heating sap under a vacuum

[penfrydd]

Okay...it's been 50 years since I took a physics course.

If I some sort of pressure cooker and put it under a slight vacuum, will it not remove more water faster than an open pan? I feel like I'm not considering something major here...

[RileySugarbush]

You remember your physics correctly. The problem is in practical application of the idea. It takes less heat energy but needs a vacuum pump and pans that can handle the vacuum and a way to get finished product out and lots of other details. It's been done but not worth it so far by all the abandoned attempts. Maybe on a large scale it would be more attractive.

[DrTimPerkins]

If I some sort of pressure cooker and put it under a slight vacuum, will it not remove more water faster than an open pan?

You are correct. This is one way cane and beet sugar industries remove water from the "juice". It is economical to do at the large-scale, but is not commercially viable (due to cost) for smaller evaporators. A thin-walled evaporator simply will not stand up to the vacuum involved, thus you need very heavy-walled units and multiple pumps and automation to open-close doors and to move the fluid around. Much more complicated (and expensive) than a simple gravity feed and float system used in maple evaporators. Vacuum-evaporators are the size of buildings.

Another consequence of doing it this way (and one reason it is used by the cane/beet sugar industry since their end product is WHITE sugar) is that without much heat added, there will be very little color and flavor formation.

[berkshires]

Put your sugar shack on the top of the tallest mountain around. You'll have lower air pressure. Of course getting the sap there... is another question.

Cheers!