Type of Fruit
As with most flavor related questions...it depends; sorry. There are generally four or five fruit sources: Fresh Fruit, Frozen Fruit, Fruit Puree, Fruit Juice, and Fruit Extract. Each is going to give you a different flavor, and depending on the fruit flavor you're looking for, one or the other might be the best. The real answer is that you should test all of them.
But, going from my experience, I'd narrow it down to Fresh, Frozen, or Puree.
- The Oregon Puree's are very good products and you generally don't need to worry about sanitation. There are probably other puree's out there, too.
- Fresh fruit is great. I live in Michigan, so I've only done my cherry beer in Cherry Season so I can get the fruit absolutely fresh. If you get Fresh Fruit, I would recommend freezing it before using it to break the cell walls within the fruit which will allow the beer to gain access to sugars and flavors easier.
- Frozen fruit is usually fine, too. It's often frozen very soon after picking, so if you can't get fresh fruit, frozen is the next best thing. For beer you tend to want Tart Cherries, as you mentioned, so you might have to look around to find frozen tart cherries (I've never looked; I don't know how difficult this would be).
I haven't used Cherry extract before. Cherry candy doesn't really taste like Cherry fruit, so I imagine the extract will probably taste like cherry candy instead. I might be wrong; I often am. I think for Cherries you'd really want to use the fruit or puree, but since I haven't actually tried an extract you'll have to tell me. The people I've talked have said that for some fruits, extract is great, and it's super easy (see below). But other extracts aren't complex enough. You'll have to test it out.
You might decide to use a combination, too. You could use fresh fruit for flavor, and extract for some extra aroma (or, maybe it's the other way around...). Experiment!
You should decide what kind of fruit you'll use first; sourcing the fruit gets easier after that.
- I believe you should be able to find fruit purees at your homebrew shop; if not, most of the online homebrew shops sell them. You can probably get fruit extract through the same sources.
- You should be able to get the fresh and frozen fruit at your local grocery store. You'll probably only be able to get fresh cherries in season, but your mileage may vary.
How to Use the Fruit
I said earlier that it depends. It still depends. Look around at other fruit questions on the board and you'll see various methods. That being said, I think there are three popular methods:
- During the boil One of the concerns with fresh fruit is sanitation, so brewers will often add fruit for the last 5-15 minutes of the boil to attempt to sanitize it. It will drive off some aromatics, but it shouldn't be too bad. The Pro to this method is sanitation, the Con is knowing how much fruit to add and giving the yeast too many simple sugars up front; it can make yeast lazy.
- Secondary After the beer finished fermenting, rack it off to a second container on top of the fresh (that you've frozen after purchasing), frozen, or puree'd fruit. This is true secondary fermentation, because the yeast will begin eating fresh sugars. The Pro to this method is that you can periodically taste the beer and rack off when you get the flavors you're looking for. The Con is potential sanitation issues with the fruit and multiple rackings, and the need for more fermentation vessels.
- Dosing before bottling/kegging The last method is for the fruit juice or fruit extract. After your beer is done fermenting, take several 2oz samples of beer and dose them with various amounts the juice or extract. When you find the right amount, scale that up to your batch size and you'll have the fruit flavor you want. The Pro to this is complete control over flavor. The Con is depending on a fruit extract.
All in all, I've preferred the secondary method with fresh/frozen/puree'd fruit. Start by brewing a great beer then rack onto cherries and taste periodically until you get the flavor you want. I used 7 pounds of cherries for a 5 gallon stout.
You'll probably have to experiment until you find something you like. Be sure to tell us what you did.