Combine all ingredients in a small saucepan, mixing the arrowroot into the cool liquid before heating.
Bring to a boil, stirring, then reduce the heat and simmer for about 5 minutes.
Remove from the heat. Transfer to a clean, covered container and refrigerate at 40°F or below within 2 hours (1 hour above 90°F). As a conservative guideline, use within 3–4 days. This is not a tested canning or shelf-stable recipe.