From: slk6p@cc.usu.edu
Newsgroups: rec.crafts.brewing
Subject: Oh what a dreg it is! Culturing
Date: 18 Aug 92 21:47:54 MDT
Organization: Utah State University

>and bought some Potato Dextrose plates (pre-prepared and sterilized)
>for $0.36 each (package of ten.)... They also threw in a handful of
>plastic pipettes at 5 cents each..

*This could become kinda pricey if you start doing this regularly.
You could try pouring your own plates.  Glass or presterilized plastic.
Agar can be used at 1.5% (ie 15 gms per liter) or gelatin.

>Now, onto my questions:   

>How long can I leave them in the fridge (they're stored upside down with the
>agaron top to alleviate moisture from dropping on the plate).   

*They would probably last at least a month, maybe longer.  But to maintain
happy yeast, and minimize mutations while maximizing viability, I would
recommend aiming at transfering them at least every month, or two weeks.  Pick
one big homogenous, healthy looking colony.  Pick it and streak to a new plate
for single colonies.  Grow this up a couple of days, then refrigerate. (lid
side down is good!)
    If you want to store them safely (sterily) for longer periods-
might I suggest Agar Slants (or tubes of agar).  In a tube there is 
less chance of contamination.  On the other side of the coin there 
is less opportunity to observe contaminating colonies/growth.  
    My suggestion:  Streak to a plate for single colonies.  Pick one 
isolated colony- inoculate a slant tube with it.  Then- inoculate a 
plate from the slant a week before you brew.  Use growth from the plate
(doesn't have to be one colony) to make a starter culture a day before 
you brew.  Brew/pitch/ferment.


>Is there any easy way to tell how pure of a strain I got? I mean, if I 
>got bacteria in with the yeast on the plate, would it look any different?
*Things to look for:
    Colors: other than a creamy off white. (red, yellow.. etc)
    Textures/Shapes: You want a consistent texture/shape of colonies.
        Mostly roundish, with somewhat wavy edges as they grow older.
    Light Transmittance:  Hold the plate up to the light- passing through-
        Look for colonies which are transluscent- let light pass.  If 
        there are opaque ones (darker) consider them contaminants.  You
        can still pick a pure colony off of a plate with a contaminant  
        elsewhere on the plate (unless you have fuzzy fungal hyphae and 
        spores all over)
The main thing is that you want homogeneous growth on the plate. Variation is
something to be cautious of.  Do streak for single colonies for best identiftn.

>Is it valid to use Potato Dextrose for culturing?    
*Sounds like a good one.  Used commonly for yeast.  Sauberaund Dextrose is
another goodie (sp?) for yeast.  Basic Malt extract/yeast nutrient agar is also
an ideal growth medium for yeast (as our brews will attest to!)


>Should I be super sterile? (ie: I have access to a class 100 clean room   
in which they normally do wire-bonding onto dies.) Should I bother with
the clean room and masks/rubber gloves?          
* Try not to spit on the plate.  Wearing a common dust mask and painters hat
won't hurt. (I don't bother- I'm just careful)
You shouldn't need the lab unless your apartment is really nasty!
Avoid the kitchen/bathroom locales for culturing.  I use the desk in my
bedroom.  I spray it down with lysol, close the door (drafts) and make sure my
hands are washed (you can rinse them with bleach soln, or EtOH if you want.
Then just be careful.  Use the lid as a shield when you do transfers.  Don't
leave the lid off any longer than you have to.
    And ultimately- DON'T TRUST ANY SURFACE IN YOUR HOUSE TO BE STERILE!

    Don't worry- be hoppy.
_____________________________________________________________________
                                J. Wyllie (The Coyote)
                                SLK6P@cc.usu.edu

"As long as he's got 8 fingers and 8 toes, he's alright by me."  H.J.S.
_______________________________________________________________________