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.