John S. Lioumbas

Effect of potato orientation on evaporation front propagation and crust thickness evolution during deep-fat frying.

UNLABELLED The various theoretical approaches that have been proposed for modeling heat and mass transport during deep-fat frying of potatoes do not take into account the effect of potato orientation with respect to gravity. This can be partly attributed to lack of systematic experimental information at different orientations. The objective of the present work is to experimentally study the effect of potato orientation on the evaporation front propagation and crust thickness evolution and how this effect varies with frying conditions. To achieve this goal, a special device has been constructed which, among others, permits: (a) exposure of only one surface of a potato stick to hot oil, (b) rotation of this potato surface at 0°, 90°, and 180° with respect to gravity, and (c) accurate placement of miniature thermocouples under-but very close to-the exposed potato surface. Crust thickness is determined by 2 independent methods: (a) microphotography and (b) a micrometer. It is found that the evaporation front propagation and crust thickness evolution are different among the examined surface orientations. The fastest heat penetration and thickest crust are measured at vertical (90°) surfaces. The implications of this finding regarding potato texture and energy consumption are discussed. PRACTICAL APPLICATION Understanding the role of surface orientation on the crust evolution and the propagation of the evaporation front inside the food is of particular value to: • Deterministic modeling efforts of the coupled heat and mass transfer phenomena during deep-fat frying, and • food industry; the present data suggest that crispier food is produced and less energy is consumed when the food is placed at a nonhorizontal position inside the fryer.

Evaporation front compared with crust thickness in potato deep-fat frying.

UNLABELLED 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.

Spatial considerations on electrical resistance tomography measurements

This work reports experimental evidence of the effect of certain geometrical parameters on the accuracy of electrical resistance tomography (ERT) measurements in two-phase systems. Emphasis is given to millimetre-size electrodes that suffer from significant electric field distortion due to (a) extension of the measuring volume beyond the electrode plane (fringe effect) and (b) the non-uniform distribution of field strength at the electrode plane. Water constitutes the continuous (conductive) phase, whereas Teflon rods constitute the dispersed (non-conductive) phase. The examined parameters include the diameter of the cylindrical test vessel, the size of electrodes and the number and size (radius and length) of the submerged Teflon rods. The variable in these tests is the axial and radial position of the Teflon rods inside the test cell. It is found that for homogeneously (axially and radially) dispersed rods, the void fraction measured by a set of electrodes at a plane agrees pretty well with 2D theoretical predictions. However, in cases of non-homogeneously dispersed rods, void fraction measurements deviate considerably from theoretical values. This is a manifestation of severe electric field distortion associated with the employed small electrodes. Moreover, evidence is provided that in the examined system, the fringe effect is more significant than the topography of the field strength in distorting measurements. To allow a quantitative analysis of the present data, nonlinear regression combined with dimensional analysis is conducted to derive an expression that describes the void fraction measurements by a set of electrodes at a plane for different axial and radial positions of the submerged rods. A parametric analysis of this expression illustrates the significance of different parameters on ERT measurements.