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A comprehensive study of the isothermal crystallization kinetics of tripalmitin-tristearin mixtures was carried out using DSC, with data fitted to the Avrami equation. Polymorphs were identified by subsequent melting of samples in the differential scanning calorimeter, with additional confirmatory information obtained from wide-angle X-ray diffraction. It was found that α-, β′-, and β-forms require small (<1.0°C), moderate (3.5-8.5°C), and large (9.0-13.0°C) amounts of subcooling below their respective polymorph melting temperatures for nucleation to occur. Concurrent crystallization of β and β′ polymorphs was not observed. The β polymorphs exhibited sharper heat flow exotherms than β′, due to the higher crystallization driving forces experienced. Analysis of apparent induction times shows that the activation free energy of nucleation for the β-form is significantly higher than for the β′-form. Samples rich in either species crystallized faster (both shorter apparent induction times and sharper peaks) than samples with equivalent compositions. Driving-force arguments do not fully explain this behavior, strongly suggesting that mass transfer resistances (greatest for equivalent compositions) have a significant effect on kinetics. Multiple crystallization events were observed for 50-80% tristearin samples between 56 and 60°C and were attributed to a demixing of tripalmitin-rich and tristearin-rich β phases, in line with established phase diagrams.The phase behavior and crystallization kinetics of TG have been extensively studied (1), and these studies have generally concentrated on either single TG or natural fats. Natural fats are complex multicomponent mixtures of TG and other minor components, and their crystallization behavior is poorly understood in comparison with well-defined systems of pure TG. A better understanding is required. One approach to the behavior of real fats is to build up an understanding of the interactions between individual TG components. A number of workers have studied the equilibrium phase behavior of mixed TG, and this has resulted in the publication of phase diagrams for a number of binary and ternary systems (2,3). Much rarer, however, are studies where the crystallization kinetics of mixed systems are examined (4,5). This paper seeks to study such behavior in tripalmitin-tristearin binary mixtures.The polymorphic and phase behavior as well as the crystallization kinetics of pure tripalmitin and tristearin has been reported previously in the literature (6-10). However, relatively few studies have been performed on blends of tripalmitin and tristearin, although a phase diagram is well established (11-13). In this work, the isothermal crystallization and subsequent melting of tripalmitin-tristearin mixtures covering the whole composition range were studied using DSC. This technique is able to provide accurate and reproducible kinetic data and also yields information on the identity of the resulting polymorphs by a subsequent melting of the crystallized material, which can be compared wi...

Starches from four varieties of West African yams were extracted and characterised. The physicochemical properties investigated (granule size and morphology, amylose content, crystal form, gelatinisation and pasting behaviour) depended strongly on the yam variety. The starch granules extracted from water yam (Dioscorea alata), white yam (D rotundata) and yellow yam (D cayensis) varieties showed mononodal particle size distributions centred between 31 and 35 mm, while the bitter yam (D dumetorum) exhibited a binodal size distribution of starch granules centred at 4.5 and 9 mm. Light microscopy con®rmed the variation in starch granule size and shape with yam variety. The X-ray diffractogram of yellow yam was of the B type, while bitter yam showed an A pattern. The starches extracted from the white and water yams were of the intermediate C-type patterns. The temperatures of onset of gelatinisation were derived from DSC and RVA measurements; values of 69.4 and 75.0°C for the yellow yam, 71.5 and 78.2°C for the white yam, 76.5 and 79.8°C for the water yam and 78.1 and 83.1°C for the bitter yam were obtained.

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Starches from four varieties of West African yams were extracted and characterised. The physicochemical properties investigated (granule size and morphology, amylose content, crystal form, gelatinisation and pasting behaviour) depended strongly on the yam variety. The starch granules extracted from water yam (Dioscorea alata), white yam (D rotundata) and yellow yam (D cayensis) varieties showed mononodal particle size distributions centred between 31 and 35 µm, while the bitter yam (D dumetorum) exhibited a binodal size distribution of starch granules centred at 4.5 and 9 µm. Light microscopy confirmed the variation in starch granule size and shape with yam variety. The X‐ray diffractogram of yellow yam was of the B type, while bitter yam showed an A pattern. The starches extracted from the white and water yams were of the intermediate C‐type patterns. The temperatures of onset of gelatinisation were derived from DSC and RVA measurements; values of 69.4 and 75.0 °C for the yellow yam, 71.5 and 78.2 °C for the white yam, 76.5 and 79.8 °C for the water yam and 78.1 and 83.1 °C for the bitter yam were obtained. © 1999 Society of Chemical Industry

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