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This study investigates the crystallization kinetics of palm stearin (PS), a palm oil fraction, in blends with sesame seed oil. The results indicate that the crystallization behavior of PS in sesame oil is mainly associated with the crystallization of tripalmitin. Therefore, crystallization of blends of 26, 42, 60, and 80% (wt/vol) PS in sesame oil was described by equations developed for simpler systems (e.g., Fisher and Turnbull equation). The isothermal crystallization, melting profile, and fitting of the kinetics of nucleation to the Fisher and Turnbull equation showed that the 26, 42, and 60% PS/sesame oil blends crystallized mainly in the β 1 ′ polymorph state. In contrast, the 80% blend crystallized in two different polymorph states (i.e., β 1 ′ at T ≤ 307.6 K and β 1 at T ≥ 308.2 K). The data indicated that, in spite of the higher concentration of PS in the 80% PS/sesame oil system, crystallization in the β 1 state required more free energy for nucleation (∆G c ) than β 1 ′ crystallization in the 26, 42, and 60% PS/sesame oil. At the low cooling rate used (1 K/min) it was observed that, for a particular PS blend, the higher the effective supercooling the higher the viscosity of the oil phase and the smaller the induction time of crystallization (T i ). Additionally, the β 1 ′ crystals from PS, developed at the highest effective supercooling investigated, were smaller than the β 1 crystals obtained at lower effective supercooling. FIG. 1. Determination of reduced crystallinity, F, from exotherm obtained at isothermal conditions.

In this work the association between polymorphism and the crystal network structure developed by the TAG of cocoa butter (CB) was investigated under static and stirring crystallization conditions using a dynamic mechanical spectrometer. The results obtained showed that parameters obtained through oscillatory rheometry (i.e., phase shift angle, δ) followed the polymorphism of CB during static crystallization. Although standard DSC was not capable of differentiating the α to β′ phase transformation from the direct β′ crystallization from CB melt, δ rheograms measured these two processes separately. Additionally, through oscillatory rheometry, we followed the dimensionality of the crystal network during CB crystallization. Within this context, the pre-exponential term (ln γ) from the weak-link regime equation for colloidal dispersions was much more sensitive than the fractal dimension (D) to differences in crystal size, spatial distribution of the crystal network, and melting temperature of the β′ phase of CB. On the other hand, torque measurements obtained during CB crystallization under stirring conditions showed a shear rate effect that favored TAG development in the β phase at temperatures of 19, 22, and 26.5°C, particularly at shear rates of 120 and 400 rpm. In contrast, under static conditions CB did not develop in the β phase at any of the crystallization temperatures investigated (i.e., 18 to 26.5°C).Paper no. J10685 in JAOCS 81, 195-202 (February 2004). KEY WORDS:Avrami index, cocoa butter, fractals, stirring crystallization.Crystallization of cocoa butter (CB) provides unique characteristics of texture, mouth feel, and flavor release to chocolate and other confectionery products. Although CB is composed of various TAG, more than 75% exhibit a symmetrical conformation with oleic acid in the sn-2 position. Because of this homogeneity in TAG conformation and composition, polymorphism in CB is quite complex, i.e., at least six polymorphic forms have been observed. According to Wille and Lutton (1), each polymorph has a unique m.p. and crystal structural properties. These authors named the polymorphs from I to VI in increasing order of m.p. However, a study with real-time Xray diffraction by van Malssen et al.(2) showed that forms III and IV according to the Wille and Lutton nomenclature exist as a range of β′ polymorphic phases rather than as two separate polymorphic phases. This property may be ascribed to cooperative interactions among the three major TAG of CB, i.e., StOSt, POSt, and POP, where St = stearate, O = oleate, and P = palmitate. Accordingly, TAG in CB crystallizes in the β′ phase as a distribution of crystallites with a particular TAG composition, providing a distribution of individual melting temperatures and diffraction patterns, all of them characteristic of the β′ polymorph (2). CB is used as the main lipid phase in chocolate manufacture, but it is also used in combination with other vegetable oils, such as hydrogenated soybean oil and palm oil, in the elaboration of confectionery products. I...

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