When making a stew, I forgot to add flour before the liquid. To thicken the stew, I added some slurry (flour + cold water). However, it was not enough. I would add more slurry, but I found this website (2012) that says
If you've prepared a dish that started out with a roux and you feel it needs a slurry at the end for additional thickening: use cornstarch or arrowroot to prepare the slurry. Why? Because a chemical reaction occurs that will prevent the same starch used twice to thicken it any further.
I could find not any other sources supporting or opposing this claim, so I'll ask here:
Can I thicken a sauce/stew twice with flour (roux+slurry or slurry+slurry)?
Best Answer
A reaction that prevents starch from the same plant thickening if you add it later instead of all at the same time seems very unlikely. And anyway, a roux could be made with non-wheat flour even though it's not traditional - what do they think would happen then? Or if you make a roux on another pan and add that. I can confirm that works.
There is a good reason not to add a slurry late in cooking - the raw flour flavour it imparts. That's why I'd use cornflour (corn starch) for late thickening.