And how is it different from a simple “boil” when making jam? That’s another question from reader Eliza who’s canning up a storm by the sound of things. I’m about done canning for the season myself, save for some tomato jam. Tomatoes are about all that’s left in the Pastry garden, not counting a few dozen peppers and some very sad looking Brussels sprouts. Actually they’re not sprouts, they’re nubs. They’re Brussels nubs. Someday we’ll learn how to grow those things properly.
But that’s a topic for another day. Eliza, I understand very well the uncertainty of jam-making. The whole process moves so quickly, it’s hard to know whether you’re boiling, full boiling, rolling boiling, or just screwing everything up. What I can say is that for most jams, a rolling boil is necessary, as it’s the only way to know — visually — whether the full volume of jam in the pot has reached the critical gelling point of about 220 degrees Fahrenheit.
You probably already know that there are various stages of boiling, usually known as simmer, low boil, full boil, and rolling boil. Each one is an expression that describes, not so much the temperature of the water in a pot, but how much of that water is actually boiling. At a simmer, very little of the water in the pot is boiling. Small steam bubbles are forming at the bottom of the pan (where the water is boiling) but the rest of the water is merely hot (somewhere around 160 degrees Fahrenheit). More of the water in the pan is boiling at the low boil, still more at the full boil, up to the rolling boil at which point all the water in the pan is boiling, and has reached the temperature 212 degrees Fahrenheit. (That’s the boiling point for water, not jam, mind you).
Know a rolling boil by the extreme motion on the surface of the liquid, where broad waves emerge before plunging downward, as freshly heated water from the very bottom of the pan — where the heat is — rises, then cools slightly, and heads back down again, where it’s reheated, rises again and so on. That motion is very apparent in a large pot of jam. The moving currents are evident in the middle of the jam, and the entire surface is covered in small bubbles of steam and syrup.
The expert jam maker knows that it’s critical to get the jam to the rolling boil as fast as possible after the acid is added, because every second of high heat comes at a cost to the pectin gel, which starts breaking apart as the mixture boils. But it’s a necessary sacrifice, as even a strong pectin gel won’t mean much if the syrup in the jam hasn’t reached the right concentration. This is why jam is always made in small amounts, ideally in broad copper pans that conduct heat well. The quick heating (and accelerated evaporation) buys the jam maker a few extra crucial seconds — seconds that can mean the difference between thick, luxurious jam and fruit soup.
I hope this helped Eliza. Me, after all this I’m ready to make some jam!