The thermal properties of different kinds of dough were investigated after different kneading times by means of Thermogravimetric Analysis (TGA). Two varieties of durum wheat semolina were used in this study: Alemanno and Cappelli. The doughs were prepared using a mixograph. The gelatinized flour fraction plays an important role on the thermal properties’ definition, while the mixing time influences the dough network building and consequently the starch gelatinization phenomena. Also, the amount of free water in the dough could be influenced by the mixing time. Thus, the TGA technique was applied in order to evidence the mass loss as a function of the increasing temperature and, from this, the free water content, the residual weight (related to the protein kind and content), and the weight loss rate, i.e. the peaks of the first derivative of the thermogravimetric curve (DTG), which appear at different temperatures and present different heights and positions, depending on the dough network force and extension. In such a way, it was possible to find some correlations between the dough characteristics, like the semolina composition (e.g. the gluten content and quality) and the mixing time, and the thermogravimetric analysis outputs. The results showed that the ratio between free and bond water is strongly dependent on both the mixing time and the semolina variety, and a clear evidence of the protein content in the dough is found because of the position and the size of the peaks in the DTG curve, in combination with the residual mass at fixed temperatures.
Proteins, starch and water are the main components of semolina dough. When the ingredients are mixed, several kinds of interactions take place. Understanding the interactions among proteins, starch and water is useful to improve the dough characteristics and, consequently, the final product quality.
Thermal analysis is an interesting tool that allows to study the interactions among the dough components in terms of bond formation. Starch gelatinization and protein coagulation phenomena are the main responsible for the thermal properties of wheat doughs. These structures show an effect on the thermal behavior approximately in the same temperature range (55-80 °C) and moisture level. Regarding the proteins, the main role is played by gluten, which is composed by two fractions: the alcohol soluble gliadins, which mainly contribute to the viscosity, extensibility, and cohesiveness of the dough, and the alcohol insoluble glutenins, which are responsible for the dough elasticity (Drabinska et al., 2016). The ratio between gliadin and gluten is also important, as if it increases, the elasticity of gluten decreases. The gluten proteins are characterized by a temperature dependent equilibrium between two phases: a semi-solid one, prevailing at high temperatures, and a glassy solid one, prevailing at low temperatures; the physical change between these two phases is considered as a “glass transition”. The glass transition temperature (Tg) is the main parameter for understanding the mechanical properties of gluten proteins (Leòn et al., 2003). Starch, the major component of wheat flour, making up about 80% of its dry weight, influences the dough rheological properties, especially upon heating in the presence of water when starch gelatinizes (Li and Yeh, 2001). The gelatinization process causes the transition of insoluble starch granules to a solution composed of individual molecules (León et al., 2003). Gelatinization of starch is a cooperative process, in such a way that structural relations between amorphous and crystalline regions within the starch granules are responsible for the sharpness of thermal transition and the temperature at which it occurs (Romano et al., 2015).
The quality of semolina doughs is related to the interactions between starch and proteins in the presence of water (Güler et al., 2002). Starch gelatinization is influenced by the presence of other ingredients that affect the water activity. Some ingredients, such as sugar, salt, and proteins, compete with starch for the available water in the system and affect its gelatinization (Avramenko et al., 2018). Protein and gluten influence the gelatinization parameters of starch and the water behaviour (Romano et al., 2015). Furthermore, starch gelatinization and protein coagulation are competitive and antagonistic (Huault et al., 2019). The interactions between starch and proteins are a consequence of the attraction between positively and negatively charged colloids in an acidic environment, and the modification of wheat proteins due to heating result in a loss of protein binding to the starch and in a decrease of the interactions (Mohamed and Rayas-Duarte, 2003). So the starch gelatinization peak temperature increases and the enthalpy decreases, in the presence of gluten proteins. In addition, the peak temperature increases as the ratio between gluten and starch increases (Mohamed and Rayas-Duarte, 2003). The thermal stability of the gluten decreases with increasing in the level of gliadins and their transition also shifts to lower temperatures; the changes in the starch gelatinization parameters are believed to be due to the less amount of available water in the presence of gluten (Khatkar et al., 2013)Water is added to hydrate the flour, for the gluten formation and to hydrate the starch so that it can gelatinize. These events result in the formation of the basic structure of a baked product. Starch and gluten are complex chemical polymers and hence their interaction with water is complex (Li and Yeh, 2001). Both the amount and the mobility of water are recognized to have a crucial role in starch gelatinization, in the formation of the gluten network (Fessas and Schiraldi, 2001), in the thermal stability of proteins (Romano et al., 2015) and in the glass transition temperature (Khatkar et al., 2013). Also, the mixing process and its conditions are important to determine the amount of free and binding water, because all the interactions among the dough components are established during mixing. An insufficiently developed dough results in a higher free water content because the latter has no time enough to react with the flour proteins and with the soluble components in the system. Therefore, the thermal properties are influenced by these interactions and, consequently, by the free water amount. The time required for the optimum dough development is also positively correlated with the polymeric protein composition and the ratio between protein polymers and monomers (Angioloni and Dalla Rosa, 2005).
To summarize, the flour components influence the properties of the finished product. Also, the process conditions affect the quality of the final product. TGA measures the change in weight caused by the loss of water and by the product degradation due to the structural interactions breaking. The TGA can be a useful tool to characterize the dough and to understand which kind of bonds and consequently of structures were formed during kneading, and how the relative amount of the different components influences their building.