Monster killing is complemented by a cooking component in Bloodstained Ritual of the Night but ingredients like the Soy Sauce can prove to be more elusive than you'd expect.
Here's everything you need to know about where to find Soy Sauce in Bloodstained Ritual of the Night.
Soy Sauce Item Location
It's fairly pointless to expect Soy Sauce to drop until you're rather far into Bloodstained Ritual of the Night. Even then, if you don't know where to look, getting it can be a bit of a hassle.
The only current source of Soy Sauce we're aware of are blue chests in the Oriental Sorcery Lab area. The bad news is that the ingredient isn't a guaranteed drop.
This likely means that you'll have to run between rooms with blue chests in the Oriental Sorcery Lab area while hoping that luck chooses to be on your side. With enough persistence, you're bound to get it eventually.