Each breakfast we have coffee made from a pint of semi-skimmed milk and about 2/3 cup of water. The mixture is then heated in a microwave for 6 minutes before ground coffee is added and stirred in.
The query concerns the milk/water when it comes out of the microwave. Sometimes it has an obvious skin on top, sometimes it has quite thick foam with a light skin, and sometimes just foam. Why the differences? All the milk is delivered by the same milkman who owns the business. There are 3 deliveries per week (Mondays, Wednesdays, and Fridays) and the differences do not appear to be related to the delivery days.
Any ideas?