Effect of water and water migration on starch retrogradation and thermomechanical properties of bread during staling
Effect of water migration on physico-chemical properties of bread during storage was investigated. When breadcrumbs were stored with crust, moisture and aw decreased significantly, whereas for those stored without crust, these properties remained unchanged (no moisture loss from crumb). Addition of glycerol retarded amylopectin recrystallization but increased breadcrumb firmness suggesting that breadcrumb firming in this case was not directly related to amylopectin recrystallization. Biopolymers in bread might be locally dehydrated (osmotic dehydration) in the presence of hygroscopic solutes.
Stress-strain curves showed the typical sigmoidal shape described by three-parameter empirical model with C1, C2, and C3 representing scale factor, prominence of the shoulder and densification level, respectively. C1 correlated well (r 2 = 0.96) with firmness, whereas recoverable work decreased over the first 3 days of storage but did not show the effect of glycerol and crumb moisture loss effect.
The self-diffusion coefficient (D) of water (by PFG-NMR) in fresh breadcrumbs increased with increasing glycerol content at the same moisture content suggesting an increased water partitioning with glycerol as compared to a starch and gluten matrix and more water was free to diffuse. Although dependent on moisture, the diffusion coefficient did not proportionally change with moisture content during storage suggesting other factors (water-biopolymer interactions, geometry, restricted diffusion in complex domains etc.) might be related.
13C CP/MAS NMR peak intensity indicating relative amount of 13C solids was found to increase with increasing storage time in all cases. A lower 13C CP/MAS NMR peak intensity was observed in breadcrumb stored without crust than that of breadcrumb stored with crust at given glycerol content because less disordered-to-ordered chain rearrangement occurred. This confirmed that an amylopectin ordered structure (observed by DSC) was formed contributing to rigid crystalline solids ( 13C CP/MAS NMR).
Data confirmed that the hardening effect of added glycerol during storage was unrelated with the amylopectin recrystallization. Therefore, the hardening of bread during storage in this case was related to other changes such as local dehydration and stiffening of the amorphous domains. Future investigation is warranted on the molecular origins of these changes. This could lead to further development in engineering physical properties of many bakery products.