Sprinkle baking soda on the chili peppers and soak them in cold water for 10 minutes to remove impurities. Rinse and drain, then remove the stems.

Slice the chili peppers diagonally, rinse to remove seeds, and dry them using a vegetable spinner. Place them in a sterilized glass container.

In a pot, combine water, soy sauce, mirin, and plum syrup. Bring to a boil over high heat. Once boiling, turn off the heat and add apple vinegar. Pour the hot mixture over the chili strips.

Allow the mixture to cool completely, then cover the container and refrigerate for one day before serving.