About a month ago, I had on-the-verge-of-going-mushy strawberries and a Saturday afternoon with nothing scheduled, which I figured meant it was a perfect day to make strawberry jam.
I chose to make no-pectin-added jam, because I had the berries, sugar and lemon juice handy.
Pectin is a naturally occurring gelling agent, and its presence in jams and jellies is what makes them thicken. Very few fruits have enough pectin in them naturally to set up into a thick jam or jelly.
When you add sugar, it combines with the naturally occurring pectin. When heated, they thicken the jam or jelly.
2 pounds fresh strawberries (capped, sliced and crushed)
4 cups white sugar
1/4 cup lemon juice
Fill your canner about two-thirds full of water and bring to a boil. Sterilize your canning jars (I usually use half-pints, because jam makes a great present and a half-pint is the perfect size to give away).
Put the lids in a pot of water and let simmer.
While that's heating, put the sliced strawberries in a heavy-bottomed pot over low heat and add the sugar and the lemon juice. Stir until the sugar has dissolved, then turn up the heat.
Bring to a boil and boil until the mixture is thick. When you pull the spoon out, the mixture should stick to the spoon a little before sliding off.
The longer you cook it, the thicker it will get; sugar, when heated, releases water.
When your jam is as thick as you'd like it, remove from heat. It's normal to have a white, foamy layer on top of the berry-sugar mixture.
The foam is just bubbles caused by the boiling liquid breaking through the top of the berry mixture. Skim it off with a spoon.
Ladle the jam into jars, leaving 1/4-inch head room. Put lids on the jars, then screw on bands finger-tight.
Put the jars in the canner and process for between 5 and 10 minutes. Too long and your jam will darken, but obviously you want to process long enough to get a seal.
Five minutes is sufficient for half-pint jars; allow 10 minutes for pints or quarts.
Remove the jars from the canner and set on a stable surface to cool. Let cool for 12 to 24 hours.
Flex-test lids to ensure a seal, label the jars and store them.
Jam can be kept for up to 12 months; after that, it starts to get runny.
• You can also run the strawberries through the blender to get them into chunks — use the "chop" setting — or use an immersion blender to chop them up. Crushing them releases the natural pectin.
•The difference between "jam" and "jelly" is that jam still has chunks of fruit in it and jelly is the strained liquid.
•Jam made without pectin will not be as thick as jam made with pectin. It's going to be softer, almost the consistency of Jell-O that's been left sitting out.
•Unripe fruit contains more pectin than ripe or over-ripe fruit. You need three ingredients — pectin, sugar and acidity — to make jam, so if your fruit is too ripe (as my strawberries were), you'll have to add more sugar, more pectin or more lemon juice.
•After the jars have cooled to the point where I can touch them, I always remove the rings, wipe the excess water from the lids and sides, and put the rings back on. The worry with leaving tight lids on the jars is that there will be trapped water, which will cause rust.
— Lauren Gross, a York transplant, has long been fascinated by the science of cooking. Her column, Preserving the Harvest, runs seasonally in The York Dispatch food section.