1 - It was as I was actually making this that I discovered that the real secret was just getting the fruit up to the required temperature for jamming, and this seems to depend on two factors: the amount of sugar added, which affects how long it will take to reach the gelling stage. Less sugar means that it will take longer, while more sugar means it won't take as long.
I'd like to make this an easily scalable recipe. So I took a recipe and tried to convert it. So I started out with :
9 cups blackberries
5 cups sugar
3 Tbsps lemon juice
and I did this calculation :
9x10 - 90/9 - 10/10 - 1 cup blackberries
5x10 - 50/9 - 5.5/10 - .55 - 1/2 cup sugar
3x10 - 30/9 - 3.33 - .33 - 1 tsp lemon juice
It seems to hold up. Now, to test it.
- Run the blackberries through a food mill to remove the seeds, continuing until you have 1 cup of puree. Reserve the blackberry puree and discard the seeds (I give them to our chickens).
- Put the blackberry puree, sugar and lemon juice in a saucepan. Be sure there are several inches of headspace to allow for foaming.
- When it starts to boil, clip on a candy thermometer, reduce it to a rapid simmer and continue to cook, uncovered, stirring occasionally, until the thermometer reads 220F/105C. This can take upwards of 30 minutes, less if you add more sugar, longer if you have less sugar.
- You can also do the plate test.
- Ladle the jam into sterilized jars leaving 1/4 inch headspace and wipe the rims clean before sealing.
- If you're not sealing the jars the jam will keep in the fridge for several months.
- If you're using the water bath canning method process the jars for 10 minutes. Carefully remove the jars, let them sit undisturbed for 24 hours, then store in a dark cool place. Jam will keep for up to a year.
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