The pH check on a cultured product
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Calibrate every session
Meters drift and buffers age. This is the measurement that decides whether your product meets the standard it is labeled against, so calibrate against fresh buffers before the readings that matter rather than at the start of a season.
Measure the finished product, within 24 hours of filling
The standard specifies a pH of 4.6 or lower measured on the finished product within 24 hours after filling. That means the measurement is taken on what is in the container, in the window named, rather than on the vat before filling.
Measure it cold, consistently
Temperature affects the reading, so take it at a consistent temperature and record which. A reading taken warm on one batch and cold on the next produces a difference that has nothing to do with the fermentation.
Use it as the endpoint, not only as a check
The real value is deciding when the fermentation is finished rather than confirming afterwards that it was. Finishing on the number instead of the clock removes the largest source of batch-to-batch variation in the craft.
Record it against the batch
The reading, the time after filling, the temperature, the meter and the batch code. That record is what supports the name on the label, and a number remembered is a recollection.
If it did not reach the number
It is not yogurt as the standard defines it, and that does not change by taking a second reading later. It may be a perfectly good cultured milk product under a different name, and that is the honest route rather than relabeling the measurement.
Clean the probe properly, every time
Milk protein and fat foul a glass electrode quickly and a fouled probe reads slowly and wrongly. Rinsing and the occasional proper clean is the difference between a meter you can trust and one that quietly drifts through a season of readings.