pH of potato dextrose broth (PDB) - (Jun/22/2008 )

Hallo all,

I suppose all of you have worked with PDB before, but has anyone ever wondered why there is a drop in pH when you heat it or sterilize it in an autoclave?

I wonder whats the exact reason for this drop. ANyone here that can give a chemical explenation for it?

And I am not talking about reheating it after you acidified it.



I think it has something to do with the dextrose, that you make it caramelize, but I am not sure about this.

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good question - write the media houses (BBL) - maybe they have an answer.

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Maybe I will be attacked for this answer, as I have no primary literature for this, but I take the bull by the horns

and hope it helps you pito !

As far as I remember it has to do something with agar-hydrolysis caused by the autoclaving temperature which is supported by the relatively low pH of fungal agar-media (PDB usually 5.8 +/- 0,2)
or you have overheated (longer autoclaving time, long time at 50 °C) your medium --> non-enzymatic browning reactions (like the caramel, but there are others too).

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I you use potato dextrose broth , then there is no agar.
So no agar hydrolysis.

Secondly, the hydrolysis thing is mainly an issue or problem when you acidify your broth or agar. YOu can NOT reheat your media after acidifying it because then you can hydrolyse your agar (in combination with the acid) ==> check this, page 3 , top corner: http://www.bd.com/ds/technicalCenter/inser...xtrose_Agar.pdf

or page 1 , right corner at the bottom.



And I know there is such thing as the maillaird reaction wich is a form of an non enzymatic browing reaction. But for this reaction you need reducing lower sugars (dextrose is a lower sugar) and aminogroups or proteins wich are probably present in the potato starch?

So yeah, you can have the maillard reaction.
But I am not sure if this is the only cause.

and yeah, maybe I better simply write to some producer of pdb to ask this questions.
I indeed might as well just do that.

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sorry , I use PDB only for the production of agar plates (and maybe once a year for broth), so I did not think one second of you using it as "broth"....

let me know if you found out more (although I like the maillaird reaction as explaination)!

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Spare us gerb - you know nothing of the medium. Pity you can't keep your silly ignorance to yourself.

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as jorge1907 allready stated, I contacted a few of the producers and got an answer back from 1 of them ===>

answer: "Please try to autoclaved the medium as said in the label: Suspend 26,5 grams of the medium in one liter of distilled water. Boil for one minute and sterilize at 121°C for 15 minutes.

The drop of pH is due to a chemical reaction, nor enzymatic.

The Maillard reaction ( non enzymatic proteins glucosilation), is a complex combination of chemical reactions that take part between proteins and reductive sugars when you heat the medium. Basically it is a caramelization.

In one of the phases, intermediate products are created and although the mechanisms are not completely known, it could change the pH.

The NaOH should not be autoclaved, the medium should be sterilized and when it has been cooled to 45-50 ºC add the NaOH."



Intersting to see: autoclave only for 15 minutes, I always have been toled to autclave 20 minutes at 121°C, how about you guys? How long do you autoclave?


And I also sometimes added NaOH before autoclaving , anyone here worked with NaOH before to make make the pH more base?

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