Mechanical properties of fats in relation to their crystallization

<p>Crystallization in bulk fats is always initiated by a heterogeneous nucleation process. Homogeneous nucleation conditions for fully hydrogenated palm oil (HP) in sunflower oil (SF) could be obtained by emulsifying the fat phase into very fine droplets and using sodium caseinate as an emulsifier. The crystallization kinetics of such a dispersion was determined by means of ultra-sound velocity measurements and could be modeled by a classical nucleation model. The temperature dependency of the kinetics yielded a surface free energy for nucleus formation of about 4mJ×m <sup>-2</SUP>and almost the whole triglyceride molecule should be in the right conformation for incorporation in a nucleus.</p><p>The isothermal crystallization kinetics of bulk solutions of HP in SF in theβ' polymorph could be described well by combination of a classical nucleation function and an empirical crystal growth rate equation taking into account the decrease of supersaturation during crystallization. This yielded a surface free energy for heterogeneous nucleus formation of 3.8 mJ×m <sup>-2</SUP>and about 80 % of the triglyceride molecule to be in the right conformation for incorporation in a nucleus.</p><p>Fat crystals in oil exhibit Brownian motion and attract each other due to van der Waals attraction that results in dispersions with fat crystal aggregates. The structure of the fat crystal aggregates could be described well by a fractal approach as was shown by light scattering and modeling of the viscosity as function of shear rate. Low volume fraction HP/SF dispersion yielded fractal dimensionalities of about 1.7 - 1.8 which is indicative for rapid diffusion limited aggregation and increased with increasing shear rate. Volume fraction of solids of about 0.005 appeared to be sufficient to form a weak gel built from aggregates with a dimensionality of about 1.7 - 1.8.</p><p>Simultaneous crystallization and aggregation of high volume fraction HP/SF dispersion leads to compaction of aggregates after primary network formation. The elastic moduli of these dispersion can be described by higher apparent dimensionalities. Crystallization after gelation also makes the stress carrying strands less flexible and leads to sintering of the fat crystals. The latter occured more easily for fats having a broad fatty acid distribution. Sintering of fat crystals was shown by comparison of elastic moduli and loss tangents before and the development of moduli after mechanical treatment. From model calculations, it was shown that for dispersions in which van der Waals forces are the only interaction forces, crystals can easily bend on deformation of the crystal network.</p><p>Application of shear during crystallization leads to more compact crystal aggregates as was shown by determination of elastic moduli in compression on dispersions that were crystallized in a surface scraping heat exchanger.</p&gt