2002 Annual Report
Research
Ohmic Heating of Particulate Foods
Roger Ruan, Professor
Paul Chen, Senior Research Associate
Xiaofei Ye, Research Assistant
Kehua Chang, Graduate Student
Funding Source
U.S. Army Natick Soldier Center; Minnesota Agricultural Experiment Station;
Private Gifts
Objective
The objective of the project is to develop a technique that enables process
developers to determine the level of thermal sterility rendered on foods
during ohmic heating. A magnetic resonance imaging (MRI) technique will
be used to non-destructively produce temperature maps of particulate foods
undergoing ohmic heating.
Project Description
The application of ohmic energy in heating low-acid food products is
producing considerable interest in the food industry. When electric current
passes through a multiphase system consisting of food particles suspended
in liquid, uniform heating of the system cannot be expected due to the
inhomogeneity of electrical properties and particle distribution and orientation.
It is essential to know the temperature distribution in the system, especially
the particles, in order to calculate the lethal treatment delivered to
potentially harmful microorganisms during the ohmic heating process. In
this study, hardware and software are being developed for rapid and reliable
temperature mapping. The key issues to be addressed are (1) structural
dependence of MRI parameters, (2) imaging artifacts, (3) fat interference,
(4) data acquisition time, and (5) reliable mathematical modeling.
Results
Proton resonance frequency (PRF) shift MRI thermometry was incorporated into a Fast Low Angle SHot (FLASH) imaging sequence to acquire two-dimensional temperature maps of two model liquid-particulate mixtures undergoing ohmic heating process. The samples used consisted of two phases, namely whey gel and NaCl solution, arranged in serial or parallel configuration, and enclosed in a custom-fabricated ohmic heating device driven by an 143 V AC power supply. Salt was added to the gel and solution to alter electrical conductivity. The PRF shift was used as a temperature indicator, which was linearly and reversibly proportional to the temperature change. The specific PRF shift for the whey gel was determined as -0.0098 ppm/°C and -0.0097 ppm/°C for the two samples. For the NaCl solution, the values were -0.0096 ppm/°C and -0.0102 ppm/°C respectively. Detailed temperature maps with spatial resolution of 0.94 mm and temporal resolution of 0.64 s were obtained. The temperature uncertainties were about ±1°C for the whey gel and ±2°C for the solution. The ease in interpretation, the speed, and the accuracy of this new technique justify its applications in dynamic food process.