I've recently frozen some guacamole with listeria in it. The original pH was about 6.5 then we added some citric and ascorbic acid, bringing the pH down to about 4.9 - tested several times.

When removed and thawed out the listeria died off more than expected so we tested the pH again to find that it had gone below the critical level of 4.1. What might make it drop by this much? I know pH is affected by temperature, but could it have a lasting effect even once thawed and back at room temperature? Obviously everything was not back in its pre-frozen state so I'm thinking it was permanently damaged and that's perhaps why the pH was different. But do -10 and -20 degrees take the pH down or is that more likely to raise it and something else was a factor in lowering it? Or perhaps the acid had more of an effect once the product was frozen? Could the pH actually go up by a lower temperature?

Really don't know how to even test which theory is accurate without repeating the experiment with no acid added - but unfortunately that's not possible at this stage.

 

Any ideas?

The pH of pure water varies with temperature from about 7.5 (at ice point) to about 6.5 boiling.

On the other hand, the pH should get back to 7 if you get it back to 20C.

 

It's possible that freezing the stuff broke up some cell membranes from the plant and let their contents out so they could react with the bulk of the material and alter the pH.

The pH of pure water varies with temperature from about 7.5 (at ice point) to about 6.5 boiling.

On the other hand, the pH should get back to 7 if you get it back to 20C.

 

It's possible that freezing the stuff broke up some cell membranes from the plant and let their contents out so they could react with the bulk of the material and alter the pH.

 

Ah ok, I know very (very) little about plants and hadn't really thought about the contents of the membranes being released. Something to consider in future! Thanks:)