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I know that veloute/espagnole is fundamentally roux and stock. Stock is basically salt, flavoring, and water. I was building a Japanese-inspired recipe, and I was curious how replacing stock with soy sauce would affect the resulting mixture.

I made a roughly 1:1 white roux with AP flour and neutral oil (a random mixture of LT olive and canola), heated approximately 1 part soy sauce in the microwave, added a spoonful of roux to the soy sauce to temper it (probably unnecessary since the soy was already hot) then combined all ingredients and stirred.

The mixture immediately thickened up into a paste. It was not clumpy and still had a good texture on my palate. I was easily able to get the mixture to relax back into a gravy by adding vegetable stock.

I recall seeing the mixture strongly foam up when I added the soy sauce, which is interesting. Kikkoman mentions that their soy sauce contains around 2% ABV, and I know that evaporative action can cause foaming. I doubt whatever chemical change happened was primarily heat-mediated--the thickening happened rather quickly. I had done this once before, but using cold soy sauce, and the foaming happened but the roux broke from the cold.

So what happened? I'm looking for answers that explain the underlying mechanism, so I can generalize and apply the knowledge in the future.

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    Doesn't roux always go paste-like when you add a small amount of liquid? That's the magic of it. I make mine on the stove, with butter, and it's pretty runny before I gradually add liquid (usually milk), but it turns into something resembling mashed potato as soon as I add the first milk (or stock). Then as I add more it starts to thin, starting from about when there's more liquid than roux. You never reached that stage, so it doesn't sound like what happened was at all unusual (maybe the foaming but I'm not sure).
    – Chris H
    Commented Mar 16 at 10:43
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    Ah, so it's not just soy sauce. "Mashed potatoes" is a good texture descriptor for what I got, so the action must be between the water in the added liquid and something about the roux. Commented Mar 16 at 17:35
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    That's right. Of course the point is to thicken liquid, and it thickens small initial amounts of liquid very quickly. If you're used to adding a lot of liquid in one go, it's not obvious.
    – Chris H
    Commented Mar 16 at 17:39
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    "Soy sauce thickens roux. Why?" It's the other way around: the roux is thickening the soy sauce. You just don't think of the roux as "thick" to start with because it's so thick that it doesn't behave like a liquid any more. Commented Mar 16 at 18:35
  • Wait, roux isn't supposed to behave like a liquid? My roux was about the viscosity of gravy before I added soy sauce, which is why I was surprised to see it seize up when I added a small amount of liquid. Commented Mar 16 at 21:40

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Roux always goes paste-like when you add a small amount of liquid. That's the magic of it. I make mine on the stove, with butter, and it's pretty runny before I gradually add liquid (usually milk), but it turns into something resembling mashed potato as soon as I add the first milk (or stock). I would expect this effect to be even more noticeable with an oil-based roux, which is runnier to start with.

Then as I add more it starts to thin, starting from about when there's more liquid than roux. You never reached that stage, so it doesn't sound like what happened was at all unusual (except maybe the foaming).

Essentially, roux thickens small amounts of liquid very quickly. If you normally use a method that dumps in all the liquid at once, you won't notice this. I suspect you might, given all the hot stock and tempering; if you add small but increasing amounts you can add cold liquid.

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