You may remember from Lesson 1.2 that HTST pasteurisation protocols require milk to be held at or above 72°C for at least 15 seconds. This temperature exceeds the normal serving temperature of milk drinks. Beverages above 70°C will scald the tongue; a comfortable serving temperature is between 55°C and 60°C. This discrepancy in temperature targets for hygiene and for comfort means that it is essential for a barista to maintain rigorous hygiene standards during the milk-steaming process. Most baristas do not usually bring milk to a temperature high enough to deactivate pathogens.
The cloth that the barista uses to wipe the steam wand is a particular source of potential contamination.