Incorporation of modified okara-derived insoluble soybean fiber into set-type yogurt: Structural architecture, rheological properties and moisture stability

In the present work, modified okara-derived insoluble soybean fiber (ISF) was incorporated into set-type yogurt to investigate its effect on structure, rheological properties and moisture stability. Low concentrations (0.25% and 0.50%, w/v) of ISF induced a more homogeneous network structure with embedded pores or cavities compared to plain set-type yogurt. At high concentrations (0.75%, 1.00% and 1.25%, w/v), a unique structure of ISF attached to aggregated casein micelles was formed. Oscillation time sweep indicated the onset of the casein micelle network appeared earlier as the ISF concentration increased from 0% to 0.50% and then became later as it continued to increase to 1.25%. Rheological measurements showed that the addition of ISF reinforced the structural strength of the yogurt gel to cope with the applied stress, and the consistency index (Pa·sn) fitted by flow curves increased significantly (p < 0.05) from 11.79 to 51.14 at 1.25% ISF addition. Furthermore, the values of the storage modulus and apparent viscosity were dependent on the addition of higher concentrations of ISF, which was related to the viscos and viscoelastic properties of ISF in the aqueous phase. The yogurt gel matrix bound more tightly to water and also exhibited significantly (p < 0.05) higher water holding capacity than the plain yogurt in the case of centrifugation (1000 g, 20 min), up to 84.5%. This study confirms the benefits of ISF in fortifying yogurt and has important implications for the production of the fiber-rich ‘clean label’ set-type yogurt.