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.