Development and application of novel ultrasound/MRI and rheological techniques to evaluate physical properties of foods during processing.

摘要

Nuclear magnetic resonance (NMR), ultrasound, and rheological tests, namely impulse viscoelastic, were applied as rapid and non-destructive methods to food systems to investigate their physical properties during processing. For several frozen samples, the content of liquid-like components such as unfrozen water was measured using NMR. One- and two-dimensional images of proton distribution were obtained by single point ramped imaging with T 1 enhancement (SPRITE), which showed a decrease of intensity of brighter areas in the image indicating a loss of available unfrozen water. The use of SPRITE to examine frozen foods allowed a spatial evaluation of liquid-like spins during freezing and storage. In addition, ultrasonic properties of frozen foods were characterized and compared with NMR data. Over temperatures, the physical changes in frozen samples influenced the ultrasonic velocities from which elastic moduli were calculated. Also, the ultrasonic waves propagated through the frozen sample were highly attenuated due to ice formation. Wavelet analysis was introduced and carried out on the waveforms of the interfacial waves that propagated through frozen samples to investigate the variation of frequency content as a function of time, which showed the dispersion of frozen foods.; In addition to the freezing process, the ultrasonic technique was used to study changes on the rheological properties of dough during fermentation. The ultrasonic properties of dough were characterized and compared with extensional properties of dough obtained with a Universal Testing Machine. From the ultrasound measurements, the decrease in the ultrasonic velocity and the increase in the attenuation coefficients were observed during fermentation. Based on the measured ultrasonic properties, the viscoelastic moduli of dough were obtained. Wavelet analysis showed also the frequency dependence of ultrasonic velocity during fermentation. A technique, named Impulse viscoelastic technique, was introduced and used to characterize the extensional properties of dough during dough fermentation. Small extensional strains of short duration were applied to the dough samples mixed for different times and fermented at 37°C. It showed the maximum resistance to extension for the optimally mixed dough and the loss of elastic properties of dough during fermentation, which was in great agreement with ultrasonic and uniaxial extensional testing.