Food Chemistry

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 elas-
ticity 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 in-
crease. 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 peakaat 60◦C caused
by the melting of crystalline amylopectin is ab-
sent in the thermogram of gelatinized starch (II).
However, the melting point of amylose/lipid
complexes (ca. 110◦C, peakbin curve II) is not
reached in the crumb on baking.

Fig. 15.51.DSC thermograms of wheat starch in wa-

ter (45:55, g/g) I: native starch, II: gelatinized starch

(according toSlade, Levine, 1991)

Fig. 15.52.DSC thermograms of white bread: I: fresh

from the oven, II: after storage for 1 week at room tem-

perature (according toSlade, Levine, 1991)

Staling of white-bread crumb begins with the for-

mation 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 amor-

phous starch and proteins. The crumb loses its

elasticity and becomes stale. On storage of white

bread, the amount of water that can freeze de-

creases corresponding to the conversion to non-

freezing crystal water (Fig. 15.53).

The formation of crystal nuclei, which proceeds

very rapidly at 0◦C and does not occur at tem-

peratures 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).