3.3.1 Apparent viscosity of walnut butter
During the ball milling process, MCT, DG and CLA formed different emulsification structures with the defatted walnut powder, which resulted in different textures of walnut butter. As the viscosity of the oil increased, the stability and sensory acceptability of walnut butter was improved. However, when the viscosity of oil beyond a certain range, it could have a negative impact on the spreadability of low-fat walnut butter (Fernandes and Salas Mellado, 2018). The viscosity of WO, MCT, DG, and CLA does not vary with shear rate (Fig. 2A). The viscous of CLA was the largest and was closer to walnut oil, while MCT and DG were less viscous.
The rheological properties of walnut oil play a crucial role in food processing. Fig. 2 showed the results of the apparent viscosity measurements of commercial walnut butter and functional lipids walnut butter at a temperature of 25°C. The results showed that the apparent viscosity of all samples showed a gradual decrease with increasing shear rate at the same temperature. This phenomenon indicated that walnut oil has non-Newtonian shear thinning properties (Mostafa et al., 2019). Similar phenomena have been found in sesame butter, peanut butter and mayonnaise (Loncarevic et al., 2016; Muresan et al., 2014; Yang et al., 2020). The presence of high molecular weight substances and aggregation of polymers, such as xanthan gum, walnut proteins and polysaccharide bonds, were also responsible for their high shear thinning behavior (Vardhanabhuti and Ikeda, 2006). In addition, with the increase of external shear force, the agglomerated particles in the emultion system could continue to deform and break. This could reduce the flow resistance of lotion, which also resulted in the reduction of its apparent viscosity (Mun et al., 2009). The apparent viscosity of MCT-6.5 was significantly higher than that of MCT-6 and MCT-7 (Fig. 2D). It may be due to the better stability of the formed emulsion agglomerate structure at the ratio of oil: defatted walnut meal was 6.5:3.5, exhibiting a higher apparent viscosity at low shear rates. The apparent viscosity of DG-WB or CLA-WB decreased with the increase of oil addition ratio (Fig. 2B and 2C). It may be due to that more oil acted as the mobile phase of the system, which resulting in a more effective arrangement of small particles in the flow direction and a lower degree of intermolecular entanglement (Marti, 2004). Fig. 2 showed that, the apparent viscosities of CLA-6.5, CLA-7, DG-6.5 and MCT-6.5 are closer to those of commercial walnut butter.