapes in ziplocks. Since I've got no more space in my freezer, I just put them in the garage and that was about 3 weeks ago. I still have about 4 bags ziplocks left. They're moldy and fermenting. Before feeding the grapes to the worms, I sqeezed out the juice (goes to my potted plants) and let it air for a while. I feed my FT 1 small ziplock at a time and so far it's OK. And yes, I put lots of shredded newspaper around it.

helencolby: you're right about the gluttons. With the rotten grapes, I also got one (rotten) of those oversized grapefruits (but this kind tasts bitter). I cut it in half and after 10 days, both halves were full with worms, just worms, no VC. The worms are happy, I am too, LOL.

Worms will eat the grapes. As you continue with your vermicomposting, you will find that every worm colony develops its own particular tastes - it's always interesting to see what fruits and veggies they like and will eat first and which they don't like as much. Some worm colonies will wolf down the grapes and others might not eat them in any kind of hurry. I have two worm bins right now and one colony doesn't care for grapes at all and they digest them slowly and begrudgingly only when the rest of their favorites are all eaten up but these guys love oranges which I find surprising! My other worm bin likes grapes just fine though and eats them quickly; conversely that bin doesn't seem to like some of the things the other colony favors!