E. A. Davis

The dielectric properties of whey protein as indicators of change in polymer mobility

Abstract The dielectric properties of whey proteins were measured at 30–90°C, 36–75% moisture and changing salt concentrations to determine the effects of these variables on the dielectric properties. The dielectric properties were found to be more sensitive to different changes in mobility occurring during hydration than were viscosity measurements. The dielectric properties were also sensitive to the changes in water and protein mobility that occurred during protein denaturation. Theoretical heating rates based on the measured dielectric properties were compared to heating rates calculated assuming the dielectric properties retained their room temperature values and were found to differ by 0–8.5%, illustrating the minor importance of using temperature dependent values in modeling. The dielectric properties also were found to be dependent on both the concentration and type of ions present.

Microwave thawing of lossy dielectric materials

Microwave thawing of lossy dielectric materials was examined theoretically. A “temperature” approach was used to model the microwave thawing of frozen slabs composed of beef or water and multilayer beef/water slabs. The heat conduction equation and Stefan equation were solved using a finite element front-tracking method and Newton iteration. The microwave power deposition in the slab was determined by an analytical solution to Maxwells equations. Solutions to the governing equations provided transient temperature and power deposition profiles, and the position of the phase-change interface with respect to time. Dirichlet (essential) and Robin (convection) boundary conditions were investigated and thawing times were calculated for incident power levels up to 2·5 × 104 Wm−2 and slab thicknesses of 5, 10 and 15  cm. The beef samples thawed more quickly and for all situations there was less of a change in the thawing time as the power increased above 5000 Wm−2. The thawing time versus sample thickness was fo...

Cryomicrotomy applied to the preparation of frozen hydrated muscle tissue for transmission electron microscopy

A method is described by which frozen hydrated bovine muscle is prepared for the transmission electron microscope. The importance of maintaining specimen temperatures at −110°C (163°K), or below, during the preparation of frozen thin sections and during specimen transfer and microscopical observations is emphasized and documented.

Specimen Preparation of Raw and Cooked Carrot Phloem and Xylem for the Scanning Electron Microscope1

Xylem and phloem tissues of raw and cooked carrots were prepared (1) by mechanical excision and (2) by cryofracturing the tissue in liquid nitrogen. Both sets were subse quently fixed with gluteraldehyde and osmium tetroxide and critical point dried aftersuc cessive dehydrations with acetone. It was concluded that for these particular tissues and sample preparations the cryofractured-critical point drying technique was less damaging, more preserving with fewer artifactual distortions, and gave more information on the sur face structure and interfacial properties than did mechanical excision of these tissues.

Scanning Electron Microscope Studies on Carrots: Effects of Cooking on the Xylem and Phloem1

Scanning electron microscopy (SEM) was used to characterize the microstructural changes of xylem and phloem tissue of mature carrots when cooked to doneness by the home preparation methods of steaming, boiling, and pressure cooking. This study yielded strong evidence for noncellulose polysaccharide fibrillar material between cells as theo rized by Colvin and Leppard (1973). This material was susceptible to degradation during cooking and resulted in varying degrees of disruption of cellular integrity. The data obtained clearly show the value of SEM in studies of this type.

Application of low temperature microscopy to food systems

Low temperature scanning electron microscopy (SEM) and X‐ray fluorescent microanalysis were applied to bulk potato tissue samples. Morphological changes and relative distributions for K, P, S and CI were evaluated at various stages of starch gelatinization. Mineral movements into the starch granules were observed as gelatinization took place. Commercially extracted potato starch granules were observed and analysed at low temperatures. Also, these granules were cracked by raising the stage temperature to 153 K. Ice crystal growth appears to induce cracks in a radial and tangential manner along sites of weakness in a pattern that is supportive of the theory of complex starch granule formation.

Morphological Comparison of Two Varieties of Carrots During Growth and Storage: Scanning Electron Microscopy

The morphological development and response to storage of two varieties of carrots, both healthy and aster yellows-diseased, were evaluated using the scanning electron microscope. Instrumental textural profiles were evaluated for samples stored two weeks and then cooked by three different methods. Morphological and textural differences were found between the varieties of carrots, and these were attributed to differences in growth rates. Diseased carrots had unique cellular development but, except for the hardness parameter, showed few differences in textural properties when compared to healthy carrots.

EVAPORATION FROM POROUS MEDIA: A CAPILLARY MODEL OF A PENETRATING FRONT

A capillary tube model was solved to investigate the influence of mass transfer coefficient, temperature, and front depth on the evaporation rate during the penetrating-front period of water-filled porous media dried in hot air. The results show that increasing the flow rate of the drying air is not so efficient as increasing the sample temperature. Due to attenuating diffusion rate, the rate of liquid front migration decreases with time. The calculations explain the falling rate period behavior of sandstone heated at 121°C. Trends depicted by the model may be useful for the design of heating conditions for drying processes.

Influence of sample width on deducing capillary pressure curves with the centrifuge

Abstract In deducing capillary pressure curves by the centrifuge technique, the conventional Hassler-Brunner method assumes that the cylindrical sample is slender enough to neglect variation of capillary pressure across the sample width. In this work, the method for deducing capillary pressure curves for an arbitrarily shaped sample is evaluated. Capillary pressure curves are generated using model average saturation “data” and the error involved in neglecting the sample width by solving the Hassler-Brunner integral equation is determined. Neglect of width effects is seen to predict capillary pressure curves with lower fluid saturations. Errors associated with neglecting width effects are larger for larger diameter samples and for samples placed closer to the axis of rotation.

Water Loss Rates and Temperature Profiles in Dry Heated Normal and PSE Porcine Muscle

Small cylinders of longissimus porcine muscles from normal and stress susceptible animals and two groups with intermediate characteristics were heated from the frozen state, past sen sory doneness and well into the dehydration period. The water loss rates and temperature pro files of the muscle cylinders were monitored throughout the heating period. Pale, soft, ex udative muscle had a greater maximum moisture emission at an earlier time and lower temperature than the normal samples. When the sample interiors were between 60°C and 100°C, the centerline thermocouple frequently registered higher temperature than those ther mocouples closer to the surface. Implications of these results for interpreting heat and mass transfer mechanisms and for cooking recommendations are discussed.