Originally Posted by
berkshires
In a nutshell:
"Foam" refers to two things:
1 - There is a type of foam formed of big light bubbles that form and pop quickly. It forms all the time, but some sap does this more than others - it may depend on the tree and or the time of the season. The main downside of this foam is that it leaves residue on the side of your pan, which (especially if you have high heat on the sides of your pan) can scorch a little. This is not a very big deal, but can lead to off-flavors in your syrup - mostly a burnt-caramel type flavor. And it's typically very subtle. The solution to this can involve scooping foam if you like, but I always ignored it. The thing is, if you don't have heat on the sides of your pan it won't scorch, so it's just not an issue. I guess if you have a super-hard boil you could get enough of this type of foam that it could cause some of the foam to boil over the sides, which would not be good.
2 - The second type of foam does not happen all the time. It only happens when sap gets close to syrup. Then the consistency changes. It gets much thicker, and when it boils, you get lots of small bubbles that don't pop as quickly, but instead tend to build up. Think of a liquid flowing sponge. I know you recently made maple candy, so you may have noticed that if it boils hard it will boil over. On the stove you can just turn the heat down, but on the evaporator, not only you can't you, but you don't want to. Instead if you add defoamer it can make the bubbles pop faster. So what is the risk of this kind of foam?
The risk is huge. First of all, this foam is progressive. The foam is an insulator. The more it foams up, the less heat gets released, causing even more of the syrup to foam. So bad goes to worse very fast. Second of all, down at the level of the pan, once the foam gets past a certain point, there's nothing but foam touching the surface of the pan. Foam is not a good heat conductor, so your pan can quickly go from 219 to 800 degrees. You now have horribly scorched sugar on your pan, and these temperatures will permanently warp your pan.
In addition to destroying your pan, you can also have all your syrup destroyed by having all that horribly burnt sugar (and nitre) in it. If that's not bad enough, your syrup can also boil over, making a big sticky mess, and even catching on fire.
The key thing about foam type 2 is that it typically only happens as your sap gets to high sugar percentage. So if you only go to 30 - 40% sugar on your evaporator, and then finish elsewhere, you may not need to worry too much about it. On my old setup, I didn't use a drop of defoamer for 5 years.
Hope that helps,
Gabe