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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The solid fat content (SFC), Avrami index (n), crystallization rate (z), fractal dimension (D), and the pre-exponential term [log(γ)] were determined in blends of cocoa butter (CB) with canola oil or soybean oil crystallized at temperatures (T Cr ) between 9.5 and 13.5°C. The relationship of these parameters with the elasticity (G ′) and yield stress (σ*) values of the crystallized blends was investigated, considering the equilibrium melting temperature (T M°) and the supercooling (i.e., T Cr°− T M°) present in the blends. In general, supercooling was higher in the CB/soybean oil blend [T M°= 65.8°C (±3.0°C)] than in the CB/canola oil blend [T M°= 33.7°C (±4.9°C)]. Therefore, under similar T Cr values, higher SFC and z values (P < 0.05) were obtained with the CB/soybean oil blend. However, independent of T Cr TAG followed a spherulitic crystal growth mechanism in both blends. Supercooling calculated with melting temperatures from DSC thermograms explained the SFC and z behavior just within each blend. However, supercooling calculated with T M°e xplained both the SFC and z behavior within each blend and between the blends. Thus, independent of the blend used, SFC described the behavior of G ′ eq and σ* and pointed out the presence of two supercooling regions. In the lower supercooling region, G ′ eq and σ* decreased as SFC increased between 20 and 23%. In this region, the crystal network structures were formed by a mixture of small β′ crystals and large β crystals. In contrast, in the higher supercooling region (24 to 27% SFC), G′ eq and σ* had a direct relationship with SFC, and the crystal network structure was formed mainly by small β′ crystals. However, we could not find a particular relationship that described the overall behavior of G ′ eq and σ* as a function of D and independent of the system investigated.

Heavy crude oil processing leads the way in current refining. These crudes yield larger amounts of distillable heavy fractions such as vacuum gas oil (VGO). VGO must be treated in at least two refining units: a hydrotreating unit where sulfur, nitrogen, and other heteroatoms are removed, and a hydrocracking unit where suitable fuels are obtained. Removal of heteroatoms during hydrotreating, particularly, nitrogen, dictates the efficiency of hydrocracking. In the first part of this work, the nature of refractory nitrogen-containing compounds on the performance of a hydrotreating catalyst was evaluated. To achieve this goal, both a VGO and its hydrotreated counterpart were studied using electrospray ionization with Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS). Weakly basic N-containing compounds, namely, heavy pyrrolic-like compounds and their partially hydrogenated derivatives, were found to be the most refractory to hydrotreating. These compounds are weakly basic compared to most nitrogen compounds present in VGO. Considering this finding, the second part of the work was devoted to assessing the effect of pyrroles on the reactivity of phenanthrene over a Ni–MoS2/Y-zeolite–alumina two-stage hydrocracking catalyst. Tests were carried out in a fixed-bed reactor using mixtures of carbazole and tetrahydrocarbazole. Results showed that these compounds can affect the catalytic performance of Ni–MoS2/Y-zeolite–alumina by reducing its activity and inhibiting its selectivity to hydrocracking products. These findings draw attention to the possible role of weakly basic nitrogen compounds in the catalytic performance of materials employed for two-stage hydrocracking units.