Study of the crystallization behaviour of mixtures of triglycerides for the production of dairy-free chocolate

The replacement of milk fat with mixtures of triglycerides from different sources (e.g palm oil or shea butter) is of particular interest for the chocolate manufacturing, due to low environmental sustainability of milk fat and also increasing consumer demand for dairy-free products. Understanding the crystallization behaviour of the mixtures of triglycerides used as milk fat replacers is crucial in chocolate production, because the crystal stucture of the triglycerides can strongly affect the properties of the final product. Triglycerides molecule have the ability to pack in different solid crystalline arrangements, called polymorphs. Generally, lipids crystallize in three basic polymorphs, called alpha, beta-prime and beta, although crystallization in more than these three basics forms is also possible. The aim of this work was to investigate the polymorphic behaviour of two mixtures of triglycerides and compare it with that of cocoa butter and milk fat. The crystallization behaviour of the samples was studied using polarized light microscopy, powder X-ray diffraction, synchrotron small-angle X-ray scattering, differential scanning calorimetry and Raman spectroscopy. Different composition, in terms of fatty acids and triglycerides, contributed to different crystallization behaviours. Cocoa butter is composed of three main triglycerides: 1(3)-palmitoyl-2-oleoyl-3(1)-stearoyl glycerol (POS), 1,3-dipalmitoyl-2-oleoyl glycerol (POP), and 1,3-distearoyl-2-oleoyl glycerol (SOS). Cocoa butter, upon heating from melt, crystallized in the unstable alpha form, but it progressively transformed into the most stable beta polymorph, whereas MF, due to its more complex composition, showed the presence of more immiscible polymorphic form, compared to CB. MF resulted stable in the beta-prime form. Different ratios of the three main triglycerides of CB (POP, POS and POP) in the milk fat replacers led to different polymorphic behaviours. The SOS rich sample presented two metastable phases which were not detected in the other samples. Both the milk fat replacers resulted stable in the beta form, but the SOS rich sample showed faster kinetic of polymorphic transformation. The presence of minor triglycerides also contributed to diffrences in the crystallization behaviour. The faster polymorphic tranformation in MF3 was, infact, attributed to the presence of higher amount of triglycerides with one or two unsaturated fatty acids (di-unsaturated and tri-unsaturated triglycerides). The effect of the addition of the milk fat replacers was also investigated, and it was found that MF3 had an enhancing effect on polymorphic transformation also when added to CB. The crystal morphology and the effect of the cooling rate was investigated with polarized light microscopy. It was found that faster cooling rates led to lower crystallization temperatures and to a finer network of cystals, whereas with slower cooling rates nucleation occurred at higher temperatures and the crystals appeared to form larger clusters.