Soy Reduction (Video)

YouTube Link: Japanese Soy Reduction
More Videos: Helen's Kitchen Channel

Soy Concentrate
(adopted from Washoku Cookbook by Elizabeth Andoh)

5 g kombu (20 square inches)
8 g dry shiitake mushrooms (1/2 cup)
340 g soy sauce (1 and 1/3 cup - that's one 10 oz bottle)
157 g sake (2/3 cup -- buy the real stuff, don’t use “sake for cooking”)
65 g mirin (1/4 cup)
80 g water (1/3 cup)
75 g granulated sugar (1/3 cup, plus 2 tsp)
10 g thin bonito flakes (1 cup lightly packed) (or 25g thick bonito flakes, atsu kezuri)

1. In a small saucepan, combine everything together except for katsuo-bushi (unless you are using atsu kezuri, in which case, add it right in). Let sit for 1 to 12 hours at room temperature. 
2. Bring to a boil on the stove top and regulate heat so that the mixture bubbles, but doesn’t bubble out of the pot (watch out, this sauce gets foamy, so don’t leave it unattended). Simmer until syrupy. Take off heat.
3. If using thin bonito flakes, stir them into the sauce and let sit for 3 minutes.
4. Strain the sauce through a fine mesh strainer forcing it through with the back of a ladle. 
5. Cool until barely warm and move to a jar or squeeze bottle. When the sauce cools completely, cover tightly and store in the fridge. This sauce doesn’t spoil, but is best if used within 3 months.