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.