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How the pH value of a bread could be measured since it is not a liquid and the possibility of such a process came up and has been questioned repeatedly in the newsgroup.

Bread- and sourdough books refer to pH values of bread products and dough additives - how do they measure the pH value?

I think, that as soon as there is humidity in a substance, there are ions and a pH value can theorectically be measured. Totally dry substances are very rare on this planet.

I am using *) my pH meter to monitor the pH value of starters and doughs to determine the state of the fermentation in my sourdough/bread undertakings. The pH meter was resently recalibrated with buffer solutions to the two high (7.0 pH ) and low (4.0 pH ) reference points. Before the calibration, it was off minimally by 0.1 and 0.2 pH - so the instrument is fairly accurate.

Below is how I determined the pH for the crumb of a freshly baked bread.

I am sure that microbiologists with lab equipment have a better method.

*) - actually I have not used it for quite a while now, since December 2002. The sourdough pH cycle is pretty much the same - in general from 4.6 - ish down to 3.6-ish and it works like a clockwork. I use the characteristics of the growthcurve to determine in which state the starter is in, and it works great.

I cut a slice of freshly baked bread. This bread is described on this page and had a dough hydration of 66 % and a theoretical bread hydration of 46 % (final bread weight 1048 g / total flour weight of 714 g). Some of the water was absorbed into the bread substance during baking and is not available as liquid. Also, the humidity inside the bread varies considerably with the crust and most outer crumb areas as being much dryer than the inner part.
I cut off the outer crust as well as the next layer of dryer crumbs. Then I made a ball of crumb, trying to avoid touching too much material (like kneading thouroughly) with my hands which would possibly throw off the result due to changing the chemistry of the sample mingling my hand. I folded it from the outside in and pressed it to make a ball. Then I gently stuck the probe in.
This is a close up picture to see the actual measurement value of pH 3.70.
This is a close up picture of the crumb ball and the electrode part. One can see how the material is smeared inside against the electrodes (not visible here but at the tool/equipment page). Not the nicest way and definitely a high stress procedure for the instrument.
This are all the leftover bread parts involved in this event. One can see the cut away outer crust and the next dry crumb layer as well as crumb ball used for the measurement.