There are a lot of things that can cause bitterness in cheese. You may be running into using too much rennet, or too much calcium chloride. Some recipes are really poor. Always taste your curds as they are going in the mold. If they taste at all bitter, then it's too much calcium chloride. Too much rennet will make cheese bitter over time. I've used as much as 60 IMCU per liter of milk, but usually I use about 46 IMCU per liter of milk. Single strength liquid rennet is usually about 200 IMCU per ml and there are 20 drops per ml. So that means there are about 10 IMCU per drop. So normally I use 4.5 drops per liter of milk and I haven't had troubles up to 6 drops per liter of milk. A tsp is 5 ml, so often you see recipes for things like X tsp per gallon :-P Basically a tsp is 1000 IMCU so you can do the math :-)
But with your washed rinds, the problem is almost certainly b. linens. In high moisture cheeses, it can make them *very* bitter. Basically, you have to slow it down or it will basically rot your cheese. Especially if you are adding geotrichum to the milk (or spraying it on), the rind will get to a high pH quickly and the b. linens will show up quickly. As soon as it is established, put it into the normal fridge and finish aging it that way. I honestly recommend *not* adding geotrichum to washed rinds, though, because things just grow way too fast. It will show up on its own anyway.
Mixed rind cheeses (where you want a mix of geotrichum and b. linens) are tricky in my opinion. But again, once you notice b. linens at all, you probably want to get it in the normal fridge to finish aging. This is actually also true of bloomy rinds. As soon as you have full white coverage, they should be aged at 6 C most of the time, IMHO. The exception is when you are planning to eat them very early.
Or, at least, that's my experience.
