Agriculture Reference
In-Depth Information
Measuring extensional properties of
developed doughs
In commercial practice once a dough is developed
in the mixer it is often subjected to a number of
additional unit operations that extend the dough
in length. In baking applications the most impor-
tant is the biaxial extension encountered during
fermentation, proofi ng, and oven rise. Noodle
dough is not developed during mixing, but rather,
dough development occurs during the uniaxial
extension imposed during sheeting operations
(Ross and Ohm 2006). Hence the outputs of
RDMs do not necessarily describe dough attri-
butes important to these extensional phases of
processing. To do this dough attributes need to
be measured while the dough is being stretched
or otherwise extended and often after a specifi ed
period of rest that allows internal stresses in the
dough to dissipate (relax). Dobraszczyk et al.
(2000) reviewed the fundamentals of a number of
extensional and dynamic instruments that are
used to do this.
ments of the maximum force required to extend
the dough (maximum resistance, MXR, and often
used as a descriptor of dough strength) and the
distance the dough can be extended before rupture
(extensibility, EXT) (Fig. 20.5). Further infor-
mation can be gathered by measuring the work-
to-rupture via the area under the force vs. time
curve or other manipulation of the raw data, for
example, the MXR : EXT ratio. Anderssen et al.
(2004) advocated the use of EXT at MXR, and
EXT between MXR and rupture as more useful
measures of dough extensional rheology, as it
relates to glutenin composition of wheat cultivars.
This work was done with the CSIRO prototype
microextension tester, but the conclusions may
also be valid for the Brabender extensograph
where MXR occurs at a variable point during the
extension of the dough. However, for the Kieffer
dough rig, MXR occurs very close to EXT at
rupture and this type of analysis may lack utility
for that variant of uniaxial dough extension. Mann
et al. (2005) showed a fi gure specifi cally compar-
ing Brabender extensograph output with output
from a Kieffer extension rig that highlights this
difference.
There has been interest in using uniaxial exten-
sion test results as the basis for examining funda-
mental dough rheology parameters. The complex
geometry, reduction in cross-sectional area of the
dough piece, and constant velocity extension
(therefore, steadily decreasing strain rate) mean
that the extensograph does not perform a genuine
tensile strength test. However, MacRitchie and
Lafi andra (1997) contend that elements of the
molecular level phenomena can be elucidated
using this type of uniaxial constant velocity
testing. Nevertheless the utility of these empirical
tests has been proven with regard to their abilities
to categorize dough samples as, for example,
strong and inextensible or weak and extensible in
ways that are meaningful to plant breeders and
commercial users of wheat fl our.
Uniaxial extension
The type of dough deformation most familiar to
wheat technologists is the extension or stretching
applied during uniaxial extension tests. Two basic
scales of testing are applied: large (150 g fl our)
and small (10 g or less). The large-scale tests are
done using the Brabender extensograph (Braben-
der Instruments, Inc., South Hackensack, New
Jersey), and small-scale extension tests can be
done using the Kieffer dough extensibility rig for
the Stablemicrosystems TA texture meter series
(Stable Micro Systems Ltd., Godalming, Surrey,
UK). A custom-made prototype microextension
tester was also reported by Australian CSIRO
wheat researchers (e.g., Anderssen et al., 2004).
The Simon research extensometer was also men-
tioned in other reviews (e.g., Rasper and Walker
2000). However, no studies using this instrument
can be found in a search of the Food Science and
Technology Abstracts between 1990 and Novem-
ber 2007.
At the most basic level the outputs of all the
uniaxial extension tests can provide measure-
Biaxial extension
It can be argued that the biaxial extension applied
by the increasing pressure in gas cells during
fermentation and initial heating in the oven is
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