Dear Anna Salamero Boix, thé way mentionned by Dr. Manuel Gómez though it is used it is only and indication of H+ at the surface, the bulk may be something different.

Another technique uses thé relationship between pH and electrical conductivity, but again strong disagreement, since all sorts of conductive spices may contribute. Please have a look at these links. My Regards

https://doi.org/10.1002/jps.21052

https://iopscience.iop.org/article/10.1088/0967-3334/17/1/002/pdf

https://www.researchgate.net/post/How_to_measure_pH_in_a_solid_state_fermentation

https://www.researchgate.net/post/What_is_a_standard_procedure_for_measurement_surface_pH_of_solid

https://ourdailybrine.com/how-to-test-the-ph-of-food-and-drink/

Dear Anna Salamero Boix , you can use a pH indicator like SNARF-1, a dye that shows different colors depending on the pH. Its a family of flourophores and requiere a confocal microscope and calibration. It allows to measure intra and extra-cellular pH.

Here you can find some information:

http://tools.thermofisher.com/content/sfs/manuals/mp01270.pdf

https://www.nature.com/articles/s41598-017-10348-4

Hope it helps.

Dear Anna Salamero Boix, thé way mentionned by Dr. Manuel Gómez though it is used it is only and indication of H+ at the surface, the bulk may be something different.

Another technique uses thé relationship between pH and electrical conductivity, but again strong disagreement, since all sorts of conductive spices may contribute. Please have a look at these links. My Regards

https://doi.org/10.1002/jps.21052

https://iopscience.iop.org/article/10.1088/0967-3334/17/1/002/pdf

https://www.researchgate.net/post/How_to_measure_pH_in_a_solid_state_fermentation

https://www.researchgate.net/post/What_is_a_standard_procedure_for_measurement_surface_pH_of_solid

https://ourdailybrine.com/how-to-test-the-ph-of-food-and-drink/

Thank you a lot for your ideas.

Kindly check https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4175927/