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.