High pH Milk

[Smurfmacaw]

All the non-PH milk I can find here in San Diego, (three different brands, two different stores - not sure if they are all from different dairy's though) has what seems to me an inordinately high pH.....6.9+, even the raw milk is over 6.8.

My question is how to handle the acidification curve.  I started using a mother culture to eliminate the latency period from DVI cultures.  Generally the renneting pH target seems to be around 6.5 but it takes a long time to get there.  Yesterday my make required an hour to get from 6.93 to 6.7 with a 2% innoculation rate with fresh MA-4002 culture (pH of the mother culture was 4.3).

Should I shoot for pH 6.5 for rennetting or is the absolute drop more important?  It seems if I wait until 6.5 the train will be speeding down the tracks (ie. the slope of the acidification curve will be very high (negatively or course)) and I start running into the danger of overshooting the finishing pH before proper coagulation, cooking etc.

As it is, I rennetted at 6.7 and continued with the make and in the end I achieved the correct pH at the end of cheddaring pretty much on time. 

Thoughts?

[Sailor Con Queso]

For renneting you are looking for a "delta" (change) of around .05 to .1 depending on the cheese. So, if you start at 6.7 you should rennet around 6.65. Otherwise, as you said, you will have a runaway acid train. You should not use absolute values.

Now, that being said, the fact that you are seeing high pH values on 4 different store bought and raw milks strongly suggests that your problem lies in your meter and/or calibration. Store bought is often standardized so you should not see pH values that high. Normally, a pH of 6.9 would suggest an elevated somatic cell count and indicate that an infectious process (usually mastitis) is either in the works or coming. So, I personally won't make cheese if the pH really is that high. (I don't think that's the case with your milk).

You should do at least a 2-point calibration at pH 4 and pH 7 every time before starting a make. I also do a third point at 10. If your meter will only do a 1-point, then it may not be accurate enough. Some inexpensive meters are only accurate to within .1 anyway. Your calibration fluid may be contaminated. If you have a hydroponics store near by, they might check your calibration for you (and try to sell you new fluid).

[Smurfmacaw]

I was considering that the pH meter might be the issue....I'm on my third exstick.  I've calibrated it (right before starting the make each time) and I'm positive my reference solutions are clean (I've got pretty good lab technique.)  I don't know though, things that I have a pretty good idea of what the pH should be measure out correctly....deionized water measures out 7.00 plus or minus .01 and my tap water measures out right at 8.05 which is what is expected.  I'm wondering if there is a notch in the measurements around 6.5-7.0.  Or, once my new meter gets here we'll see if there is a difference in the measurements.  Maybe the milk really is less acidic than normal.  I've pretty much decided to spend the money for a professional dairy pH meter (Hanna 99161).  What the heck, hobbies are supposed to keep your wallet from swelling up too large. 

[Ekomilk]

Referring to your assumption of mastitis, did you know that you can measure the somatic cell count in fresh (raw) milk yourself, digitally and reliably? It's easy and only costs around 0.03$/test. www.ekomilk.eu The pH value is not a reliable indicator for mastitis.

[pfabsits]

Every combination pH electrode is made of two half cells. One is the pH indicating cell and the other is the reference. In the old days it was common to use these two probes separately. Today it is very common to use a combination electrode in which the two half cells are combined into a single body hence the name combination pH electrode.

If you had a traditional glass body pH electrode it would be easy to see these two components in which the outer glass body is the reference and the indicating probe is glass tube inside the glass body that ends with the indicating pH bulb.

The problem is that if the inner glass tube has a fracture then a short will occur between the indicating probe and the reference probe.  When this happens a 0 mV response is observed. A pH electrode in pH 7 buffer theoretically generates 0 mV. There is a little tolerance in the 0 mV due to the fact that the glass is hand blown and it changes over time. But regardless a pH probe should generate close to 0 mV in pH 7.

So if a fracture in the glass causes 0 mV and a probe should read close to 0 mV in pH 7 then it is difficult to know if your probe is working correctly f you only perform a pH 7 calibration. A broken electrode will generate pH 7 regardless of the actual pH of the solution that you are measuring. It sounds like you have a broken probe on the inside that you cannot see and it needs to be replaced. This can be confirmed by placing in an acidic solution such as pH 4. If you do not have pH 4 buffer you could check with orange juice. The reading should be close to pH 4 as well.

The take away points are:
1) Perform two point calibration, this will ensure that the probe has a slope.
2) A crack in the pH indicating bulb/stem will result in a reading always around pH 7.

Working for one of the manufacturers I am surprised of the tolerances required in cheese making. Believe it or not it is very difficult to get to accuracy better than 0.1 pH. It is critical that the probe is clean and within user tolerances. That is pH 7 must be +/-30 mV. This is known as an offset and meters are programmed to accept up to +/-60 mV or greater for a pH 7 buffer. The other factor is called slope. Many companies state to have a slope greater than 85% (~150 mV difference from pH 7 to pH 4). An 85% slope is low. Many Government regulated industries require a slope greater than 90-92%. If a proeb is within theses specs then we can achieve a high accuracy measurement.

It is important to note that an accuracy statement of a meter is for the meter and not the probe + meter.

It is also important to note that performing a three point calibration will not increase your accuracy. It is important to bracket your calibration around the expected value. So if your readings are less than pH 7 then calibrate to pH 7 and 4. Many meters use a segmented slope in which a reading is based on slope of the bracketed calibration. That is if I calibrate to pH 4, 7 and 10 then there are actually two segments pH 7 to 4 and pH 7 to 10. These slopes can be different from each other (slight but still different). Most meters will only use the segment that brackets the reading. The other segment is not used.