pH measurement during yogurt fermentation - fluctuations

Hi everyone! 

I was hoping one of you could help me. I am in the final stages of developing a fermented maize drink - and I am now struggling with a few practical issues regarding the pH measurements etc. 

We incubate our maize slurry at 45 deg C for around 6-7 hours, upon which we achieve a pH of about 4.3. The starter culture we use is a mixture of S.thermophilus and L.bulgaricus. Our final product's pH should be 3.7-3.8 - which we will achieve with the addition of Citric acid. 

When the slurry has reached a pH of 4.3 via fermentation at 45 deg C, we stop fermentation and add citric acid to further decrease the pH to our desired pH. We do this before the other ingredients are added, mainly because of our production process: We thermally package the product at 75 deg C, thus the other ingredients (sugar, flavours etc) are added at 65 deg C. Acidifying the product at this point will be even more inaccurate because of the high temperature. 

The problem I am experiencing is that upon adding citric at 45 deg C, the pH drops significantly (from 4.3 to 3.0). Once the other ingredients are added, and the product has cooled overnight, the pH increases again to pH 4.0. This varies every time. 

I have done a trial to see what effect each additive has on the pH, and compensated for that increase accordingly - however I am still not sitting with a final pH which is in spec. 

I make use of a Hanna Yogurt pH meter which can compensate for temperature up to 50 deg C, with an Open Junction Electrode. 

Is there a general rule of thumb, or any tables, which will allow me to calculate what pH the maize slurry at 45 deg C should be, in order to result in a pH within spec in the final product at room temperature? 

I can't check the final pH before packaging, as due to us thermal packaging the product, it is too hot!

Any advice will be greatly appreciated! 

TIA 

Anna :)

Hi Anna,
The challenge with pH is that at face value it is a relatively simple measurement, but actually represents a fairly complex system of interactions.

Higher temperature would generally be expected to reduce the pH - broadly, increased temp = more energy in the system = more hydrogen ions = lower pH (since this is proportional to

How do the measurements look if you draw a sample and cool it to 20C/25C (or whatever reference point your spec/targets are set around) before measuring?

I'd also be inclined to discuss the nature of the temperature-correction with your probe supplier, just to make sure you're not expecting this to do something that it can't/isn't. My recollection is that most of these systems only correct for the effect of temperature on the probe itself, and may not be able to correct for the affect of temperature on the actual product, as the temperature vs. pH relationship for the specific substance being tested will be unknown.

Indeed it's worth noting that pH at the higher temperature is "right" at that temperature, as long as the probe has been corrected for the effect of temperature on the measurement apparatus, so there isn't necessarily anything about this to "correct". Consider e.g. pure water with a pH of 7 at 25C and a pH of 6.63 at 50C - both values are correct, but also somewhat meaningless unless quoted with respect to the reference temperature at which they're taken.

I'd be looking at cooling samples before measuring them, and then seeing how the results compare with your spec. You may then be able to define a temperature vs. pH relationship for the various stages of your production process (as adding ingredient may theoretically affect this) if you need to do more time-critical online monitoring, and in effect define a second set of spec criteria based on pH at a reference temperature of 45/65/75C as necessary, but I don't think you can achieve that until you've characterised how these relate to your tolerance for the product at final storage temperature.