It went dark
Updated
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The causes
- Time at temperature. Reduction is heat, and so is processing.
- Brass, iron, copper or zinc in contact with an acid product.
- Oxygen in the headspace, working across the shelf life.
- Light on the shelf, for a product in clear glass.
The metal rule is absolute and is usually broken by accident
Not by a deliberate choice of pot, but by a stray utensil: a brass skimmer borrowed once, an old iron pan used because the stainless one was in the sink. One use is enough. The practical form of the rule is to keep the wrong metals out of the room rather than out of the process.
Time at temperature is the one you can design around
A sauce reduced hard for a long while will be darker than the same sauce thickened partly by formulation. That is a recipe decision with a color consequence, and it is worth knowing which lever you are pulling when a batch comes out darker than the last.
It shows up late
Darkening is a month-four complaint, by which time the decisions that caused it are several batches behind you. That delay is why this is worth fixing by buying the right equipment and fixing the process once, rather than by watching for it.
Clear glass is a commitment
This failure is only visible because the product is sold in something you can see through, which is a packaging decision taken long before the first batch. Clear glass is usually right for a sauce, because the color is part of what a buyer is choosing, but it means every cosmetic change is on display for the whole shelf life rather than hidden. Choosing it is choosing to solve the headspace and storage questions properly.
Light is the one cause that acts after the jar leaves you, which makes it partly a decision about where the product is displayed rather than only about how it was made.