Roux Foams Up When Trinity is Added

It was off the heat completely at the time, but it had not cooled. I took it straight from the oven and then added the onions a little at a time, stirring between additions. I could only add a few cups because it started foaming. Once that calmed, I was able to add the rest. I assumed the roux was too hot, but I've just not had this happen but a few times and I do it the same way every time. Same flour. Same oil.

I add all my trinity at once, and it bubbles somewhat (I wouldn't call it a foam) and I just assumed it was the water evaporating off of the veggies. No much different than dropping potatoes in hot oil for fries. It normally subsides quickly in both instances.

Mine always does this and I assumed like posted above it was water evaporating from the veg. Not really a foam but just bubbles

What's the issue?

I use fresh vegetables only.

Yes, it's more of a bubbling than foam, I guess, but if I don't stir a lot, it will bubble over. The bubbling rose up quickly when this happened. The first time, it bubbled over because it started after I'd mixed in all the veggies and the roux temp had gone down, so I turned around to do something else while it simmered a little. Next thing I knew roux was overflowing from the pot.

I would think you can avoid or minimize this by adding more at once to cool off the roux rapidly. You are making steam from the moisture in the onions and not adding enough to cool off the roux quickly enough. You need to add enough at one time to cool down the roux/vegetable mixture to below the temp where steam is being generated so rapidly. And stir immediately to get it mixed in quickly and cooled down.

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I would think you can avoid or minimize this by adding more at once to cool off the roux rapidly. You are making steam from the moisture in the onions and not adding enough to cool off the roux quickly enough. You need to add enough at one time to cool down the roux/vegetable mixture to below the temp where steam is being generated so rapidly. And stir immediately to get it mixed in quickly and cooled down.

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I had about a 5 cup:6 1/2 cup oil to flour roux in a big deep cast iron skillet. It was about half full of roux. I added a gallon ziploc bag of onions which was about 3/4 full. I started with probably about 2 full cups of onions and it started bubbling pretty immediately. Stirred as soon as they were added to distribute them. Added the rest as soon as I got the bubbling to calm down. Maybe I should have added the entire bag. Normally, I add all the trinity at once. This time, I was trying to caramelize the onions first. Sounds like I didn't put enough in at once.

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Dat's a lotta roux!!!

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I was making a lot of gumbo!

Actually, I forgot that I took out about 2 cups of the roux after it was done but before adding the veggies to use for something else.

At this point I'm guessing it was due to the starch in the flour. Your description makes it sound like it boiled over in a similar way to how pasta water or potato water can easily "bubble" over.

Hot roux (probably still close to boiling temperature of water) + not enough onions to drop the temp + moisture coming out of onions + flour starch = bubble over

???

You're not supposed to put Dawn dish soap in your roux