A narrow window of heat
Why Does Custard Suddenly Scramble?
Egg proteins thicken a sauce over a few degrees, then curdle it over the next few.
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Step 1 of 5
Egg proteins start folded up tight.
In a raw egg each protein is a coiled ball that slides past its neighbours. That is why raw egg pours.
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Step 2 of 5
Heat unfolds them.
Warming shakes the coils loose. The unfolded strands catch on each other and the liquid gets thicker as they tangle.
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Step 3 of 5
A little more heat and they grip too hard.
Past about 82 degrees the strands bond tightly to each other, squeezing out the liquid they were holding. That is scrambling.
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Step 4 of 5
Sugar and starch widen the window.
Both get between the proteins and slow them from finding each other, which is why a sweet or floury custard is far more forgiving.
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Step 5 of 5
Which is why it is cooked slowly and stirred.
A water bath cannot exceed 100 degrees and stirring stops any one spot racing ahead, so nothing reaches the scrambling point first.
A scrambled custard can sometimes be rescued by blitzing it — the clumps break small enough to feel smooth again.
The short version
Egg proteins unfold and tangle to thicken custard, but a few degrees more makes them clump and squeeze the liquid out.
Try it yourself
Heat egg and milk in a pan with a thermometer. Watch it coat the spoon around 80 degrees and grain a few degrees later.
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