Check some of the recipes in the recipe section. I have used fresh raspberries and there are a few recipes for that. It seems most use 3-4 pounds. You could try 2 techniques: one, peel peaches and cut into small pieces, pasteurize, then pour into secondary on top of the beer; two, peel peaches and put in a blender to make your own puree, then pour into secondary. Either way, put them in the secondary because I believe the yeast activity in the primary eats up the flavor. I had a peach ale once a long time ago and really liked it. Can't wait to do my own.
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Drinking: Fat Guy Porter, Fat Guy Tire, AJ's Raspberry, SWMBO Slayer
Up Next: Xmas Ale