pH data on souring milk

The label on my milk bottle tells me how long the milk will ‘last’ at different temperatures. What do they measure to make this assessment? Is it the pH? I summoned patience, collected a glass of milk, pH electrode, pH sensor, data logger, data logger power supply and temperature sensor.

First run: leave the milk on a shelf above the heating radiator (about 29 degrees Celsius). The milk bottle label says that milk can be kept for a few hours at this temperature. 

Insight file: milk_ph1.isa Excel or SID file: milk2.sid

Last run: leave the milk at room temperature in a heated room (about 21 degrees Celsius). The milk bottle label says that milk can be kept for half and day at this temperature.

Insight file: milk_ph2.isa Excel or SID file: milk2.sid