1.4. Empirical Rheometry
1.4.1. Sensory Testing
There can be no doubt that sensory testing to assess the
characteristics of dough and the crumb and crust of bread is one of the classic
and oldest uses of empirical rheometry. The baker uses his hand and his
acquired experience to judge the viscosity and elastic properties of his
intermediate product (the dough) and his end product (the crumb of the bread).
A baker's tester (a specialist trained in sensory testing) works almost like an
instrument; both the production manager and the customer rely on his "measurements".
A tester assesses the nature of the dough according to its components:
"elasticity" (from unsatisfactory and soft to short) and
"surface" (from slimy and wet to dry). He assesses the crumb
according to elasticity and chewability; his verdict can range from "soft"
to "firm". A baker's language is more flowery and describes the viscoelastic
properties of the dough more aptly. He would distinguish more precisely between
"soft" and "weak" on the one hand and "firm" on the
other by describing the dough as "sticky", "pliant" or
"pliable", "silky" or even "bucky" (if it
"springs back").
These ratings describe the desirable, positive characteristics of the
dough and its undesirable, negative characteristics. In their flowery terms
they also describe the shred of an RMT 17 roll. The shred of an RMT
roll is an important indicator of the viscoelastic properties of the dough, and
vice versa. The shape of the shred and the appearance of the roll can be
predicted from the known properties of the dough. A dough portion shaped by
flattening and rolling in the RMT standard baking test should open up in the
oven to form a "normal" shred.
In the case of doughs with short, dry properties the shred tends to be
wide open; besides genetic and environmental factors the causes of this may be
unsuitable drying and heat damage to wheat that has been harvested wet. A
"sewn up" roll with a shred that is stuck together and unopened is
the result of a wet dough surface. An unopened shred in a small, irregularly
shaped roll in which the baker's fingers have left clearly visible marks may
indicate weak flour quality. In conjunction with a large baked volume the
unopened shred shows that the flour has quality reserves; such flour can be
used for blending with and improving flours of weak quality. The conclusions
drawn from the quality characteristic "shred" in the RMT standard
baking test can also be applied to loaves of bread and other baked products.
The ratings acquired by sensory testing are underpinned by
measurements carried out on doughs with the Farinograph and the Extensograph.
These instruments of empirical rheometry are used in large mills and industrial
bakeries.
To a certain extent a baker can correct his dough, made with a small
amount of flour, in the course of preparation. But an industrial bakery must
have formulations it can rely on; they must ensure that the products turn out
properly, since no corrections can be made to a large batch of dough. For
large-scale baking, objective measurements of the physical and rheological
properties of the dough are essential. Results obtained by empirical rheometry
are used effectively for this purpose too.
1.4.2. Instruments of Empirical
Rheology
As we know, the process of bread making consists of two phases: a cold
phase in which the dough is prepared by mixing and left to ferment, and a hot
phase in which the dough is transformed into bread in the oven. Monitoring the
process means showing and checking the viscosity and viscoelastic properties in
both phases. For technical reasons empirical rheometry can only carry out the
measurements separately.
1.4.3. In the Cold Phase of
Bread Making
Bread baking starts with mixing flour and water to form a dough,
followed by fermentation ("rising") in a fermentation chamber at
controlled, slightly elevated temperatures similar to those used in practice.
Mixing and kneading is simulated with recording mixers under laboratory
conditions; the condition of the dough during and after fermentation is shown
and described by means of stretching tests. The experience of the baker
(laboratory worker, production manager, shift supervisor) enables him to read
the measurements and curves thus obtained in order to determine the optimum
flour for a particular product and adjust the recipe accordingly.
1.4.4. Recording Mixers
Generally speaking, modern laboratories use two types of recording
mixer: the Brabender Farinograph and the Swanson Working Mixograph. These two
mixers differ fundamentally in the way they mix and thus the mechanical stress
to which the dough is exposed, i.e. in the ratio of flour to water and the
amount of water added at the start of mixing. The sigma-shaped paddles of the
Farinograph squeeze and stress the dough relatively little compared to other
types of mixer (Weipert, 1987b). The amount of water that has to be added to
achieve a constant consistency of the dough is determined in a preliminary test
before the main test. The working parts of the Mixograph are vertical pins that
exercise a planetary, rotating motion and stretch, squeeze and fold the dough;
mixing of this kind subjects the dough to greater mechanical stress than that
of the Farinograh mixer. The doughs are prepared with the same amount of added
water irrespective of the water absorption capacity of the flours. This means
that evaluation and interpretation of the curves resulting from the
measurements differs. For the sake of completeness it should be said at this
point that the one-arm Alveograph mixer stresses the dough less than the
Farinograph. With their sharp-edged tools, which result in very intensive
mechanical stress, the mixers of the Brabender Do-Corder and the Resistograph
damage the dough to the point where it completely loses its structure
("fatigue point"). It is deformed until it becomes liquid and is
literally destroyed.
Note :
17 Rapid Mix Test; See Number 3
إرسال تعليق