Differential Scanning Calorimetry Heating Curves Research Articles

Thermal measurements of curdlan in aqueous suspension during gelation

Differential scanning calorimetry (DSC) measurements of curdlan in aqueous suspension were carried out in a temperature range of 40–200°C by using silver pans with and without heat treatment, respectively. Silver pans with heat treatment were heated at 200°C and then cooled. Heating DSC curves of curdlan in aqueous suspension showed an endothermic peak around 60°C and were identical below 120°C, but quite different above 120°C. A large exothermic peak was observed at 140–190°C in a heating DSC curve when a non-heated pan was used, whereas a small endothermic peak was observed at 140–160°C and no exothermic peak was observed at higher temperatures when a heated pan was used. It was suggested that the large exothermic peak was attributed to an interaction between the non-heated silver pan and water above 120°C. Only the endothermic peak at 140–160°C reflected the reality of the thermal property of curdlan in aqueous suspension, which was caused by the melting of the curdlan gel formed.

Thermal studies on the gelatinisation and retrogradation of heat–moisture treated starch

The gelatinisation and regelatinisation of heat–moisture treated corn starches with different heating temperatures (120 and 130°C) and corn starch without treatment were studied by differential scanning calorimetry (DSC). The gelatinisation peak in the heating DSC curves shifted to higher temperatures with increasing temperature of heat–moisture treatment. The endothermic enthalpy of the gelatinisation decreased with the severity of heat–moisture treatment whilst those for the disintegration of amylose–lipid complex increased. The retrogradation ratio increased faster for heat–moisture treated corn starch than for corn starch without treatment.

Polymorphism of milk fat studied by differential scanning calorimetry and real‐time X‐ray powder diffraction

AbstractThe crystallization behavior of milk fat was investigated by varying the cooling rate and by isothermal solidification at various temperatures while monitoring the formation of crystals by differential scanning calorimetry (DSC) and X‐ray powder diffraction (XRD). Three different polymorphic crystal forms were observed in milk fat: γ, α, and β′. The β‐form, occasionally observed in previous studies, was not found. The kind of polymorph formed during crystallization of milk fat from its melted state was dependent on the cooling rate and the final temperature. Moreover, transitions between the different polymorphic forms were shown to occur upon storing or heating the milk fat. The characteristic DSC heating curve of milk fat is interpreted on the basis of the XRD measurements, and appears to be a combined effect of selective crystallization of triglycerides and polymorphism.

The influence of locust bean gum and dextran on the gelation of κ-carrageenan

The interaction of κ-carrageenan with locust bean gum and dextran has been studied by rheology, differential scanning calorimetry (DSC), and electron spin resonance spectroscopy (ESR). Rheological measurements show that the carrageenan gel characteristics are greatly enhanced in the presence of locust bean gum but not in the presence of dextran. Carrageenan/locust bean gum mixtures show two peaks in the dsc cooling curves. The higher temperature peak corresponds to the temperature of gelation and its intensity increases at the expense of the lower temperature peak as the proportion of locust bean gum in the mixture increases. Furthermore, the DSC heating curves show enhanced broadening when locust bean gum is present, indicating increased aggregation. These results are taken as evidence of carrageenan/locust bean gum association. The gelation process has also been followed by ESR using spin-labeled carrageenan. On cooling carrageenan solutions, an immobile component appears in the ESR spectra signifying a loss of segmental mobility consistent with chain stiffening due to the coil → helix conformational transition and helix aggregation. For carrageenan/locust bean gum mixtures, carrageenan ordering occurs at temperatures corresponding to the higher temperature DSC setting peak and the temperature of gelation. Similar studies using spin-labeled locust bean gum show that its mobility remains virtually unaffected during the gelation process. It is evident, therefore, that carrageenan and locust bean gum interact only weakly. It is proposed that at low carrageenan concentrations the gel network consists of carrageenan helices cross-linked by locust bean gum chains. At high carrageenan concentrations the network is enhanced by the additional self-aggregation of the “excess” carrageenan molecules. For carrageenan/dextran mixtures, only one peak is observed in the dsc cooling curves. The onset of gelation shifts to higher temperatures only at very high (20%) dextran concentrations and this is attributed to volume exclusion effects. Furthermore, there is no enhanced broadening of the peaks in the DSC heating curves as for the carrageenan/locust bean gum systems. It is therefore concluded that carrageenan/dextran association does not occur. The difference in behavior between locust bean gum and dextran is attributed to the greater flexibility of the dextran chains. © 1996 John Wiley & Sons, Inc.

Structure of stratum corneum lipids characterized by FT-Raman spectroscopy and DSC. IV. Mixtures of ceramides and oleic acid

The thermotropic and lyotropic phase behaviour of mixtures of ceramides type IV (CER) and oleic acid (OA) was investigated using Fourier transform (FT) Raman spectroscopy and differential scanning calorimetry (DSC). The Raman spectra of the mixtures in the solid state exhibit sharp bands associated with trans sequences of the alkyl chain residues of both lipids, indicating highly ordered chains. The DSC heating curves for the dry and the fully hydrated mixtures show three endothermal transitions. The events at T=−5.5 and 12.3°C do not depend on the composition of the mixtures. As in the case of pure OA, they are associated with the solid–solid transition of OA from γ to α modification and the melting of the α phase, respectively. The temperature of the third event at higher temperatures decreases continuously with increasing OA content. This transition involves the melting of CER. Both DSC and Raman spectroscopic studies reveal that CER and OA are immiscible in the solid state. The phase diagram is a monotectic type one. Calculations of the melting curve of CER support this interpretation. Full hydration of the system causes a decrease of the temperature of the upper transition, but, the general feature of the phase diagram remains.

Effects of polyols and sugars on the structure of water in concentrated gelatin gels as studied by low temperature differential scanning calorimetry

Phase-transition temperatures, glass-transition temperatures, melting temperatures, gel-sol transition temperatures in differential scanning calorimetry heating curves of gelatin solutions with and without various sugars and polyols quenched by liquid nitrogen were studied. Both sugars and polyols added to concentrated gelatin solutions shifted the glass-transition temperatures to lower temperatures, and it was attributed to the increase of unfreezable water which acts as a plasticizer. The mechanisms of the increase in unfreezable water, however, seem to be different for sugars and polyols; sugars increase unfreezable water by increasing the number of junction zones which hold unfreezable water, while polyols by themselves increase the amount of unfreezable water.

DSC and rheological studies on aqueous dispersions of curdlan

Differential scanning calorimetry (DSC) and dynamic viscoelastic measurements of aqueous dispersions of curdlan were performed to clarify the gelation mechanism. Second run DSC heating curves for aqueous dispersions of curdlan showed a single endothermic peak for dispersions heated to temperatures lower than 53°C, and splits into two peaks for dispersions heated to temperatures higher than 53°C, indicating that the structural disorders of at least two different energies are involved. Dispersions heated at temperatures higher than 120°C did not show any endothermic peak in the second run DSC heating curves indicating that structures formed at temperatures higher than 120°C are stable and thermo-irreversible. Mechanical spectra of the aqueous dispersions of curdlan showed solid-like behaviour both at 40 and 70°C although the mechanical loss tangent for the dispersion heated to a higher temperature was much smaller. All these results are consistent with previous findings that gels formed at higher temperatures are thermo-irreversible, and that gels formed at lower temperatures are thermo-reversible.

Structure of stratum corneum lipids characterized by FT-Raman spectroscopy and DSC. II. Mixtures of ceramides and saturated fatty acids

Fourier transform (FT) Raman spectroscopy and differential scanning calorimetry (DSC) were used to study the thermotropic phase behaviour of mixtures of ceramides type IV (CER) and stearic acid (SA). For comparison the melting behaviour of SA was re-examined. The Raman spectra of all mixtures in the solid state show sharp bands associated with trans sequencies of the alkyl chain residues of both lipids. These features demonstrate that the hydrocarbon chains are highly ordered in the mixtures, too. The temperature dependence of the conformationally sensitive bands is used to estimate the degree of order in terms of the relative population of trans and gauche conformations. The DSC heating curves for the mixtures show two endothermic transitions which are typical for eutectic melting. The factor group splitting of the CH2 scissoring mode, arising from the orthorhombic subcell packing of SA, disappears in the course of the eutectic melting of samples with a SA content lower than 90 mol%. Both DSC and Raman spectroscopic studies reveal that CER and SA are immiscible in the solid state. The phase diagram of the system is a simple eutectic type one. The addition of SA to CER shifts the melting temperature of ceramides to lower values. However, though SA is a major component of stratum corneum (SC) it is not efficient enough to increase the fluidity of ceramides.

Suppressive Effects of Salts on Droplet Coalescence in a Commercially Available Fat Emulsion During Freezing for Storage

To assess possible improvemets in the practical use of long-term freezing storage for commercially available fat emulsions, the effects of salts on coalescence (fusion) of emulsions during freeze–thawing cycles were studied. For this purpose, apparent size changes were measured by dynamic light scattering, and the dispersed droplets were directly observed by freeze-fracture electron microscopy before and after the freeze–thawing processes. Intravenous high-calorie nutrient fluid, based on soybean oil in water (2.2wt% glycerin) and emulsified by egg phosphatides, was selected as a representative commercially available fat emulsion. Possible mechanisms of the suppressive effects of various salts (Li, Na, K, Rb, and Cs chlorides) on coalescence during repeated freezing with liquid nitrogen and subsequent thawing were examined in terms of the zeta-potential of the emulsion droplets and differential scanning calorimetry heating curves of the water-salt-glycerin ternary solutions. The suppressive effects of salts on the coalescence of the emulsion droplets during freeze–thaw cycles could be ascribed to the suppression of ice crystal formation and the condensation of the droplets in unfrozen aqueous channels between the ice crystals during the freezing cycle.

Characterization of thermotropic block copolyetheresters by x-ray diffraction and DSC

The block copolyetheresters with hard segments of poly(alkylene-p, p'-bibenzoate) and soft segments of poly(tetramethylene ether) have been investigated by two x-ray diffraction techniques and differential scanning calorimetry (DSC). The results from the counter diffractometry technique indicate the effect of composition on the diffraction peaks and relative crystallinity of the block copolyetheresters. The results from the photographic technique not only show the presence of the diffraction peaks qualitatively, but also indicate the layer spacings relating to the smectic phase for certain block copolyetheresters. The successive DSC heating curve after cooling from the isotropic (I) state to a temperature below the I-S (smectic) transition but well above the S-K (crystalline) transition indicates the nature of the S-I transition for some block copolyetheresters. The S-I transition peak overlaps with the K-S transition peak, or is even replaced by a K-I transition peak. Thus it is difficult to distinguish between the K-S, S-I transitions and a K-I transition in second-run DSC heating curves for the block copolyetheresters.

Rheological and DSC studies on the interaction between gellan gum and konjac glucomannan

Interaction between gellan gum (GELL) and konjac glucomannan (KGM) has been studied by dynamic viscoelastic measurements and differential scanning calorimetry (DSC). Three fractions of konjac glucomannan with different molecular weights (1.17 × 10 6 (ND), 9.5 × 10 5 (LM-1) and 2.51 × 10 5 (LM-2)) were used in this study. Mechanical spectra showed that 1.6% solutions of GELL alone and of LM-2 alone behaved like a dilute polymer solution whilst 1.6% solutions of ND alone and LM-1 alone behaved like a concentrated polymer solution where molecular entanglements play an important role. Thermal scanning rheological measurements showed a step-like change in the loss modulus which is attributed to helix-coil transition, and subsequent aggregation of helices, and an exothermic and endothermic peak appeared in cooling and heating DSC curves at the same temperature of transition. The transition temperature shifted to lower temperatures, and the transition enthalpy decreased with increasing KGM content in the mixture. The elasticity of mixtures originates mainly from aggregated helices of gellan gum molecules for GELL/LM-2 systems whilst the contribution from KGM is more important for GELL/LM-1 and GELL/ND systems. Although the storage modulus of GELL/LM-1 and GELL/ LM-2 as a function of mixing ratio showed a maximum, it is concluded that KGM inhibit the formation of the ordered structure of gellan gum.

A mixed system composed of different molecular weights konjac glucomannan and κ-carrageenan. II. Molecular weight dependence of viscoelasticity and thermal properties

Rheological and thermal properties for 5:5 mixed gels of konjac glucomannan (KGM) with five different molecular weights and κ-carrageenan (CAR) were studied. Molecular weight distribution was analysed by gel permeation chromatography (GPC). Dynamic and static viscoelastic moduli for the mixed gels were much smaller than those for gels of CAR alone and became larger with increasing molecular weight of KGM. Gel—sol transition for mixed systems with total gum content 1.5% w/w occurred at the temperatures between the transition temperature for 0.75% w/w CAR and that for 1.5% w/w CAR. Gel-to-sol and sol-to-gel transition temperatures for the mixed systems were slightly increased with increasing molecular weight of KGM in the mixture observed in differential scanning calorimetry (DSC). One exothermic peak was observed in cooling DSC curves, while two endothermic peaks appeared in heating DSC curves for 5:5 mixed systems. The intensity of the lower peak increased with increasing KGM proportion in the mixture and with increasing molecular weight of KGM at a given mixing ratio. This fact confirms that the mixed gels involve two crystalline regions: some are made by CAR molecules in the helical structure and others are formed by the interaction between helical CAR and KGM. On heating, the latter is broken at a lower temperature and then the former melts at a higher temperature. The junction zone formed by CAR and KGM is easily made in a gel containing KGM with a higher molecular weight or in a higher ratio to CAR, or a system of lower total gum content.

Calorimetry of apolipoprotein-A1 binding to phosphatidylcholine-triolein-cholesterol emulsions

The thermotropic properties of triolein-rich, low-cholesterol dipalmitoyl phosphatidylcholine (DPPC) emulsion particles with well-defined chemical compositions (approximately 88% triolein, 1% cholesterol, 11% diacyl phosphatidylcholine) and particle size distributions (mean diameter, approximately 1000-1100 A) were studied in the absence and presence of apolipoprotein-A1 by a combination of differential scanning and titration calorimetry. The results are compared to egg yolk PC emulsions of similar composition and size. Isothermal titration calorimetry at 30 degrees C was used to saturate the emulsion surface with apo-A1 and rapidly quantitate the binding constants (affinity Ka = 11.1 +/- 3.5 x 10(6) M-1 and capacity N = 1.0 +/- 0.09 apo-A1 per 1000 DPPC) and heats of binding (enthalpy H = -940 +/- 35 kcal mol-1 apo-A1 or -0.92 +/- 0.12 kcal mol-1 DPPC). The entropy of association is -3070 cal deg-1 mol-1 protein or -3 cal deg-1 mol-1 DPPC. Without protein on the surface, the differential scanning calorimetry heating curve of the emulsion showed three endothermic transitions at 24.3 degrees C, 33.0 degrees C, and 40.0 degrees C with a combined enthalpy of 1.53 +/- 0.2 kcal mol-1 DPPC. With apo-A1 on the surface, the heating curve showed the three transitions more clearly, in particular, the second transition became more prominent by significant increases in both the calorimetric and Van't Hoff enthalpies. The combined enthalpy was 2.70 +/- 0.12 kcal mol-1 DPPC and remained constant upon repeated heating and cooling. Indicating that the newly formed DPPC emulsion-Apo-A1 complex is thermally reversible during calorimetry. Thus there is an increase in delta H of 1.17 kcal mol-1 DPPC after apo-A1 is bound, which is roughly balanced by the heat released during binding (-0.92 kcal) of apo-A1. The melting entropy increase, +3.8 cal deg-1 mol-1 DPPC of the three transitions after apo-A1 binds, also roughly balances the entropy (-3 cal deg-1 mol-1 DPPC) of association of apo-A1. These changes indicate that apo-A1 increases the amount of ordered gel-like phase on the surface of DPPC emulsions when added at 30 degrees C. From the stoichiometry of the emulsions we calculate that the mean area of DPPC at the triolein/DPPC interface is 54.5 A2 at 41 degrees C and 54.2 A2 at 30 degrees C. The binding of apo-A1 at 30 degrees C to the emulsion reduces the surface area per DPPC molecule from 54.2 A2 to 50.8 A2. At 30 degrees apo-A1 binds with high affinity and low capacity to the surface of DPPC emulsions and increases the packing density of the lipid domain to which it binds. Apo-A1 was also titrated onto DPPC emulsions at 45 degrees C. This temperature is above the gel liquid crystal transition. No heat was released or adsorbed. Furthermore, egg yolk phosphatidylcholine emulsions of nearly identical composition were also titrated at 30 degrees C with apo-A1 and were euthermic. Association constants were previously measured using a classical centrifugation assay and were used to calculate the entropy of apo-A1 binding (+28 cal deg-1 mol-1 apo-A1). This value indicates that apo-A1 binding to a fluid surface like egg yolk phosphatidylcholine or probably DPPC at 45 degrees C is hydrophobic and is consistent with hydrocarbon lipid or protein moities coming together and excluding water. Thus the binding of apo-A1 to partly crystalline surfaces is entropically negative and increases the order of the already partly ordered phases, whereas binding to liquid surfaces is mainly an entropically driven hydrophobic process.

Effects of Sodium Chloride and Calcium Chloride on the Interaction between Gellan Gum and Konjac Glucomannan

Interaction between gellan gum (GELL) and konjac glucomannan (KGM) with and without sodium chloride and calcium chloride has been studied using dynamic viscoelastic measurements and differential scanning calorimetry (DSC). With progressive addition of NaCl, storage shear modulus G‘ and loss shear modulus G‘‘ for mixtures increased, and not only the helix−coil transition temperature of GELL molecules in mixtures but also the sol-gel transition temperature for mixtures shifted to higher temperatures with increasing concentration of salts. In the presence of CaCl2, G‘ increased up to a certain concentration and then decreased with increasing concentration of salts, although the helix−coil transition temperature of GELL molecules in mixtures monotonically increased with increasing concentration of salts. The exothermic and endothermic peak temperatures for mixtures in the cooling and heating DSC curves shifted to higher temperatures with increasing concentration of both NaCl and CaCl2. In the presence of CaCl2, the exothermic enthalpies for mixtures showed a maximum at a certain concentration and then decreased with increasing concentration of CaCl2. KGM is attached to the surface of large aggregates of GELL helices, and since cations promote GELL self-aggregation by a screening effect, the synergistic interaction between GELL and KGM was promoted with increasing concentration of salts. However, sufficient CaCl2 formed various aggregates of GELL helices with different thermal stabilities, so that the phase separation in GELL/KGM mixtures was promoted in the presence of sufficient CaCl2. Keywords: Gellan gum; konjac glucomannan; rheology; differential scanning calorimetry; helix−coil transition; sol−gel transition; salts

Solid fat content determination: Comparison between pNMR and DSC techniques

Differential Scanning Calorimetry (DSC) has been one of the most popular thermal analysis techniques during the last two decades. The present work shows the utilization of DSC on the determination of solid fat content in fats. Solid fat content values were calculated through the partial areas of DSC heating curves. In a comparison with pulse NMR values, DSC values were generally higher than the pNMR ones. Differences between samples and the techniques themselves seems to be responsible for this fact. DSC is a fast technique on solid fat content, characteristic temperatures and phase transitions enthalpies determinations besides giving a characteristic curve of each sample.

Gel—sol transition in gellan gum solutions. II. DSC studies on the effects of salts

The thermal properties of sodium form gellan gum solutions with and without sodium chloride, potassium chloride, calcium chloride and magnesium chloride were studied by differential scanning calorimetry (DSC). The DSC cooling or heating curves for 1% gellan gum solutions without salt showed a single exothermic or endothermic peak at ~30°C. DSC cooling curves showed a single exothermic peak, with the setting temperature ( Ts) shifting to progressively higher temperatures with increasing concentration of the added NaCl or KCl. At low concentrations of NaCl or KCl, DSC heating curves showed a single endothermic peak; however with more addition of salt the endothermic peak gradually developed a bimodal character and eventually split into more than two distinct peaks. The onset of detectable splitting occurred at a high salt concentration which coincided with that at which elastic gels are formed at even a higher temperature as was observed by viscoelastic measurements. With a sufficient addition of monovalent cations the endothermic curve became again a single peak shifting to higher temperatures. In the presence of divalent cations, although Ts shifted to higher temperatures with increasing concentration of added CaCl 2 or MgCl 2, the melting temperature ( Tm) in heating DSC curves shifted to higher temperatures (up to a certain temperature) and then shifted to lower temperatures with increasing concentration of salt. With increasing concentration of CaCl 2 or MgCl 2, the exothermic and endothermic enthalpies estimated for a main peak increased up to a certain salt concentration and then decreased; however many other peaks were observed at higher temperatures. The endothermic peaks for gels with excessive divalent cations were too broad to be resolved from the baseline; in contrast the exothermic peaks were much sharper and readily recognized. In comparing thermal properties with rheological properties, gellan gum solutions with excessive divalent cations form firm gels on cooling to below the setting temperature, and then it was difficult to remelt them. This was quite different from the behaviour of thermoreversible gels formed in the presence of monovalent cations. It seems that the mechanism of gel formation in gellan gum with divalent cations is markedly different from that with monovalent cations.

Structure and thermotropic properties of 1-stearoyl-2-acetyl-phosphatidylcholine bilayer membranes

The structural and thermotropic properties of 1-stearoyl-2-acetyl-phosphatidylcholine (C(18):C(2)-PC) were studied as a function of hydration. A combination of differential scanning calorimetry and x-ray diffraction techniques have been used to investigate the phase behavior of C(18):C(2)-PC. At low hydration (e.g., 20% H2O), the differential scanning calorimetry heating curve shows a single reversible endothermic transition at 44.6 degrees C with transition enthalpy delta H = 6.4 kcal/mol. The x-ray diffraction pattern at -8 degrees C shows a lamellar structure with a small bilayer periodicity d = 46.3 A and two wide angle reflections at 4.3 and 3.95 A, characteristic of a tilted chain, L beta' bilayer gel structure. Above the main transition temperature, a liquid crystalline L alpha phase is observed with d = 53.3 A. Electron density profiles at 20% hydration suggest that C(18):C(2)-PC forms a fully interdigitated bilayer at -8 degrees C and a noninterdigitated, liquid crystalline phase above its transition temperature (T > Tm). Between 30 and 50% hydration, on heating C(18):C(2)-PC converts from a highly ordered, fully interdigitated gel phase (L beta') to a less ordered, interdigitated gel phase (L beta), which on further heating converts to a noninterdigitated liquid crystalline L alpha phase. However, the fully hydrated (> 60% H2O) C(18):C(2)-PC, after incubation at 0 degrees C, displays three endothermic transitions at 8.9 degrees C (transition I, delta H = 1.6 kcal/mol), 18.0 degrees C (transition II), and 20.1 degrees C (transition III, delta HII+III = 4.8 kcal/mol). X-ray diffraction at -8 degrees C again showed a lamellar gel phase (L beta') with a small periodicity d = 52.3 A. At 14 degrees C a less ordered, lamellar gel phase (L beta) is observed with d = 60.5 A. However, above the transition III, a broad, diffuse reflection is observed at approximately 39 A, consistent with the presence of a micellar phase. The following scheme is proposed for structural changes of fully hydrated C(18):C(2)-PC, occurring with temperature: L beta' (interdigitated)-->L beta (interdigitated)-->L alpha(noninterdigitated)-->Micelles. Thus, at low temperature C(18):C(2)-PC forms a bilayer gel phase (L beta') at all hydrations, whereas above the main transition temperature it forms a bilayer liquid crystalline phase L alpha at low hydrations and a micellar phase at high hydrations (> 60 wt% water).

Effect of potassium ions on the rheological and thermal properties of gellan gum gels

To clarify the effect of cations on the gelling ability of gellan gum, the cation-free gellan gum was prepared by treating with ethylenediaminetetraacetic acid and ethanol. Storage Young's modulus E′ and mechanical loss tangent tan δ were observed for cation-free gellan gum (F1) gels, non-treated gellan gum (F2) gels and potassium added F1 gels. E′ increased and tan δ decreased in the presence of potassium ions indicating the increase of the number of junction zones, and the critical polymer concentration below which no gelation occurs shifted to lower concentrations by adding cations. An endothermic peak accompanying gel-to-sol transition in heating differential scanning calorimetry curves shifted to higher temperatures by adding potassium ions. It was confirmed that potassium ions increase the number of junction zones and make the junction zones more heat resistant. F2 gels containing sodium, calcium and magnesium ions in addition to potassium ions showed multiple broad endotherms in addition to a main endothermic peak indicating that junction zones are made from various aggregations of double helices, with different bonding energies and/or different degrees of rotational freedom of parallel links consisting a single zipper.

Effects of pH and DMSO content on the thermal and rheological properties of high methoxyl pectin-water gels

It was shown that high methoxyl pectin forms a gel with a high elastic modulus in the presence of dimethyl sulfoxide (DMSO). Differential scanning calorimetry (DSC) and dynamic viscoelastic measurements were carried out for high methoxyl pectin-DMSO gels prepared at pH 2·4 and 6·86. Elastic moduli of pectin gels as a function of DMSO content showed a maximum around 0·2 mole fraction (mf) DMSO; however, acidic gels (pH = 2·4) needed less DMSO to form a gel and the elastic moduli for the acidic gels were larger than those for neutral gels (pH = 6·86). Endothermic peaks accompanying the gel-to-sol transition observed in heating DSC curves were sharpest for the acidic pectin gels with 0·168 mf DMSO and for neutral pectin gels with 0·277 mf DMSO. A small amount of DMSO, less than 0·3 mf, promotes gel formation, whilst excessive amounts of DMSO lower the gelling ability. The mean end-to-end distance r m of chains which connect junction zones decreases, and the bonding energy ε increases with increasing DMSO content up to 0·277 mf; then r m increases and ε decreases beyond this DMSO content. Judging from the heat absorbed on forming 1 mole of junction zones, junction zones in acidic gels are more heat-resistant than those in neutral gels. The electrostatic repulsion between carboxylate ions is reduced at lower pHs because of the suppression of the dissociation of carboxylic groups, hence the elastic modulus, the endothermic enthalpy accompanying gel-to-sol transition, and gel-to-sol transition temperature increase with decreasing pH.

Chemical and physical properties of the solid fats in commercial soft margarines

The fats of ten Canadian soft margarines were crystallized from acetone at 15°C to obtain the high‐melting glycerides (HMG). The solid fats in the margarines were extracted with isboutanol at 5°C. X‐ray diffraction showed that the canola margarines were in the β crystal form, the soybean and sunflower‐palm‐palm kernel margarines in the β′ form, while those of canola‐palm and another sunflower‐palm‐palm kernel margarines contained a mixture of β′ and β forms. X‐ray diffraction of the isolated solids showed additional short spacings compared with those of the original margarines. Differential scanning calorimetry heating curves of the solids were compared with those of the HMG. The melting temperatures of the HMG were 10°C higher than the solids. It is suggested that the polymorphic behavior of soft margarines is related to the chemical composition of the HMG and the solids. Solids in margarines can also be provided by interesterification of palm oil products.