It set too hard
Updated
The three
A stiff set has fewer causes than a soft one and they are easier to tell apart, which is the good news. The bad news is that it is harder to correct after the fact: a soft set has several second lives, and an overcooked batch has fewer.
| Cause | What actually did it |
|---|---|
| Overcooking | Too much water driven off, so the gel is over-concentrated |
| Too much pectin in the fruit | Underripe fruit, or a high-pectin fruit treated as a low one |
| Too little sugar | Which forces a long cook to reach concentration, and the long cook stiffens it |
Why the third one matters most
Two of the three are judgment calls and the third is a measurement nobody took. It accounts for most stiff sets and it is entirely preventable, which is why it is worth separating out rather than listing alongside the others.
It is the failure mode of trying to reduce sugar without changing anything else. The concentration still has to be reached, so the only lever left is time on the heat, and that produces a stiff, darkened product. Reducing sugar properly means a different standard and a different pectin, not a longer boil.
What it does to the jar
Beyond texture, an overcooked batch changes in ways that show on a shelf and in the mouth, and neither improves with time. Color darkens and the flavor moves towards caramel, which some people like and none of them ordered. A jar that is noticeably darker than the rest of the run will be noticed on a shelf beside them, and a customer who bought the lighter one first will notice the difference before you do.
A stiff set will not level in the jar, which leaves an uneven surface and a headspace that reads as underfilled. That is a presentation problem on top of a texture one, and it is visible through glass.