Overripened fruit contains less pectin, which could prevent a jam from fully setting.

The size and shape of the pot affect the jam’s rate of evaporation; an underevaporated jam will be watery and won’t set, while an overreduced jam will have a thick, gluey texture. We like to use a 7-quart Dutch oven for larger batches because its sides are shorter than those of a tall stockpot and better encourage evaporation. For most small batches, we prefer to use a 4-quart saucepan because the wide bottom of the Dutch oven causes these jams to overreduce quickly. We use a 12-inch nonstick skillet for a few simple jams and chutneys; the nonstick surface prevents the jam from sticking and scorching as it reduces and thickens.

In our jam recipes, we often call for bringing the ingredients to a boil over high heat and then maintaining that boil for up to 15 minutes. Once the jam mixture starts to boil, you’ll see a layer of foam begin to build up and bubble on the surface. Don’t be tempted to lower the heat at this point; the jam needs to rise above the boiling point of water, and it will take a lot longer to reach that point when cooking at a simmer. Instead, just stir or whisk the jam for a few minutes until the foam dissipates.

When making jam without commercial pectin, we like to monitor the jam’s temperature as it cooks. The set point for jam is roughly 8 degrees above the boiling point of water, but as a double check, we strongly suggest removing the pot from the heat when the jam is 2 to 3 degrees below the target temperature and performing the frozen plate test. The temperature inside the pot can vary by up to 10 degrees; to get an accurate reading, whisk the jam thoroughly and move the thermometer back and forth as you take the temperature.

We don’t often use commercial pectin in jam; the fruit itself often has sufficient stores, or we add pectin-rich apple to the mix. But when we do, we use Sure‑Jell for Less or No Sugar Needed Recipes, which we find works well in any jam recipe. When adding the pectin to the pot, we like to mix it with a small amount of sugar to help it disperse evenly. After whisking in the pectin, it’s important to return the mixture to a boil and let the pectin dissolve. Only then can you whisk in the remaining sugar and boil the jam 1 to 2 minutes longer to activate the pectin and dissolve the sugar.