There appears to be conflicting advice over whether heating milk will reduce its B12 content.
For example, this healthcare site says:
Vitamin B12 is stable at room temperature and therefore does not need to be refrigerated. It is not destroyed by cooking. Vitamin B12 resists breakdown even at boiling point of water for several hours.
On the other hand, a poster in this Vegan forum provides some (referenced) quotes which claimed that boiling milk reduces B12 content:
Prolonged cooking, including boiling of cow's milk, destroys B12.
Similarly, about 70% of B-12 is still present after cow's milk is boiled for 2-5 minutes.
Does boiling of milk reduce its B12 content?