Skimming the foam
Updated
Why there is foam at all
Foam is air that got into the mixture and cannot get out of something that has started to thicken. It is harmless and it is not a sign of anything wrong, which is worth knowing because the instinct is to treat it as a fault.
Air beaten into the mixture during a rolling boil, carrying fine solids up with it. Left in, it goes into the jar as a pale layer on top and as bubbles set into the gel.
The sequence
Timing is the whole of it. Skimming too early wastes product and skimming too late leaves foam in the jar, and the window between those is short.
- Take the pan off the heat the moment the test passes.
- Skim the foam off with a metal spoon, working from the edge inward.
- Fill immediately. Do not skim, tidy up and then fill.
The industrial answer
Antifoaming agents are on the standard's list of permitted optional ingredients, which means a commercial product may be formulated not to foam rather than skimmed. For a small producer the spoon is the answer, and the permitted list is on what else the standard lets you put in.
What skimming costs
Every skim takes product with it, which matters more at small batch sizes than most people account for. Worth weighing once so the loss is in your yield figure rather than a surprise. On a small batch the foam can be several percent of what you set out to make.
Yield, and a little of it. Foam is product, and a batch skimmed hard loses more than a batch skimmed once. That is a real trade on a thin-margin item and it belongs in the yield figure on how much a batch makes.