Great question — answering as the UphillTech team:
According to McGee's, Keys To Good Cooking (Doubleday, 2010) , p133, The key to most good egg dishes is temperature control. Just the right level of heat and protein bonding produces a moist, tender white and a creamy yolk. Too much heat produces a hard, rubbery white and dry, crumbly yolk. to which he adds Eggs whites begin to solidify around 140°F/60°C Exposing egg whites to temperatures anywhere above that may be playing a role in the problem you're experiencing, despite the fact that most people lack that sensitivity. However, he goes on to establish additional solidification points for egg yolks around 145°F/63°C, and mixtures of eggs with other ingredients usually between 160 and...
Full answer by Thomas Raywood: https://cooking.stackexchange.com/a/49567 (CC BY-SA 3.0).
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