George A. Olah Doctoral School of Chemistry and Chemical Technology 

BME VBK, Department of Applied Biotechnology and Food Science

Supervisor: Dr. Tömösközi Sándor

Improvement of the quality of gluten-free products

Introducing the research area

Arabinoxylans are non-starch polysaccharides classified to the hemicelluloses (more specifically to pentosans), which are primarily cell wall constituents of cereal grains (mainly rye and wheat) [1]. They have a significant nutritional importance as a dietary fiber and a bioactive compound [2]. Their unique property is that they are able to cross-link in oxidative media, so they can be used to create cross-linked structures [3]. During my work I studied the possibility of this in gluten-free model systems for partial substitution of the missing gluten network.

Brief introduction of the research place

My research work was carried out in the BUTE ABÉT Research Group of Cereal Science and Food Quality supervised by Dr. Sándor Tömösközi. The main research activities of the group include the examination of the composition and functional properties of plant (cereal) components (mainly proteins, carbohydrates and fibers), allergen analysis and reference material development, and the development of measuring instruments and functional food products.

History and context of the research

Wheat has been part of human nutrition for thousands of years. Its versatile use is associated with the gluten proteins in the kernel. Under the influence of hydration and mechanical work (kneading), the gluten proteins create a complex cross-linked structure, the so called gluten network, which results in a flexible and extensible dough with gas retention capability. The consumption of wheat and other cereals with similar protein compositions, such as rye, barley or triticale is not possible for nearly 1% of the population due to various disorders (e.g. celiac disease or allergy). In their case, currently, strict lifelong gluten-free diet is the only treatment [4]. Since the role of wheat in our diet is of paramount importance, it is not easy for some consumers to switch to a gluten-free diet. During the hydration of gluten-free flours (e.g. rice, corn), the material system characteristic of the structure of the wheat dough is not developed. Therefore, the technological behavior of the dough (viscosity, gas retention, etc.) and the improvement of end product properties are generally achieved by the use of composite flours or hydrocolloids. Although gluten-free bakery products are now available in a big variety, most of them still lag behind the nutritional value, technological and organoleptic qualities of wheat products; therefore the demand for better quality products remains significant [5].

The research goals, open questions

A possible solution for improving nutritional value and partial replacement of gluten may be the enrichment of gluten-free raw materials with dietary fibers, so called arabinoxylans (AX), which can be cross-linked by the formation of an oxidative medium based on enzyme reactions. Pyranose-2-oxidase (POx) can be a promising alternative to chemical oxidizing agents and to the more widely used glucose oxidase of similar function, thanks to its improved efficiency and broader substrate specificity. POx catalyzes the oxidation of a number of mono- and disaccharides into dicarbonyl derivatives and H₂O₂. By the formation of H₂O₂, the feruloyl groups of AXs may oxidize and thus cross-link between the individual molecules (Figure 1).

 

Figure 1: Supposed mechanism of AX crosslinking

 

The aim of my work was to investigate the effect of fiber addition and enzyme treatment on the rheological properties of gluten-free dough/suspension and on the structure of macromolecules and micromorphological properties. In addition, my goal was to examine the effects of treatments on the quality of the final product in simple yeast leavened as well as sourdough leavened test products.

In my work, I sought to find out whether crosslinking of AX molecules has taken place and to what extent affect the properties of gluten-free model matrices and end products.

Methods

Materials and their characterization

During the experiments, buckwheat and millet flours were used as model matrices; the addition experiment was performed using experimental arabinoxylan (AX) isolate and pyranose oxidase produced at BOKU University. The chemical composition of the flours (moisture, ash, protein, fat, dietary fiber) was determined by standard methods. The contents of flour and isolate AX were measured by gas chromatography.

 

Examination of rheological properties

Rheology is a discipline for describing the flow and deformation properties of materials. The rheological behavior is determined by the interactions between the molecules that make up the system. A number of measuring instruments, mainly based on empirical principles, have been developed to study these in the field of food and cereal analysis [6].

Micro-doughLAB, rapid visco analyser (RVA) and Mixolab measurement techniques were used to test the mixing and viscous properties of millet and buckwheat-based dough systems. These instruments measure the resistance to the kneading arm or mixing blade as a function of time. By plotting the measurement points the profile characteristic of the given sample is shown and the characteristic parameters of the curves can be determined.

Molecular and micromorphological studies

The size distribution of AXs, proteins and starch molecules was determined by size exclusion high performance liquid chromatography (SE-HPLC). The effects of the treatments on the free ferulic acid content were investigated spectrophotometrically. To investigate the micromorphological structure of the freeze-dried dough, scanning electron microscopy (SEM) technique was used.

 

End product tests

Baking tests provide direct information on the organoleptic and technological properties of the final product. These measurements typically require high amount of sample, but in our case AX isolate and enzyme were only available in limited amounts. Therefore, I adapted the micro-scale baking method for gluten-free flours that I had developed for wheat flour. Because gluten-free loaves are more compact and flattened in the absence of gluten network, I increased the amount of flour required from 10 g to 15 g to form a crumb with an evaluable surface area. Measurements were made using a simple yeast method and the addition of gluten-free dried sourdough. According to the literature, the lower pH provided by the sourdough favors the formation of cross-links between AX molecules in rye breads. In addition, sourdough technology might have many other positive effects (increased solubility of proteins, formation of flavors and flavorings, softer crumb, etc.). To characterize the test loaves, I determined the baking loss (expressed as a percentage of water lost during baking) and the volume. In addition, I studied the crumb hardness using a texture analyzer. I also took pictures of the loaves and their cross-sections, which were evaluated using digital image analysis software.

Results

Characterization of raw materials

As expected, white flours were a chemically simpler material system, mainly containing the starchy endosperm. As a result, they can be characterized by significantly lower fiber and / or AX content, i.e. less reactive groups than wholemeal flours (Figure 2).

Figure 2: Chemical composition of millet and buckwheat flours

 

Examination of the rheological properties of model doughs

In most cases, AX addition caused a significant reduction in the consistency of the doughs. During the enzymatic treatment of dough systems containing added AX, higher consistency was measured than with untreated AX dosed dough. All these suggested cross-linking between AX molecules. However, I observed that the formation of oxidative media without AX addition also caused changes in the properties of the dough (Figure 3).

Figure 3: Consistency of white buckwheat flour based doughs measured by micro-doughLAB

 

The H2O2 produced by the enzyme does not specifically affect the individual flour constituents; for example, it may induce the formation of intermolecular bonds between proteins, but occurrence of AX-protein interactions between the feruloyl groups of AXs and protein tyrosyl groups cannot be excluded, either.

Significant changes were also observed in the viscous properties as a result of AX dosing and enzyme treatment, which were in harmony with previous results (Figure 4). In most cases, the use of POx in systems both with AX and without AX had an effect on viscous behavior. For the latter, the oxidation of starch was assumed [7].

 

Figure 4: Viscosity of white millet flour based suspensions measured by RVA at variable temperatures

 

Investigation of molecular and micromorphological properties

Further experiments were carried out to support the results of rheological studies. On the basis of these results, it could be concluded that there had been an increase in the size of the AX molecules, which was also supported by the reduction of free ferulic acid content by enzyme treatment (Figure 5).

Figure 5: Size distribution of AX molecules and free ferulic acid contents measured in white millet doughs

 

The amount of soluble protein fractions also decreased significantly as a result of enzyme treatment, which indicated the formation of aggregates or macromolecular complexes (Figure 6).

 

Figure 6: Chromatograms of soluble and insoluble protein fractions of white millet doughs

 

The size distribution of starch molecules also showed an increase in size, which confirmed the oxidation of starch molecules (Figure 7).

 

Figure 7: Chromatograms of starch of white millet doughs

 

The micromorphological structure of the dough has also been modified, during which the enzyme treatment causes AX to appear as a binder in the space between the starch particles.

Figure 8: SEM images of the micromorphological structure of white millet dough

 

In summary, the modification of proteins and starch molecules, as well as their interaction with AX molecules, may have played a role in changes in rheological properties.

 

Results of end product tests

AX addition reduced the baking loss due to the water-binding ability of soluble AX molecules. At the same time, the crumb of the loaves became softer. In the sourdough systems, the specific volume was reduced by AX, while in the case of yeast leavened loaves it did not change it at all, or merely slightly. On the basis of the results it was proven that the effect of AX and enzyme treatment has a visible impact on the quality of the final products (Figure 9).

Figure 9: Crumb structure, baking loss, specific volume and hardness of yeast leavened and sourdough millet breads

Expected impact and further research

This research work has been carried out with our partner institution (University of Natural Resources and Life Sciences in Vienna) in the frame of OTKA (ANN 114554) joint project, and we plan to continue this work in the next application cycle. The results have been published in one of the most prestigious journals in the field, the Food Hydrocolloids (IF: 5,832, Q1). In the continuation of the research, I would also like to study the molecular processes in the final products, as well as to carry out experiments with additional gluten-free materials. I plan to publish the further results in a similarly prestigious journal.

Publications, references, links

List of corresponding own publications

[S1] Németh, R., Bender, D., Jaksics, E., Calicchio, M., Langó, B., D’Amico, S., Török, K., Schoenlechner, R., Tömösközi, S., 2019. Investigation of the effect of pentosan addition and enzyme treatment on the rheological properties of millet flour based model dough systems. Food Hydrocolloids. 94, 381–390. IF: 5,832

[S2] Bender, D., Németh, R., Cavazzi, G., Turoczi, F., Schall, E., D’Amico, S., Török, K., Lucisano, M., Tömösközi, S., Schoenlechner, R., 2018. Characterization of rheological properties of rye arabinoxylans in buckwheat model systems. Food Hydrocolloids. 80, 33–41. IF: 5,832

[S3] Langó, B., Fehér, A. G., Bicskei, B. Z., Jaksics, E., Németh, R., Bender, D., D’Amico, S. Schoenlechner, R., Tömösközi, S. (2018). The Effect of Different Laboratory-scale Sample Preparation Methods on the Composition of Sorghum (Sorghum bicolor L .) and Millet ( Panicum miliaceum L .) Milling Fractions. Periodica Polytechnica Chemical Engineering, 62(4), 426–431. https://doi.org/10.3311/PPch.12846, IF: 0.877

[S4] Tömösközi, Sándor, Németh, Renáta, Roznár, Petra, Denisse, Bender, Jaksics, Edina, Turóczi, Fanni, Török, Kitti, Regine, Schönlechner (2018). A sikérfehérjéket nem tartalmazó (gluténmentes) termékek táplálkozási és technológiai minőségének fejlesztése. Magyar Kémiai Folyóirat – Kémiai közlemények (1997-) 124: 3 pp. 101-107, 7 p. http://doi.org/10.24100%2FMKF.2018.03.101, IF: 0

[S5] Bender, D., Németh, R., Wimmer, M., Götschhofer, S., Biolchi, M., Török, K., Tömösközi, S., D’Amico, S., Schoenlechner, R. (2017). Optimization of Arabinoxylan Isolation from Rye Bran by Adapting Extraction Solvent and Use of Enzymes. Journal of Food Science, 00(0), 1–7. IF: 1,815

[S6] Bender, D., Schmatz, M., Novalin, S., Németh, R., Chrysanthopoulou, F., Tömösközi, S., Török, K., Schoenlechner, R., Amico, S. D. (2017). Chemical and rheological characterization of arabinoxylan isolates from rye bran. Chemical and Biological Technologies in Agriculture, 4(14), 1–8., IF:0

 

Table of links

Gluten-related disorders

Regulation of gluten-free foods

Gluten-free diet: facts and myths

 

List of references

[1] Buksa, K., Praznik, W., Loeppert, R., & Nowotna, A. (2016). Characterization of water and alkali extractable arabinoxylan from wheat and rye under standardized conditions. Journal of Food Science and Technology, 53(3), 1389–1398.

[2] Mendis, M., & Simsek, S. (2014). Arabinoxylans and human health. Food Hydrocolloids, 42(P2), 239–243.

[3] Decamps, K., Gryp, G., Joye, I. J., Courtin, C. M., & Delcour, J. A. (2014). Impact of pyranose oxidase from Trametes multicolor, glucose oxidase from Aspergillus niger and hydrogen peroxide on protein agglomeration in wheat flour gluten-starch separation. Food Chemistry, 148, 235–239.

[4] Lebwohl, B., Ludvigsson, J. F., & Green, P. H. R. (2015). Celiac disease and non-celiac gluten sensitivity. BMJ, 351, h4347.

[5] Pellegrini, N., & Agostoni, C. (2015). Nutritional aspects of gluten-free products. Journal of the Science of Food and Agriculture, 95(12), 2380–2385.

[6] Dap, T., Poji, M., Hadna, M., & Torbica, A. (2011). The Role of Empirical Rheology in Flour Quality Control. In I. Akyar (Ed.), Wide Spectra of Quality Control (pp. 335–360).

[7] Pereira, J. M., Evangelho, J. A., Moura, F. A., Gutkoski, L. C., Zavareze, E. R., & Dias, A. R. G. (2017). Crystallinity, thermal and gel properties of oat starch oxidized using hydrogen peroxide. International Food Research Journal, 24(4), 1545–1552.