Triacylglycerol composition
The TAG compositions of the original MKF and the solid fraction of MKF that crystallized at 5 min of isothermal time for the experiment both without and with the application of HIU (70% amplitude) are given in Table 1. The main TAG species in MKF were SOS (45.5%), SOO (27.3%) and POS (12%). Similar results were observed by Sneha and Jeyarani (2018) and Jin et al. (2016). In the solid fractions crystallized without and with HIU, the contents of triunsaturated, diunsaturated and monounsaturtaed TAGs (i.e., OOO, POO, POP, SOO, POS and OOAr) decreased whereas the contents of trisaturated TAGs (i.e., SSP and SSS) and the high-melting monounsaturated TAGs (i.e., the symmetric SUS TAGs like SOS and SOAr) increased (p < 0.05). The contents of both SSP and SSS in the solid fraction crystallized with HIU were higher than those in the solid fraction crystallized without HIU (p< 0.05), suggesting that HIU increased the crystallization of high-melting TAGs at the early stages of crystallization (Kadamne et al., 2017). Frydenberg et al. (2013) suggested that HIU could affect the crystallization of specific TAGs. These high-melting TAGs had a tendency to crystallize into β structure leading to the acceleration of β′→β in the sample.
There is most likely more than one mechanism to explain the effect of HIU on crystallization. This includes the increase in the melting point in the vicinity of the collapsing cavity due to the local pressure changes, leading to an increase in the degree of super cooling. The cavitation bubbles and violent collapse can form active sites for secondary nucleation. In addition, the shear forces and the turbulence generated by sonication can influence the crystallization kinetics (Chandrapala and Leong, 2015). The faster polymorphic transition could be due to the shear forces generated during HIU application. Shear could influence the structure of the nanoplatelets formed upon nucleation which then act as templates for higher, more-organized polymorphs (Maleky et al., 2011). This work has demonstrated that HIU can affect the crystallization of MKF and the effect was dependent on the amplitude of the HIU applied. The crystallization induction time reduced, the crystal size decreased and the crystal number increased with the HIU application. In addition, the polymorphic transition of the fat was accelerated by HIU. This suggests that HIU can be used to temper MKF when it is being used as a confectionery fat in food products such as chocolate.