Amaranth, quinoa and oat doughs: Mechanical and rheological behaviour, polymeric protein size distribution and extractability
Introduction
Wheat flour is the only cereal flour that can form a three-dimensional viscoelastic dough when mixed with water. This unique ability of wheat to suit the production of leavened and pasta products is due to the gluten, a cohesive, viscoelastic proteinaceous material prepared as a by-product of starch isolation from wheat flour. The proteins that form gluten are storage proteins which consist of two major fractions: the monomeric gliadins and the polymeric glutenins (Schofield, 1994). The latter are known to be the most important determinants of pasta and bread-making quality (D’Ovidio and Masci, 2004, Lindsay and Skerritt, 1999) and one group of these, of 3–6 proteins, is largely responsible for the elastic properties (Thatam et al., 2001). The high Mr subunits, in fact, possess the characteristics of a putative elastomer, with N- and C-terminal domains containing residues for covalent cross-linking and a central domain that can potentially undergo deformation (Shewry and Thatam, 1990, Belton, 1999). In terms of its nutritional value, gluten (or wheat proteins) is considered to be poorer than proteins from animal sources and can cause allergic reaction and intolerances (Gallagher et al., 2004). Amaranth, quinoa and oat have attracted many interest because of their high nutritional value and for the absence of gluten. In spite of this, the absence of gluten, in these flours, results in major problems for many pasta and bakery products. Their utilization as food ingredients in the production of pasta and bakery products depends largely on their functional properties, which are related to protein structural characteristics. Attempts to use proteins from alternative flours as a partial substitute in wheat products have generally been unsuccessful, because of the contrasting differences between proteins such as the water-solubility, differences in primary structure and their size distributions, accounted for viscoelastic properties that are unique to wheat gluten proteins. 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 issues that cause weakening, such as competition between 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.
The major seed protein fraction of amaranth, oat and quinoa is represented by globulin, which does not possess the requisites to confer dough elasticity (Belton, 1999, Thatam et al., 2001) and of these only the amaranth one have been extensively studied. Amaranth globulins are composed of 11S-globulin, globulin-P and a small amount of 7S-globulins (Marcone, 1999, Martinez et al., 1997, Segura-Nieto et al., 1994). It was shown that the 11S-globulins have molecular characteristics similar to those of other legumes (Chen and Paredes-Lopez, 1997, Marcone et al., 1994, Marcone et al., 1997, Segura-Nieto et al., 1994). Most of cysteine residues of the globulin-S are involved in disulfide bridges required to maintain the quaternary structure, although their cleavage does not mainly affect the protein secondary structure (Marcone and Yada, 1997). In addition, globulin-P is composed of unitary molecules of molecular weight and polypeptide composition similar to those of 11S-globulin, but it tends to polymerize, thus showing different solubility (Castellani et al., 1998, Martinez et al., 1997). Furthermore, globulin-P molecules have been reported as being composed of dimeric subunits linked by disulfide bonds, since their polymers are stabilized by SS linkages (Martinez et al., 1997). Oat globulins are mainly composed of salt soluble globulin (11S-globulin), and in contrast to other cereals such as wheat, barley and rye, whose storage proteins are generally alcohol soluble prolamins, they represent the major protein fraction. Oat also contains prolamins, called avenins, that account for only 10–15% of total protein, whereas those of wheat, rye and barley account for 40–50%, 30–50% and 35–45% of total protein, respectively (Moulton, 1959, Peterson and Brinegar, 1986).
Quinoa globulins represent ∼77% of total proteins while the percentage of prolamins is low (0.5–0.7%) (Koziol, 1992).
In this type of flours, the absence of gluten represents a formidable challenge to the cereal technologist in pasta products preparation. An effective instrument in predicting the processing behaviour and in controlling the quality of final pasta is the characterization of rheological properties of non-conventional doughs.
Farinograph, mixograph and extensograph are the most common empirical instruments used for characterizing dough rheology. Tests based on these instruments are useful for providing practical information for the pasta industries, while they are not sufficient for interpreting the fundamental behaviour of dough processing and pasta quality. Dynamic rheological testing, especially in the linear viscoelastic region, has been used to follow the structure and properties of doughs and to study the functions of dough ingredients (Janssen et al., 1996, Miller and Hoseney, 1999). This testing simultaneously measures the viscoelastic parameters of dough expressed in storage and loss moduli, G′ and G″, and loss tangent tan δ. It is generally found that doughs made from good quality flour have tan δ values lower than doughs made from poor quality flour. High G′ and G″ values in pasta dough can be related to good structure (Song and Zheng, 2007).
The technological properties of doughs and the quality of the final products are affected by both the modification of polymeric protein size distribution and the protein polymerization through cross-linkage and it is well known that polymers aggregation leads to a significant rise in elastic plateau modulus of the network (Cornec et al., 1994, Popineau et al., 1994). Two types of polymeric proteins can be separated by their solubility in SDS–phosphate buffer: the soluble fraction and unextractable polymeric proteins (UPP). Only UPP percentage is well correlated with dough strength (Rmax and extensograph tests) and with mixograph peak time (MPT), indicating that the highest polymeric fraction is the major contributing factor to variations in dough properties (MacRitchie and Lafiandra, 1997, Weegels, 1996).
Dynamic rheological and static-mechanical tests are good ways to fundamentally study the changes in product characteristics due to both processing and formulations. Moreover, the dough components (starch, proteins and water) and their interactions play an important role on the conformational structure as well as the rheological properties (Shiau and Yeh, 2001). The dynamic viscoelastic behaviour of doughs can be understood by taking into account the dual role of water that behaves as an inert filler reducing the rheological properties proportionally and as a lubricant enhancing the relaxation (Masi et al., 1998). Starch is able to form a continuous network of particles together with the macromolecular network of hydrated gluten. This interaction gives rise to rheological properties of doughs. Though the interaction plays an important role, the relative contributions of the two sources are difficult to resolve. The component interactions depend on stress level. The starch–starch interactions dominate over protein–protein interactions at low stresses, while the protein–protein interactions play a dominant role at large deformations (Khatkar and Schofield, 2002). Gluten contributes to the viscoelastic properties of dough to varying degrees depending on its source differing with both gliadin/glutenin ratio and LMW–GS (Edwards et al., 2001, Edwards et al., 2003). Gliadin enhances viscous flow of dough. Glutenin addition results in a more elastic dough in comparison with gluten and gliadin additions (Edwards et al., 2001). Increasing the glutenin/gliadin ratio improves maximum shear viscosity and dough strength (Uthayakumaran et al., 2000).
At our knowledge there are no works about the influence on the rheology of proteins different from gluten.
The aim of this work was to study the rheological characteristics of amaranth, quinoa and oat crumbly dough for pasta making. In addition, the molecular size distribution of the non-conventional dough polymeric proteins and their extractability were also evaluated.
Section snippets
Materials and preparation of dough samples
Amaranth, quinoa, oat wholemeal flours and semolina were purchased from Bongiovanni Mill (Molino Bongiovanni, Mondovì, Cuneo, Italy). For each flour, 300 g of dough crumbly samples were prepared using ordinary tap water and a fresh pasta home appliance (Pastamatic, Simac 1400N, Treviso, Italy). The kneading time was 15 min for non-conventional dough samples and 20 min for semolina ones. The water added to non-conventional flours and semolina to prepare dough samples was of 30% (Chillo et al., 2008
Chemical analysis
The flours examined had very similar protein content while significant differences could be detected for the ash and total fiber content (Table 1). The lowest ash content value was recorded for semolina sample (0.68%) while the highest were shown by amaranth (2.38%) and quinoa (2.17%) flours. On the contrary, oat flour showed the highest total fiber content (11.33%) followed by quinoa (9.86%), amaranth (8.83%) and semolina (3.8%) samples.
Static and dynamic mechanical properties
Fig. 1 reports the stress–strain curves for amaranth,
Conclusion
The tenacity of amaranth, oat and quinoa doughs was lower than that of semolina dough sample. The elastic modulus of amaranth, oat and quinoa dough samples was higher than that of semolina dough. The G′ values of amaranth and quinoa were similar but significantly higher (p < 0.05) respect to that of oat dough. G″ for the three non-conventional doughs showed different values: the highest was recorded for the amaranth dough while the lowest was shown by oat. The semolina dough showed G′ and G″
Acknowledgement
This research work was financially support by Italian Puglia Region, Strategic Project “Process innovation for production of functional pasta”, PS_003.
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