The effects of cellulose nanocrystals (CNC) and microcrystalline cellulose (MCC) on the gel properties and microstructure of glucono-δ-lactone-induced soy protein isolate (SPI) gels were investigated. The water-holding capacity, gel strength, and viscoelastic modulus of CNC-SPI gels were positively associated with CNC concentration from 0 to 0.75% (w/v). In contrast, MCC-SPI gels exhibited decreased water-holding capacity, gel strength, and viscoelastic modulus. All composite gels displayed high frequency dependence and the typical type I (strain thinning) network behavior. Changes in viscoelasticity under large strain were correlated with differences in the microstructure of SPI composite gels. Scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) showed that CNC were more evenly and steadily distributed in the protein matrix and formed a compact network structure. In contrast, MCC-SPI gels exhibited a discontinued and rough gel network with some large aggregates and pores, in which MCC was randomly entrapped. Fourier transform infrared spectroscopy (FTIR) and molecular forces results revealed that no new chemical bonds were formed in the gelation process and that the disulfide bond was of crucial importance in the gel system. With the addition of CNC, electrostatic interactions, hydrophobic interactions, and hydrogen bonds in the SPI gel network were significantly strengthened. However, the incorporation of MCC might obstruct the connection of the protein network. It is concluded that both cellulose type and concentration affect gelling properties.
Keywords: CNC; MCC; acid-induced soy protein isolate gelation; microstructure; rheology.