Agriculture Reference
In-Depth Information
tents. If the percentage of polar to nonpolar lipids
in a fl our is varied from 0 to 100, at a constant
lipid level, test-bake loaf volume increases approx-
imately linearly (MacRitchie 1977). Higher loaf
volume and better crumb texture in bread are
favored by a high ratio of polar-nonpolar lipids
and a higher content of native fl our lipids (McCor-
mack et al., 1991). However, the variation in loaf
volume that can be attributed to lipids is relatively
small, and by far the greatest variation is imposed
by the effect of gluten protein quality.
Similar effects by lipids were also found for
Arabic bread, steamed bread, cakes, and biscuits
(Papantoniou et al., 2003). Despite differences in
processing, the general features with respect to
lipids have much in common. Crumb texture is
infl uenced most and, like bread, differences in
lipid have not been found to account for major
variations in quality.
form monomolecular fi lms at the gas-liquid inter-
faces of the dispersed gas bubbles. Competition
for the interface occurs between polar lipids and
soluble proteins. Since proteins form rigid fi lms
at the interfaces, the highly mobile polar lipids
destabilize the protein fi lms fi rst. Thus increased
polar-lipid content leads to the formation of con-
densed monolayers at air-water interfaces, pro-
viding more stable interfacial fi lms between the
gas and the liquid phases, and stabilizing the gas
cells and the dough structure (MacRitchie 2003).
This mechanism explains why, when adding polar
lipids in a defatted fl our, there is fi rst a decrease
of bread volume (caused by the instability of the
gas cell structure). Then (above a threshold
content) polar lipids improve the stability of the
gas-cell structure during dough mixing and
expansion, and thus the fi nal loaf volume and
texture.
Certain interfacially active proteins in wheat,
such as nonspecifi c lipid-transfer proteins, puro-
indolines, and various α-amylase-trypsin inhibi-
tors (CM3 proteins), may also be expected to
form thin protein-protein or protein-lipid fi lms
lining the gas cells (Jones et al., 2006). Puroin-
dolines are able to form very stable foams which
have high resistance to destabilization by both
neutral and polar lipids (Marion et al., 2003;
Rouillé et al., 2005). Furthermore, puroindoline
foams show a synergistic enhancement of the sta-
bility in the presence of polar lipids. They can act
cooperatively with polar lipids in interfacial fi lms
resulting in increased foam stability. However,
the involvement of these lipoproteins in gluten
structure has not yet been investigated.
Dough structure and gas cell stabilization
A simple dough is made by mixing wheat fl our
with water. Doughs contain roughly 45%-50%
water, in which approximately 35% water is taken
up by binding to gluten protein and starch gran-
ules. The additional 15% water is in the forma-
tion of a separate liquid phase, “dough liquor,”
which can be separated from dough solids by
ultracentrifugation. Air is also incorporated into
the dough in the form of tiny bubbles during the
later stages of dough mixing. No more air is intro-
duced and no new bubbles are formed in the
dough after mixing, although subdivision of exist-
ing bubbles can occur during subsequent molding
steps. Cell rupture and coalescence may occur
during proofi ng and baking. During fermenta-
tion, the gas produced by yeast dissolves in the
liquid phase and diffuses into the gas cells. The
liquid phase is essential for expansion and stabil-
ity of the gas cells formed during dough mixing.
Therefore the fi nal loaf volume and texture is
largely determined at the mixing stage.
The gas cells are surrounded by thin fi lms with
an aqueous interlamellae phase (Gan et al., 1995).
Surface-active compounds (mainly the water-
soluble proteins and polar lipids) are adsorbed to
STARCH COMPOSITION AND
WHEAT QUALITY
In contrast to lipid, a minor component of the
grain, starch is the most important by amount,
comprising almost two-thirds of the grain dry
matter. However, for many years starch was
considered to play only a minor role in wheat
product quality. It has been only recently that the
contribution of starch to fl our processing quality
