Malt vinegar doesn't spoil. However, the manufacturers place a "best by" or "best consumed before" date on the packaging. This is because the integrity of the vinegar might change. Once opened, the appearance of malt vinegar starts to change. Malt vinegar appears clear brown, but it may become darker as oxidation occurs. You might also notice the consistency changing. From clear, it becomes opaque. To prevent these from happening, store your malt vinegar in a cool, dry place while keeping it sealed after opening.