READING UNIVERSITY - BUBBLES IN BREADMAKING
THE PREDICTION OF BREADMAKING QUALITY USING THE NEW STABLE MICRO SYSTEMS TA.XTPLUS
Baking is about bubbles: the size, distribution, growth and failure of these bubbles during proof and baking has a major impact on the final quality of the bread in terms of both appearance (texture) and final volume. During proof and baking these bubbles slowly expand, producing a large increase in volume. In order for the bubbles to grow, the surrounding matrix must retain gas (CO2) for long enough to allow a considerable amount of expansion before the structure is set by baking.
The limit of expansion of these bubbles is related directly to their stability, due to coalescence and the eventual loss of gas when the bubbles burst. The rheological properties of the bubble walls will be important in maintaining stability against premature failure during baking. Conventional dynamic rheological tests carried out in shear under small deformation and high strain rates are inappropriate to baking, because they are carried out at rates and conditions very different from those experienced by the dough during baking expansion, and these sort of tests have not been able to predict the baking quality of different flours of varying quality. In terms of deformation, the relevant conditions around an expanding bubble wall during breadmaking are biaxial extension, large strains and low strain rates at elevated temperatures.
Work at Reading University has shown that the stability of failure in single dough bubble walls is related directly to the extensional strain hardening properties of the dough, and that strain hardening plays an important role in the stabilisation of bubble walls during baking. We are evaluating rheological methods to predict breadmaking performance using a modified bubble inflation technique, developed by Stable Micro Systems Ltd., Godalming, UK, called the D/R Dough Inflation System which measures the extensional rheological properties of dough and gluten in biaxial extension under conditions of deformation, strain rate and elevated temperatures relevant to breadmaking. The latest version of the instrument (TA.XTplus) has the option of continuously variable speed, which allows tests to be performed at constant strain rates and higher temperatures more relevant to proof and baking conditions. In the previous method, bubble inflation tests were limited to a fixed rate of inflation, and both strain and strain rate varied during the test. Because dough is viscoelastic, its rheological properties vary with both stain and strain rate. Therefore it is necessary to separate out the effects of strain and strain rate by keeping one constant while varying the other.
In the new dough inflation test strain is varied and measured as the bubble inflates, and by continuously changing the speed at which the bubble is inflated, strain rate is kept constant.
Strain hardening measured at 50°C and constant strain rate for a number of commercial flours of varying quality using the new TA.XTplus system is shown related to commercial breadmaking performance. This clearly shows that a strain hardening value of around 1.2 discriminates well between flours of poor to moderate quality and those considered to be good and excellent.
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