A method for preparing a reversed boiled egg introduced in an old Japanese recipe book has been a mystery for a long time until we recently succeeded in reproducing it 220 years later. The recipe book was published in 1785, in which 103 recipes for cooking eggs were introduced. Although 102 recipes have been recreated, only the reversed boiled egg failed to be reproduced, despite numerous challenges had been carried out. The recipe book reads, “Prepare a fresh egg and pierce the eggshell by about 3 cm depth with a needle. Leave it in fermented, salted rice-bran paste for three days, and then boil the egg to ensure the yolk in-side out”. The key to solving the mystery lay in the idea that the eggs used for cooking in such an old period could be mostly fertilized eggs. We finally reproduced the reversed boiled egg by using fertilized eggs incubated for three days at 38ºC. The objective of this study is to reveal the cooking principle of a reversed boiled egg. Fertilized and unfertilized eggs were incubated for seven days at 38°C with a relative humidity of 70% (hatching condition). The changes in fertilized and unfertilized eggs (such as a ratio of egg yolk and egg white, water content, pH, yolk viscosity, and gelation temperature by heat treatment) were compared. For 3-4 days incubation of fertilized eggs, water in the egg white is transferred to the egg yolk. As a result, the weight of the egg yolk doubled, and the viscosity of the egg yolk significantly decreased by one hundredth, becoming watery. On the other hand, the egg white weight halved, and it got thick. During this process, the vitelline membrane became very weak and easy to be broken. Upon being ruptured the membrane, the runny yolk diffused inside the eggshell, and the watery yolk gelled outside the thick egg white during heated. Thus, the reversed boiled egg (the Yolk Inside Out) was reproduced.
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