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The staling of long shelf-life bread as determined by thermal analysis
Abstract
The glassy-rubbery state of bread polymers has been proposed as a key factor contributing to staling. Glassy-rubbery states of bread components as measured by Dynamic Mechanical Analyzer (DMA) in meal-ready-to-eat (MRE) bread exhibit three thermal transitions: two moisture dependent ($-70\sp\circ$C and $-12\sp\circ$C) and one moisture independent (10 to 40$\sp\circ$C). One moisture dependent transition ($-70\sp\circ$C) was attributed to the humectant. The other had a tan delta peak and dropped in storage modulus (E') at $-12\sp\circ$C. The independent transition, was attributed to fats. Over storage, the MRE bread significantly decreased in the tan delta peak height but the Tg temperature remained relatively unchanged, indicating no sign of maturation of the polymer network. Standard white pan bread (SWB) showed only one moisture-dependent transition (T1) and over storage (with moisture loss), there was a dramatic decrease in the tan delta peak amplitude and an increase in the T1 peak temperature with firming. T1 has a relationship with the presence of unfrozen water detected by Differential Scanning Calorimeter (DSC). The amount of "freezable" water did not change in MRE bread during storage, however, SWB lost almost 50% of its "freezable" water, had an increase in Tg, and a dramatic increase in Instron compression resistance. MRE bread displayed only a small increase in firmness, until 82 months. Fifteen month old MRE bread was found to be softer than SWB stored 3 months. The MRE curve was linear (within 0-50% compression range) and required much less stress to compress 50% ($<$10KPa), whereas the SWB curve was sigmoidal and required approximately 30KPa. Both aged MRE and SWB showed a significant increase in amylopectin crystallization with time compared to fresh (MRE, slightly higher). MRE bread remained soft over time (at least 47 months) indicating that the softening effect from added glycerol and retention of moisture over storage had a greater impact on the texture than amylopectin crystallization. Thus, it could be concluded that firming of bread during staling is not mainly caused by starch retrogradation. This is based on the experimental bread formula, processing, and storage conditions done in this work.
Subject Area
Food science
Recommended Citation
Hallberg, Linnea May, "The staling of long shelf-life bread as determined by thermal analysis" (1996). Doctoral Dissertations Available from Proquest. AAI9638967.
https://scholarworks.umass.edu/dissertations/AAI9638967