2.6. Gluten Aggregation Test with
Whole Wheat Meal
Much work has recently been done on wheat flours with the
"KO" (knock-out) test (Gluszcynski et al., 2001).
For wholemeal analyses we see further possibilities of characterizing
the raw material after grinding wheat in the Falling Number mill. The author
feels certain that the use of this method alongside NIR and Falling Number
determinations, for example, will enable wheat to be allocated more precisely
to quality groups. The method cannot replace the standard tests, nor is it
meant to do so; its usefulness lies chiefly in providing quick information on
delivered lots of quality wheat. The test itself, on a milled sample, takes
only about 15 min.
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| Fig. 73: Flow chart for the Gluten Aggregation Test |
The procedure is shown in the flow chart in Fig. 73. The most
important evaluation criteria are aggregation time and aggregation area, and
also power uptake (Tab. 62). Moreover, the temperatures should be recorded
continuously throughout the test. They are closely related to viscosity
behaviour.
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| Tab. 62: Gluten Aggregation Test |
The main purpose of this test is to confirm the stated quality classes
of delivered lots of wheat. However, at the points where wheat qualities
overlap, this quick test of the raw material also produces ambiguous results,
as the diagram of typical GAT (Gluten Aggregation Test) curves shows (Fig. 74).
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| Fig. 74: Typical curves from the Gluten Aggregation Test on wholemeal wheat |
This method is especially suitable for determining the gluten
properties of special types of wheat, e.g. for biscuits, at an early stage.
Unlike protein analysis, the GA Test indicates characteristics that would not
otherwise be discovered until the baking test, in spite of possible
overlapping. Tab. 63 shows the possibilities for evaluation. At present these
values are only national, i.e. geared to German requirements, for example for
acceptance at mills.
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| Tab. 63: Average results of the Gluten Aggregation Test (wholemeal and extracted flours) |
The first part of the curve with aggregation times up to about 60 s –
i.e. those of wheat with medium baking potential such as the German B varieties
– is dominated by soft gluten structures with high water absorption. These are
revealed analytically by relatively low gluten index values.
This is followed in the range of aggregation times from about 100 to
250 s by patterns showing higher protein, moisture and gluten levels if the
corresponding gluten structures (meaning better baking functionalities) can be
derived from them.
Wheat with poorer baking properties, i.e. weaker B varieties and also
C and feed wheat and the biscuit wheats so important for the durable baked
goods industry, that usually have low protein and gluten values and low water
absorption, are then characterized by aggregation times of > 300 s or even
> 700 s for soft biscuit wheat. Unfortunately the wheats produced by organic
farming also fall into this category of inferior baking wheat. Although the
protein they contain bakes fairly well, they have very poor aggregation
properties because of their lower wheat protein levels and especially their low
gluten content.
The information derived from the GAT is made more reliable by the fact
that the maximum and mean power uptake and above all the aggregation area
circumscribed by the curve are measured in addition to the aggregation time.
But before we go into these general trends in greater detail we should
explain the typical differences between the GAT values for extracted wheat
flours with mineral contents of about 0.5 to 0.7% in the dry matter and
wholemeal flours from a Falling Number mill (Kamas mill).
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| Fig. 75: Gluten Aggregation Test: results of tests with wheat, Falling
Number meal and classified flours of various qualities |
The GAT curves for whole wheat meal (Falling Number mill) show more
rapid aggregation than those of the extracted flours of the same patterns (0.5
to 0.7% mineral content) because of greater natural viscosity, but the peak
value of the curves is lower. This tends to apply to biscuit wheat too. But
here a certain amount of aggregation is observed with wholemeal flours which no
longer occurs with the extracted flours. This shows that the extraction rate of
the cereal and the ingredients of the outer layers of the grains influence the
viscosity of the suspension in the GAT. Fig. 75 shows typical curves from the
Gluten Aggregation Test carried out on wholemeal wheat flours and extracted
flours produced from the same wheat, with a mineral content of about 0.55 to
0.65% of the dry matter, from the various German quality groups. Quality group
K characterizes "biscuit wheats" for the durable baked goods industry,
and group C "other wheats", including feed wheat. The distinctions
between the various quality groups result from comparison and combination of
the most important GAT data – aggregation time and area, the course taken by
power uptake, and in future perhaps a measurement of the temperature of the
suspension as the difference between the initial and final temperature or a
temperature at certain points in time.
If we compare all the tests carried out to date on wholemeal flours
and the corresponding extracted wheat flours, we find that the wholemeal
product yields rather less information on the properties of the gluten because
the outer layers of the cereal always reduce gluten formation. When testing
wholemeal flour to identify biscuit wheats it is important not to be misled by
the fact that certain aggregations may occur after about 400 to 500 s. When
extracted flours from the same wheats are tested, these do not occur.
The following GAT trends have been ascertained to date:
• Short aggregation times (up to about 80 s) generally mean high water
absorption.
• Soft gluten indicates only moderately good baking properties even if
the peak values of the curves are fairly high, i.e. if the protein and gluten
content is possibly elevated.
• Late aggregation (periods over 300 s) means lower water absorption,
normal to firm gluten but also short gluten, with a generally low protein and
gluten content.
• No aggregation or very late aggregation (over 400 s with wholemeal
flour or 700 s with extracted flours) indicates very low water absorption,
firm, short or even crumbly gluten, and varying but generally low protein and
wet gluten content (also organically grown wheat).
Moreover, increasing aggregation areas mean stronger gluten complexes
with gluten properties that are generally more suitable for bread baking. These
trends are the same whether wholemeal wheat flours or extracted flours are
tested. The better possibility of prediction offered by extracted flours is
already explained by the fact that their good baking properties have been
deliberately concentrated in their design and production. So it is not
surprising that wheat with very good baking properties is characterized best;
and it is this that makes the GAT valuable for the acceptance of cereals at a
mill.
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| Tab. 64: Variety-specific trends in the GAT on wheat (wholemeal and extracted flours) |
But besides the general trends there are also variety-specific
observations, especially concerning the results obtained with wholemeal wheat
flours; these are then also reflected in the GAT with extracted flours. Tab. 64
shows that a knowledge of varieties is an advantage in this analysis as it is
in the assessment of cereals in general. So far these special features have
only been followed up with German wheat varieties.
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| Fig. 76: Wheat characterization by GAT values and baked roll (RMT) volumes |
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| Fig. 77: Wholemeal wheat flours compared with the volume yields (mL/ 100 g) of various quality groups in the RMT |
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| Fig. 77: Wholemeal wheat flours compared with the volume yields (mL/ 100 g) of various quality groups in the RMT |
There are promising correlations between GAT values and baked volume when
an extracted flour (in Germany Type 550, for example) is used in the Rapid Mix
Test, a baking test for bread rolls (Fig. 76 - Fig. 78). Fig. 76 shows the
average GAT value ranges obtained to date with German E, A and B wheat with the
analytical means of all the samples tested. For example, the E wheats had an
average aggregation time of 87 s, an aggregation area of 45 cm2 and a baked
volume of 732 mL/100 g flour in the RMT. Fig. 77 shows the scatter, especially
of the baked volume in the RMT, in comparison with the GAT measurements. The
most important correlations between the aggregation area in the GAT and baked
volume in the RMT can be seen from Fig. 78. In the E-wheat range this includes
some quality wheat varieties from abroad, for example the USA and Canada, with
extreme GAT aggregation areas. Tests have also been carried out with wheat from
four non-European and five European growths, in which the quality rating given
by the sender was compared with our GAT readings. Requests for samples were
made largely according to the following criteria: high and medium baking
quality and – especially important – so-called biscuit wheat. The comparability
of baking quality was good to very good, with a few deviations that plainly
resulted from differences in the definition of good or medium baking quality in
Germany and other countries. All the wheats intended for durable baked goods
(hard biscuits and wafers) were identified very reliably. A further noticeable
feature was that very strong baking wheats from the USA and Canada sometimes
produced double peaks, which resulted in extremely large aggregation areas. But
in general it was possible to maintain a value greater than 50 cm2 for the
aggregation area to differentiate between high and medium baking quality.
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