OK, it sounds like some of you know the more technical end of WHY
certain ingredients react the way they do in cooking. So here's my
question.

What changes in an egg if you beat it for a long time? Specifically, I
made an upside-down cake the other day. The recipe stated, "Beat 2 egg
yolks for 5 minutes, until thick and lemon yellow." Thises then get
sugar, flour, juices, etc added for cake batter.

Also, I have a recipe for a non-baked creamy pie filling that states,
"Add 1 egg. Beat for 2 minutes. Add second egg. Beat for 2 minutes."
This is a filling that is made of butter, sugar, cocoa and instant
coffee flakes. No, the eggs never get cooked. Yes, I know about
salmonella risk. But thats not the point; it was grandma's recipe and
I'm gonna make it and eat the darn raw eggs.

What changes when you beat an egg for longer, rather than just a short
turn with the mixer to combine white and yolk, or froth it a bit? And
what happens that causes the yolks to look so thick and lemony after 5
minutes of beating?

Thanks for tellig me in plaing English. I'm not a pro chef, just a
curious home baker.