Sheared edible oils studied using dissipative particle dynamics and ultra small angle X-ray scattering: TAGwood orientation aggregation and disaggregation

Abstract

Previous work on the computer simulation of edible fats and oils showed that triglyceride crystalline nanoplatelets (CNPs) aggregated into cylindrical structures dubbed "TAGwoods". This was experimentally verified using Ultra Small Angle X-ray Scattering experiments. In this paper, the aggregation of these TAGwoods was studied using the fluid simulation technique, Dissipative Particle Dynamics. The intent was to predict the TAGwood aggregation structures which arise via the application of a series of shear rates, [small gamma, Greek, dot above]. The effect of shear on TAGwood orientational order was also investigated. Three aggregation regimes were identified: At shear rates below a certain critical value, 0 < [small gamma, Greek, dot above] < [small gamma, Greek, dot above] aggregation was enhanced. The value of the critical shear rate depended on the size of the CNPs. With large CNPs possessing a side length of ∼500 nm, the critical shear rate was [small gamma, Greek, dot above] ≈ 0.6 s. However, if the CNPs were smaller with a side length of ∼100 nm, then [small gamma, Greek, dot above] ≈ 75 s. For shear rates above the critical shear rate, [small gamma, Greek, dot above] > [small gamma, Greek, dot above] aggregation was inhibited. The USAXS data was analyzed using the Unified Fit model and the observations were in accord with the simulation results. Three regimes were identified based on the values of the linear slope P of the USAXS data. P increased as [small gamma, Greek, dot above] increased, indicating increased aggregation of the TAGwoods as the shear rate was increased. P ceased increasing and began to decrease when [small gamma, Greek, dot above] ≈ [small gamma, Greek, dot above]. With further increases in [small gamma, Greek, dot above], P decreased as [small gamma, Greek, dot above] increased further, which is indicative of a decrease in aggregation. The orientational quadrupole order parameter, S = 〈Q〉 = 1/2〈cosθ - 1〉, was computed, where θ is the angle between the axis of the TAGwood and the axis of flow, and showed that, for large [small gamma, Greek, dot above], it achieved a near-maximum value. This indicates that at high shear rates, the long axis of the cylindrical TAGwoods aligns in a direction parallel to that of the fluid flow.

Manipulation of Recrystallization and Network Formation of Oil-Dispersed Micronized Fat Crystals

“…By means of the box-counting method, , fractal dimensions of 2.6–2.8 were obtained, which did not vary during network formation under stirring conditions in the first 12 h (data are not shown). This was in line with observations made for the MFC network formed under static conditions, demonstrating that the aggregation of MFC nanoplatelets was via a particle-cluster mechanism and that the involvement of TAGwood aggregate clustering could be excluded. MFC nanoplatelet aggregates were clearly observed after 4 h, which is in line with the slowing down of recrystallization and network formation (Figure A,B).…”

Networks of micronized fat crystals grown under static conditions

“…Under slow crystallization conditions PPP can form pure and smooth crystals, which are prone to stack into TAGwoods. 9,24,47,48 The higher s values for MFC dispersions can be explained by rapid crystallisation during Table 1 Overview of structural parameters derived from 20% MFC and MC dispersions by means of WAXS, SAXS and USAXS using the Scherrer equation (SE) to the first order diffraction line, the Unified Fit (UF) and Guinier-Porod (GP) models. The UF model covered two structural levels and provided the Porod slope, P, and the radius of gyration, R g for each of them.…”

“…9,18,[49][50][51] Such mechanisms are in line with the previously observed stacking of nanoplatelets in TAGwoods. 24 The aggregation of TAGwoods will then form networks with a low fractal dimension (D m < 2). 47,48 The nanoplatelets that are dispersed in MFC dispersions have rough surfaces, which will impede stacking interactions between them.…”

“…At the nanoscale, Wide and Small Angle X-ray Scattering (WAXS, SAXS) were used for the assessment of fat crystal polymorphs and the thickness of the nanoplatelets. Their possible arrangement in stacked aggregates (TAGwoods) 23,24 was studied by means of Ultra Small Angle X-ray Scattering (USAXS). Confocal Raman imaging was deployed to obtain a micron scale view of the fat crystal networks.…”