The physics of maple sap reduction
March 15, 2021 9:31 AM   Subscribe

We use a wood stove for heat. We tap a single maple tree for fun. Before boiling the sap down in the kitchen, we passively reduce it most of the way in a pot on the wood stove. I'm wondering whether it reduces more if we do it in big batches or sequential small ones, and with or without a heat-powered fan.

We use the wood stove to heat the house, and we don't change the temperature or run time for the benefit of the sap -- we just sit a pot of sap on top when we have a fire going anyway. On the wood stove, the sap evaporates, steams, and gets little bubbles on the bottom of the pot, but it doesn't boil. I'm wondering...

1. Is it more efficient to have a gallon or more of sap in the pot at once? Or to reduce one fraction, and then replace it with the next fraction? How many fractions would be most efficient? A couple of assumptions:
- We start the first batch of sap from refrigerator-temperature in the morning. We could queue up subsequent batches at room temperature first. But if batches are more efficient, is that the only reason why?
- We're using the same pot either way; it takes up most of the available flat surface we've got.

2. Is it more efficient to point our heat-powered fan (like this) at the pot?

This is just for fun, so we're not looking to buy anything or do anything less convenient to eke out more efficiency. But these couple of variables seem worth playing with, and they've also just piqued my curiosity.
posted by daisyace to Science & Nature (16 answers total)
The total energy required to bring a liquid up to a boil (and then maintain that boil) varies linearly with mass, so from an energy perspective it makes no difference whether you do one large batch or several smaller ones. Total energy efficiency would be affected by losses from your cooking vessel to the atmosphere and any energy "lost" in the transfer from your stove to the cooking vessel.

The rate of vaporization is affected by the partial pressure of the vapor above the liquid, which is why, for example, sauces will reduce more quickly in a shallow saute pan than in a saucepan (larger surface area means you have a lower partial pressure directly over the liquid). I imagine blowing across the top of your vessel will allow the vapor pressure to lower (good for increasing vaporization rate), but blowing it directly at the pot will simply pull heat energy out of the pot and will reduce your overall energy efficiency.

So, the way to maximize efficiency given your current setup would be to choose a pot that covers as much of the hot surface of your wood stove as possible with as much surface area above it as possible to allow for maximum heat transfer to the pot and maximum vaporization to the atmosphere. Blowing a fan over the top of it might help to increase the rate of vaporization, but be careful not to blow directly at the pot. If you can insulate the side wall of the pot, even better.
posted by backseatpilot at 9:54 AM on March 15 [6 favorites]

the main factor if you don't control the heat will be the surface area exposed to the air.
if two vessels have the same volume, more evaporation will happen in the one that is wider and shallower.
posted by ArgentCorvid at 9:55 AM on March 15 [4 favorites]

Yeah was coming here to say what ArgentCorvid said, this is why professional sap boiling operations have these super low and flat pans.
posted by jessamyn at 10:03 AM on March 15 [3 favorites]

I think the fan would actually hinder your efforts of heating the sap, under any circumstance.

I think there are a few more factors at play here than just the temperature. If the less-filled pan boils more rapidly, then it's possible that it would lose more moisture per second than the larger pan. I would just do two tests - half full and full. Time both of them while trying to keep the heat consistent. If you wanted, you could test with something smaller like a coffee mug instead.

I would expect the result of two halves- vs one full being very similar, but it's possible there's a few other factors at play making the halves faster.
posted by bbqturtle at 10:46 AM on March 15

Response by poster: Since we use the same pot either way, sequential batches do mean each batch is shallower. So I think we've got a disagreement here -- ArgentCorvid and jessamyn, you'd expect that means batches would be more efficient, but backseatpilot, you'd expect it to be equal either way?

I can't just run the experiment because the conditions vary too much -- our fire and our room temp and humidity aren't constants.

The fan's center lines up with the top rim of the pot, so half of the blade is over the top and half is pointing at the pot wall.
posted by daisyace at 10:55 AM on March 15

it's not the depth that matters for speed of evaporation, it's the surface area. If the pot is a cylinder then the surface area should be the same at whatever volume. But if the pot has sloped sides, or is a cauldron shape, there will be some difference.

Also, of course it takes less time and energy to evaporate a small batch, but then you have to do another batch, so it should be equal for the total quantity, assuming the same pot and the same efficiency of heating. If it takes a while to build up the heat for some reason - like if you're using an electric stove - then you're better off doing it all at once.
posted by fingersandtoes at 11:03 AM on March 15

The surface area of liquid exposed to the air at any one time is going to be the main determinant. If the pot widens at the top, use as much sap at one time as you can. Once that is as large as it gets, yes, you can blow air over the area in order to get rid of the humid air a bit faster.
posted by tigrrrlily at 11:23 AM on March 15

Since you're doing this as a secondary use of the wood stove's heat, the best approach might depend on how big / long-lasting a fire you intend to make. As the weather warms up, you'll probably be making smaller, quicker fires because that's all that will be needed to heat the house. A small, quick fire might never get a large volume of sap up to the temperature needed for efficient reduction.
posted by jon1270 at 11:29 AM on March 15

It's unclear that this question is worth thinking about to such an extent. Could it be the case that some advantage might be gained by placing smaller amounts of sap in the vessel? Or by blowing hot air over the vessel? I mean... maybe? But if there is any effect it is likely to be infinitesimal, and it will be offset by the time and work spent switching out or topping up the vessel and/or the energy cost of running the fan. As it stands right now, you are getting the reduction more or less for "free" because the evaporating molecules help the stove heat and humidify your house. It is also requires very little effort on your part. Why would you want to increase your workload and/or electricity bill for... what, exactly? It's not like any of the things you've suggested would reduce the sap in 50% of the time. Let's be optimistic and suppose that running a heated fan on full blast and maintaining no more than an inch of sap in the pan would allow you to get a four-fold reduction 10% faster. Would that be worth it? To me, the easy answer is no.
posted by slkinsey at 11:33 AM on March 15 [3 favorites]

Since this is sap, there's another factor to consider, which is the end product. Do you like your syrup light amber or darker and more robust? If you like light amber, go for more frequent, smaller batches. If you like robust, go for longer boil-down time with larger batches.
posted by pie ninja at 11:52 AM on March 15 [2 favorites]

I agree with backseatpilot and others; the surface area exposed to air is the most important variable. To add more detail, though:

The main benefit of using multiple small batches is that it takes time for sap to heat up to the point where it starts vaporizing, and if you are heating up less sap then it will get to that point faster. If you only have the stove on for an hour at a time and a full batch takes 45 minutes to get to steaming, then doing it in smaller batches that get up to temp in 15 minutes mean that it spends 3 times as long boiling off. If, however, a full batch takes 5 minutes to get steaming and spends 12 hours on the stove, then going to smaller batches will provide almost no benefit on this front whatsoever. (Small batches will also technically be slightly more thermally efficient because you lose heat out of the sides of the vessel and a smaller batch has less side depth to lose heat, but this change will be negligible, especially since the air on the top of a wood stove is pretty warm.)
posted by Superilla at 11:56 AM on March 15

I would expect that convection currents above the stove would pull the vapor-laden air up and away from the pot pretty efficiently, so the fan might not make that much of a difference in the rate of evaporation. The thermocouple fans like you have are mainly designed to circulate air horizontally in the room; you get vertical air circulation pretty much for free.
posted by Johnny Assay at 11:57 AM on March 15

I was curious about this question because we did a boil this past weekend. We use a ~50-gallon evaporation pan, and our biggest challenge seems to be getting a hot-enough fire to keep it boiling rapidly. A low simmer seems to take forever to significantly reduce the volume.

Contra @pie ninja, I've always heard that the color/grade has more to do with when you tap: early season sap comes out more golden and late-season sap is darker and more flavorful.
posted by libraryhead at 1:15 PM on March 15 [1 favorite]

Response by poster: A low simmer seems to take forever to significantly reduce the volume.

This is an interesting part of the question. Ours never even simmers (until the kitchen step). Sometimes there is visible steam and little bubbles on the bottom of the pot, and sometimes not. So it’s not just a matter of units of heat energy, but their impact on evaporation rate.

It's unclear that this question is worth thinking about to such an extent.

It's fun. And my other main activity at the moment is watching sap evaporate, so...
posted by daisyace at 1:22 PM on March 15 [8 favorites]

My grandparents just covered the top of the stove with full roasting pans, the idea being to get as much as possible done and out of the way in one go.
posted by The Underpants Monster at 3:48 PM on March 15

In a boiling liquid, you lose heat out the sides of the pan at a constant rate because the liquid in the pan is at the boiling point: this is wasted heat. You lose heat out of the top of the pan at a rate that varies with how much heat is being applied to the bottom of the pan: this is work heat, it's reducing the liquid. You could apply just enough heat to the pan to only bring the liquid to a simmer and no more, and a large potion of that heat would be wasted out the sides of the pan. That is why if you want to evaporate a liquid quickly, you apply as much heat as possible, so a larger fraction of heat goes out of the top of the pan. It's also why shallower pans are better, because there is less side to radiate heat. If you insulated the sides of your pan this would mean very little burner heat being wasted and almost all of it coming out of the top as steam.
posted by seanmpuckett at 5:34 AM on March 16

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