Contactless measurement of gas retentivity Dough aeration behavior

Functionality of an analysis instrument developed at the TUM for contactless height measurement and determination of the gas formation rates of yeast-based doughs

+Yeast fermentation is a fundamental step in bread manufacture that affects the bread’s characteristics, e.g. volume, pore structure, flavor, color and texture. Based on the biological conversion products of baker’s yeast (Saccharomyces cerevisiae ), input of gas into the cereal dough matrix takes place, which is mainly influenced by three factors: the type of yeast, the age of the yeast cells, and the presence of substrate (sugar, inorganic substances etc.). However, the cereal dough matrix’s ability to retain the gas that has been put in is decisively determined by the raw materials and process conditions used, e.g. mixing/kneading time or the energy input during mixing/kneading. Thus, the raw materials and the process conditions in this complex system can both permanently affect product quality. These factors can normally be analyzed on a laboratory scale by using a rheofermentometer. This involves using a plunger to record the growth in height of samples in a container over a defined period of time. As a result, the growth of the sample matrix is restricted as uniformly as possible, and a more accurate determination of the height is enabled, however the influence of the plunger’s own weight and of any possible additional weights on the height growth is unclear. Gas retention and the time of first outgassing are determined based on the total pressure of the gas phase and the partial pressure of the proportion of the gas phase excluding carbon dioxide. For example, this allows the gas retention of cereal doughs to be recorded as a function of raw material variations. However, temperature variation in this system is impossible, and studies can only be carried out at 30°C. Because in practice most yeast fermentations are carried out at different temperatures, and there is a need to study the effect of various process parameters on the gas retention kinetics, e.g. freezing or baking, a highly flexible controlled-temperature system to record the gas formation and retention kinetics of microbiologically (yeast, sourdough) or chemically (baking agent) leavened dough was developed at the Chair of Brewing and Beverage Technology.

The analysis instrument (Figure 1) consists of three fermenters whose temperature can be controlled in a range from –15 to 150°C by a heating and/or cooling medium in the double jacket. On the one hand, the temperature regulating medium in an externally attached water bath can be held constant at an adjustable temperature in a range between –15 and 150°C, thus enabling research into gas formation rates in high and low temperature regions. On the other hand, the apparatus can be heated and cooled dynamically, thus allowing analysis of the gas formation rate in ramps. The maximum possible sample heating rate is 4.25 °C/min, thus allowing the baking process to be followed in the stated temperature range. Temperature sensors are installed to control the heating/cooling process, to record the temperature of the gas phase and to record the dough temperature. The growth in height of the samples is monitored continuously for the duration of the experiment by up to three laser sensors, mounted at three points on the diagonal of the fermenter’s cover. Due to the intentional omission of a plunger and the associated uniform limitation of the surface profile, greater dough heights were

++ measurable in the center of the diagonals compared to the outer points, as illustrated diagrammatically in Figure 1. Sensors to record the gas phase pressure are also installed in the fermenters. Achievement of a freely selectable gas phase pressure is the decisive criterion for controlled release of the fermentation products that are formed. Initially the gas volume flowrate was calculated using the pressure differences and the ideal gas equation.

Initial analyses were designed to compare the fermenter’s functional capability with that of commonly available analysis systems. First of all, the optimum kneading/mixing time and water absorption of the wheat flour (Type 550, Rosenmühle Landshut DE) were determined. A dough was mixed and kneaded based on these results. The samples contained dry yeast (Saccharomyces cerevisiae, Casteggio Lieviti IT) at levels of 1, 2 or 3% based on flour weight. Dough samples each weighing 315 g were examined in the fermenters over a fermentation time of 3 hours with regard to gas formation and profile height, and compared with the results from the plunger-based system. The samples in the plunger-based analysis system were loaded with a 1 kg weight on the plunger.

Figure 2 illustrates a diagrammatic ideal profile height curve over the whole time of the experiment. Sample volume increases at the start of the experiment due to gas formation. Large proportions of the gas can be retained in the matrix at this point. After a certain time, depending on the choice of raw materials and process conditions, the matrix’s gas retention capacity decreases at a constant gas formation rate. Consequently, the dough sample is no longer able to fully retain the gas that is formed. However, the sample’s volume can continue to increase for as long as the gas formation rate is still greater than the gas escape rate. As soon as the gas escape rate exceeds the gas formation rate, the sample volume decreases. This is characterized in the diagrammatic profile in Figure 2 by reaching the maximum height Hmax during the experiment, whereby the height after reaching the end of the experiment is designated as h. The proportion by which the profile height has subsequently decreased after reaching Hmax is called the Weakening Coefficient, and is calculated as the quotient of Hmax-h divided by Hmax.

The dough height developments for experiments using different dry yeast concentrations based on flour weight at a constant temperature of 30°C over an experimental period of 3 hours are plotted in Figure 3. On the one hand, a faster increase, a higher Hmax and a smaller Weakening Coefficient

++ was recorded for the average profile heights in the fermenter. The fermenters that were designed have a diameter that is 2 cm less compared to the containers in the plunger-based system. This is why the samples in the fermenter record a larger profile height increase and can thus be distinguished better, e.g. with different raw materials. The key characteristic figures Hmax and the Weakening Coefficient were supplemented by multiplication by the respective cross-sectional areas and were calculated over the volume change. This results in the calculation of the maximum volumes and percentage weakening of the volume after reaching the maximum volume, thus ensuring better comparability between the systems. When considering the maximum dough volumes in Figure 4 A, there is a clear difference between the plunger-based system and the fermenters despite identical temperature conditions and experiment runtime. The samples in the fermenters showed a larger maximum volume at all dry yeast contents, and also a smaller rising trend with increasing dry yeast content. No sample volume increase with increasing dry yeast content was detectable in the plunger-based system. A rising trend in sample volumes with increasing yeast content was detectable in the fermenters, in which only the maximum gas phase pressure of 0.05 bar acted on the surface profile. Based on the Weakening Coefficient illustrated in Figure 4 B, it is also possible to observe an effect of additional weights and/or of the plunger on the development of the height profile. For better comparability, the Weakening Coefficients were calculated using volumes recalculated based on the smaller diameter of the fermentation containers. A clearly larger collapse of the samples after reaching maximum sample volume was measurable for the plunger-based system. Hence, in practice, it was possible to describe the values measured for maximum dough volume increase and Weakening Coefficient closer to reality in the fermenter.

The whole of the gas formed cannot be retained in the dough sample over the experimental period of three hours. This is why the gas phase pressure is monitored continuously over the entire experimental period. The attainment of a freely adjustable gas phase pressure, in this case Δp = 0.05 bar, acts as a criterion for opening the magnetic valves and venting off the atmosphere until ambient pressure is reached. During this process, the gas phase volume can be calculated via the respective pressure differentials, temperatures and compositions. The gas volume retained in the dough sample during the growth phase or the volume of gas that has escaped from the dough sample can be calculated via the optical height monitoring.

A maximum gas production in the range between 1,500 and 1,600 ml was determined in numerous experiments carried out at the Institute using plunger-based systems, irrespective of substrate availability and yeast content, which indicates a method-dependent limitation. An examination of this situation is planned in the new fermentation system with a smaller pressure effect. Figure 5 illustrates the total measured gas volumes of the two measuring systems for the above-mentioned samples. Almost identical gas volumes were recorded in the plunger-based system and in the fermentation container for the samples with wyeast = 1%. However, as the dry yeast level rose, a rising trend in gas production was detectable in the fermentation container, whereas the gas volume in the plunger-based system showed no differences for a dry yeast content between 2% and 3%. This raises the suspicion that the previous equipment exerted a methodological effect on the results. This could influence results depending on changes in recipes or process parameters. Ultimately it was impossible to prove through the plunger that the above-mentioned limitation is induced via the effect of pressure on the sample, although the rising trend in gas volumes in the fermentation container shows that the absence of the pressure effect on the dough sample results in greater gas production. The fermentation containers will be supplemented by adding a CO 2 detector and a volume flowmeter to allow more detailed examination of the gas volumes. Overall, it was possible on the one hand to establish comparability of the results relative to plunger-based systems for the series of experiments. The absence of additional weights acting on the samples enables a method for analyzing dough development and fermentation that is closer to reality and which is allowed by the contactless distance measurement of the surface profiles of the samples and was demonstrated in the key figures for Vmax und Weakening Coefficient when using fermentation containers compared to the plunger-based system. It was possible to verify the uniform distribution of the temperature control medium across all the containers by recording the temperature in the container outlet, in the samples and in the gas phase. This enables a simultaneous triple determination with constant monitoring of the fermentation conditions in all the fermentation containers. This results in a saving of time, which either allows a larger number of experiments to be carried out or enables the results to be made available faster.

Gas formation rates in extreme temperature regions and by using dynamic temperature variations will be studied in continued experiments in order to obtain a more detailed overall picture of the effects of process parameters and choice of raw materials during baking, and of freezing or proofing, on product quality. The design and construction of the fermenters also allows fluids, e.g. pure yeast suspensions, to be examined in isolation and thus without the interfering effects of dough matrices. Consequently, the bandwidth of parameter variations will enable cross-sectoral studies using this newly-developed analysis instrument to optimize a variety of process parameters. +++

Authors

Florian Lücking, Mario Jekle, Thomas Becker Technical University of Munich, Institut of Brewing and Beverage Technology, Research Group of Cereal Technology and Process Engineering, Contact: mjekle@tum.de