Agriculture Reference
In-Depth Information
endosperm, which constitutes the bulk of fl our
and semolina (Mares and Campbell 2001).
Brightness (L*) is determined by the overall
light refl ectance of the sample and is a function
of particle size, particle-size distribution, and the
absorption of light by the fl our or semolina. The
creaminess of bread wheat fl our and the yellow-
ness of durum semolina are measured as positive
values of b* and are primarily related to the
content of xanthophyll (Mares and Campbell
2001), mainly lutein and its mono- and di-fatty
acid esters, and to the inherent color of the
starchy endosperm, which constitutes the bulk of
fl our and semolina (Mares and Campbell 2001).
During the development of modern bread-wheat
cultivars, selection has been towards a white-
creamy fl our and relatively low levels of lutein.
In contrast, the opposite is true of durum wheat,
for which bright yellow pasta is preferred.
Red-green pigments are not considered appro-
priate for wheat-based products, and materials
with a* values substantially different from
zero are generally discarded during the breeding
process.
End-product color depends on the color of the
fl our or semolina, together with reactions and
interaction with ingredients that occur during
mixing with water, as well as subsequent process-
ing and cooking. The process of mixing with
water itself results in a lower brightness due to a
reduction in the light refl ecting capacity. In Asian
alkaline noodles, the interaction between ingredi-
ents, alkaline salts and high pH, and fl our con-
stituents (fl avone-C-diglycosides) results in the
development of the yellow color that is consid-
ered desirable for this end-product (Asenstorfer
et al., 2006). Variation in brightness between
samples and change in brightness with time can
be related partly to protein content and partly to
the activity of enzymes.
living tissues of the embryo, scutellum, and aleu-
rone, but in addition some enzyme activities are
retained in the dead tissues of the seed coat and
the starchy endosperm. Many of the enzymes in
the living tissues are essential for general cellular
metabolism and function, typical of all plant cells,
and will not be discussed here. Other enzymes
that may have had specifi c roles during grain
development are retained in ripe grain. Some of
these may interact with endogenous substrates
during mixing and processing to produce
compounds that affect product color or color
stability.
Finally, there are enzymes that are normally at
low levels (or not present) in dry, ripe grain but
are activated during germination to remobilize
the starch and protein stored in the starchy endo-
sperm to feed the new seedling. If, for example,
the grain becomes wet prior to harvest and starts
to germinate (preharvest sprouting), or if the
cultivar contains a genetic defect such as late-
maturity α-amylase (LMA, otherwise referred to
as prematurity α-amylase), these enzymes can be
present at unacceptable levels in harvested grain.
The result is a failure to meet receival standards,
price dockages, and unsuitability for many end
products.
Lipase and lipoxygenase
Lipases release free fatty acids from triglycerides
and lead to the development of rancidity in whole-
meal, germ (embryo), and products that contain
germ that has not been stabilized or deactivated.
Free fatty acids such as linoleic and linolenic acids
can be oxidized by lipoxygenase (LOX) to produce
hydroperoxides (Hessler et al., 2002). These in
turn can initiate oxidative degradation of pig-
ments such as lutein and β-carotene. As a conse-
quence of this co-oxidation, lutein and lutein-ester
content may decline during dough mixing, giving
poorer colored end products, especially noodles
and pasta. In addition to its effects on color, lipox-
ygenase has been associated with the formation of
off-fl avors and unpleasant odors. Substantial
variation for LOX activity, including near-zero
levels within bread and durum wheat cultivars,
means that the effects of LOX may be addressed
ENZYMES AND WHEAT QUALITY
Enzymes are biological catalysts that facilitate
complex biochemical reactions at temperatures
that are relevant to living tissues. Wheat kernels
contain a vast array of enzymes, not only in the
