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I've tried twice to make fruit preserves using only the natural pectins in the fruit. The first time with plums where the jam was much to thick, and yesterday orange marmalade which came out tasting great but a bit on the runny side.

In both attempts I tested for doneness by letting a few drops of hot jam fall onto a cold plate and letting it cool before pushing it around with one finger to see if a crinkly skin has formed.

In my hands this test is clearly not fool-proof. Are there alternative techniques for those of us who have not yet developed an eye for crinkliness?

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up vote 2 down vote accepted

Have you tried using a candy thermometer and testing the jam when it's at 220°F?

An alternative to the method that you use is to use a spoon and do the 'two drop test'. If you dip a cold metal spoon into your jam mix and then lift it. When the mixture is only just boiling it will drip off and be light. As the mixture continues to heat the drops that fall from the spoon will be heavier When the two drops form and fall off the spoon it should be ready. I'll be honest though I much prefer the method you use and this is just an alternative.

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Great answer. Is it enough for the jam to reach 220°F or does it need to stay there a while? – Chris Steinbach Mar 9 '11 at 3:54
@chris the temperature is a function of how much water is in the jam. The less water there is, the higher the boiling point. So you are effectively using the thermometer to gauge how much water is left in your jam. You need to take it off the boil as soon as it reaches the right temperature, because if it gets any hotter, your jam will end up thicker. – slim Mar 9 '11 at 15:05
I'd add - just like oven thermometers and meat thermometers, sugar thermometers are pretty cheap and provide a way to get reliable, repeatable, foolproof results. – slim Mar 9 '11 at 15:06
I agree with 'slim', it's best to wait until it's just at the right temperature – nixy Mar 10 '11 at 12:17

The pectin temperature is 104°C (220°F) (adjust -4°C per km in elevation). It can take an hour of simmering to get to this temperature

To test the pectin level add one teaspoon of jam to three teaspoons of methylated spirits in a cup swirl the solution. If it forms a single clump you have enough pectin

To fix low pectin levels, simmer for longer and add a little acid (lemon juice is fine)

Old preserving stories say using plum stones or apple cores will increase pectin levels, YMMV on this

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Apple is high in pectin, so the apple core story may be using that. – Martha F. Mar 9 '11 at 0:02

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