How do I use beer barm to make sourdough? I have been making sourdough now for the last six months or so, fairly successfully, using a starter that I created on the bread-making course that kicked this all off.

While on the course, I mentioned to the instructor that I also brew beer and he said I could use the beer barm to make bread. However, I'm not sure exactly how to do that.

Barm is defined as


  the foam or scum formed on the top of a fermenting liquid, such as
  beer[...]


Now, the first problem I have is that I do not have easy access to the top of my fermenting beer. In order to keep things sterile, the lid of our fermenter is kept firmly on until after bottling. I am therefore left with whatever we have left over after bottling. I have emptied this into a tub and the result is a lot of sediment at the bottom with fermented beer on top. I'm not even sure if this qualifies as "barm", although it must contain whatever was on the top of the liquid before we bottled.

This is a side view that shows the separation.



and here is some of the sediment scooped off the bottom.



Question: Can I use this to make a starter? If so, how? Which part do I use, and what do I do with it? I'm not even sure exactly what the sediment is.

And is there any part of it I shouldn't use, or can I just bung it together with some flour and see what happens next? :-) I'm more than happy to experiment, as long as it's safe!
What you've got there is a mix of precipitated proteins from the wort (the "trub") mixed with Saccharomyces cerevisiae spores, and also various compounds from the hops. It's on the bottom of the beer because you're making an ale; only lagers have the yeast floating on the top, and then only during active fermentation.

Taste a bit. Then drink something to get the awful taste out of your mouth. :-)

You can absolutely use those spores to start a sourdough starter. The hop compounds may slightly retard the growth of the bacteria which you want in a sourdough starter, but after a couple of feedings they'll be basically gone. If you're worried about that (or the taste) you can use a process called "yeast washing" to get most of the trub out of the yeast.

As for whether you should do this? I wouldn't bother. The ultra-pure strain of S. cerevisiae that's been selectively bred for beer fermentation is eventually going to be crowded out by some other, scrappier wild yeast. Honestly, the most successful sourdough cultures I've started have been made of flour, water, and kismet.

Historically, yeast cake from beer brewing has been used for baking bread directly (rather than through propagating a starter over time), because it was an easy way to get a large quantity of a pure strain of yeast. That is, it was used to make non-sourdough bread. Now we have packets of instant yeast, for beer and for bread. Since you're using the instant yeast for beer, I don't know why you wouldn't use (a different) instant yeast for bread.