Abstract
An attempt was made to compare the phenomenon of cracks which occurred on the surface of doughnuts in the process of steaming and baking with that in the frying system. The results obtained can be summarized as follows :
1) Heat exchange and transfer to the samples from the medium during heat treatment progressed rapidly in the steaming system more than baking one. This resulted in an increase of the coefficient of expansion for the samples prepared with the steaming system, while a reduction of this coefficient appeared in the samples provided with the baking system.
2) The two different systems of heating were effective to form the crusts, which on the surface of the samples in either system were featured by the thick and much pliant texture more than those formed in the frying procedure.
3) The occurrent mechanism of cracks could be characterized by the condition of pk<pi, where pk is the pressure of crack and pi is the internal pressure of the sample. The basic principle of this condition is identical with the other heat treatments including frying system with any type of the samples.
4) We estimated theoretically the place of cracks occurring top or inside surface of the ring-shaped samples in either heating system. This was also confirmed experimentally with a variety of samples to be tested in this study.
