Does the foam on top of boiling maple sap affect the rate of evaporation? 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
 A: It could go either way. If you are heating the liquid to the boiling point, then the foam will not limit boiling (unless it raises the pressure), but will limit convection/advection of air near the surface. Note that latent heat of water vapor is not the only method of heat loss from your pot. If air advects/convects over the surface, you are also heating air molecules. Also some heat is being lost by the surface via thermal radiation (probably roughly a kilowatt per meter squared). So the bubbles provide insulation, so that the heat loses other than into latent heat of water vapor are reduced..
But, if it is not actually boiling, but the temperature is controlled to be some value below boiling, then it loses water via evaporation, and that requires fluid to flow to and away from the surface, and the foam would seriously inhibit that.
A: Yes and No,
The evaporation is controlled by the heat flow into that evaporator. 
(How does that evaporator look like? Could You post/link a picture?)
So foam is not the control, but the limiting factor, because You have 
to reduce heat flow to avoid the foam spilling over. 
To "fight" that foam, there are several possibilities: 


*

*Kind of stirrer/cam agitating above the surface of liquid. 
Whether this is helpful, You know better than me, just 
try the foams reaction to some agitation by a stick/paddle. 
Coating the upper part of the evaporator and that foam breakers 
with some hydrophobic plastic (eg PTFE) might help.

*Big diameter flat evaporation vessel with ample space above liquid. 

*Antifoam agents: more theoretically, because those might leave 
some unpleasant smell/taste in the syrup. You needed a food 
grade defoamer, I don't know whether such are available. 
Some hints to defoaming: most defoamers are some hydrophobic 
substances like petroleum (that used for lamps) or fat. 
This fatty substances  will disperse in the foaming liquid as 
fine droplets, which act as rupture initiation points on the foam 
lamellae. 
If the syrup was for my own use only, I'd try to add some paraffine wax 
(food grade) for a trial. (one gram on 100 ltr of syrup, not more!)
The wax will separate after cooling down, (and swim on top I assume?). 
Just try that in a pan with a small amount of syrup. The critical 
factor is this removal, maybe the paraffin remains dispersed as tiny 
droplets in the syrup. Removal by adsorption filtration is possible, but 
that is more messy/lossy than slow enough evaporation. 
Another point: some of the taste of maple syrup comes from heating 
for some time (caramel reaction). In case You reduce this time by 
reduced foaming, You might get a very light (color) syrup missing the 
"right" flavor.  
PS 
foam is not just a question of surface tension, think of water, which has 
the highest surface tension of all liquids (exept molten metals). 
Foaming is a very complicated process, viscosity, not too high 
surface tension are the main "incredients".
Edit: 
Thank You for the picture! A evaporator more flat than this one 
is not possible, I think. 
In chemical technology there are a number of machines which were developed 
to evaporate/concentrate viscous and foamy liquids. But those are big machines,
not for such a small scale business. 
A: I'm new at making syrup but the foam issue seems to relate to boiling water in a pot.  A pot of hot water will rapidly boil with a lid, in this case a layer of foam, while an uncovered pot will barely form bubbles at the bottom of the pot.  The foam seems to form a layer of insulation that allows the sap to reach a higher temperature to increase the rate of evaporation. 
A: I am too lazy to compute anything, but consider what is the foam?
The foam is a bubble of steam covered with a layer of water and sugar solution.  If there is no foam the area of the pot exposed to air or evaporation  is two dimensional, with maybe some big bubbles from the boil. Each little bubble in the multitude  contributes more to the area exposed to the air where the surface cover water can evaporate, burst and release trapped steam, making way for the next bubble. It is a fractal problem,but the dimensions of the available evaporation surface are larger than 2 by far when there is foam.
Lets put it another way. The foam raises a film of hot water to contact with the air,  without obstructing the trapped steam, so when a foam bubble breaks there is water out of the pot that would have been in the pot if there were no foam,  the extra surface of the foam increases the  evaporation.
So the oldtimers are probably correct.This argument depends on the surface of the foam bubbles being a solution with water.Of course an experiment should be carried out with the specific liquid to decide on this.
A: The energy you input ends up as steam (less heat loss through the container's walls).  540 cal/gm water turns one gram of 100 C water into one gram of 100 C steam.  A thin foam layer increases the evaporative surface area.  A thick foam layer impedes evaporation, and probably raises the temp of the liquid a bit as the steam cannot get out.  You may want a foam layer to interact with air to oxidize to flavor compounds (maple lactone).
Lard is foam breaker.  A mist spray bottle of ethyl alcohol will also do it (but it is flammable).  Wine or whiskey adds flavor, vodka probably not. You don't get to be be clever with thin silicone oil (re bovine bloat) in food applications.  A cooking whisk on a drill shaft can break foam, or not.  So,
1) Alcohol.  Fermentation ethanol only for food.  Flammable and expensive.
2) Esters.  Food grade only, and will impart flavor.  Soybean oil in a mister bottle?  Use lard.
3) Fatty acids.  Imparts (nasty) flavor.
4) Fatty acid derivatives.  Food grade?
5) Silicones!  Foam breaker champs!  Not really for food.
6) Sulfites and sulfonates.  Allergic reactions.  Not for food.
7) Other stuff.  "Generally Recognized As Safe" (GRAS) in the US.
A: foam will not decrease the rate of evaporation but surely will make it difficult to cool(it'll behave like an insulation) so if you are cooling it(after boiling) then remove the foam
A: What about this: To make foam much more heat must be imparted to the near or just boiling point sap. A nice thin layer of foam is nice to ensure a "proper" boil is being achieved. However increasing heat to cause a stack-up of foam impedes the evaporation by trapping the newly formed steam which returns to the rolling mass and the increased heat becomes aided by this trap to concentrate on scorching the sap and further causing the formation of Geraldsugar sand. There will be plenty of those maple-caramel like texture and flavors achieved in the time it takes to get to be syrup.
