Blackcurrant jam that set like rubber, and the batch that fixed it
Blackcurrants are the easiest fruit to set, which is exactly why it is so easy to overdo them. The first batch of this season was 1.2 kilograms of fruit picked on the 1st of August, topped and tailed over an hour at the kitchen table, simmered in 600 millilitres of water for 20 minutes until the skins burst, then boiled with 1.5 kilograms of sugar. I had the thermometer in and I was watching it, and I still let it climb to 106 degrees because the surface did not look as if it was setting. It was. The jam in the jars is firm enough to cut with a knife, dark, and a little caramelised at the edges of the flavour. Perfectly edible, and not what I wanted.
The second batch a week later was the same weight of fruit and sugar, but this time I did three things differently. I used 700 millilitres of water, because blackcurrant skins are high in pectin and the extra liquid gives a slightly softer set. I took the pan off the heat the moment the thermometer read 104.5, which it reached after six minutes of hard boil, a full three minutes earlier than the first batch. And I did the wrinkle test on a cold saucer as the deciding check rather than the thermometer: a teaspoon of jam, 30 seconds in the freezer, a push with a fingertip. It wrinkled and held. That was it.
The difference in the jar is complete. The second batch is spreadable straight from the cupboard, holds its shape on a spoon but slumps on toast, and tastes like fruit rather than like sugar that has met fruit. Yield was eight 340-gram jars from the second batch and seven from the first, the missing jar being the water that boiled off in those three extra minutes.
What I have written in the book for next year: with blackcurrants, trust the saucer, not the surface of the pan, and start testing at 104. The thermometer reads half a degree low against boiling water, which I have known for two years and keep forgetting to allow for. The firm batch has been labelled honestly and will go into almond tarts, where a jam that holds its shape is actually a virtue.