What are the differences between fermenting a beer solely with Brettanomyces (or some blend of sour yeast) versus adding it to the secondary after primary fermentation with a non-sour yeast?
I'll have to do some checking, but from what I understand, the biggest difference is time, the next being a vast difference in sourness. If you ferment first with a non-souring yeast the majority of the fermentable sugars will be converted quite quickly. Brett is very slow to ferment and also to multiply. If it has to ferment all of the sugars alone it will take a considerable amount of extra time to complete fermentation, months longer. As far as flavor is concerned, I'll have to get back to you. I think the Pediococcus and Lactobacillus are mostly responsible for the sour taste but I may be wrong, but I'm pretty sure that the Brettanomyces is responsible for the "barnyard" flavor. With all of the sugars being fermented by only souring yeasts I think you would end up with a significantly more sour beer, one I may be hesitant to drink. Something like vinegar maybe. If the temperature gets to hot in Belgium, these bacteria "yeast" increase their fermentation rate and actually yield a vinegar like product that the brewers will actually use to clean brewing equipment in the brewery. I've been wanting to try this myself though and just be cautious of the temperature, keep it below 75 to be safe and see how it turns out but I'm worried about ending up with vinegar regardless.