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).