The yolk (yellow portion) makes up about 33% of the liquid weight of the egg. It contains all of the fat in the egg and slightly less than half of the protein. With the exception of riboflavin and niacin, the yolk contains a higher proportion of the egg's vitamins than the white. All of the egg's vitamins A, D and E are in the yolk. Egg yolks are one of the few foods naturally containing vitamin D. The yolk also contains more phosphorus, manganese, iron, iodine, copper, and calcium than the white, and it contains all of the zinc. The yolk of a large egg yields about 250 kJ of energy. Egg yolks have a pH of about 6.0 which stays relatively constant as the egg ages as there is no CO2 loss. Coagulation occurs in the range 65° to 70°C.