Abstract
The mechanism of the rupture in the crust of frozen croquettes during frying is investigated.
(1) The crust becomes fragile and weak after freezing. Water holding capacity of the contents decreases after freezing and thawing, so that thawed water is isolated just under the crust during frying.
(2) The frozen crust shrinks during frying and is subjected to tensile stress because of the expanded frozen contents, so that it becomes easy to rupture. The croquette with 2 mm thick crust becomes very hot during frying locally at the thinnest part of the crust, and the vapor pressure of the thawed water rapidly increases just under the crust, which causes explosion with pinholes.
(3) In the croquette with 3 mm thick crust, vapor pressure does not increase enough to make pinholes because the crust is so thick that the temperature does not become very high.
Because of the fragility of crust, however, even the slightest expansion of the contents cracks the weakest part of crust, which causes the whole rupture of the croquette.
