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Temperature: apply enough heat to the fruit and sugar to raise the temperature over the boiling point of 212 degrees and alter the structure of the sugar; jam reaches its ideal set point at 220 degrees (a beeping thermometer is great!)
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Sugar: if you reduce the amount of sugar in your recipe too drastically, you may not be able to get your jam to the set point
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Plate Test: take an ice-cube tray (or a frozen plate ready for this test) from the freezer); place a small dollop of jam on it
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After a minute, gently prod the puddle of jam with your finger; if a surface skin seems to be developing a certain solidity, it is done (If it is runny, keep cooking a bit longer)
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Sheet Test: (This is the one Grandma Maude taught me.) Using a wooden spoon, stir the jam and lift it up side ways, or horizontally over the jam and watch the jam drip off of the spoon
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If the drips come together in a fall of the spoon separately, the jam needs to be cooked longer; if the drips join together in a "sheet" or a thick wide drop, the jam is finished