Answer to Question 1
The ferrous sulfide ring in boiled eggs happens when eggs are overcooked or heated at too high a temperature. The sulfur in the egg white combines with the iron in the yolk, the cooking water, or other iron sources. To prevent this from occurring, use fresh eggs, use stainless steel equipment, use low cooking temperatures, avoid overcooking, cool hard-cooked eggs quickly in cold water, and serve the eggs immediately.
Answer to Question 2
Rennin is an enzyme that promotes coagulation; coagulation results much more quickly and the curd texture is tough and rubbery and high in calcium. The acid-coagulated curd occurs over a longer period of time, loses more calcium, and results in a soft and spongy texture. The texture is influenced by pH.