I really didn't think about the pH of the stout reduction, and I realize that this was a mistake. I think I ran into a similar issue with my horseradish Cheddars, adding pickled horseradish and not accounting for the acidity I was introducing. It fused OK, but not great. It fractures easily along the curd lines. Judging from the flavor that a Gouda that I made picked up when I soaked it in stout, I think that soaking may be the best way to add some flavor. I've been following another thread where the cheese has been soaked in wine for a day, then dried for a day, then soaked again. For flavor, I think that soaking may be a better method, but I'm trying for that pretty effect of the dark lines outlining the curds in the cheese; I may be better off just focusing on making better tasting cheese, and ignoring the glitz factor. Honestly, the Caerphillys that I have made have been the best - just good simple cheese. But I love to tinker!