# How to control pH in a fermenter based on the acid produced by mircoorganism

I have got a bioreactor with an aqueous medium which produces biomass (yeast; S.cerevisiae) after inoculation. I am observing and collecting data over the whole process via probes/etc. on different process parameters such as pH, temperature, offgas of $$\ce{O_2}$$ and $$\ce{CO_2}$$, base uptake ($$\ce{NaOH}$$), weight of bioreactor and some little more like stirrer speed and so on.

How can one calculate or make a model on adding base to control pH based on the produced acid
(for now I've got no information on which one that is exactly) of the yeast or other factors?
(Is there a mechanistic or only a deterministic solution?)

Edit:
I can indirectly calculate how much acid is produced because that corresponds to the biomass whose growth data I collect. I don't know the exact acid that is produced, f.ex. $$\ce{HCl}$$ or so, but I think that the molar ratio between my used base and the produced acid is either 1:1 or 2:1, based on my assumption that yeasts will not produce acids with 3 or more protons in their metabolism.

The medium used for growing:
I am currently using glucose as the main substrate, the fermentation itself lasts only about 5-7 hours which means the $$\ce{EtOH}$$ that is produced can be "neglected" as second substrate. Furthermore I use $$\ce{(NH4)2SO4}$$ and $$\ce{(NH4)H2PO4}$$ as N-source and P-source respectively.
[Also some trace elements but those are in minor concentrations/ negligible for my pH model.]

My question is if there is a way to make a model on the acid production to control the base feed to stabilize the pH.

• So let me get this straight: you don't currently have a way of measuring how much acid the yeast produces, but wonder if, knowing this, you can predict how much base you'd need to add to keep the pH at a given value? Jul 23 '19 at 8:28
• No, I can indirectly calculate how much acid is produced because that corresponds to the biomass whose growth data I collect. I meant that I don't know the exact acid that is produced, f.ex. $\ce{HCl}$ or so. But I think that the molar ratio between my used base and the produced acid is either 1:1 or 2:1, based on my assumption that yeasts will not produce acids with 3 or more protons in their metabolism. My question is if there is a way to make a model on the acid production to control the base feed to stabilize the pH. Jul 23 '19 at 9:06
• What is the medium on which the yeast grows? Does it contain sucrose or starch or something else? Jul 23 '19 at 10:07
• You should always edit in the additional information, not just include it in the comment sections. Aug 6 '21 at 20:28
• That said, simple feedback control connected to a continuous pH probe is much more reliable and is standard practice for well controlled fermentation. Aug 9 '21 at 20:44