Biomedical Engineering Reference
In-Depth Information
frozen storage at −25 °C, hardness remained constant while the moisture content
decreased with the time of frozen storage (Barcenas and Rosell
2006b
). Nevertheless,
aging studies of full baked bread showed progressive increase in the retrogradation
in amylopectin, and great energy was required for amylopectin melting at longer
frozen storage period. Simultaneously, crumb hardness of the full baked bread
increased with the time of frozen storage, while moisture content of the crumb
decreased. Also the hardening rate during aging was dependent on the time of fro-
zen storage (Barcenas et al.
2003a
; Karaoglu
2006
). Therefore, some structural
changes are produced during the frozen storage that affect the fi nal quality of the
full baked product. Those changes have been associated to the damage of bread
structures produced by the ice crystallization. This was supported by the results of
cryo-scanning electron micrographs (Barcenas and Rosell
2006b
). Alpha-amylase,
sourdough and HPMC have been proposed as bread improvers because they
decreased the retrogradation enthalpy of the amylopectin, retarding the staling
(Barcenas et al.
2003b
). Some deep modifi cation at molecular level, for instance
on starch chains arrangement, has been observed on scanning electron micrographs
(Barcenas and Rosell
2006b
). Indeed, a signifi cant increase of resistant starch in
frozen stored part baked wheat rolls was observed by Borczak et al. (
2008
). This
could subsequently reduce the glycaemic index. The low glycaemic index diets
have been associated to reduce the insulin resistance syndrome, cardiovascular
disease, type 2 diabetes and certain cancers (Holm and Björck
1992
). Any strategy
to decrease the glycaemic index of the bakery products is welcomed.
Hamdami et al. (
2004
) simulated the freezing process in an infi nite two-layer
cylinder to understand the phenomena governing the heat and mass transfers during
the freezing of par-baked bread. The model considered the apparent specifi c heat,
enthalpy, thermal conductivity, and water activity, and predicted the temperature
profi les and weight losses. They also validated the freezing of par-backed breads
with cylindrical shape.
Freezing and frozen storage provoke a retraction of part-baked bread derived
from the contraction stress in the matrix during the freezing process due to a rapid
deformation during ice-crystallization (Ribotta and Le Bail
2007
). Those changes
affect water properties and modify the shrinkage tendency of part baked crumb
(Ribotta and Le Bail
2007
).
Crust fl aking has been related to excessive drying of the bread surface at the end
of the post-baking chilling and freezing process (Lucas et al.
2005
). Crust fl aking is
produced due to the concentration of ice below the crust during freezing and the
mechanical damages induced by the thermo-mechanical shock during chilling,
freezing and fi nal baking (Hamdami et al.
2007
). Specifi cally, the thermal stress
induced by the volume expansion of water during freezing process of part-baked
bread has a great impact on fl aking. Consequently, it is very important to understand
the thermo-mechanical changes associated to freezing in order to control and pre-
vent strains within the loaf. Expansion due to phase change and thermal contraction
play a fundamental role to explain cracking patterns (Ben Aissa et al.
2008
). During
freezing, cracking is likely due to tensile tangential stresses caused by thermal
contraction. Presumably, vitrifi cation at the surface induces crack initiation and this
could propagate towards the centre (Ben Aissa et al.
2008
).
