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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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    $\begingroup$ 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? $\endgroup$
    – Edward
    Commented Apr 4, 2011 at 9:37
  • $\begingroup$ 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. $\endgroup$
    – anna v
    Commented Apr 4, 2011 at 15:54
  • $\begingroup$ @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). $\endgroup$
    – Jimbugs
    Commented Apr 4, 2011 at 20:03
  • $\begingroup$ Old timers are notoriously unreliable :) I'd guess it actually makes no difference. $\endgroup$
    – Mark Rovetta
    Commented Feb 12, 2014 at 4:15

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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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  • $\begingroup$ ""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? $\endgroup$
    – Georg
    Commented Apr 5, 2011 at 16:44
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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.

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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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  • $\begingroup$ ""I am too lazy to compute anything, but consider what is the foam?"" Wat do You think could be calculated in this business? $\endgroup$
    – Georg
    Commented Apr 4, 2011 at 15:26
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    $\begingroup$ 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. $\endgroup$
    – anna v
    Commented Apr 4, 2011 at 15:55
  • $\begingroup$ ""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. $\endgroup$
    – Georg
    Commented Apr 4, 2011 at 16:08
  • $\begingroup$ 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. $\endgroup$
    – anna v
    Commented Apr 4, 2011 at 16:24
  • $\begingroup$ 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!) . $\endgroup$
    – Georg
    Commented Apr 4, 2011 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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  • $\begingroup$ btw we've moved to shortening because someone was concerned about "animal products" - we have also tried liquid oils but the convenience of the solid matters too $\endgroup$
    – Jimbugs
    Commented Mar 26, 2021 at 20:31
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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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  • $\begingroup$ cooling isn't an issue - finished syrup is removed to another container and then packaged while still warm to produce a seal $\endgroup$
    – Jimbugs
    Commented Mar 26, 2021 at 20:29
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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.

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  • $\begingroup$ the amount of heat is pretty variable - but not because of the foam. It is a wood fired evaporator so it cools off quite a bit when we open it to add fuel - then gets very hot (and indeed this is when the most foam is produced) - then gradually less hot until it is time to add more fuel. $\endgroup$
    – Jimbugs
    Commented Mar 26, 2021 at 20:33

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