The kinetics of crystallization of high-melting fraction (HMF) and a mixture of 40% HMF and 60% low-melting fraction (LMF) of milk fat were studied at 5 degrees C by time-resolved in-situ synchrotron X-ray diffraction. HMF crystallized in the alpha polymorph, had a longer lifetime than the ones previously reported in pure milk fat, and was almost completely solid. The HMF/LMF mixture crystallized initially in the alpha form and transformed into the beta' polymorph, with a solid fat content much lower than that of HMF. The polymorphic change was therefore attributed to a delayed sudden formation of beta' mixed crystals from the uncrystallized melt. These findings are important for the food industry and as fundamental knowledge to improve our understanding of the origin of the macroscopic physical properties of solid milk fat fractions used in many manufacturing processes.
Control of heat and mass transfer conditions during chocolate manufacture is a critical factor influencing product quality and performance. The molecular basis for specific tempering protocols developed by the chocolatiers, and used widely by the food industry, is poorly understood. Here we show that cooling and shear rates applied during cocoa butter crystallization affect the incorporation of specific triacylglycerol (TAG) molecular species onto the surface of growing seed crystals, thus affecting the different structural levels in a cocoa butter crystal network. In this work, the effects of shear work on the different structural levels in cocoa butter are determined. Results from this research show different compositions during the early stages of static crystallization at different cooling rates. In other words, there is selective attachment of TAG species onto growing crystal surfaces, leading to fractional crystallization. With the application of shear, differences between cooling rates became negligible. Shear appears to enhance the formation of mixed crystal in the growing crystals, leading to faster crystallization kinetics, the formation of a higher number of smaller crystals, and a mechanically stronger crystal network. ■ INTRODUCTIONIn the manufacturing of chocolate products, the molten chocolate mass is processed under specific shear and cooling conditions with the aim of crystallizing cocoa butter into an ordered crystalline structure. Cocoa butter is chocolate's main fat component; therefore, its structure will directly affect the product's final physical properties such as contraction, snap, gloss, melting properties, and bloom resistance. This controlled crystallization is known as tempering. The purpose of tempering is to produce 1−3% of the desired highly ordered β V seed crystals, which will subsequently serve as nuclei for the rest of the mass to crystallize. 1−4 If molten chocolate mass were to be cooled directly at cooling tunnel or ambient temperatures, cocoa butter would crystallize in the metastable α II and β′ III−IV forms. 5−8 This results in a dull, soft, low melting point chocolate, prone to bloom.During tempering, the chocolate mass is cooled to 27−29°C to induce nucleation of both stable and unstable crystals. The temperature is then raised to 30−33°C to promote transformation of seed crystals to the stable β V form and melt unstable crystals. The temperature set points used vary by recipe. The tempered chocolate mass is then rapidly cooled to complete crystallization of the fat phase, yielding chocolate with desired quality attributes. 1−4 All methods of tempering, from small-hand tempering to large industrial scales, involve controlled cooling and some element of shear. Shear improves heat and mass transfer of molten cocoa butter, helping overcome kinetic barriers for nucleation and growth. 9 Shorter induction times for crystallization in cocoa butter 10 and chocolate 11,12 when processed under shear suggest that shear affects nucleation. The effect of shear on secondary nucleati...
The present study endeavors to understand how small changes in the composition of cocoa butter affect its crystal habit, crystallization behavior, microstructure, and mechanical properties. Such compositional variations were attained by blending cocoa butter with 1 and 5% 1,2,3-tristearoyl-glycerol (SSS) or 1,2,3-trilinoleoyl-glycerol (LLL). Structural parameters such as crystallization kinetics, crystal structure, microstructure, and mechanical strength were obtained via differential scanning calorimetry (DSC), pulsed nuclear magnetic resonance (pNMR), X-ray diffraction (XRD), polarized light microscopy (PLM), and texture analysis. Changes in the triacylglycerol (TAG) profile of cocoa butter affected its crystal structure and therefore its functionality. The high melting saturated SSS becomes rapidly undercooled, reducing cocoa butter’s onset crystallization times and temperatures. SSS molecules are spatially bigger (i.e., straighter) relative to cocoa butter’s symmetric monounsaturated TAG (the unsaturated oleic acid in the sn-2 position introduces a kink into the TAG structure), and hence infringe upon the order of the crystal domain, reducing the crystal−melt interfacial tension, and delaying polymorphic transformations. On the other hand, the limited molecular compatibility between cocoa butter’s TAGs and the fully unsaturated low melting LLL prevents it from co-crystallizing with the bulk of cocoa butter’s TAGs, only slightly affecting the crystallization behavior, increasing the liquid fraction, having no impact on crystal structure yet accelerating polymorphic transformations into the stable β form.
Fractionation of milk fat by short-path distillation changes the chemical composition and physical properties of the resulting fractions. Increases in distillation temperature from 125 to 250 degrees C increased distillate yield from 0.3 to 42.7% (wt/wt). The distillate was enriched in short- and medium-chain fatty acids and low molecular weight acylglycerols, while the retentate was enriched in long-chain saturated and unsaturated fatty acids as well as high molecular weight acylglyerols. As distillation temperature increased, dropping points of the distillate increased. Relative to native milk fat, the solid fat content (SFC) vs. temperature melting profile of the distillate was depressed and that of the retentate was augmented, which correlated with the saturated long-chain fatty acid content in the fractions. Retentate crystallization parameters obtained by fitting the Avrami model to SFC-time data, did not change as a function of distillation temperature, but varied as a function of the degree of undercooling. Changes in microstructure observed by polarized light microscopy also appeared to be solely a function of the degree of undercooling, with no observable differences between retentates obtained at the different distillation temperatures. In addition, no changes in the retentate's free energy of nucleation (deltaGc) as a function of distillation temperature were found. The compressive storage modulus of the crystallized retentate increased as a function of increasing distillation temperature.
Since the velocity of an ultrasonic wave through a material depends on its density, bulk modulus (K), and shear modulus (G), a new approach to determine the shear elastic modulus and the mass fractal dimension (D) in a fat crystal network was developed. An ultrasonic chirp wave containing a range of frequencies and amplitudes, was used to estimate the structural and mechanical properties of palm oil based fats, crystallized under shear at three different temperatures (20, 25, and 30°C). Considering the fat crystal network as a two-phase system (i.e. liquid and solid fat) the velocity of sound in both phases was obtained separately, assuming that the speed of sound in the oil phase was inversely dependent on the temperature. A constant shear modulus for the solid fraction was obtained experimentally by rheology, which was independent of the sample's nature. These parameters were used for the determination of sample compressibility and its corresponding shear modulus by ultrasonic velocimetry. In addition fractal dimensions (D) were determined by using the relationship of the shear elastic modulus (G) to the mass fraction of the solid fat (/) in a weak-link regime. The obtained results are comparable and consistent with previously reported fractal dimension values. This method allows online determination of the shear modulus of fats and could be potentially applied for quality control purposes in manufacturing.
Here we investigated the effects of applied shear and temperature during the early stages of nucleation on the isothermal crystallization behavior and microstructure of cocoa butter (CB). Results showed that the composition of nucleating triacylglycerols (TAGs) as well as crystalline microstructure and polymorphism of CB were affected by mixing and temperature gradients while still in the molten state. The initial crystalline material isolated from CB after it had been subjected to shear had a similar TAG composition as native CB. However, in the absence of shear, high melting TAGs such as trisaturates (SSS) along with lower amounts of monounsaturated TAGs (SUS) were present, possibly due to fractionation. After subjecting CB to shear in its molten state, crystallization rates were faster due to the cocrystallization of different TAGs into a mixed crystal; however, the polymorphic transition into the more stable β-V form was found to be slower due to inherent complexity in TAG composition. Under static conditions, the presence of high amounts of homogeneous TAGs (SSS) was correlated to faster polymorphic transformations possibly due to a templating effect.
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