Did he really need to taste the yogurt?
The cup was in the trash, so there were probably flies and insects in there, as well as other pieces of germy garbage. Plus, Mills daughter drank from the cup (more germs), and the yogurt, being perishable, likely started turning bad by the time Whitaker finds it.
In addition, the note already indicated that Byran drugged it. Why, therefore, does Whitaker feel it necessary to confirm that it was drugged?
At worse, he could have just sniffed it, not tasted it.