He Jiguo, Head of the Department of Food Nutrition and Food Safety at China Agricultural University: The hard core in a boiled egg is caused by the egg being cooked for too long, leading to the coagulation and denaturation of its protein. While such an egg is harmless, its nutritional value is lower and it is less digestible.
Generally, 5 minutes is the optimal time for boiling an egg. At this point, the egg is semi-cooked, with most microorganisms and bacteria killed. It is soft and tender, has a rich egg aroma, high nutritional value, and is easy to digest. Furthermore, it should be boiled over low heat. This is because the yolk coagulates at 68°C–71°C, while the egg white coagulates at 62°C–64°C. If the heat is too high, the egg white with its lower coagulation point will quickly solidify and harden, thereby hindering the transfer of heat to the yolk. The result is an egg that is "cooked white, uncooked yolk".
Beer-boiled eggs
In terms of nutrient absorption from eggs, boiled eggs have a 100% absorption rate, while scrambled eggs have 97%. Boiling is the best cooking method, but it should be chewed slowly; otherwise, digestion will be affected. For children and the elderly, steamed egg custard and egg drop soup are more suitable. These two methods break down the protein, making it easier to absorb and digest. Eating too many eggs can increase the burden on the kidneys. Therefore, it is recommended that children and the elderly eat one egg per day, and adults should not exceed two.
Beer boiled eggs