I have about 5lbs of Cabernet grapes from a vineyard on Howell Mtn (Calistoga CA). I just picked them this AM and they still have their dusty yeast cover. I am brewing a Belgian Bruin and was going to add Roeselare blend in secondary. I am know thinking of 1. Adding the grapes to the wort for a wild primary ferment or 2. Adding the grapes to secondary for flavor and souring. This is an experiment, any thoughts of what will work best?
I harvested yeast off of a peach before. I used a couple different small containers to see how they fermented out. A few them got very gross. I would do a test ferment and see what you can grow before adding to a whole batch.
I’ve done something similar with Zinfandel grapes. Primary ferment with the Roselare and added the grapes in secondary. It worked well.
Generally wild yeast are not great attenuators and not great at fermenting complex sugars well so personally I wouldn’t do either expecting much in the way of results from yeast on the grapes. If you add them to primary you’ll have to follow it with your pitch fairly quickly and then you have to consider issues regarding whether you are blowing off some of the more delicate grape flavors/aromas and how you will deal with the extended contact time with the skins. Add them later and you’ll have more control over the grape attributes but highly unlikely you will get anything fermentation-wise at that point.
I think your best path here is to think about the beer on its own and the grapes as a secondary addition so you can make a great beer with a great grape addition.