Place sugar in an ovenproof shallow pan and warm in a 250°F (120°C) oven for 15 minutes. (Warm sugar dissolves better.)
Place berries in a large stainless steel or enamel saucepan. Bring to a full boil over high heat, mashing berries with a potato masher as they heat. Boil hard for 1 minute, stirring constantly.
Add warm sugar, return to a boil, and boil until mixture will form a gel (see tips, below), about 5 minutes.
Ladle into sterilized jars and process as directed for Shorter Time Processing Procedure .
Tip: To make a small boiling-water canner, tie several screw bands together with string or use a small round cake rack in the bottom of a large covered Dutch oven. Be sure the pan is high enough for 2 inches (5 cm) of water to cover the jars when they are sitting on the rack.
Notes
To determine when the mixture will form a gel, use the spoon test: Dip a cool metal spoon into the hot fruit. Immediately lift it out and away from the steam and turn it horizontally. At the beginning of the cooking process, the liquid will drip off in light, syrupy drops. Try again a minute or two later — the drops will be heavier. The jam is done when the drops are very thick and two run together before falling off the spoon.
“The intensity of this jam is due to the fact that it has no added fruit pectin,” says Topp. Adding pectin helps the jam jell, but necessitates more sugar, which dilutes the natural flavor of the fruit. Making jam without added pectin requires more careful cooking (see notes about the spoon test, above), but the extra effort pays off in a deliciously old-fashioned, fruity product.