Differential Scanning Calorimetry Melting Curves Research Articles

Application of Differential Scanning Calorimetry and Thermogravimetry for Thermal Analysis of Dark Chocolates

Dark chocolate is a confectionery product traditionally made from cocoa beans, sugar, and vanilla essence. The aim of the study was to investigate the thermal properties of dark chocolates and fats extracted from these chocolates using thermal methods of food analysis, such as differential scanning calorimetry (DSC) and thermogravimetry (TG). The profile of fatty acids in the fat extracted from the chocolates was also determined. The presence of three fatty acids (palmitic P, stearic S, and oleic O) constituting triacylglycerols—SOS, POP, POS, POO, and SOO—was observed in all the samples. The presence of linoleic acid (L) was also found, which forms triacylglycerols such as PLP and PLS. The researched chocolates were characterized by a diverse composition of fatty acids. In all the obtained DSC melting curves of fats, the presence of endothermic peaks was observed. The peaks, appearing at negative temperatures, may be caused by the transition of low-melting triacylglycerols. The differences between the melting curves for the obtained dark chocolate fats may have resulted from the presence of less stable polymorphic forms of cocoa butter. Based on the shape of the TG and DTG curves, it could be possible to indicate the adulteration of chocolates.

Thermal and spectroscopic profiles variation of cold-pressed raspberry seed oil studied by DSC, UV/VIS, and FTIR techniques

The characteristics of six cold-pressed raspberry seed oils, investigated by melting and crystallization phase transition analysis using differential scanning calorimetry (DSC), UV–VIS and Fourier transform infrared (FTIR) spectra, as well as the fatty acid composition, was the aim of this study. Two groups of oils samples were investigated: commercial (I-group) and laboratory-pressed (L-group). UV/VIS and FTIR spectra of raspberry seed oil revealed important wavelength and wavenumber ranges to indicate qualitative differences between I- and L-group. Regardless of the scanning rates, DSC melting curves were more suitable for detecting differences in raspberry seed oils than crystallization profiles. Based on the Principal component analysis of UV–VIS, FTIR and DSC data two groups of oils were distinguished. Fatty acids by GC analysis confirmed differences between oils, since the commercial oils exhibited a lower ratio of n-3/n-6 fatty acids than L-group oils. The obtained results provide practical information on the usefulness of DSC profiles and FTIR spectra for the comprehensive characterization of cold-pressed raspberry seed oils, especially in the field of B2B industries that need rapid and non-targeted instruments to confirm the authenticity of the purchased product.

Differential scanning calorimetry of cocoa butter and chocolate glaze

Nowadays there is a wide market for cocoa butter equivalents, substitutes and improvers for the confectionery and dairy industries. An urgent task is the development of operational instrumental methods for cocoa butter and its substitutes quality control. Thermophysical parameters are among the most important characteristics of the fat phase for the food technology. Differential scanning calorimetry (DSC) is becoming one of the most promising methods for analytical control of fat and oil products. Thermophysical data (temperatures of the maximums of endothermic peaks and their areas) for cocoa butter and chocolate glaze typical samples applied at dairy processing enterprises of the Central black soil region for the production of chocolate glazed curd bars were obtained in the work performed with its help. DSC data were compared with chromatographic data on triglyceride composition of the fat phase of cocoa butter, cocoa butter equivalents, lauric and non-lauric substitutes, and POP and SOS cocoa butter improvers. It was shown that the DSC method can control the quality of cocoa butter and chocolate glaze, identify chocolate products of different origin and triglyceride composition. Melting thermograms obtained by DSC are highly sensitive to the fat phase triglyceride composition. DSC allows reliable identification of samples of cocoa butter and glaze by melting curves in the temperature range from -100 to +50 ° C. It was found that the main melting peak of cocoa butter and its substitutes, due to the presence of a certain set of triglycerides, is observed in the temperature range from -5 to +30 °C. When examining glazes, the melting peak changes: it bifurcates, expands or narrows. Additional application of computer separation of the unseparated peaks superposition on the DSC melting curves increases the information content of the method and improves the reliability of the fat phase identification. The DSC method is characterized by sample preparation simplicity, has good reproducibility and other metrological characteristics and can be an independent method for fat and oil products identifying and quality control..

A comparative study of thermal and textural properties of milk, white and dark chocolates

The effect of different composition of dark, milk and white chocolate samples on their thermal and textural characteristics was investigated. Thermal behaviour was evaluated by means of differential scanning calorimetry (DSC) and thermogravimetry (TG). The research included melting characteristic of chocolates and fats extracted from chocolates. Additionally, TG and DTG curves of tested samples have been analysed.Mechanical texture parameters (hardness and work) have been determined on the basis of a penetration test by strength machine (ZWICK). The melting characteristics determined for cocoa butter and milk fat before the addition to chocolate and after extraction from chocolate was very similar. The course of DSC melting curves of chocolates depend on numerous factors such as the presence and quality of fat, addition of sugar and/or emulsifiers and particle size distribution of chocolate. White chocolate was characterized by mechanical parameters significantly lower than milk and dark chocolate.

Lipophilic compounds and thermal behaviour of African mango (Irvingia gabonensis (Aubry‐Lecomte ex. O'Rorke) Baill.) kernel fat

SummaryIrvingia gabonensis kernels are a promising oleiferous source. Their total lipid content was 72.3%. C14:0 and C12:0 represented the most abundant fatty acids. Triacylglycerols with ECN 32 and 46–48 were identified for the first time by HPLC‐DAD‐ESI‐MSn. Comprehensive GC‐FID and GC‐MS analyses revealed novel insights into minor lipids like phytosterols and tocochromanols. Among the latter, γ‐tocopherol was found to be the major vitamer. β‐Sitosterol and stigmasterol were the prevailing phytosterols in the kernel fat. The high saturation level of the fat resulted in a sharp differential scanning calorimetry melting curve with a high melting temperature of 42.1 °C. The fat remained solid over a wide temperature range and still contained 66.6% solid fats at 35 °C. Consequently, kernels of the African mango provide a viable source for the recovery of solid fats applicable in the food industry as sustainable alternatives to replace palm‐based fats or hardened vegetable oils.

One pot synthesis of bimodal UHMWPE/HDPE in‐reactor blends with Cr/V bimetallic catalysts

ABSTRACTBimodal ultra‐high‐molecular‐weight polyethylene (UHMWPE)/high‐density polyethylene (HDPE) in‐reactor blends (IRBs) are produced by the bimetallic catalysts, which are synthesized through co‐supporting silylchromate and vanadium‐oxide‐based catalysts on silica or alumina, zirconia and titania‐modified silica. After support modification, the activities of the catalysts for ethylene polymerization are substantially enhanced. The IRBs produced by the modified catalysts also contain more UHMWPE and low‐molecular‐weight polyethylene (LMWPE) fractions, and have much broader molecular weight distribution (MWD). The homogeneous nature of the IRBs is preliminarily confirmed by the differential scanning calorimetry melting curves, showing unique melting peak in both nascent and recrystallized states. The rheological results reflect that the viscosity of the IRBs is reduced more or less when compared with UHMWPE. The distinct elastic dominance of the IRBs is also observed, implying the UHMWPE characteristics in the IRBs. In addition, the intimate mixing of the IRBs is further verified by the similar slopes of Han curves for the polyethylene (PE) samples. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3404–3412

Grain boundary wetting transition in Al–Mg alloys

The melting of coarse- and fine-grained Al–Mg alloys was studied by the differential scanning calorimetry (DSC). The transition from incomplete to complete wetting of grain boundaries (GBs) takes place in binary Al–Mg alloys. The heat effect of GB wetting manifests itself in the asymmetric shape of the DSC melting curve. By decreasing grain size (and increasing specific area of GBs) the GB “shoulder” in DSC curve becomes more pronounced. Basing on DSC data, the position of GB solidus (or premelting) line for Al–Mg alloys has been estimated. Contrary to the Al–Zn alloys, the difference between bulk and GB solidus does not exceed 2°C.

Characterization, thermal properties and phase transitions of amazonian vegetable oils

Thermal profiles of buriti pulp oil (Mauritia flexuosa Mart.), tucuma pulp and kernel oils (Astrocarium vulgare Mart.), rubber seed oil (Hevea brasiliensis), passion fruit oil (Passiflora edulis) and ucuuba butter (Virola surinamensis) were analyzed by thermogravimetry (TG/DTG) and differential scanning calorimetry (DSC). Gas chromatography and calculated iodine values were performed to determine the fatty acid profile and to measure the degree of unsaturation in these oils, respectively. The TG curves showed three steps of mass loss, which can be attributed to the degradation of polyunsaturated, monounsaturated and saturated fatty acids. The DSC crystallization and melting curves are reported and depended on the fatty acid composition. Usually, oil samples with a high degree of saturation showed crystallization and melting profiles at higher temperatures than the oils with a high degree of unsaturation. The data obtained by physicochemical analysis of oil samples were analyzed by principal component analysis and hierarchical cluster analysis to increase understanding of the data set, examining the presence or absence of natural groupings between samples.

Modeling Thermal Properties for Copolymers Covering a Comonomer Composition Range up to 20%

Differential scanning calorimetry (DSC) analysis for ethylene/propylene copolymers synthesized in the whole composition range with a supported metallocene catalyst has been carried out. The melting/crystallization process can be determined both in the ethylene and propylene rich regions covering a comonomer composition range of up to 20%. Two peaks are detected showing two different transitions (Tm1 and Tm2). A procedure is developed for modeling DSC melting curves by using a model previously reported by Kissin. Two contributions are considered for the copolymer crystallization in connection with a random comonomer distribution. Such model favourably describes experimental DSC curves and Tm1 and Tm2 values. The obtained parameters follow a regular trend in accordance with its physical meaning. Standard deviation for melting temperatures reveals accurate calculated values. The analysis of the parameter trend allows a fit against composition and predicting melting temperatures, yielding also accurate values.

Grain boundary wetting and premelting in the Cu–Co alloys

The complete and incomplete wetting of Cu/Cu grain boundaries (GBs) by the Co-containing melt has been observed. The GB wetting in a peritectic system, where the GB wetting layer is depleted by a second component and not enriched like in the conventional cases, has been observed for the first time. The submicrograined structure in Cu–2.2wt.% Co and Cu–4.9wt.% Co alloys has been produced by the high pressure torsion. The melting process of both HPT-treated alloys was studied by the differential scanning calorimetry (DSC). The DSC melting curves were very asymmetric and have been deconvoluted into 2 and 4 different components, respectively. The positions of onsets and minima of high-temperature component peaks for both alloys correspond well with respective liquidus and solidus temperatures in the Cu–Co bulk phase diagram. The low-temperature component peaks were interpreted as GB premelting. The respective GB solidus line was constructed. The step-wise viscosity change of Cu–Co solid solutions was previously observed below the bulk solidus line. The viscosity changes 40–50°C below the GB solidus line observed in this work. This difference can be explained by the difference in the GB character distribution or (alternatively) by the premelting in dislocations cores.

Effect of diacylglycerol addition on crystallization properties of pure triacylglycerols

The objective of this study was to investigate the effects of blending triacylglycerols (TAGs) and diacylglycerols (DAGs) on the melting and crystallization properties in a fat system. To this end, differential scanning calorimetry (DSC), X-ray diffraction (XRD) and polarized light microscopy (PLM) methods were used. Different DAGs (diolein—OO, dipalmitin—PP and distearin—SS) were added at 5% to each TAG (triolein—OOO, tripalmitin—PPP and tristearin—SSS). DSC results showed that the addition of DAGs delayed the onset of crystallization of saturated TAGs (PPP and SSS). By contrast, the addition of DAGs to unsaturated TAG (OOO) accelerated the onset of crystallization, with the appearance of an extra crystallization peak upon the addition of SS and PP. PLM results revealed that the addition of OO affected the polymorphic transition of the TAGs studied findings were consistent with DSC melting curves and XRD results.

Heating effect on the DSC melting curve of flaxseed oil

Flaxseed oil is rich in the alpha-linolenic acid. The effect of heating on the thermal properties of flaxseed oil extracted from flax seeds has been investigated. The flaxseed oils were heated at a certain temperature (75, 105, and 135 °C, respectively) for 48 h. The melting curve (from −75 to 100 °C) of flaxseed oil was determined by differential scanning calorimetry (DSC) at intervals of 4 h. Three DSC parameters of exothermic event and endothermic event, namely, peak temperature (Tpeak), enthalpy, and temperature range were determined. The initial flaxseed oil exhibited an exothermic peak, two endothermic peaks, and two endothermic shoulders between −68 and −5 °C in the melting profile. Heating temperature had a significant influence on the oxidative deterioration of flaxseed oil. The melting curve and parameters of flaxseed oil were almost not changed when flaxseed oil was heated at 75 °C. However, the endothermic peaks of melting curve decreased dramatically with the increasing of heating time when heating temperature was above 105 °C. There is almost no change of melting heat flow of flaxseed oil when heating time exceeded 32 h at 135 °C. The preliminary results suggest that the DSC melting profile can be used as a fast and direct way to assess the deterioration degree of flaxseed oil.

83 Application of differential scanning calorimetry to measure the differential binding of ions, water, and protons in the unfolding of DNA molecules

The overall stability of DNA molecules globally depends on base-pair stacking, base pairing, polyelectrolyte effect, and hydration contributions. In order to improve our understanding of the role of ions, water, and protons in the stability and melting behavior of DNA structures, we report an experimental approach to determine the differential binding of ions (Δn ion), water (Δn W), and protons (Δn H+) in the helix-coil transition of DNA molecules. A combination of differential scanning calorimetry (DSC) and temperature-dependent UV and CD spectroscopic techniques to investigate the unfolding of a variety of DNA molecules: S.T. DNA, two dodecamers, one undecamer, nine short hairpins as a function of the GC content of their stem, and two triplexes. We determine complete thermodynamic profiles, including all the three linking numbers, for the unfolding of each molecule. The CD spectra indicated that all molecules adopted the B-conformation at low temperatures. Thermodynamic profiles obtained from the DSC curves indicate that the favorable folding of each molecule results from the typical compensation of favorable enthalpy and unfavorable entropy contributions, and negligible heat capacity effects. UV and DSC melting curves as a function of salt, osmolyte, and proton concentrations yielded releases of ions, water, and protons (for the triplex with C+GC base triplets). Therefore, the favorable folding of each DNA molecule results from the formation of base-pair stacks and uptake of water and counterions. The thermodynamic data will be discussed in terms of the effects of DNA length, loop contributions and type of water molecules.

Thermodynamic contributions of 5’- and 3’-single strand dangling-ends to the stability of short duplex DNAs

Differential scanning calorimetry (DSC) melting analysis was performed on 27 short double stranded DNA duplexes containing 15 to 25 base pairs. Experimental duplexes were divided into two categories containing either two 5’ dangling-ends or one 5’ and one 3’ dangling-end. Duplex regions were incrementally reduced from 25 to 15 base pairs with a concurrent increase in length of dangling-ends from 1 to 10 bases. Blunt-ended duplexes from 15 to 25 base pairs served as controls. An additional set of molecules containing 21 base pair duplexes and a single four base dangling-end were also examined. DSC melting curves were measured in varying concentrations of sodium ion (Na+). From these measurements, thermodynamic parameters for 5’ and 3’ dangling-ends were evaluated as a function of dangling end length. 5’ ends were found to be slightly stabilizing but essentially constant while the 3’ ends were destabilizing with increasing length of the dangling-end. 3’ ends also display a stronger dependence on Na+ concentration. In lower Na+ environment, the 3’ ends were more destabilizing than in higher salt environment suggesting a more significant electrostatic component of the destabilizing interactions. Analysis of thermodynamic parameters of dangling ended duplexes as a function of Na+ concentration indicated the 3' dangling ends behave differently than 5' dangling ended and blunt-ended duplexes. Molecules with one 5' and one 3' dangling end showed variation in excess specific heat capacity (ΔCp) when compared to the blunt-ended molecule, while the molecules with two 5’ ends had ΔCp values that were essentially the same as blunt-ended duplexes. These observations suggested differences exist in duplexes with 3’ and 5’ dangling ends, which are interpreted in terms of composite differences in interactions with Na+, solvent, and terminal base pairs of the duplex.

Characterization of Five Natural Resins and Waxes by Differential Scanning Calorimetry (DSC)

Thermal properties of 5 natural resin and wax samples (shellac, rosin, shellac wax, beeswax, Chinese insect wax) were examined by differential scanning calorimetry (DSC). The DSC melting and crystallization curves of the samples were presented in this paper. Three DSC parameters, To, Tf and ΔT (difference between To and Tf), were selected from each curve. Evaluation results of the parameters showed that they were statistically significant with individual excellent reproducibility. Information was provided by evaluation of changes among thermal absorption or release peaks of the curves in differentiating the five resins and waxes. It was demonstrated in this paper that DSC is rapid, convenient, reliable and accurate to qualitatively identify the resins and waxes mentioned above.

Modeling differential scanning calorimetry melting curves of ethylene/α‐olefin copolymers

AbstractA procedure is developed for modeling differential scanning calorimetry (DSC) melting curves of linear polyethylene and ethylene/α‐olefin copolymers produced with single‐center and multicenter polymerization catalysts. The modeling demonstrates that the use of the statistical description of random copolymers and the introduction of several assumptions about the crystallization pattern of long ethylene blocks in ethylene copolymers are sufficient for the representation of most characteristic features of DSC melting curves of the polymers. This type of modeling can be applied to ethylene/α‐olefin copolymers containing up to 10 mol % of an α‐olefin. The approach is, in principle, similar to modeling the results of other analytical techniques used for the characterization of copolymer heterogeneity, such as Tref and Crystaf. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Solid-liquid phase separation of poly-4-methyl-1-pentene/diluent system via thermally induced phase separation

Poly-4-methyl-1-pentene (PMP) microporous membranes were prepared via thermally induced phase separation (TIPS) with dioctyl adipate (DOA) as the diluent. Solid-liquid phase separation happened to this PMP/DOA system because of the weak interaction between them, which resulted in the spherulitic structure. Unique and obvious spherulites were formed at low PMP concentration system or low quenching temperature, while the spherulites were destroyed and impinged at high PMP concentration or high quenching temperature. Differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) were used to analyze the crystallization of PMP in the PMP/DOA system via TIPS. The increase of the PMP concentration delayed the crystallization of PMP, which was inferred from the results of both the isothermal and non-isothermal crystallization of the PMP/DOA system. The crystallization was faster and had more effect on the ultimate membrane morphology when the quenching temperature decreased. Double endotherm peaks emerged on DSC melting curves of PMP/DOA quenched samples were owing to the crystals of different extent formed in primary crystallization and further crystallization during TIPS.

High-Pressure Analysis of the Multiple Melting Endotherms of Poly(ethylene succinate) and Poly(butylene succinate)

The origin of the multiple melting peaks in two linear polyesters, poly(ethylene succinate) (PES) and poly(butylene succinate) (PBS), of isothermally crystallized samples was investigated by differential scanning calorimetry (DSC) at atmospheric pressure and high-pressure differential thermal analysis (HP-DTA) at elevated pressures. In PES, the DSC melting curves showed three endothermic peaks at slow heating rates, which decreased to two with increasing heating rates. The HP-DTA curves showed that the area (qualitative) and peak height of the high-temperature peak decreased with increasing pressure and merged with the low-temperature peak at pressures above 450 MPa. This behavior supported the melting, recrystallization, and remelting model for the observed multiple melting endotherms. In PBS, the DSC melting curves were similar to those seen in PES. The HP-DTA curves were also similar to PES up to 400 MPa, but above this pressure the area and the peak height of the high-temperature peak and the temperature difference between the high- and low-temperature peaks remained unchanged. This observation suggested that the two peaks in PBS were due to the melting of two populations of crystals with different lamellar thickness originally present in the sample. The multiple melting behavior in isothermally crystallized PBS is proposed to incorporate both the melting of two populations of crystals and melting, recrystallization, and remelting.

Targeting DNA Hairpin Loops with their Partially Complementary Strands

Antisense, antigene and siRNA strategies are currently used to control the expression of genes. To this end, our laboratory is currently mimicking the targeting of mRNA by targeting DNA hairpin loops with their partially complementary strands. We use a combination of differential scanning calorimetry (DSC) and temperature-dependent UV spectroscopy to investigate the reaction of a variety of DNA hairpin loops (single end loops, dumbbell, three-way junction with two loops and a hairpin with a bulge of 5 nucleotides) with single strands that are complementary to the bases in the loop and to one strand of their stem. The resulting reaction products form duplexes with dangling ends, nicks or with a displaced strand. We determine standard thermodynamic profiles for the unfolding of the reactants (hairpin loop) and products (duplex) of each reaction. The DSC and UV melting curves show monophasic transitions for the unfolding of all DNA single hairpin loops (reactants) and biphasic transitions for the unfolding of the double hairpin loops (reactants) and duplex products. The resulting unfolding data is then used to create thermodynamic (Hess) cycles that correspond to each targeting reaction. All eight targeting reactions investigated yielded favorable free energy contributions that were enthalpy driven. These favorable heat contributions result from the formation of base-pair stacks involving the unpaired bases of the loops, indicating that each single strand was able to disrupt the hairpin loop structure. Supported by Grant MCB-0616005 from NSF.

Melting Behavior of DNA Complexes With Joined Triple and Duplex Motifs

One focus of our laboratory is to understand how sequence, duplex and triplex stabilities, and solution conditions affect the melting behavior of complex DNA structures. We used a combination of UV and circular dichroism (CD) spectroscopies and differential scanning calorimetry (DSC) techniques to obtain a full thermodynamic description of the melting behavior of seven DNA complexes involving joined triplex and duplex motifs. Six of these complexes are formed intramolecularly while the seventh forms an intermolecular complex. The circular dichroism spectra at low temperatures indicated that all complexes maintained the “B” conformation. UV and DSC melting curves of each complex show biphasic or triphasic transitions. However, the transition temperatures, TMs, of the intramolecular complexes remained constant with increasing strand concentration, while the TM of the intermolecular complex did not. This confirms their molecularity. Deconvolution of the DSC thermograms allowed us to determine standard thermodynamic profiles for the transitions of each complex. For each transition, the favorable folding free energy terms result from the characteristic compensation of a favorable enthalpy and unfavorable entropy contribution. The magnitude of these thermodynamic parameters (and associated TMs) indicate that the overall folding of each complex depends on several factors: a) the extent of the favorable heat contributions (formation of base pair and base triplet stacks that are compensated with both the ordering of the oligonucleotide and the putative uptake of protons and ions; b) inclusion of the more stable C+GC base triplets; c) stabilizing the duplex stem of the complex; d) complex molecularity; and e) solution conditions, such as pHand salt concentration. Overall, the melting behavior of each complex corresponds to the initial disruption of the triplex motif (removal of the third strand) followed by the partial or full unfolding of the duplex stem.