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Help! My plates won't gel

Bratty John bratty at MEDCN.UMontreal.CA
Thu Aug 25 20:26:48 EST 1994

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                               

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