Crystal microstructure
The amount of crystalline material in a fat system influences the
texture and hardness of a fat crystal network (Frydenberg et al., 2013).
HIU can be used to control the crystal habit and the crystal size
(Ruecroft et al., 2005). The evolution of crystal morphology with time
for MKF crystallized without and with HIU (30% and 70% amplitudes) at
25 °C are shown in Fig. 2. At 1 min of the isothermal time, the sample
without HIU application was still a complete liquid whereas the
HIU-treated samples with 30% and 70% amplitudes had already started to
solidify with small crystals (5.6 ± 1.18 and 6.3 ± 0.84 µm,
respectively) scattering against the dark liquid background. At 10 min,
the MKF from the experiment without the HIU treatment showed a few
crystals (19.6 ± 1.92 µm) and the crystals of the HIU-treated samples
were larger (20.9 ± 2.37 and 23.4 ± 3.72 µm for HIU amplitudes 30% and
70%, respectively) with an increase in the number of crystal compared
to 1 min. At 30 and 60 min, the crystals of the samples from all
treatments were spherulites and the crystal number and the crystal size
had increased with time. The MKF crystallized without the application of
HIU had the largest crystal size (97.9 ± 6.47 and 114.3 ± 4.08 µm for 30
and 60 min, respectively) but with the lowest number of crystals,
suggesting that crystal growth process was dominating. In contrast, MKF
crystalized with the application of HIU with 70% amplitude exhibited
the smallest crystal size (33.5 ± 2.18 and 43.8 ± 1.83 µm for 30 and 60
min, respectively) but the highest number of crystals, indicating that
crystal nucleation process was dominating. The decrease in the crystal
size could be related to the shear forces associated with ultrasound
that act to slow growth processes (Nalajala and Moholkar, 2011). This
was similar to the effect of HIU on the microstructure of anhydrous milk
fat reported by Frydenberg et al. (2013) and Martini et al. (2008). In
addition, higher viscosities were recorded when samples crystalized
after HIU application (Martini et al., 2008), which could delay the
crystal growth process, leading to small crystal size. Luque de Castro
and Priego-Capote (2007) reported a decrease in crystal size and an
increase in crystal number, with a clear involvement of an abrasion
effect, when the power of the applied HIU increased. It was possible
that the formation and collapse of cavitation provided the increasing
possibility of nucleation for the samples. An increase in the number of
smaller crystals typically results in increased fat crystal network
strength (Tran and Rousseau, 2016). Changes observed in crystal
morphology due to sonication can be attributed to the formation of
different molecular compounds or to the presence of different
polymorphic forms, hence, the TAG composition and the polymorphic form
of MKF that crystallized without and with the application of HIU were
investigated in the following sections.