Introduction

Magnetic resonance imaging (MRI), originally developed for medical applications (Morris, 1986), has recently been exploited for observation and characterization of foodstuffs and their manufacture (McCarthy and Kauten, 1990; Schmidt and Lai, 1991; Schräder et al., 1992). MRI and its related techniques have already proven useful in non-invasive observations of fruit and vegetable quality, e. g., ripening (Chen et al., 1989 & 1993) and fruit defects (Wang et al., 1988; Wang and Wang, 1989; Chen et al., 1989). Lipid/water content has been visualised in beef and pork (Groeneveld et al., 1984), salad dressing (Heil et al., 1990), and fish (Winkler et al., 1991). The value of MRI has been demonstrated in several processes involved in cheesemaking: syneresis (Ozilgen and Kauten, 1993); formation of eyes during ripening of Swiss cheese (Rosenberg, et al., 1991, 1992); fat droplet size determination in the finished product (Callaghan et al., 1983); and diffusion of salt into cheese (brining). MRI observations of water and oil phase changes include drying (Perez et al., 1988; Ruan et al., 1991; Song, et al., 1992; Ruan, et al., 1992; Schräder and Litchfield, 1992), fat crystallisation (Simoneau et al., 1991 & 1992), and freezing (McCarthy et al., 1989; Özilgen et al., 1993). Transport phenomena evaluated by MRI range in mobility from mm/day in a creaming emulsion to as fast as 3 meters per second: foam stability (German and McCarthy, 1989); diffusion of water and oil (Callaghan et al., 1983; Ruan et al., 1991; Watanabe and Fukuoka, 1992) and its relation to temperature variations (Sun et al. 1993); creaming of emulsions (Kauten et al., 1990); and flow in aseptic processing (McCarthy et al., 1992a) and extruder models (McCarthy et al., 1990b). Mobility serves both as the origin of many aspects of the magnetic resonance phenomenon, and as an MRI-observable attribute of the fluids within foods. After a brief introduction to the theory underlying the exploitation of the magnetic resonance phenomenon, the discussion moves from low to high mobility processes, from crystallisation, through diffusion, creaming emulsions and syneresis, and concludes with more rapid flow in aseptic processing and extrusion.

1.1. Theory of magnetic resonance imaging

The magnetic resonance (MR) phenomenon, and its utilisation in exploring the properties of food materials, depends on the inherent magnetic properties of certain atomic nuclei in a magnetic field. The hydrogen nucleus, a proton, behaves as a spinning charged particle; it possesses angular momentum and generates a polar field. In the earth's weak magnetic field, incoherent motion caused by thermal energy prevents any significant population of protons from aligning with the magnetic field. Little difference exists between energy levels of the numbers of protons aligned with or against the field; thermal energy equalizes the two populations.

Placing hydrogen nuclei in a stronger magnetic field imposes a larger difference in energy levels between protons aligned with and those aligned against the field. Thermal motions still tend to equalize the populations, but the population of nuclei aligned with the magnetic field will very slightly outnumber those aligned against it. A sudden pulse of energy at a frequency and amplitude precisely adjusted for a given nucleus will move the protons from the lower energy level (aligned with) to the higher level (aligned against). Energy depends on frequency:

h denoting Planck's constant and CD frequency in radians per second. As mentioned above, a stronger magnetic field imposes a larger energy difference between the aligned and opposed populations, and the relationship is linear:

where y is the gyromagnetic ratio, a constant unique for each MR-sensitive nucleus, and Bo is the strength of the applied field in gauss or Tesla. Equation 2 forms the basis for understanding the behavior of nuclei in any magnetic field, whether uniform or nonuniform.

Perturbing the alignment of hydrogen nuclei in a magnetic field allows observation of the state of the nuclei; molecular structure, temperature, mobility, and even position influence the properties of the signal emitted by the nuclear system as it relaxes, or returns to equilibrium (aligned with the magnetic field). The signal decay, or free induction decay (Figure 1), contains all of the information available by MR. The signal of Figure 1 consists of two components, water resonating at 25 Hz and oil at 100 Hz. The reference frequency of 21.4 megahertz has been subtracted, leaving audio frequencies to represent the small differences (parts per million) between the transmitter frequency and the resonance frequencies. Oil and water resonate at somewhat different frequencies due to the variation in their electronic environment; protons have bonded to oxygen to form water, and to carbon chains to form oils. Chemical shift results from shielding by volts volts volts volts

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