Rheological characterization of thermal gelation of cowpea protein isolates: Effect of processing conditions.
Introduction
Thermal gelation of globular proteins is important to generate texture in food. Protein gels simultaneously retain water, fats, flavor, pigments and other ingredients and stabilize them in three-dimensional matrix, thus they allow an interesting platform to generate new food products (Hugo, Pérez, Añón, & Speroni, 2014; Shand, Ya, Pietrasik, & Wanasundara, 2007). Thermal gelation of globular proteins is a multi-step process requiring heat-induced unfolding of polypeptides to expose interaction sites, intermolecular interaction or aggregation of unfolded proteins, and agglomeration of aggregates to form a network (Clark, Kavanagh, & Ross-Murphy, 2001). Thus, gel forming ability and viscoelastic properties of globular proteins largely depend on the nature of interactions, such as hydrogen and covalent bonds, and electrostatic and hydrophobic interactions. The better understanding of these interactions will allow the modification and the control of the textural properties of the food (Matsumura & Mori, 1996; Clark et al., 2001, O’Kane, Happe, Vereijken, Gruppen, & Boekel, 2004). Temperature, heating and cooling rates, protein and salts concentrations and pH are processing conditions that affect those interactions and modify final gel properties. Many plant proteins have been reported to have good gelation properties, including soy protein (Renkema & van Vliet, 2004; Speroni, Jung, & de Lamballerie, 2010; Wu, Hua, Chen, Kong, & Zhang, 2017), pea protein (O’Kane, Vereijken, Gruppen, & van Boekel, 2005; Sun & Arntfield, 2011), gluten (Wang et al., 2017), and sweet potato protein (Zhao, Mu, Zhang, and Richel (2018). O'Kane et al. (2004) compared the processes of thermal gelation of pea legumin and soybean glycinin from rheological and molecular basis, these authors concluded that a common model of gelation cannot be built despite the structural similarities between those hexameric globulins. Therefore, although certain behaviors can be similar, the effect of processing conditions should be analyzed for each protein system.
Legumes are one of the most promising economic crops and source of vegetables protein. According with the percentage of total pulses worldwide production, cowpea (Vigna unguiculata) is the fourth main legume (FAOSTAT, 2016). Cowpea seeds contain about 24–27 g/100g crude protein on a dry weight basis and their proteins have a high content of essential amino acids (Avanza, Acevedo, Chaves, & Añón, 2013; Horax, Hettiarachchy, Chen, & Jalaluddin, 2004a). Most essential amino acids of purified cowpea vicilin and cowpea protein isolate are present in acceptable levels as compared to the FAO/WHO/UNU reference pattern for preschool children and adults (Rangel, Domont, Pedrosa, & Ferreira, 2003). Functional properties of cowpea protein isolate have been studied and encouraging data were reported (Horax, Hettiarachchy, Chen, & Jalaluddin, 2004b; Ragab, Babiker, & Eltinay, 2004). Peyrano, Speroni, and Avanza (2016) found that solubility of cowpea protein isolates was high (72–97%) after different denaturing treatments, which may contribute to good gelling ability. However, only few studies on the gelation properties of cowpea protein isolate are currently available and basic information including rheological characterization is still limited.
In a previous work, we investigated the effect of different treatments and found that irreversible changes in protein structure may be induced by exposure to pH 10.0 during protein isolation, such as increase in surface hydrophobicity and change in sensibility to further treatments (Peyrano, de Lamballerie, Avanza, & Speroni, 2017; Peyrano et al., 2016). The aim of this work was to characterize thermal gelling ability and rheological behavior of untreated and pH-shifting-modified cowpea protein isolates under different processing conditions and pretreatments. This work is presented in two parts. In the first part, the effect of maximal temperature, protein concentration and heating and cooling rates is characterized. In the second part, the influence of calcium addition and high hydrostatic pressure pretreatment will be described. The whole study can be a contribution for the utilization of cowpea proteins to conceive foods with specific texture properties.
Section snippets
Materials
Cowpea seeds variety Cuarentón were provided by Estación Experimental El Sombrero Corrientes (Instituto Nacional de Tecnología Agropecuaria-INTA). Shrunken, discolored and insect-infested seeds were eliminated. Seeds were sun-dried and stored in a hermetic vessel at 10 °C until use.
Preparation of cowpea protein isolate
The cowpea protein isolate were prepared according to Peyrano et al. (2016). Cowpea seeds were ground (Braun KSM2, coffee grinder, Mexico) and sifted through an 80 ASTM sieve (177 μm). A 10 g/100 mL dispersion of the
Rheological behavior during thermal cycle
At the beginning of the cycle, G″ was higher than G′ for both A8 and A10. The values of the moduli were low, decreased down to 52.4 ± 0.7 °C and then started to increase up to 74.3 ± 0.3 °C (Fig. 1a). The initial decrease seemed to be due to a disruption of a weak viscoelastic structure. A partial maximum (such as the one we found at 74.3 ± 0.3 °C) was also reported for soybean proteins and explained as a reordering of polypeptides (Renkema, Knabben, & van Vliet, 2001; Renkema & van Vliet, 2002
Conclusions
Gelation started with a low DD in both A8 and A10, thus cowpea proteins exhibited an important ability to establish protein-protein interactions, which corresponded to interesting gelation ability.
The pH shifting during protein extraction resulted in a simple and inexpensive way to induce structural modifications, which improved gel forming ability of cowpea protein isolates in terms of CPC, CT and G′. Moreover, at high temperatures A10 gels exhibited lower tan δ values (and higher G′) than A8
Acknowledgment
The authors wish to thank Delphine Queveau for her excellent technical assistance with the rheometer, and Gina Villamonte, Cecilia Arnaud and Anja Rakotondramavo for their kind help and suggestions during experiments. The stay of Felicitas Peyrano in ONIRIS was funded by BEC. AR program from Argentina.
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2023, Food Research InternationalCitation Excerpt :Gelation is one of the most important functional properties of proteins, which has been widely used to provide texture modification and organoleptic properties in the manufacturing of many food products. Generally thermal gelation of legume proteins involves protein unfolding, aggregation and association of aggregates, and a three-dimensional gel network is formed via hydrogen bonds, disulfide bonds, electrostatic and hydrophobic interactions (Peyrano, de Lamballerie, Speroni, & Avanza, 2019a, 2019b). Therefore, gel strength of proteins largely depends on protein–protein interactions that occur in gels after heating-induced denaturation.