Use of X-ray Micro-CT to characterize structure phenomena during frying

Abstract

It is the microstructure that gives the desired textural characteristics to food products. Thus it is of our interest to understand structuring mechanisms in order to be able to design food products with specific textural properties. Furthermore, in the case of frying, consumers demand for healthier foods has initiated interest in understudying the underlying physical phenomena of oil absorption during frying. A number of techniques have been used to quantify internal structures of fried foods (e.g. optical microscopy, confocal laser scanning microscopy, Bouchon et al., 2001) but there is a need of methods that would allow detailed 3D, non-invasive quantitative characterization of food microstructures. Recently X-ray microcomputed tomography (X-ray micro-CT) has been introduced as a new method to investigate food structures (Mousavi et al., 2005). X-ray micro-CT is a combination of X-ray microscopy and tomographical algorithms that can resolve details as small as a few microns in size. It is based on differences in X-ray attenuation (absorption and scattering) arising principally from differences in density within the specimen.In this work the effect of frying time and frying temperature on the structure of potato strips was investigated. Three dimensional images of samples at different frying times and temperature were obtained. It was also possible to quantify parameters such as pore size distribution and crust development during frying. This work demonstrates the capabilities of X-ray micro-computed tomography as a non-invasive technique for the study of the internal 3D microstructure of food materials and relate it to processing parameters in a way that allows not only a fundamental understanding of the process but also a process design that would result in specific microstructures. Deep-fat frying of potatoes can be defined (Miranda, ML Aguilera, JM 2006)) as the process of cooking foods by immersing them in an edible oil or fat which is at a temperature above the boiling point of water, typically 150-200oC. Upon addition of the French fries to the hot oil, the surface temperature of the fries rises rapidly. The water at the surface of the fries immediately starts boiling, the surrounding oil is, in the process, cooled to lower temperatures, but this is only a temporary effect as it is quickly compensated by convection.In the event that the amount of added French fries exceeds a critical value, only then will the temperature of the oil be significantly affected (Mellema, 2003). As the boiling commences, the convection will be further intensified by the turbulent water vapour; due to the evaporation, surface drying will occur along with shrinkage and the development of surface porosity and roughness. Mellema (2003) also adds that explosive evaporation can lead to the formation of large pores. As frying time increases moisture content in the crust decreases, thereby reducing the amount of steam leaving the surface. In some circumstances, the surface temperature may rise above the boiling point of water, but i...

Direct observation of the surface structure of French fries by UV–VIS confocal laser scanning microscopy

“…Oil was shown to be mainly localized in the surface but was also present in deeper cell layers with the persistence of oil in samples after hexane washing. Miri, Bakalis, Bhima, and Fryer (2006) used an other non invasive technique, X-ray tomography, and made volumetric reconstructions of the microstructure of a fried potato of 1 cm of diameter and 2 cm of length. They showed that pore interconnectivity was limited in the surface of the product, considering the axial and lateral resolution of the X-ray measurement technique of about 6 lm (van Dalen, Nootenboom, van Vliet, Voortman, & Esveld, 2007).…”

3-D Imaging, Analysis and Modelling of Porous Cereal Products Using X-Ray Microtomography

“…porosity assessment) of the non-hydrated material before and after certain durations of hydration. µCT has proven to be a very useful technique for the non-invasive visualisation and measurement of the internal microstructure of cellular food products, such as porous rice kernels and whipped cream (van Dalen et al, 2003), aerated chocolate and muffins (Lim et al, 2004), bread (Falcone et al, 2004;Lassoued et al, 2007), cornflakes (Chaunier et al, 2007), dough (Mousavi et al, 2005), extruded starches (Babin et al, 2007), French fries (Miri et al, 2006) and biopolymer foams (Trater et al, 2005).…”

Effects of Initial Pore Diameter on the Oil Absorption Behavior of Potato Chips during Frying Process

“…According to Fig. 2, as the pore diameter increased, the drying rate increased and evaporation of water was more intense, so more pores, cracks, voids and open channels formed, which contribute to the space available for oil absorption 29 .…”

“…Huizenga and Smith 17 observed that the effective vapor diffusivity in porous solids increased with the rise of pore size, which was responsible for the above phenomenon. At the beginning of frying 2 min , moisture evaporates at a very fast rate for all sample with different pore diameters samples, which created an overpressure inside the pores 29 . So the STO contents were all low at 2 min, which was less than 10 .…”