Evaporation Front Compared with Crust Thickness in Potato Deep-Fat Frying



Abstract:  The various theoretical approaches that have been proposed for modeling heat and mass transport during deep-fat frying of potatoes provide a rather ambiguous view of the relation between the propagation of the evaporation front inside the food and the evolution of crust thickness. This can be partly attributed to the unavailability of detailed experimental information concerning the temperature field inside the developing crust to validate the models. The objective of the present work is to experimentally study the relation between crust thickness evolution and evaporation front propagation and how this varies with frying conditions. To achieve this goal, a special device has been constructed that permits (1) only 1 side of a potato stick to be exposed to hot oil, and (2) accurate and stable placement of miniature thermocouples in prescribed positions under but very close to the potato surface. Temperature recordings inside the developing crust allowed identification of different heating regimes during frying and a rough estimation of the evaporation front propagation. In addition, crust thickness was determined at intermittent time intervals by 2 independent methods (1) microphotography and (2) a micrometer. Comparison of the evaporation front propagation with crust thickness evolution indicates an interrelationship roughly up to the end of the boiling regime (bubble-end point). After this moment, the propagation of the evaporation front is faster than the evolution of crust thickness.

Practical Application:  Understanding the role of parameters that determine crust formation is of paramount importance since crust characteristics such as thickness and texture dictate the sensory perception of fried foods. This study aims to quantify the relationship between such parameters (that is, crust evolution and the propagation of the evaporation front inside the food) and to examine how frying conditions (oil temperature and frying duration) affect it. In addition, the present findings may be of particular value to deterministic modeling efforts on the coupled heat and mass transfer phenomena during deep-fat frying.