Based upon my experience with the use of UV in the lab, I do not think you have anything of which to be concerned. Back in the 50s, 60s and early 70s, UV lights were commonly set up in many labs for pass through areas; doorways, change rooms, pass-through boxes, etc. to control/reduce bacterial contamination. It was found that even a thin layer of dust on the bulb’s surface reduced the efficacy significantly to eventually warrant the elimination of these devices in most areas. Directional airflow; positive air pressure and air showers were found more effective.

Even if there were some penetration of the UV through the plastic, only the milk along the surface where the UV light hits would be exposed.

Pete