Thursday, 29 July 2010

Purifying a culture

Reading Amber, Gold and Black has inspired me to try brewing a few beers. In fact I haven't got as much inspiration from a book since I read Radical Brewing

For one of the beers I needed a Brettanomyces yeast and thanks to the wonder of the internet I got a culture from a home brew shop. The culture was in liquid medium, which I have to say made me a little wary. With liquid cultures all you get to see is a turbid suspension, but you have no idea which organisms might be in it. Before I used it for brewing I thought it prudent to streak it out on an agar plate so I'd be able to look at the colonies of whatever was in there and see if they were what they were supposed to be. It was a good job too, as when I stuck a loop in the vial I could feel there was something a bit solid in there, most likely a filamentous fungi. I streaked it out and sure enough what mostly grew was mould. 

On the plus side there were some yeast colonies in there as well, and thankfully some of them had separated out from the mould (the dots on the right). I picked a couple off and subcultured them onto a fresh plate with gratifying results:

The culture is a bit older than in the first picture so the yeast colonies are larger but I think the difference in pretty clear. I've saved the Brettanomyces on an agar slope as I don't need it just yet but I now feel confident about using it. 


  1. Awesome stuff. Wish I had the kit to do this sort of thing. How long will you be able to keep it on the slope for?

  2. Should be fine for at least six months. Any culture you're after a particular? I should be able to help.

  3. This is an excellent post, I've not seen this sort of thing before, really interesting!

  4. Good stuff Ed. What are you going to brew with it?

  5. Now this is beer blogging! More yeast porn please Ed!!!

  6. If I'd known you were all so keen on microbiology I'd have tried to get some better pictures and explained in more detail.

    The beer I'm planning to do is something inspired by Colne Spring Ale

  7. It's disturbing to think that commercial vendors are selling yeast cultures contaminated with other microorganisms. Most buyers would just have used what was in the vial and ended up with mouldy beer.

  8. It would have been a disaster if I hadn't cleaned up the culture and I don't really know what I would recommend non-microbiologists do when buying new cultures. Liquid ones in particular are prone to contamination, but I've bought a liquid saison yeast culture as well and that was fine. Maybe always make it up in a small starter culture first and don't use it if it doesn't seem right.

    I was so pleased I could actually get the rather obscure culture I was after I'm not worried that it took a little more work and I haven't complained to the shop.

  9. Damn!
    Interesting stuff. Up to this point I'd just assumed that the cultures are simply what the label says they are, and that's all.

  10. I don't think any are totally pure, though as I've said liquid cultures are the ones I worry about most.

    Here's some of the specifications for a Fermentis dried yeast:

    Viable cells at packaging: > 6 x 109 / gramme
    Total bacteria*: < 5 / ml
    Acetic acid bacteria*: < 1 / ml
    Lactobacillus*: < 1 / ml
    Pediococcus*: < 1 / ml
    Wild yeast non Saccharomyces*: < 1 / ml

    So they're saying the contamination level is very low but guaranteeing it's not totally absent.