In a large glass jar or non-reactive container, combine the soy sauce, fresh lemon juice, white grapefruit juice, lime juice, rice vinegar, and mirin. Be sure to include any fruit pulp for extra flavor.
Cover the container tightly and let the mixture rest in a cool, dark place for at least 24 hours. This allows the flavors to meld and deepen.
After resting, strain the mixture through a fine-mesh sieve into a clean bottle or jar to remove the pulp.
Store your homemade ponzu in an airtight container in the refrigerator. For the best flavor, allow it to come to room temperature before using.
Don't discard the citrus pulp after straining!
Freeze it in ice cube trays and add it to marinades, soups, or stir-fries for a zesty flavor boost.
