Jam

Jam, jelly, marmalade, preserve

These names are sometimes used interchangeably but mean specific things:

• Jam — fruit pulp cooked with sugar; cloudy, spreadable, with chunks.
• Jelly — clear fruit juice cooked with sugar; firmer, no fruit pieces.
• Marmalade — usually citrus-fruit jam, often with shredded peel.
• Preserve — generic term, often refers to whole-fruit-in-syrup styles.
• Conserve — fruit preserved in larger pieces, sometimes with nuts.
• Compote — looser, less sweet, typically eaten fresh.

How it sets

Jam thickens through a chemistry of three components:

• Pectin — a polysaccharide naturally present in fruit, especially in pips, peel, and underripe fruit.
• Sugar — typically 60-65% by weight; both preserves and helps pectin gel.
• Acid — lemon juice or natural fruit acid; activates pectin gelation.

Low-pectin fruits (strawberries, peaches, cherries) need added pectin or pectin-rich pairings (like apple peels or commercial pectin) to set.

The 220 °F set point

The traditional test for a finished jam: heat to 104 °C (220 °F) — the temperature at which sugar concentration is correct for set. The “wrinkle test” (drop a bit on a chilled plate, push with a finger) gives a visual confirmation: if it wrinkles instead of running, the jam will set on cooling.

Storage

Properly canned jam (water-bath processed, sealed) lasts a year or more at room temperature. Once opened, it goes in the fridge and is generally good for several months. The high sugar content acts as a preservative — historically, jam was a way to keep fruit through winter without refrigeration.