| Rheological (small strain, fracture) and water holding properties of myofibrillar protein gels derived from Alaska pollock (a poikilotherm) vs. breast meat of chicken (a homeotherm) were measured as affected by heating rate (0.5 vs. 90 °C/min) and time of isothermal holding (5-15 min) over a range of endpoint temperatures (45 - 90 °C). Properties of chicken gels were most affected by heating rate and endpoint temperature. Heating conditions could be optimized to produce gels from either species/meat such that the advantages of rapid heating technologies (e.g. reduced processing time, smaller equipment footprint and better energy efficiency) could be realized while producing gels with desirable water holding and rheological properties.;Capillarity, according to the Young-Laplace equation, is the prevailing theory for explaining how gels hold water. Accordingly, water should be more tightly held in gels with smaller pore size, more hydrophilic network surface (small contact angle), and higher solvent surface tension. Only qualitative evaluation of pore size has been studied by previous workers. We endeavored to determine whether water holding in model polyacrylamide and protein (fish or chicken) gels could be related to quantitative measurements of pore size (quantified by image analysis of scanning electron microscopy micrographs) and contact angle (obtained by the captive bubble method). Cook loss of fish gels correlated with larger mean pore size as expected, and smaller contact angle (greater surface hydrophilicity) also correlated with better water holding of these gel systems. However, expressible water of polyacrylamide and chicken gels was greater as pore size decreased, contrary to capillarity theory.;The mobility of water in gels has been studied by low field time-domain nuclear magnetic resonance (NMR) T2 relaxation experiments. It has however been suggested that pore size imposes artifacts in the measurement such that T2 relaxation times do not reflect changes in water mobility alone. Thus, it is unclear whether the various water 'pools', associated with differing T2 relaxation times, obtained by processing the multi-exponential decay of water, represent discrete locales within gels where water is more or less mobile (structured). In the present work, longer T2 relaxation times correlated with larger pore diameters in some, but not all gels. Relaxation times of gels immersed in increasing amounts of pure water did not relax on the same order of pure water alone until a critical amount of pure water was added, suggesting significant water structuring at the protein-water interface. Water holding properties of these gels exhibited some relationships with water pools when processed by the distributed continuous approach often used by other workers, and a two-state model for water structure was used for explaining these associations. |
