The evolution of molecular dynamics during freezing and thawing of strawberry tissue studied by variable temperature proton NMR
Abstract
Changes in molecular state of water and sugars in intact tissue strawberry were monitored in terms of changes in proton NMR chemical shift, line widths, intensities, and longitudinal relaxation times during stepwise freezing and thawing over the temperature range from $20\sp\circ\rm C$ to ${-}60\sp\circ\rm C.$ The concentrations of water, glucose, fructose, sucrose, citrate, and lipids were quantified at room temperature by high resolution proton NMR with magic angle spinning. Sugar concentrations and mobility in the freeze-concentrated phase could be followed from the freezing event at ${-}7.0\sp\circ\rm C$ to about ${-}15\sp\circ\rm C$ and during thawing from about ${-}15\sp\circ\rm C$ to complete melting at about $0\sp\circ\rm C.$ Broad ($>$30 kHz), intermediate (1 to 10 kHz), and narrow (3 to 1000 Hz) components of the water-dominated wide-line solid echo NMR spectra were attributed to solid (ice and glassy freeze-concentrated phase), high viscous (freeze-concentrated), and low viscous liquid, respectively, with the help of spectrum simulation. Direct evidence has been obtained to demonstrate, for the first time, the coexistence of three different molecular mobility states (solid, high viscous phase, and low viscous liquid) over a certain temperature range in a freezing and thawing system. The distribution of the intensities of the different NMR phase components may lead to a rough estimation of the mass distribution among different molecular mobility states. NMR results were compared with literature thermal analysis results in order to relate molecular dynamics to processing and stability properties. The rotational and translational correlation times were estimated from NMR line width for sugar and water. Temperature dependence of micro-viscosity in the freeze concentrated phase was also calculated using Debye-Stroke's theory. A model was proposed to illustrate NMR phase distribution among solid, high viscous, and low viscous states and to relate the phase transition and coexistence to the cell structure of freezing and thawing strawberry tissue.
Degree
Ph.D.
Advisors
Eads, Purdue University.
Subject Area
Food science|Chemistry|Materials science
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