How should I store the jar after opening?

The label on the jar is the final authority on best-before dates and any specific opening instructions. That said, this type of sugar-based preserve follows the same general rules as jam or honey.

• Before opening, keep the jar in a cool, dry cupboard away from direct sunlight.
• After opening, store it in the refrigerator to slow down fermentation or mould growth. The sugar content helps preserve the walnuts, but a cold fridge adds extra margin.
• Always use a clean, dry spoon. Introducing water or food residue into the jar is the most common reason a preserve spoils sooner than expected.
• Close the lid firmly after each use. A loose lid lets air in, which can dry out the syrup or change its flavour.
• Check the product before eating: if you see mould, an off smell, or fizzy bubbles, discard it. These signs mean the preserve has spoiled.

As with any sugar-based product, the syrup may crystallise over time. If that happens, you can warm the jar briefly in a bowl of hot water and stir — a routine trick used for jam and honey, and it does not change the flavour.