The roux breaking down in the stock is likely due to overbrowning, choice of white roux, and use of cake flour.
The most likely cause for the roux breaking down in the stock is that the roux was browned too long. Browning the roux too much can diminish its thickening ability.
White roux is typically used for its strong thickening power with minimal color. A white roux is cooked only until the raw flour taste is removed, making it less likely to break down than a darker roux.
The type of flour used, cake flour, also affects the roux's thickening properties. Cake flour is lower in protein than all-purpose flour, resulting in weaker thickening abilities.
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