Rice Vinegar — 1 tbsp rice vinegar + 1/2 tsp salt per 1 umeboshi plum. Rice vinegar with salt provides similar salty-sour flavor for rice and onigiri.
Ponzu Sauce — 1 tbsp ponzu sauce + 1/2 tsp salt per 1 umeboshi plum. Ponzu provides citrus acidity and soy umami similar to umeboshi's sour-salty profile. Best in Japanese dressings and dipping sauces where fermented funk is acceptable.
Lemon Juice — 1 tbsp lemon juice + pinch salt per 1 umeboshi plum. Lemon juice with salt mimics umeboshi's sour-salty profile; lacks the fermented depth.
Apple Cider Vinegar — 1 tbsp apple cider vinegar + 1/2 tsp salt per 1 umeboshi. Apple cider vinegar has a fruity sourness that loosely approximates umeboshi's tang.