Is casein a buffer, in regards to a 0.1M HCl titration into milk?

We did an experiment where we titrated 0.5mL of 0.1M HCl into milk and recorded the pH difference. The pH difference of milk had stayed constant at roughly 5.5 throughout the six time we dispensed 0.5mL of acid in. Are there any chances that casein was the reason for this consistency?