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| Fig. 133 : Gluten extraction by the Martin process (modified from Tegge, 1984) |
To describe all the aspects of gluten and its production would fill a
separate book, so this section will concentrate on the issues relating to flour
improvement.
Information on the rheological properties of gluten is also given
in The Role of Gluten Elasticity in the Baking Quality of Wheat .
But
like many other articles, that articles only deals with native gluten, i.e. as it is extracted from wheat
flour. If wheat gluten is to be used commercially as an additive, it first has
to be extracted from wheat and converted into powder.
This is a multiple-step
process (Fig. 133), starting with the milling of the wheat grains.
Milling is
followed by aqueous separation of the starch and soluble substances from the
aggregated gluten, disintegration of the gluten in a pin mill or the like and finally
hot air drying, for example in a ring dryer.
Only about 82% of the protein of flour is insoluble in water and
contributes to wet gluten formation. Furthermore, some of the watersoluble
proteins are trapped by the insoluble proteins.
But wet gluten is not just
water and protein; it also contains about 5 - 10% lipids (d.b.) and a
significant amount of non-starch carbohydrates (Pomeranz, 1988). It is clear
that the functionality of extracted and dried gluten that is re-added to flour
will differ from the gluten naturally present.
Most of the changes, especially
in the water absorption and rheological properties, are caused by the drying
process. Drying will always result in a (limited) denaturation of the protein,
i.e. careful drying is a key to highquality vital wheat gluten.
To facilitate
blending with flour, the dry gluten particles should not be larger than the
largest flour particles. The smaller size improves rehydration (after
incorporation into a flour).
The water absorption of dry gluten is less than that of native gluten.
It is typically 1.3 - 1.5 parts of water to 1 part of dry gluten when added to
flour.
The water absorption of native gluten is about 2.5 - 3 times its dry
weight. Interestingly there have been reports that low dosages of fungal protease
can improve the water absorption of vital wheat gluten in baking applications,
but this does not necessarily coincide with improved baking performance.
Drying results in a shorter gluten structure, i.e. lower
extensibility.
Although it is possible to test the rheological properties of
rehydrated gluten as it is, for instance with the Texture Analyser equipped
with a Kieffer rig, it is better to add the gluten to flour or to prepare a
reconstituted flour using starch as a "diluent".
The latter has the
advantage that starch is a better defined raw material than flour. Because of
the possible and variable interactions of wheat starch with wheat gluten, corn
starch is even better.
When added to wheat flour, vital wheat gluten increases the strength
of the protein, i.e. in the Extensograph and Alveograph the resistance will be
higher and the extensibility lower. The Farinogram shows a broader and more
stable curve.
Because of the denaturating action of the drying process and the
resulting shorter gluten properties, wheat varieties with a softer gluten are
more suitable for gluten production.
But the author has noticed several times that the effect of the gluten
also depends on the flour to which it is added: for instance vital wheat gluten
from German wheat added to U.S. wheat flour seemed to be more effective than
using gluten from U.S. wheat.
On the other hand, U.S. wheat gluten produced
better results in European bread applications when added to French or German
wheat flour. It can be speculated that in both cases, mixing with foreign
gluten improved the total gluten composition. Unfortunately, scientific
evidence is still lacking.
The colour of the gluten also matters. In many cases, extracted wheat
gluten has a greyish appearance which will show up in the final application,
especially in noodles.
It should therefore be as bright white or yellowish as
possible. The wheat variety, and also the extraction and the drying process,
have an influence on the colour.
The protein content of vital wheat gluten is about 80%. Unfortunately,
different methods and factors are used to determine the protein content. When
using the Kjeldahl method, a nitrogen: protein factor of 5.7 should be applied.
This typically results in a protein figure of about 78 - 79%. Sometimes the
factor for feed wheat, 6.25, is used. The protein figure will then be about 10%
higher.
Although the Kjeldahl method is the reference, the largescale
production of vital wheat gluten will always involve the much faster NIR
method, using Kjeldahl only for calibration purposes.
Trials using reducing agents such as L-cysteine in the gluten
extraction process in order to improve the gluten extensibility were
unsuccessful (Hedwig, 1996).
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| Fig. 134 : Impact of lecithinated vital wheat gluten (EMCEvit plus) on dough stability (Dirndorfer, 2000) |
Phospholipids have long been known to protect protein or micro-organisms
against detrimental impacts such as temperature, pH conditions or mechanical
stress (Höfer et al., 1996).
Incorporating lecithin into the wet gluten before
the drying process should therefore improve the "vitality" of the
protein. Hydrolysed lecithin does, in fact, improve the rheological properties
of wheat gluten, for instance extensibility.
Even quite a small proportion of
these protected gluten proteins probably decelerates the agglomeration of the
endogenous flour proteins during dough preparation. So the protein network only
reaches perfection at the end of the mixing process (Fig. 134).
As compared to
standard vital wheat gluten, the addition of lecithinated wheat gluten results
not only in enhanced dough stability and volume yield, but also in better dough
extensibility and improved machinability.
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