Influence of okara dietary fiber with varying particle sizes on gelling properties, water state and microstructure of tofu gel

Effects of nano-sized (NDF, 370 nm) and micro-sized (MDF, 110 mu m) dietary fibers on gelling properties, water state and microstructure of tofu were investigated. Soymilk was mixed with different volumetric ratio of NDF or MDF suspension (0-50%) to make tofu. As the volumetric ratio of NDF suspension increased, yield, moisture, fiber content, syneresis and whiteness of tofu gradually increased, parameters of texture profile analysis (TPA) continuously decreased, while water holding capacity fluctuated. Measurements on relaxation times (T-21 and T-22) and corresponding peak areas by nuclear magnetic resonance (NMR) indicated more immobilized water while less free water as the ratio increased. With respect to NDF, changes of MDF suspension ratio led to the same change tendency of TPA parameters, but opposite of whiteness. The other parameters as mentioned above increased and then decreased as the ratio increased up to 50%. When added at the same ratio (> 30%), MDF tofu showed significantly lower yield, fiber content, synersis and whiteness, while higher TPA parameters. Proton density analysis showed that water was more evenly distributed in the NDF tofu than that of MDF tofu. Light microscopy and SEM confirmed that NDF tofu formed porous and disconnected gel matrix, in which the NDF particles were evenly distributed, while MDF fragments were randomly imbedded in the continuous gel matrix of tofu. Although being more uniformly and steadily entrapped in the tofu matrix, NDF more pronouncedly impeded the gelling ability of soymilk than MDF, which might be due to the stronger interactions with soybean proteins and water.