This is a serious question from someone engaged in evaporating large quantities of water to turn sap into syrup at this time of year.
Probably some background will help. When sap boils vigorously it creates quite a bit of foam, which will overflow the evaporator (incidentally filling the building with a pleasing maple caramel smell as it burns on the side of the evaporator). When the foam gets too high we touch it with a bit of lard and the foam level drops (surface tension - I know). However, it is tempting for me to give a good swipe so that the foam almost disappears (instead of just dropping). The old-timers however contend that I should just reduce the foam to the point where it isn't overflowing any more. They say that it will take longer to boil away the water if I eliminate the foam.
I fail to see how the foam will improve evaporation (although it seems to me that it might slow it down).
Edit: by request ( @georg ) , a link to the evaporator in question https://sites.google.com/site/lindsayssugarbush/_/rsrc/1240515239201/Home/2005-03-30--12-25-21.jpg