I am brewing a RIS and giing to rack it directly on to a yeast cake from a brown ale. How do I sanitize the fermentor before racking on to the yeast?
It should be sanitized already from the previous batch that was in there. As long as your previous batch wasn’t infected you should be fine just pouring the new batch on top.
You don’t need the entire cake. Sanitize three Ball jars and lids, pour a little cold sterile water on top of the cake and swirl to loosen it, then divide evenly between the jars. Clean and sanitize the fermenter, pitch two of the jars to the RIS, and put the third jar in the fridge for another batch of beer.