I had mushy plates when I first began pouring drop out plates too.
Now I always autoclave the agar separately at 2X concentration, and
combine with carbon source, YNB, drop out mix, etc. just before
pouring. I cannot remember ever having mushy plates with this method,
and I have never checked the pH of my plates in my life.
I offer the following model to explain the mushy plate problem:
- mushy plates are caused by hydrolysed agar.
- agar is hydrolysed at high temperature in acidic solution
- the hydrolysis is not rapid enough at moderate acidity to be a
problem if the temperature is not kept high for too long.
- if you keep your pH up you will have no problem.
- if you keep your autoclaving relatively short and not too hot
(say 15-20 min at 120 degrees) you will have no problem.
This also helps with caramelisation, but that's a different
story. However, if your pH is extremely low, you may still
have some hydrolysis.
- if you autoclave your agar separately you will have no
problem. (personnally, I doubt that acidity at the moment of
pouring unhydrolysed agar is a problem, but I will listen to
anybody who disagrees)
I think this is essentially in agreement with what other posters have
said, and I would say that all the solutions are probably equally
John Bratty bratty at medcn.umontreal.ca
Dept. de Biochimie
Universite de Montreal (514) 343-6111, ext. 5165
Montreal, Canada H3C 3J7