Introduction
Crystallization is a process used in many industrial domains including
food, chemical, pharmaceutical and petrochemical industries (Mulin,
2001). Recently, there has been increasing interest in the application
of ultrasonic waves to crystallization (called sonocrystallization).
High-intensity ultrasound (HIU) is an invasive technique that uses
acoustic waves operating at low frequency (20-100 kHz) and high power
(10-10,000 W/cm2) (Ye and Martini, 2015). In general,
HIU can assist various processes in food technology involving
crystallization (Mason et al., 1996). In particular, the application of
HIU to the nucleation and crystallization of fats has been a great deal
of interest because of its ability to change their functional properties
controllably (Ueno et al., 2003).
HIU causes cavitation, which refers to a series of compressions and
rarefactions leading to the formation and implosion of vapor cavities or
bubbles (Patist and Bates, 2008). Cavitation is associated with high
localized temperatures, high shear forces and high pressures that lead
to several physicochemical changes in the material (Soria and Villamiel,
2010). As for the effects on crystallization, HIU influences both
nucleation and crystal growth by creating new additional nucleation
sites in the crystallization medium (Frydenberg, et al., 2013; Silva et
al., 2017). HIU has been reported to (a) induce the nucleation at much
lower supersaturation levels, (b) shorten the induction time between the
establishment of supersaturation and the onset of nucleation and
crystallization, (c) reduce crystal agglomeration, (d) give smaller and
more uniform crystal sizes, (e) control the polymorphic crystallization
of fats, and (f) increase the hardness as a result of an altered
microstructure (Ueno et al., 2003; Luque de Castro and Priego-Capote,
2007; Suzuki et al., 2010; Frydenberg et al., 2013). HIU has also been
shown to improve crystalline products and provide a feasible method for
industrialized reaction crystallization (Li et al., 2006). The degree of
supercooling, HIU application settings and a combination of both
parameters influence the degree of the HIU effect on the crystallization
behavior (Martini et al., 2008). Chen et al. (2013) mentioned that the
effects of HIU were more significant at higher power level and longer
irradiation time. However, the amount of heat generated during long
sonication time can offset the effect of HIU. Ye at al. (2011) suggested
that the effect of HIU on secondary nucleation, where HIU was applied in
the presence of crystals, was more significant than the one observed for
primary nucleation. In addition, HIU has been shown to promote the
formation of a stable polymorphic structure in lipid systems such as
cocoa butter (CB) and tripalmitin (Higaki et al. 2001; Ueno et al.,
2003). A previous work by Higaki et al. (2001) on the effect of HIU on
the crystallization of confectionery fats has indicated the possibility
of HIU-assisted tempering of CB.
Mango is one of the most important fruit crops of Asia. The fat
extracted from the mango seeds, called mango kernel fat (MKF), has
received attention in recent years due to the resemblance between its
melting and crystallization characteristics and those of CB. MKF
contains high content of stearic (S) and oleic acids (O) with SOS as its
main triacylglycerol (TAG) component (Sonwai et al., 2014) and hence can
be used as confectionery fat. According to the EU chocolate directive
(2000/36/EC), MKF is one of only six vegetable fats which are allowed to
be used for up to 5 % in chocolate (Wilson 1999). Therefore, MKF has a
potential to be used as an ingredient for the production of cocoa butter
equivalent (CBE). CBEs are vegetable fats that have chemical and
physical properties similar to CB and have been used in chocolate
products for many years. It can be added to CB in any proportion without
causing significant softening effect, nor altering the melting,
rheological and processing characteristics of CB. By adding CBE into
chocolate to partially replace CB, both fats will go through tempering
process to induce the crystallization of the stable polymorph during
chocolate processing.
This paper presents experimental results on the effects of HIU on the
crystallization behavior of MKF. The crystallization kinetics, crystal
microstructure and polymorphic forms of the fat were studied by
controlling the duration of ultrasound application to the liquid fat
prior to crystallization. The objective of this study was to gain
insight into the possibility of tempering MKF using HIU and to provide a
rational way of controlling the polymorphism and nucleation rate by HIU
in this industrially relevant system.