This study aimed to evaluate the ability of commercial soy protein isolate (SPI) to form cold-set gels under different pHs (5–11), pre-heating temperatures (60 °C, 80 °C), CaCl2 (0–15 mM) and SPI (5–15%, w/v) concentrations, and also select a formulation for the investigation of the effects of incorporating locust bean gum (LBG) (0–0.3%, w/v) and solid lipid microparticles (SLM) on gels rheological and microstructural properties. Gels were evaluated in terms of visual aspect, water-holding capacity, microstructure (using confocal laser scanning microscopy and cryo-scanning electronic microscopy) and rheological properties. SPI showed higher solubilities at pHs 7 (32.0%), 9 (51.6%) and 11 (100%). Self-supported gels were obtained under several conditions at alkaline pHs. At pH 7, only systems pre-heated to 80 °C with 15% (w/v) SPI and 10 or 15 mM CaCl2 gave self-supported gels. At neutral pH, samples showed relative structural instability, which was minimized with LBG incorporation. Formulations GSPI (pH 7, preheated to 80 °C, 15% (w/v) SPI, 10 mM CaCl2) and GMIX (pH 7, preheated to 80 °C, 15% (w/v) SPI, 0.2% (w/v) LBG, 15 mM CaCl2) were selected for emulsion-filled gels (EFG) production. Power law parameters (K′, K″), calculated from frequency sweep results, revealed that non-filled GMIX (K′: 472.1; K″: 77.6) was stronger than GSPI (K′: 170.4; K″: 33.6). Besides, GMIX showed microphase separation. SLM stabilized with Tween 80-Span 80 were active fillers in EFG, altering microstructures and increasing G’, G” and the Young’s modulus (1.8 to 2.1 kPa for GSPI and 1.4 to 2.2 kPa for GMIX).
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The authors thank the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for fellowships for Thais C. Brito-Oliveira (grants 2014/26106-2 and 2016/03271-3) and the University of São Paulo for a fellowship for Marina Bispo. The authors also thank Agropalma, Danisco and Cargill for donating the palm stearin, xanthan gum, and locust bean gum, respectively, and the National Institute of Science and Technology on Photonics Applied to Cell Biology (INFABiC) at the State University of Campinas (Unicamp) for access to the LSM 780 NLO-Zeiss inverted microscope (Zeiss, Germany). The authors also would like to thank Dr. Robert L Seiler of the Life Science Microscopy Facility at Purdue University for his technical assistance in cryo-SEM analyses.