Temperature Mapping of Ohmically Heated Food System Using Proton MR Phase Imaging

Proton Magnetic Resonance (MR) Phase Imaging was realized using a Fast Low Angle SHot (FLASH) imaging sequence. Two-dimensional temperature maps of two model liquid-particulate mixtures undergoing ohmic heating process were acquired. The samples used consisted of two components, namely whey gel and NaCl solution, arranged in serial/parallel configuration, and enclosed in an in-house made ohmic heating device driven by an 143 V AC power supply. Salt was added to the gel and solution to alter their electrical conductivity. The Proton Resonance Frequency (PRF) shift was used as temperature indicator and was manifested as phase change, 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 to interpret, the speed, and the accuracy of this new technique justify its applications in dynamic food process.