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# Aging of starch and bread as studied by DSC, DMA, NMR and confocal microscopy

#### Abstract

Structural, thermo-mechanical and molecular changes in starch gels and bread were studied at variable moisture contents, storage times and temperature. Glassy-rubbery transition was characterized in systems with and without "freezable" water using thermal analysis and NMR (Nuclear Magnetic Resonance). Dynamic Mechanical Analysis (DMA) in conjunction with Differential Scanning Calorimetry (DSC) were used to describe structural relaxation (long range) while $\sp1$H cross-relaxation NMR and $\sp{13}$C CP-MAS (Cross-Polarization/Magic Angle Spinning) NMR were used to observe the starch molecular motions (short range). DMA results showed a transition region covering a large temperature range which was found to be moisture dependent. Overlapping tan $\delta$ (T) curves were deconvoluted into asymmetric double Sigmoidal curve (glassy-rubbery transition) and Gaussian curve (ice melting). The latter was found to highly correlate with the amount of "freezable" water (DSC data). The asymmetric curve was also confirmed as a glassy-rubbery transition on a molecular level using $\sp1$H cross-relaxation NMR and $\sp{13}$C CP-MAS NMR. Both NMR data showed a decrease in starch mobility with lowering temperature around the asymmetric (glassy-rubbery) transition observed by DMA. As the moisture content decreased so that the amount of "freezable" water was depleted, the transition temperature range increased, shifting to a higher temperature, typical of a glassy-rubbery transition.^ Firming of aging starch gels was related to amylopectin recrystallization (which followed amylose crystallization), glassy-rubbery transition or network formation (DMA), starch molecular mobility (NMR) and "freezable" water. A distribution of glassy-rubbery transition temperatures increased with storage time. The structural network was nonuniformly distributed leading to heterogeneous domains exhibiting different glassy-rubbery transition temperatures. However, only increased overall solid fractions (intensity) was observed by both $\sp1$H cross-relaxation and $\sp{13}$C CP-MAS NMR methods but no change in the molecular transition temperature range. This suggested that the "network" formation formed during the aging of starch did not occur on the molecular level but on a structural one. ^

Food science

#### Recommended Citation

Vodovotz, Yael, "Aging of starch and bread as studied by DSC, DMA, NMR and confocal microscopy" (1996). Doctoral Dissertations Available from Proquest. AAI9709664.
https://scholarworks.umass.edu/dissertations/AAI9709664

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