Improved functional properties for soy–wheat doughs due to modification of the size distribution of polymeric proteins

https://doi.org/10.1016/j.jcs.2005.10.001Get rights and content

Abstract

Physical modification of soy flour was shown to greatly improve the dough and baking qualities of soy–wheat (1:1) composite doughs, compared to raw soy flour, giving better stability and Rmax, although extensibility was still below that of the wheat dough.

Reasons for improvements caused by the physical-modification process were sought by determining the relative size distribution of proteins in the soy–wheat composite doughs by size-exclusion high-performance liquid chromatography (SE-HPLC). Results were expressed as the proportion of ‘unextractable polymeric protein’ (%UPP)—the proportion of the protein that is over 100,000 Da and only extractable after sonication. Protein extracts from the soy–wheat dough were sampled at different stages of dough mixing and fermentation, and their molecular-size distributions evaluated.

Unextractable soy proteins were lower in raw soy flour (only 8% UPP) than in two physically-modified soy flours (19 and 34% UPP, respectively). Unextractable polymeric protein was much greater for wheat flour (57% UPP). After mixing a 1:1 soy–wheat composite dough, the %UPP was 36 and 22 (for the two types) when made from physically modified soy flours, compared to 8 for a composite dough using raw soy flour, and 43 for a wheat-only dough. The higher proportion of UPP for the wheat-modified soy doughs was taken as a reason for this composite dough providing better dough and baking qualities. Prolonged fermentation time caused a decrease in UPP percentages for all composite doughs and for the wheat-only dough.

Introduction

High levels of soy flour in wheat dough are known to cause deleterious effects on rheological and bread-making properties (D'Appolonia, 1977, Lorimer et al., 1991, Pomeranz et al., 1969), but the use of soy–wheat composite doughs is desirable in regions where wheat flour is expensive, and where soy protein is nutritionally desirable. Accordingly, the studies described employed wheat-soy bread as a vehicle for soy proteins in an attempt to address Protein Energy Malnutrition in developing countries. The use of physically-modified soy flour made from optimal thermal treatment of the beans or meal is a practical strategy for implementation in developing countries, as this process also destroys lipoxygenase while retaining functional and nutritional properties.

Understanding soy- and wheat-protein interactions should give an insight into possible ways of minimising the dough-weakening effect of soy flour in wheat doughs. Soy–wheat composite doughs offer an unusual contrast of differing protein classes. Most soy proteins are globulins, insoluble in water at their iso-electric points (pH 4.2–4.6), but are extractable in water and dilute salt solutions (Hou and Chang, 2004, KeShun, 1997). They consist of four major fractions (2S, 7S, 11S and 15S globulins), of which the 7S and 11S fractions are the major components comprising about 70% of the storage proteins. The soy proteins are not suitable for pan-bread making.

On the other hand, wheat-flour proteins are divided into four main classes, of which the albumins and globulins are minor fractions, compared to the gluten-forming monomeric gliadins and the polymeric glutenins. These very large polymeric glutenins proteins are composed of high-molecular weight (high Mr) and low-molecular weight (low Mr) subunits linked together by disulphide bridges (Bietz and Wall, 1972, Fisichella et al., 2003, Grosch and Wieser, 1999, Schofield, 1986). When hydrated, the gliadin fraction behaves as a viscous liquid and the glutenin fraction contributes cohesion and elasticity (Schofield, 1986); in balance, their visco-elastic properties make wheat dough uniquely suited to bread making.

Attempts to use legume proteins for bread making have generally been unsuccessful. Lorimer et al. (1991) reported that the addition of non-gluten-forming proteins (e.g. bean-seed proteins) causes a dilution effect and consequent weakening of wheat dough. They suggested several factors that cause weakening, namely, competition between the legume proteins and gluten for water molecules, the disruption of starch–protein complexes by the foreign proteins and disruption of SS interchange by the non-gluten proteins. However, they did not produce sufficient evidence to conclude that globular proteins disrupt the disulphide-interchange system of dough.

Ryan et al. (2002) offered the hypothesis that gluten- and soy–protein interactions have the potential to provide dough improvement. They claimed that the sulphydryl groups of the soy proteins may even contribute to dough development through SS–SH interchange, and that negative effects associated with soy–wheat breads are primarily due to lack of interaction between soy and gluten proteins. Hyder et al. (1974) demonstrated, by gel electrophoresis, the interaction between soy–protein fractions, sucrose esters and a pH 6.1 gluten-insoluble fraction. Similar interactions to improve soy protein functionality in wheat bread have been reported, between sucrose esters and soy proteins (Pomeranz et al., 1969), e.g. the binding of sodium stearoyl-2-lactylate to wheat and soy proteins (Chung et al., 1981).

Contrasting differences between soy and gluten proteins are their water-solubilities, the associated differences in amino-acid composition and their size distributions, and the consequent visco-elastic properties which are unique to wheat gluten proteins, enabling the gluten proteins to stretch and retain gas bubbles during baking. The dough-making quality of gluten has been attributed to the high proportion of very large proteins with molecular weights up into the tens of millions (Southan and MacRitchie, 1999, Stephenson and Preston, 1996). This set of observations raises the possibility that soy proteins might be better suited to dough forming if the proportion of high molecular-weight protein could be increased considerably.

In this paper we explore the hypothesis that a process of physical modification of soy flour by moist heat treatment causes an increase in the proportion of high molecular-weight protein of the soy proteins, thus making them more suitable for dough formation. A 1:1 soy–wheat dough incorporating heat treated soy protein exhibits higher resistance to extension (Rmax), greater tolerance to mixing, better mixing stability, higher water uptake and better water absorption than a 1:1 soy–wheat composite dough made from raw soy flour (Maforimbo et al., 2005). Size-exclusion high-performance liquid chromatography (SE-HPLC) was used to determine the protein compositions of these composite doughs during mixing and resting, and thus to gain a better understanding of size distribution and possible interactions at the molecular level.

Section snippets

Preparation of soy flours

Whole-seed soybeans (Meriram Pty Ltd, Everton Hills, NSW, Australia) were used to produce physically-modified soy flour no. 1 (PMSF1) by immersion of the soybeans in boiling water for 3 min. The beans were then spread on stainless-steel trays and blow-dried with hot air (80 °C) to constant weight in an oven for 5–6 h. The beans were later milled to fine flour through a 0.8 mm sieve, using a hammer mill (Newport Scientific Cereal Mill 6000 model, Warriewood, NSW, Australia). Using the same whole

Rheological results of soy–wheat doughs at 1:1 ratio

The rheological parameters, shown in Table 1, indicate the considerable decreases in maximum resistance to extension (Rmax) and stability in the RSF–wheat dough, compared to the wheat dough. These changes, also seen in the actual Farinograms and Extensograms (Fig. 1), explain the difficulties in making bread using raw soy flour. In contrast, the PMSF–wheat dough has better stability and Rmax, although its extensibility is still below that of the wheat dough. The differences in dough properties

Conclusion

SE-HPLC was used to demonstrate that the improved contribution to dough properties of physical modification of soy proteins is due to changes in the molecular size distribution of the soy proteins. SE-HPLC profiles showed that raw-soy–wheat dough has much of its protein in an intermediate size range, compared to the profile for the wheat dough. The physical modification process appears to have altered this size distribution, giving the PMSF–wheat dough a SE-HPLC profile much closer to that of

Acknowledgements

The support from the University of Western Sydney in sponsoring this work is greatly acknowledged. Special thanks are extended to CSIRO, for facilitating the analysis and expertise for this work.

References (27)

  • D.H.J. Hou et al.

    Structural characteristics of purified glycinin from soybeans stored under various conditions

    Journal of Agricultural and Food Chemistry

    (2004)
  • M.A. Hyder et al.

    Interactions of soy flour fractions with wheat flour components in bread making

    Cereal Chemistry

    (1974)
  • L. KeShun

    Soybeans, Chemistry, Technology and Utilization

    (1997)
  • Cited by (33)

    • Effect of additional water supply during grain filling on protein composition and epitope characteristics of winter oats

      2022, Current Research in Food Science
      Citation Excerpt :

      SE-HPLC is a powerful technique for size-based separation of proteins providing quantitative size distribution information. Since its first application in cereal science (Batey et al., 1991; Gupta et al., 1993) the methodology plays an important role in relating techno-functional properties to the protein composition of wheat, barley and rye (Janes and Skerritt, 1993; Nilsson, 2009; Wrigley et al., 2006; Van Der Borght et al., 2006; Silva et al., 2008; Békés, 2012; Redan et al., 2017) but legumes, for example, soybean (Oomah et al., 1994) and cereal-soybean blends (Maforimbo et al., 2006; Lamacchia et al., 2010). Interestingly, no application of SE-HPLC for characterizing oat proteins is reported in the critical work of Sunilkumar and Tareke (2017) reviewing the analytical methods for measurement of oat proteins by covering 2000 works published between 1970 and 2015.

    • Selectively hydrolyzed soy protein as an efficient quality improver for steamed bread and its influence on dough components

      2021, Food Chemistry
      Citation Excerpt :

      Ryan et al. (2002) reported that the negative effects of unmodified soy protein could be due to the lack of interaction between soy and gluten proteins. However, Maforimbo et al. (2006) improved the dough and baking quality with the use of heat treatment on soy protein, which increased the size distribution of the soy protein and hydrophobicity, thereby increasing the contribution to the ‘‘unextractable polymeric proteins” in the composite dough. The secondary structure of native gluten mainly consists of β-sheets, α-helices, and β-turns (the latter being disordered structures with low stability), of which the β-sheets account for about half of the structure.

    • Influence of pistachio by-product from edible oil industry on rheological, hydration, and thermal properties of wheat dough

      2021, LWT
      Citation Excerpt :

      A decrease in alveographic parameters in dough with PBF could be attributed to the high amount of lipids remaining in this by-product and also to the fiber, as previously observed by Wang, Rosell, and de Barber (2002), Gómez, Ronda, Blanco, Caballero, and Apesteguía (2003), Anil (2007), and Pasqualone et al. (2018). The presence of non-gluten-forming pistachio proteins can also negatively influence the alveograph indices (Maforimbo, Skurray, Uthayakumaran, & Wrigley, 2006). Fig. 1 shows the farinograms obtained from the different formulations.

    • Interactions between soy protein hydrolyzates and wheat proteins in noodle making dough

      2018, Food Chemistry
      Citation Excerpt :

      When soy protein product is used as a supplement of wheat flour, the interactions between soy proteins and wheat proteins during dough mixing also have significant influence on the quality of wheat flour and products (Baiano, Lamacchia, Fares, Terracone, & Notte, 2011; Lamacchia et al., 2010; Schmiele et al., 2017). Several studies have been carried out to identify and characterize these interactions (Baiano et al., 2011; Lamacchia et al., 2010; Lampart-Szczapa & Jankiewicz, 1983; Ribotta et al., 2005b; Roccia, Ribotta, Pérez, & León, 2009; Maforimbo, Skurray, Uthayakumaran, & Wrigley, 2006). Lamacchia et al. (2010) and Baiano et al. (2011) have suggested that soy proteins denatured by heat treatment tend to cross link to semolina proteins by disulphide linkages, but the DSF (partially defatted soy flour) proteins interact with semolina proteins by other bonds other than disulphide bonds, in particular dityrosine and isodityrosine bonds.

    View all citing articles on Scopus
    View full text