1.4.8. Hot Phase of the Bread-Making Process
Whereas the focus of attention in the cold phase of bread making is on the swelling substances of the flour, particularly the proteins, it is starch and its pasting behaviour that dominate tests to show the behaviour of doughs in the hot phase, i.e. the actual baking. Starch only starts to swell intensively at elevated temperatures; it binds water and gelatinizes, losing its crystalline structure. But the gelatinizing and already gelatinized starch is exposed to enzymatic breakdown through the activity of α-amylase. In the quick breakdown process of the enzymes the starch loses its ability to bind and hold water; this results in bread with an inelastic, soft, wet and very often unchewable crumb which makes the product inedible. On the other hand, optimum enzyme activity is necessary for optimum results in the baked product. So it is essential to determine the activity of the α-amylase in a flour or any other ground product in order to achieve the desired result. If the enzyme activity is too high, activity-inhibiting agents are added or suitable measures taken; if it is too low it can be optimized by adding enzyme preparations.

1.4.9. Amylograph and Falling Number
The Amylograph is a rotational viscometer with a measuring system consisting of a round vessel, in which the flour-and-water slurry is heated under controlled conditions, and a sensor to record the changes in viscosity during the measuring time. The pins of the measuring device cause turbulences in the slurry; these are necessary to prevent sedimentation of the starch, but they make it impossible to calculate the viscosity precisely in absolute physical units. The viscosity of the slurry is therefore stated as torque in Brabender or Amylogram units. The measurements that can be read off include the temperature and the viscosity at maximum gelatinization; these provide more differentiated information than a viscosity value alone.

The Amylogram, a viscosity curve showing the gelatinization of the starch in the flourand- water slurry, reflects the changes in water binding capacity of the swelling and pasting starch and the enzymatically and mechanically decomposed starch gel. In the way the standard Amylograph method is used it offers a suitable means of describing the pasting properties of rye flour slurries. Rye starch gelatinizes at lower temperatures than wheat starch, especially if it is sproutdamaged as a result of poor environmental conditions. Then the task is to find out whether the rye is suitable for baking by determining the temperature and viscosity at the pasting peak of the Amylogram. This applies to wheat too, but only if it is assumed to be sprout-damaged. The starch of the flour slurry from a wheat lot that is not sprout-damaged normally gelatinizes later and at higher temperatures, towards the end of the temperature range of an Amylogram or the measurements from an Amylograph. However, a very high viscosity at the pasting peak yields little information in relation to the amount of time that has to be invested. For such wheat flours the quick and simple determination of the Falling Number is sufficient.

Falling Number determination is a simple and quick method in which the viscosity of a flourand- water slurry is stated as the number of seconds a pestle takes to penetrate the starch gel. Measurement of the viscosity in a Falling Number tube does not start until 60 seconds after stirring, when the viscous properties of the gelatinized starch slurry have already been changed by the α-amylase present in the flour and the mechanical force of stirring. The Falling Number is therefore a one-point measurement of the residual viscosity of the starch gel, not a continuous measurement like that of the Amylograph. The Falling Number method can be used for both wheat and rye, although the limits of the measurements differ. This results not least from the different water binding capacities of the swelling substances of wheat and rye. In rye flours too, the Falling Number can be used with sufficient accuracy for indirect determination of α-amylase activity and the suitability of a flour for baking.

There is not a close enough relationship between the Amylograph and Falling Number methods to permit a direct comparison of the measurements. Firstly, the ratio of flour to water (concentration of the slurry) and the time/temperature gradient of the heating differ; secondly, the Amylograph method is a continuous measurement over a period of up to 45 minutes, whereas in the case of the Falling Number the measurement of residual viscosity does not start until after 60 seconds of stirring. For these reasons it is not possible to allocate an Amylogram value to a corresponding Falling Number. Nevertheless, a numerical relationship between the two methods can be achieved by comparing a large number of measurements and calculating a mathematical-statistical regression. But this relationship only applies to the harvest of a single year and has to be reviewed or recalculated for each new harvest.

By using nomograms in a double-logarithmic system it is possible to make up optimized mixtures from rye flours with different Falling Number and Amylogram data (Weipert, 1987c). However, the percentages of the enzyme-active components with low data depend on the height of the data of the lowenzyme components; as a rule these percentages tend to be low.

1.4.10. Rapid Visco Analyzer
Whatever the advantages for which the Brabender Amylograph (and the Viscograph, intended chiefly for the starch industry) is appreciated, it has disadvantages too. Firstly it requires a very large sample for testing, and secondly the recording of a pasting curve is time-consuming. Several attempts have recently been made to develop and market a "micro-Amylograph". The development of the Rapid Visco Analyzer (Newport Scientific, Sydney, Australia) was and is still the biggest and most successful step towards simplifying and broadening the investigation and description of the pasting properties of starch and products containing starch (Weipert, 1998a). Because of its versatility it is in general use in the field of food analysis (milk, soups, sauces, salad dressings etc.).

The Rapid Visco Analyzer is also a rotational viscometer that unites the advantage of requiring only a small sample (2-4 g) with the possibility of setting to any desired temperature gradient. The temperature profile of a test can be adjusted in such a way that the test starts at any chosen temperature, which rises slowly or rapidly and remains constant for a time before cooling down in the desired steps. In practice this means that the test can be performed at a constantly high temperature in the manner of the Falling Number test, with slow or faster heating in the manner of an Amylograph or with controlled heating and cooling as in a Viscograph. A close correlation has been found between the measurements from these two methods and the results of the Falling Number and Amylograph tests; this correlation enables the Rapid Visco Analyzer with the already standardized methods ICC "stirring number" (similar to the Falling Number) and "rapid pasting" (similar to the Amylograms and Viscograms) to be adopted by cereal laboratories and used "seamlessly" (Weipert, 1998a).

Interpretation of Brabender viscograms and the RVA rapid pasting curves:
Fig. 60: Temperature ramps (upper set of curves) and pasting curves (lower set of curves) of a wheat flour recorded in the Rapid Visco Analyzer at different temperature gradients:
SN: constant 95 °C;
ST1: 12 °C/min;
ST2: 6 °C/min;
ST3: 3 °C/min;
ST4: 1.5 °C/min
The possibility of programming and determining the duration of a test for the pasting behaviour of starch in a starch/water or flour/water slurry oneself, according to needs, and thus monitoring the behaviour of the starch in the relevant process is very much appreciated by users of the Rapid Visco Analyzer. A quick method can doubtless yield a reliable result as a guide, but the process of making and baking bread takes rather longer. In order to describe the pasting behaviour of wheat and rye starches in flours for baking and to assess it in the manner of an Amylogram, the measuring time in which the starch swells and gelatinizes must be taken into account. The starch grains have time to absorb and bind the water, to swell, to be "annealed", and finally to gelatinize completely or incompletely, depending on the amount of free water available. One and the same flour/water slurry shows different viscosities and temperatures at the pasting peak according to the length of the measuring time. The shorter the measuring time, the higher the viscosity; it is therefore highest in the "stirring number" method, similar to the Falling Number (Fig. 60). To save time by shortening the measuring period may mean a loss of information (Weipert, 1998a), especially if the quality data from the time-consuming Amylograph method are still used on the grounds of experience. Nevertheless, a "quick test" of this kind may serve as an initial guide. Farther-reaching decisions require the introduction of new critical values for each of the suggested temperature profiles in the course of measurement. The Rapid Visco Analyzer can measure both fast and slowly.


Next Page>>>
Fundamentals of Rheology and Spectrometry :

Post a Comment

Previous Post Next Post