Making agar containing media - (Feb/18/2014 )

Dear all,

when I make media that contains agar, I always adjust the pH prior to adding the agar. I do it this way because they told me so.

Now I wonder: why is this?

Why can I not just add the agar and then adjust the pH?

No one here seems to know why, they just know you have to do it like this.

thanks in advance

one reason is that you may damage the agar. then it won't gel properly.

mdfenko on Tue Feb 18 13:49:06 2014 said:

one reason is that you may damage the agar. then it won't gel properly.

This I know, but its only for very low pH values, right?

But in general: if you dont really need to adjust the pH or just a little bit (lets say for standard agarplates, pH 7)

If you add the agar at RT and adjust pH, the dispersed powder in the liquid can make difficult to adjust the pH properly or even can damage the probe. If you dissolve the agar beforehand, the solution will be really hot, and given the pH in temperature-dependant you will be using a wrong pH-meter calibration, especially if the pHmeter does not have a temperature electrode for temperature compensation as you will be using a calibration done at RT for a hot solution and even if it has it, the temperature difference between the RT used for calibration and the solution is so different that it diminishes accuracy as the temperature compensation is not perfect, and the probe might be damaged when it cools and there are agar residues on it.  Also, dealing with hot liquids is always a pain... and the agar barely affects to the pH when you add it.

If you are lucky enough to use common agar media don't complicate yourself. I use PDA pH 3.0 to keep my fungal isolates and I have to sterilize agar and pH-adjusted broth separately and then mix them before cooling completely.

if you want to modify the pH, adjust the agar first... adding extra agar wont hurt your microbial buddies

and after autoclaving, never take your media immediately... let the steam get away first... then remove from autoclave

happen to me once, trust me ;)

I dont really understand what you mean.

You mean you can add the agar first and then adjust the pH?

Prasetyo Muhammad Dwi on Tue Mar 25 18:16:28 2014 said:

if you want to modify the pH, adjust the agar first... adding extra agar wont hurt your microbial buddies
 

 

i wouldn't add the agar first then adjust. you will probably "clog" (nice, technical term) the electrode's porous plug.

I never bother to pH my media... it always works still