Abstract
We previously reported that the potato croquette, having a thin crust of 1 mm, ruptured during frying when the vapor pressure of the surface layer exceeded the pressure-convert-value of the crust measured by a curd-tension-meter. Now in this report, we examined how the crust ruptured when it was thicker.
Potato croquette samples were made of a mixture of 40 g of dried mashed potato and 200 ml of 75°C water, which were than battered and breaded several times. This was followed by deep frying at 180°C. Thus the thickness of the crust was changed to 2, 3, and 4 mm.
The rupture of the croquette having a thick crust occurred in a different manner to that having a thin crust as we reported, in which there were a few pinholes and only a small amount of the potato exploded. In the case of the croquette having the thickness of 2 or 3 mm, the crust split along the longitudinal direction of the croquette, and potato exploded. in the frying oil. The croquette having a 4 mm thick crust did not rupture in this experimental frying condition.
We assumed the following mechanism. Potato the inner substance of the croquette, expands as the temperature increases during frying. Then, inner pressure becomes higher and gives the tensile stress to the crust. When this tensile stress exceeds the tensile strength of the crust, the crust will break and the croquette will rupture.
This assumption was confirmed by the determination of expansion of potato which corresponded to the inner pressure and tensile ratio of the crust corresponded to the tensile strength of the crust.
