Tuesday, 9 February 2010

Making Slants for Yeast Growing

I bought some glass tubes with metal, autoclavable, screw caps from ebay this week for making slants.  I already have a load of pyrex tubes with plastic stoppers but the yeast dries up very quickly in those so I thought I'd try these.

A slant is a tube containing a solidified wort (growth medium) that you innoculate with yeast in order to keep a small number of yeast cells from a starter or a batch of beer for use in a later batch. 
The growth medium is sloped up the side of the tube to make it easier to lay the yeast cells onto it quickly to limit the risk of infection.  It's a great way of making a tube of liquid yeast go a long way but it's also good fun in a sort of geeky way.

So making slants is very easy.  I use a wort of about SG 1.015 and then I add about 3-4% agar to that.  For me this means adding 260g of water to 12g of light, dried, malt extract.  I then boil this up and add about 9g of dried agar (agar agar if you want to be pedantic).

I then take a syringe and put 10 ml of this sticky worty mixture into each tube.  The tubes hold about 30 ml so by the power of mathematics I make that about a third full.  I then put the lids on loosely and cooked them in the pressure cooker for 15 minutes.

Now, I keep the lids on loosely so that the tubes don't explode when the pressure builds up in the pressure cooker.  It also helps to keep the moisture out of the tubes.  Then I put the lid on the pressure cooker.

Once they've been cooked under pressure for 15 minutes I turn off the gas and allow them to cool enough for the pressure to equalise in the cooker.  If you don't do this and you release the pressure too quickly the wort will bubble up in the tubes and you'll end up with a sticky mess in the pressure cooker.  I don't know about you but I'd be very unpopular if I did this!

While the tubes are still too hot to handle I remove them one by one and rest them on a piece of wood with the, still loose, caps higher than the closed end.  This allows the wort/agar mixture to solidify in the tube at a jaunty angle ideal for growing yeast on.  Once they're cool they can be stood upright and the lids tightened.

Before they're used for yeast it's a good idea to prove them for a few days in a warm place.  The airing cupboard is good for this but anywhere about 20-24ºC is fine.  What you're doing here is making sure that nothing grows in the tube.  If anything does grow then you've got an infection and the slant's no good.  Clean the tube and start again!

Hopefully in a few days time I can show you how I innoculate the slants.


  1. Like the tubes. Metal lids should increase the shelf life although I have used by glass bottles with plastic lids successfully to keep slants for 12 months.

    Can you send me a link to the supplier. Ta.


  2. The method you use will work for sure. The weak link is having the lids on the bottle. The air within has to be displaced by the steam in order to sterilise the inside of the bottles. The lids, although only loosly attached restrict the steam's access.

    If you want to be anal about it you could sterilise twice. Once with the lid off and then again with the lid loosely fitted.

    Alternatively: at 15psi/1Bar steam under pressure needs 15 minutes to achieve sterility but will take a finite time to displace the air from the bottles so it might be better to extend the sterilising time to, say, 20 minutes. Less arsing around like that :-)

  3. Good point, thanks. I'll see if anything grows in the airing cupboard and will modify my method next time.

    I'll send you the link via email.

  4. It will probably be OK. Most microbes are killed pretty quickly by steam under pressure. The extend time is needed to finish of more resistant stuff like some viruses and spores. As these are not usually brewery contaminants there shouldn't be a problem. But extending the sterilising time would just make sure.

  5. As it happens the pressure cooker ramained pressurised for another 10-15 minutes anyway. I don't think there'll be a problem but I'll leave them off next time if it's "Best practice".

  6. phil
    as the lids are on loosely until the jars have cooled what is to stop the cooling air in the jars 'sucking in' contaminated air as it reduces in volume. or does this just never happen. ideally the jars could be cooled a little in the pressure cooker before it is opened. then the lids tightened immediately before cooling otside on a slant. or have i missed something
    regards mark