2009_Book_FoodChemistry

15.4 Baked Products 739
Table 15.62. Concentrations of odorants in the crusts
of white bread and rye bread
Compound
Concentration (µg/kg)
White bread
Rye bread
2-Acetyl-1-pyrroline 19 0.8
3-Methylbutanal 1406 3295
Methional 51 480
(E)-2-Nonenal 56 45
4-Hydroxy-2,5-dimethyl- 1920 4310
3(2H)-furanone
15.4.4 Changes During Storage
Bread quality changes rapidly during storage.
Due to moisture adsorption, the crust loses its
crispiness and glossyness. The aroma compounds
of freshly baked bread evaporate or are entrapped
preferentially by amylose helices which occur
in the crumb. Repeated heating of aged bread
releases these compounds. Very labile aroma
compounds also contribute to the aroma of bread,
e. g., 2-acetyl-1-pyrroline. They decrease rapidly
on storage due to oxidation or other reactions
(Table 15.59).
The crumb structure also changes, although at
a lower rate. The crumb becomes firm, its elasticity
and juiciness are lost, and it crumbles more
easily. The so-called staling defect of the crumb
is basically a starch retrogradation phenomenon
(cf. 4.4.4.14.2) which proceeds at different rates
with amylose and amylopectin. On cooling bread,
the high-molecular amylose very rapidly forms
a three-dimensional network and the crystalline
states of order of amylose/lipid complexes increase.
These processes stabilize the crumb.
On the other hand, the amylopectin is in an
amorphous state because the crystalline regions
present in flour melt on baking. This is in contrast
to the behavior of crystalline amylose/lipid
complexes. Thermograms of an aqueous starch
suspension (Fig. 15.51) show the differences in
the melting points. In comparison with native
starch (I), the endotherm peak a at 60 ◦ C caused
by the melting of crystalline amylopectin is absent
in the thermogram of gelatinized starch (II).
However, the melting point of amylose/lipid
complexes (ca. 110 ◦ C, peak b in curve II) is not
reached in the crumb on baking.
Fig. 15.51. DSC thermograms of wheat starch in water
(45:55, g/g) I: native starch, II: gelatinized starch
(according to Slade, Levine, 1991)
Fig. 15.52. DSC thermograms of white bread: I: fresh
from the oven, II: after storage for 1 week at room temperature
(according to Slade, Levine, 1991)
Staling of white-bread crumb begins with the formation
of crystalline structures in amylopectin.
The endotherm peak at 60 ◦ C appears again in the
thermogram of stored white bread (Fig. 15.52).
A state of order arises which corresponds to that
of B starch (cf. 4.4.4.14.2) and binds up to 27%
of crystal water, which is withdrawn from amorphous
starch and proteins. The crumb loses its
elasticity and becomes stale. On storage of white
bread, the amount of water that can freeze decreases
corresponding to the conversion to nonfreezing
crystal water (Fig. 15.53).
The formation of crystal nuclei, which proceeds
very rapidly at 0 ◦ C and does not occur at temperatures
below −5 ◦ C (Fig. 15.54), determines
the rate of amylopectin retrogradation. The
nuclei grow most rapidly shortly before the
melting point (60 ◦ C) is reached (Fig. 15.54).