Crystallization kinetics
The crystallization of the MKF samples at 25 °C without and with the application of HIU is given in Fig. 1 as a plot of changes in the normalized light intensity versus time. Although measurements were executed in triplicate, this graph represents one exemplary curve of each experiment, as there was small variation between the repetitions. At the beginning, all samples were clear liquids hence the light intensity that went through all samples was at the maximum value of 1. Once the crystallization started, the transmitted light intensity began to drop. The HIU-treated samples with the amplitudes of 70 and 50 % of the maximum value began to crystallize first with a crystallization induction time of ∼3 min (Table 1) where the transmitted light intensity of the two samples started to decrease very rapidly to reach the minimum value of 0 at 12 and 16 min, respectively. At ∼4 min of the isothermal time, the HIU-treated sample with the amplitude of 30 % of the maximum value was next to begin the crystallization and the minimum value of the transmitted light intensity was reached at 29 min. The MKF without HIU application was the last sample to solidify at ∼6 min and the crystallization proceeded with the lowest rate to reach the plateau region of transmitted light intensity at 36 min. Fig. 1 shows that with the application of HIU at the beginning of the isothermal time, the induction time of crystallization for the MKF fat was reduced and the crystallization reached equilibrium faster, suggesting that HIU increased the driving force for the crystallization and that HIU induced the primary nucleation in the system. The results were in line with many previous published works. For example, HIU was reported to induce the crystallization of interesterified soybean oil (Silva et al., 2017), anhydrous milk fat (Martini et al., 2008) and palm oil (Patrick et al., 2004; Chen et al., 2013; Ye and Martini, 2015) and low saturated shortening (Ye et al., 2011) with reduced induction time and increased crystallization rate. It is believed that HIU induces crystallization because cavitation bubbles generated during sonication provide a large number of heterogeneous nucleation sites with reduced activation energy barrier for nucleation, leading to a reduction of the crystallization induction time (Wohlgemuth, 2009, Chen et al., 2013). In addition, it was reported that HIU enhanced the mass transfer in the system (Luque de Castro and Priego-Capote, 2007), which could also contribute to faster crystallization. Silva et al. (2017) mentioned that greater induction in crystallization was observed when HIU is applied at the onset of crystallization. However, in this work, HIU was applied before the crystallization began and it was still effective. The same authors also reported that the effect of HIU on the crystallization induction time was dependent on the supercooling used with the HIU effect being compromised under extreme supercooling.
The time duration taken for the normalized light intensity to decrease from 0.8 to 0.2 were 12, 8.2, 5 and 3.3 min for the samples crystallized without HIU and with HIU amplitudes of 30, 50 and 70%, respectively, suggesting that the crystallization rate of MKF increased as the HIU intensity increased. The increase in the crystallization rate with increasing the HIU intensity was reported in the literature. Chen et al. (2013) reported that ultrasound greatly reduced the crystallization time to reach the equilibrium without changing the SFC of the sample. Increasing the HIU intensity leads to a heavier flow pattern in the crystallizer (Luque de Castro and Priego-Capote, 2007) and this could contribute to the increase in the crystallization rate. During sonication, the sample temperature increased to approximately 25.7, 29.2 and 32.8 °C for the HIU amplitudes of 30, 50 and 70%, respectively, before decreasing to 25 °C within a few minutes. The fact that the sample with the HIU application crystallized with shorter induction time, hence at higher temperature, suggested that HIU raised the nucleation temperature resulting in the decrease in the supersaturation limit (Luque de Castro and Priego-Capote, 2007). HIU decreases the apparent order of primary nucleation rate and increases the rate of appearance of solid, leading to the reduced metastable zone width where a supersaturated solution is much more unstable under an HIU field (Luque de Castro and Priego-Capote, 2007).