Exothermic transitions of glycinin determined by differential scanning calorimetry

Abstract

Thermal properties of soy glycinin were investigated in the presence and absence of the reducing agent 2-mercaptoethanol. An endothermic transition was observed without reducing agent, but an exotherm, followed by a relatively small endotherm, was seen in the presence of 0.05 M 2-mercaptoethanol. The presence of the exotherm was dependent on the glycinin concentration and disappeared a t 6% protein concentration. The peak maximum temperatures of the endotherms in the presence and absence of 2-mercaptoethanol were similar, indicating the reduction of intermolecular disulfide bonds between acidic and basic polypeptides did not alter the thermal stability of the protein. The reason for the occurrence of the exotherm was investigated and was found to be due to the aggregation of the basic polypeptides.Aggregation, and hence the presence of the exotherm, could be prevented by the addition of soy pconglycinin. With p-conglycinin present, glycinin exhibited only an endotherm. The exotherm described in this study was considered to be thermally distinct from the exotherm normally attributed to aggregation following denaturation of a protein.Very little information is available on the thermal properties of glycinin at protein concentrations where gelation does not occur. This would include concentrations below 2.5%, a value Nakamura et al. (1986) found to be the lowest concentration of glycinin needed to form selfsupporting gels upon heating. Protein levels below 2.5% have practical value for food applications such as coffee whiteners, infant formulas, and protein-fortified beverages where gelation during thermal processing is undesirable. Thermal properties of proteins are normally investigated by differential scanning calorimetry (DSC), and most of the calorimetric studies of glycinin have been a t high (8-1070) protein concentrations. At these values, an endothermic transition is observed where the transition temperature is pH and ionic strength dependent (Hermansson, 1978). Several studies (Hermansson, 1978;Bikbov et al., 1983;Nakamura et al., 1985) have found glycinin to be a thermally stable protein with a denaturation temperature between 84 and 108 OC depending on pH and ionic strength. This heat stability has been interpreted as being due to extensive disulfide bond formation in the molecule and may be one of the factors limiting its use as a functional protein in foods (Kinsella, 1981). Reducing agents, such as 2-mercaptoethanol (2-ME), can readily reduce the intermolecular disulfide bonds between glycinin molecules (Briggs and Wolf, 1957) and potentially lower the denaturation temperature. However, Harwalkar and Ma (1987) have determined the effect of dithiothreitol on the thermal properties of oat globulin and observed little change in the transition temperature. They concluded that hydrophobic interactions, rather than disulfide bonds, contributed to the observed high heat stability of oat globulin when investigated by DSC.In a preliminary investigation using a glycinin concentration (1.0%) below that required...

Functional and molecular properties of calcium precipitated soy glycinin and the effect of glycation with κ-carrageenan

“…With increasing pH, a significant (P < 0.05) increase in the enthalpy of denaturation was observed. The enthalpies of transitions found in this study are generally in good agreement with those reported in the literature (7-17 J/g for different ionic strengths) (Lakemond, de Jongh, Hessing, Gruppen, & Voragen, 2000b;Koshiyama, 1980Koshiyama, -1981Marshall & Zarins, 1989).…”

“Structural characterization of protein isolates obtained from chia (Salvia hispanica L.) seeds”

“…Unlike CPI10, a great exothermic change of baseline was detected at °C. This exotherm could be associated with an aggregation process of previous denatured proteins (Fitzsimons, Mulvihill & Morris, 2007, Marshall & Zarins, 1989, Lohner & Esser, 1991). The energy of endotherms obtained in CPI12 was considerably lower than in CPI10, 0.7±0.5 J/g (dry basis).…”

Glass Transition Temperature of Rice Kernels Determined by Dynamic Mechanical Thermal Analysis

“…The most widely used method for this application is DSC. Most studies have focused on systems of starch in excess water (Stevens and Elton, 1971;Slade, 1984;Biliaderis et al, 1986;Yost and Hoseney, 1986;Slade andLevine, 1987, 1991;Marshall and Normand, 1991;Liu and Lelievre, 1991;Huang et al, 1994;Buera et al, 1998); only a few studies have been carried out in starch/water systems with a low MC (Slade, 1984;Slade andLevine, 1987, 1991;Zeleznak and Hoseney, 1987;Liu and Lelievre, 1991;Kalichevsky et al, 1992;Perdon et al, 2000). Slade (1984), Zeleznak and Hoseney (1987), and Slade andLevine (1987, 1991) showed that T g of wheat starch is extremely sensitive to MC.…”