Wheat bran-associated subaleurone and endosperm proteins and their impact on bran-rich bread-making
Introduction
Increasing evidence for the beneficial effects of dietary fiber consumption has raised demand for dietary fiber-enriched foods. Wheat bran constitutes a highly available and cheap source for dietary fiber fortification of staple foods, such as bread. Unfortunately, wheat bran affects processing and quality aspects of cereal-based products (Hemdane et al., 2016a). For bread, this results in an end product that is less appealing to a majority of consumers and causes a need for expensive improvers (Albers et al., 2009).
Despite extensive research on this matter, the mechanisms that govern wheat bran's impact and the individual relevance of these mechanisms remain disputed. Indeed, wheat bran's complex nature features distinct histological and (bio)chemical properties, which complicate wheat bran-related research (Hemdane et al., 2016a). Nevertheless, it is obvious that a key factor of wheat bran's detrimental impact on bread-making is related to a weakening of the visco-elastic gluten network, with related consequences for processability and resulting organoleptic properties. The most evident impact of bran incorporation on the visco-elastic network is related to dilution of gluten proteins (Hemdane et al., 2016a). Furthermore, the mere presence of bran particles may impede proper gluten development through hindrance of proper contact between flour particles (Sanz Penella et al., 2008). Wheat bran is also characterized by its peculiar ability to strongly bind considerable amounts of water, which may compete with proper gluten hydration (Jacobs et al., 2015).
Glutathione (GSH) is another wheat bran component that may weaken the gluten network through disulfide interchanges (Joye et al., 2009). Phytate has also been suggested to negatively affect oxidative cross-linking of glutenin molecules during dough mixing through chelation of iron and via interaction with glutenins (Park et al., 2016). Wheat bran-related endopeptidases may further structurally weaken the gluten network in cases of excess activity (Linko et al., 1997). The joint action, if any, of these mechanisms may be responsible for the decreased dough strength and extensibility that is typically observed upon bran incorporation (Sanz Penella et al., 2008).
However, it is conceivable that incorporation of miller's bran into the visco-elastic dough matrix may also contribute to this matrix, since it contains gluten proteins that are present in residual peripheral tissues adjacent to the aleurone tissue. These peripheral tissues consist, partially or exclusively, of residual endosperm, which may hold significant amounts of gluten, considering that wheat endosperm displays increasing protein content from the center to the periphery of the kernel (Nelson and McDonald, 1977). Moreover, wheat cultivars may possess a subaleurone layer adjacent to the aleurone layer, which constitutes a single layer of relatively small cubic cells that are differentiated from the aleurone (Kent, 1966). Whereas aleurone proteins are stacked in granular entities, the proteins present in the subaleurone layer occur in protein bodies derived from the endoplasmic reticulum or in vacuoles, as is the case for protein of the inner endosperm (Tosi et al., 2011, Zheng and Wang, 2014). Interestingly, upon fractionation, the protein content of this tissue has been reported to be as high as 54% (Kent, 1966). Limited information is available on the composition of subaleurone protein, though evidence exists that these proteins consist of more LMW-GS, α-, and ω-gliadins compared to the inner endosperm (Kent, 1966, Tosi et al., 2011).
The potential techno-functional relevance of gluten from bran-associated subaleurone and endosperm (BASE) tissues towards its bread-making potential has not been investigated, probably because its potential impact has been considered negligible. This study aimed to gain insight about the characteristics of such proteins and their functionality within the bread-making process. BASE tissue and its proteins were characterized using the Akteur and Apache cultivars, which displayed distinctive BASE tissue properties. Qualitative and quantitative protein analysis of BASE proteins was performed using BASE tissue, which was obtained following dry fractionation of roller milling-derived bran. The fractionated bran offered a means of evaluating the functionality of bran with a decreased BASE tissue content. In addition to fractionation, particle size reduction and toasting were used as tools to assess BASE functionality. It was hypothesized that particle size reduction could constitute a tool to increase the availability of BASE proteins that were possibly entrapped in BASE cells. Alternatively, toasting was introduced as a tool to induce a loss of visco-elastic properties of BASE gluten proteins. To validate these findings from Akteur and Apache BASE tissue studies, four commercial wheat bran samples were also studied.
Section snippets
Materials
Wheat (Triticum aestivum L.) grains from Akteur and Apache cultivars and from white wheat flour Crousti were provided by Dossche Mills (Deinze, Belgium). Commercial coarse wheat bran samples were obtained from various industrial mills. Amygluten, a commercial vital gluten, was obtained from Syral (Aalst, Belgium). Chemicals, solvents and reagents were purchased from Sigma-Aldrich (Bornem, Belgium).
Light microscopy
Bright field microscopy on sections (4 μm) of samples embedded in Historesin was performed using a
Screening wheat cultivars for differences in BASE tissue protein content
To explore the potential contribution of BASE proteins during bread-making, two wheat cultivars with various BASE tissue properties were selected, following microscopic analyses in which wheat kernel sections were stained with Lightgreen, which stains proteins green (Khan and Shewry, 2009). Akteur, a wheat cultivar with superior baking quality, and Apache, a wheat cultivar with poor baking quality (Krejčířová et al., 2007), were selected given their differences in BASE tissue protein occurrence
Conclusions
The findings of this work demonstrated clear differences in protein content across the kernels of wheat cultivars Akteur and Apache. For Apache, the inner endosperm displayed a protein content of 10.0%, which increased to 15.1% for the tissue adjacent to the aleurone. For Akteur, these respective protein contents were found to increase from 12.8% to 22.1%. Concerning protein composition, no differences were observed across the Apache endosperm. In contrast, the aleurone-associated tissue of
Acknowledgements
Pieter J. Jacobs acknowledges the “Vlaams Agentschap Innoveren en Ondernemen” (Vlaio, former IWT or “Agentschap voor Innovatie door Wetenschap en Technologie”, Brussels, Belgium) (grant number 121702) and the KU Leuven Special Research Fund for financial support. Sami Hemdane acknowledges Flanders' FOOD (Brussels, Belgium) for financial support within the framework of the BranTech project. The authors also gratefully acknowledge financial support from the ‘Fonds voor Wetenschappelijk Onderzoek
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