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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

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For a potentially complicated system like this, it seems the best solution would be experiment. Can't you just try it both ways for the sake of curiosity? –  Edward Apr 4 '11 at 9:37
    
With suitable approximations on the size of the foam bubbles and the size of the foam body one could get the effective dimensions for a hypothetical foam. –  anna v Apr 4 '11 at 15:54
    
@Edward - it takes hours and hours of evaporating. With a ratio (sap to syrup) of something between 30 and 40 to 1 (depending on all kinds of environmental factors that can't be easily controlled). This also means it is usually a team effort (again making it hard to measure accurately). –  Jimbugs Apr 4 '11 at 20:03
    
Old timers are notoriously unreliable :) I'd guess it actually makes no difference. –  Mark Rovetta Feb 12 at 4:15
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5 Answers 5

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.

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""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."" And in this case, not actually boiling, there is no reason for foam! The Foam is made from steam bubbles, what else? –  Georg Apr 5 '11 at 16:44
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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.

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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.

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""I am too lazy to compute anything, but consider what is the foam?"" Wat do You think could be calculated in this business? –  Georg Apr 4 '11 at 15:26
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With suitable approximations on the size of the foam bubbles and the size of the foam body one could get the effective dimensions for a hypothetical foam. –  anna v Apr 4 '11 at 15:55
    
""With suitable approximations on the size of the foam bubbles and the size of the foam body one could get the effective dimensions for a hypothetical foam"" Anna, that is so true, that it doesn't say anything. –  Georg Apr 4 '11 at 16:08
    
Georg between greek english and german english we seem to have a problem in communicating. I am talking of treating the foam as a fractal system, so instead of air.water surface contact being two dimensional, it will be two dimensions +something. The pot surface is pi*r^2. The total exposed surface of the foam is much more than that. The "more" can be calculated if one takes the trouble. –  anna v Apr 4 '11 at 16:24
    
Anna, that is all in vain. Evaporation is controlled by heat flow. This vapor will evade the vessel through some vent. Some foam will cause a lot of liquid lost along through the vent, but not change the amont of steam generated. (And vented to outside!) . –  Georg Apr 4 '11 at 16:38
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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.

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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

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