Why did this roux fail?

Question

I needed to make gumbo for someone with a gluten allergy, so I used glutinous rice flour in the roux instead of wheat flour. At first it looked ok:

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When I added the "trinity," though, this happened:

Basically, the roux split, and the rice flour formed disgusting blackened clumps. Here's a close-up:

Yeah, I didn't like looking at that either. I tried again with tapioca flour, and that did work, but I'm still wondering why that happened with the rice flour. Could it have been some kind of thermal shock when the veggies went in? Or maybe a reaction to some chemical in the onion?

Answer 1

After the starches are cooked with oil, and you added the vegetables, as the vegetables cooked they would have released moisture, which the starch absorbed and caused the formation of the lumps (like miniature dough-boys).

Some tips to prevent this from happening:

• make the roux and sweat the vegetables separately
• allow the roux to cool (off the heat) so when it is used then the absorption is less intense, you can increase the heat after the roux has had some amount of liquid to it. traditionally it is thickened continually while adding liquid but the cheating method is to leave the roux cool and add all the liquid, then keep stirring (to prevent sticking to the bottom) while over high heat and until the mixture comes to a simmer
• if browned colour or freezing (leftovers) is not required then consider using mashed potatoes, mashed potato flour (NOT potato starch) or plain rice instead.
• avoid glutinous rice starch, potato starch, and tapioca starch for roux. the end product is not as good mouthfeel as other starches, and they can make the mixture like glue/slime.