Microstructural, rheological and physicochemical properties of milk protein and rice starch during heat-induced gelation

Abstract

Physicochemical and gelatinization properties of starches obtained from different rice cultivars are responsible for their processability in food production. In this study, the relationship between the structure evolvement of rice starches upon heating and their properties was investigated with the emphasis on rice cultivars. Starches separated from 10 different Korean rice cultivars were studied with respect to their morphological, viscoelastic, hydration, pasting, and thermal properties. The granular morphology of the 10 samples was found to be mostly polyhedral but was not significantly different among the rice cultivars. The rheological behavior during gelatinization upon heating brought out differences in onset in elastic modulus (G′) and degree of steepness. Upon gelatinization, the G′ values of the rice starch pastes ranged from 37.4 to 2,057 Pa at 25 °C, and remarkably, the magnitude depended on the starch varieties. R1 showed the lowest critical strain (γc), whereas W1 and W2 possessed the highest γc. The amylose content in starch affected the linear viscoelasticity of rice starch pastes and gels. In addition, the amylose content in rice starches also affected their pasting properties

the sample possessing the highest amylose content showed the highest final viscosity and setback value, whereas waxy starch samples displayed low final viscosity and setback value. The onset gelatinization temperatures of the starches from 10 rice cultivars ranged between 57.9 and 64.4 °C, but the thermal properties did not correlate well with the amylose content. Furthermore, amylose content could be correlated to hydration (Pearson correlation coefficient (PC): -0.9269) and pasting properties (PC: 0.8514) of rice starches whereas it was insufficient in accounting for their viscoelastic and thermal characteristics (PC: -0.3745). The combined analysis of hydration, pasting, viscoelastic, and thermal data of the rice starches is useful in fully understanding their behavior and in addressing the processability for food applications. Gelation characteristics were investigated by in situ rheological test during gelation of β-lactoglobulin (BLG) system heated from 25 to 100 °C. Some valuable parameters derived to represent gelation characteristics were maximal G′ (G′max), and initial gelation temperature (Ti). Changes in rheological properties were investigated by using small-amplitude oscillatory shear (SAOS) test during gelation of BLG system heated from 25 to 100 °C. The effects of pH (3.5, 5.5, and 6.8), were studied on the viscoelastic properties during gelation. BLG solutions prepared at different conditions were poured directly on the measuring stage of the rheometer installing a 20-mm parallel-plate geometry with a 1-mm gap. During heating, rheological parameters such as elastic modulus (G′), and loss modulus (G″) were measured as a function of temperature at 1 Hz and a maximum target strain of 0.01. G′ and G″ rapidly increased with increasing temperature and then tended to reach a plateau with G′ becoming higher than G″. Some valuable including G′ and initial gelation temperature (Ti) were derived to represent gelation characteristics. Various gelation conditions yielded systems showing different rheological properties. G′max at pH 3.5 and 6.8 was much higher than that at pH 5.5. High electrostatic repulsion offered high G′max. Different gel preparation conditions endowed different electrostatic conditions to BLG system which resulted in different gel properties. Also changes in rheological properties were investigated with small-amplitude oscillatory shear (SAOS) test during shear stress sweep at a constant frequency of 1 Hz. The elastic moduli (G′) of the cured BLG gels prepared ubder different conditions were measured as a function of strain (γ) after cooling. The G′ and γ showed remarkable dependence on pH. The pH affected dominantly the critical strain (γc). At pH 3.5 or 6.8, the G′ was higher than that at pH 5.5. The texture of BLG gels at pH 6.8 was rubber-like while that at pH 3.5 was very brittle. The gels prepared at pH 5.5 were slightly rubbery but overall weak. These experimental results would provide useful information for food industry using rice starch and BLG.