Study of thermal properties and heat-induced denaturation and aggregation of soy proteins by modulated differential scanning calorimetry

The thermal properties and heat-induced denaturation and aggregation of soy protein isolates (SPI) were studied using modulated differential scanning calorimetry (MDSC). Reversible and non-reversible heat flow signals were separated from the total heat flow signals in the thermograms. In the non-reversible profiles, two major endothermic peaks (at around 100 and 220 degrees C, respectively) associated with the loss of residual water were identified. In the reversible profiles, an exothermic peak associated with thermal aggregation was observed. Soy proteins denatured to various extents by heat treatments showed different non-reversible and reversible heat flow patterns, especially the exothermic peak. The endothermic or exothermic transition characteristics in both non-reversible and reversible signals were affected by the thermal history of the samples. The enthalpy change of the exothermic (aggregation) peak increased almost linearly with increase in relative humidity (RH) in the range between 8 and 85%. In contrast, the onset temperature of the exotherm decreased progressively with increase in RH. These results suggest that the MDSC technique could be used to study thermal properties and heat-induced denaturation/aggregation of soy proteins at low moisture contents. Associated functional properties such as water holding and hydration property can also be evaluated.     

Influence of fiber on the phase transformations in the starch-water system

High-sensitivity, temperature-controlled DSC measurements at a low heating rate and creation of differential DSC traces scaled with respect to the reference material (completely dehydrated starch or completely dehydrated fiber, or their respective blends) permitted investigation of the influence of fiber on phase transformations in the wheat-starch-water system in the course of thermal gelatinization. Thermal effects associated with water interactions over the temperature range from 283 to 384 K under atmospheric pressure were determined. These thermal effects and previous structural studies permit us to make the following observations: (1) The main endothermic transition associated with melting of the crystalline part of the starch granule followed by a helix-coil transition in amylopectin occurs over the temperature range 319-333 K independent of the water and fiber contents. Adding fiber causes that transition to disappear both in the native blends and in water suspensions at low water contents. After adding more water and heating, recrystallization is observed and the transition reappears. (2) The fiber content has practically no influence on the slow exothermic transformation, which follows melting and helix-coil transition in amylopectin, proving that the slow transformation has a specific chemical character. In this reaction, the free ends of the unwound helices of amylopectin reassociate with parts of amylopectin molecules other than their original helix duplex partner, forming physical junctions and creating more general amorphous hydrogen bonded associations. (3) The high-temperature transition and small, but reproducible, distortions on the peaks of the main endothermic transition for water contents near 70-80 wt % are associated with smectic and nematic transitions, respectively. These are significantly influenced by the fiber content; higher fiber content causes an almost complete disappearance of these transitions. (4) The slow exothermic effect appearing almost from the very beginning of the heating in the starch-water system, associated with softening and uptake of water in the amorphous growth rings of the starch granule, is significantly hindered by added fiber.     

Effect of Long-Term Storage on Water Status and Physicochemical Properties of Nutritionally Enhanced Tortillas

The effect of functional ingredients (carrot juice, whole soy flour, and whole kamut flour) and storage (180 days) on physicochemical properties (texture and amylopectin recrystallization) and water status (moisture content, water activity, ice melting peak thermal properties, and proton nuclear magnetic resonance (¹H NMR) mobility) of tortillas has been studied. Different formulations significantly changed the parameters studied during storage resulting in larger changes than in the standard formulation (STD) that, therefore, may be considered the most stable product. The properties of whole kamut tortillas were very similar to those of standard sample while the formulation that contained carrot juice lead to an increased system rigidity observable both at macroscopic (textural properties), macromolecular (significantly reduced), and molecular (¹H FID) levels. A decrease of moisture content, water activity, endothermic transition ~0 °C, and an increase of ¹H NMR mobility (¹H T₂ pop A and C) were observed in soy-containing products [(soy enriched (SOY) and carrot, soy, and kamut (CSK)]. SOY and CSK had very low water activity, presented the highest ¹H NMR molecular mobility and underwent the most marked changes during storage suggesting that water activity cannot be taken as a sole indicator of food stability as very important modifications occurred in tortillas at molecular level.     

Comparison of potato amylopectin starches and potato starches — influence of year and variety

Starches from three potato varieties and their respective transformants producing amylopectin starch were studied over a period of 3 years. The gelatinisation, swelling and dispersion properties were studied using differential scanning calorimetry (DSC), X-ray diffraction, swelling capacity measurements and a Brabender Viscograph.

The potato amylopectin starches (PAP) exhibited higher endothermic temperatures as well as higher enthalpies than the normal potato starches (NPS). PAP samples gave rise to an exceptionally sharp viscosity peak during gelatinisation and a relatively low increase in viscosity on cooling. Swelling capacity measurements showed that PAP granules swelled more rapidly, and that the dispersion of the swollen granules occurred at a lower temperature (85°C). Analysis of variance (ANOVA) also revealed that the year influenced the DSC results, and that both year and variety affect some of the Brabender parameters. Furthermore, the PAP and NPS samples were subjected to heat–moisture treatment at three different moisture levels, and the Brabender viscosity properties were studied.     

Thermal properties of polysaccharides at low moisture: 1—An endothermic melting process and water-carbohydrate interactions

The thermal properties of a broad range of polysaccharides containing 5–25% w/w water have been studied by differential scanning calorimetry and dynamic mechanical thermal analysis (DMTA). Following room temperature conditioning, an endothermic event accompanied by material softening is observed at 45–80°C for all samples except those above their glass transition temperature. The temperature of the event is determined by thermal history and is apparently independent of polymer type or moisture content. The associated enthalpy increases with water content. Variable frequency DMTA analysis suggests a structural melting event rather than a relaxation process. The endothermic event is recovered over the days timescale after heating, and can be annealed to higher temperatures with increasing holding temperature.

Results are interpreted in terms of a dynamic hydration model in which specific energetic water-carbohydrate interactions occur but with a lifetime defined by their local effective microviscosity. The observation of the endotherm below glass transition temperatures suggests that in aqueous polysaccharide glasses, enthalpic structures involving the solvent can be made and broken.     

Effects of annealing on the polymorphic structure of starches from sweet potatoes (Ayamurasaki and Sunnyred cultivars) grown at various soil temperatures

Starches extracted from the sweet potato cultivars Sunnyred and Ayamurasaki grown at 15 or 33 degrees C (soil temperature) were annealed in excess water (3 mg starch/mL water) for different times (1, 4, 8 or 10h) at the temperatures 2-3 degrees K below the onset melting temperature. The structures of annealed starches, as well as their gelatinisation (melting) properties, were studied using high-sensitivity differential scanning calorimetry (HSDSC). In excess water, the single endothermic peak shifted to higher temperatures, while the melting (gelatinisation) enthalpy changed only very slightly, if any. The elevation of gelatinisation temperature was associated with increasing order/thickness of the crystalline lamellae. The only DSC endotherm identified in 0.6 M KCl for Sunnyred starch grown at 33 degrees C was attributed to A-type polymorphic structure. The multiple endothermic forms observed by DSC performed in 0.6M KCl for annealed starches from both cultivars grown at 15 degrees C provided evidence of a complex C-type (A- plus B-type) polymorphic structure of crystalline lamellae. The A:B-ratio of two polymorphic forms increased upon annealing due to partial transformation of B- to A-polymorph, which was time dependent. Long heating periods facilitated the maximal transformation of B- to A-polymorph associated with limited A:B ratio.     

Influence of amylopectin structure and degree of phosphorylation on the molecular composition of potato starch lintners

Morphology, molecular structure, and thermal properties of potato starch granules with low to high phosphate content were studied as an effect of mild acid hydrolysis (lintnerization) to 80% solubilization at two temperatures (25 and 45°C). Light microscopy showed that the lintners contained apparently intact granules, which disintegrated into fragments upon dehydration. Transmission electron microscopy of rehydrated lintners revealed lacy networks of smaller subunits. The molecular composition of the lintners suggested that they largely consisted of remnants of crystalline lamellae. When lintnerization was performed at 45°C, the lintners contained more of branched dextrins compared to 25°C in both low and intermediate phosphate‐containing samples. High‐phosphate‐containing starch was, however, unaffected by temperature and this was probably due to an altered amylopectin structure rather than the phosphate content. After lintnerization, the melting endotherms were broad with decreased onset and increased peak melting temperatures. The relative crystallinity was lower in lintners prepared at 45°C. A hypothesis that combines the kinetics of lintnerization with the molecular and thermal characteristics of the lintners is presented. © 2013 Wiley Periodicals, Inc. Biopolymers 101: 257–271, 2014.     

Thermal and rheological properties of nixtamalized maize starch

The effect of nixtamalization process on thermal and rheological characteristics of corn starch was studied. Starch of raw sample had higher gelatinization temperature than its raw counterpart, because, the Ca(2+) ions stabilize starch structure of nixtamalized sample; however, the enthalpy values were not different in both samples. The temperature of the phase transition of the retrograded starches (raw and nixtamalized) were not different at the storage times assessed, but the enthalpy values of the above mentioned transition was different, indicating a lower reorganization of the starch structure in the nixtamalized sample. The viscoamylographic profile showed differences between both starches, since raw starch had higher peak viscosity than the nixtamalized sample due to partial gelatinization of some granules during this heat treatment. Rheological test showed that at low temperature (25 degrees C) the raw and nixtamalized starches presented different behaviour; however, the elastic characteristic was more important in the starch gel structure. The nixtamalization process produced changes in thermal and rheological characteristics becoming important in those products elaborated from nixtamalized maize.     

Glass transition temperatures of cassava starch-whey protein concentrate systems at low and intermediate water content

Glass transition temperatures of cassava starch (CS)-whey protein concentrate (WPC) blends were determined by means of differential scanning calorimetry (DSC) in a water content range of 8-20% (dry basis, d.b.). Water equilibration in the samples was carried out by storing them at room temperature (25 °C) during four weeks. Physical aging and phase segregation were observed in some samples after this storage period depending on the water content. Both, first DSC heating scans and tan δ curves of CS-WPC blends with intermediate water content (10-18%), showed two endothermic thermal events. The first one appeared at around 60 °C and was independent of water content. The second one was detected at higher temperatures and moved towards the low-temperature peak as the water content increased. The results can be explained by a phase segregation process that can take place when the samples are conditioned below their glass transition temperatures. The Gordon-Taylor equation described well the plasticizing effect of water on the blends. WPC was also found to decrease the glass transition temperature, at constant water content, an effect attributed to additional water produced during browning reactions in the blends.     

Structural characteristics of low-glycemic response rice starch produced by citric acid treatment

The structural characteristics of citric acid-treated rice starch, which was subjected to autoclaving and stored at a high temperature under acidic conditions, were investigated by high performance anion-exchange chromatography, multi-angle laser-light scattering, X-ray diffraction, differential scanning calorimetry (DSC), nuclear magnetic resonance, and scanning electron microscopy. The citric acid-treated rice starch contained chains of different lengths (DP 7−70) and had an A + V type polymorphism. The DSC thermogram of acid-treated rice starch showed a broad endothermic peak, which indicated that the structure of the acid-treated rice starch contained a number of double helices with various melting temperatures. Microscopic observation showed that the internal structure of the acid-treated starch displayed more or less spherical lumps that could be composed of short chains and amylose–lipid complex. The digestive properties of acid-treated rice starch were altered by heat processing such as cooking and autoclaving because crystalline regions were converted to amorphous regions or lamellae.     

The effect of mutant genes at the r, rb, rug3, rug4, rug5 and lam loci on the granular structure and physico-chemical properties of pea seed starch

The granular structure and gelatinisation properties of starches from a range of pea seed mutants were studied. Genes which affect the supply of substrate during starch synthesis (rb, rug3, rug4) affected the total crystallinity and possibly increased the content of A polymorphs in the starch. Conversely, genes directly affecting the synthesis of starch polymers (r, rug5, lam) increased the content of B polymorphs, but had a minimal effect on total crystallinity. During gelatinisation, starches from the rb, rug3, rug4 and lam mutants had narrow endothermic peaks which were similar to starch from the wild-type, although all the starches had different peak temperatures and enthalpy changes. Starches from r and rug5 mutants were very different to all other starches, having a very wide transition during gelatinisation. In addition, the amylopectin in starch from these mutants had altered chain lengths for those parts of the polymer which form the ordered structures in the granule.     

Simultaneous and in situ analysis of thermal and volumetric properties of starch gelatinization over wide pressure and temperature ranges

A method for simultaneous and in situ analysis of thermal and volumetric properties of starch gelatinization from 0.1 to 100 MPa and from 283 to 430 K is described. The temperature of a very sensitive calorimetric detector containing a starch-water emulsion at a selected pressure is programmed to rise at a slow rate; volume variations are performed automatically to keep the selected pressure constant while the heat exchange rate and the volume are recorded. The method is demonstrated with a novel investigation of pressure effects on a sequence of three phase transitions in an aqueous emulsion of wheat starch (56 wt % water). The volume changes during the main endothermic transition (M), associated with melting of the crystalline part of the starch granules and a helix-coil transformation in amylopectin, but also with an important swelling, were separated into a volume increase associated with swelling and a volume decrease associated with the transition itself. Thermodynamic parameters for this transition together with their pressure dependencies have been obtained from four independent experiments at each pressure. The data are thermodynamically consistent, but are poorly described by the Clapeyron equation. The negative volume change of the slow exothermic transition (A) appearing just after the main endothermic transition (M) is small, spread out over a wide temperature interval, and occurs at higher temperatures with increasing pressures. This transition is probably associated with reassociation of the unwound helixes of amylopectin with parts of amylopectin molecules other than their original helix duplex partner. The positive volume change of the high-temperature, endothermic transition (N) with a small enthalpy change is probably associated with a nematic-isotropic transformation ending the formation of a homogeneous SOL phase (in the sense of Flory), and is also pushed to higher temperatures with increasing pressures. Knowledge of the state of wheat starch as a function of pressure and temperature is important in extruder processing. The data also provide a basis for the elliptic phase diagram for starch gelatinization. The method is easily adapted to determine similar data for other macromolecular materials.     

Enthalpy relaxation of bovine serum albumin and implications for its storage in the glassy state

Two endothermic peaks could be observed for five commercial samples of bovine serum albumin (BSA). The smaller peak observed by differential scanning calorimetry (DSC) corresponded to enthalpy relaxation. This peak was followed on storage of BSA, in its glassy state, after it had been heated above its denaturation temperature. Enthalpy and peak temperature increased with duration of storage. On storage for one week at 60 degrees C, a sample at 8.3% moisture showed a peak at 100 degrees C with an energy value of approximately 2 J per g protein. BSA samples were heated within the DSC sufficiently to eliminate the lower enthalpy peak but without altering the denaturation enthotherm. The amount of physical aging shown by these BSA samples was similar to that of the heat-denatured samples. It was concluded that the heating endotherms of dry BSA reflect both the storage and thermal history of the sample. Possible implications of the enthalpy relaxation of BSA on the behavior of this important protein are considered.     

Thermal behavior of fowl feather keratin

Differential scanning calorimetry (DSC) was applied to elucidate the thermal behavior of fowl feather keratins (barbs, rachis, and calamus) with different morphological features. The DSC curves exhibited a clear and relatively large endothermic peak at about 110-160 degrees C in the wet condition. A considerable decrease in transition temperature with urea and its helical structure content estimated by Fourier transform infrared spectroscopy (FT-IR), and the disappearance of one of the diffraction peaks with heating at 160 degrees C for 30 min, indicated that DSC could be used to evaluate the thermal behavior of keratin. Barbs showed a lower denaturation temperature than rachis and calamus. The pulverized samples showed a slightly higher denaturation temperature than the native samples. In the dry condition, thermal transition occurred in a markedly higher temperature region close to 170-200 degrees C. It is hence concluded that fowl feather keratins have very high thermal stability, and that the elimination of water brings about even greater thermal stability.     

Effect of microwave radiation on physico-chemical properties and structure of potato and tapioca starches

The time-temperature profiles of selected starch-water systems subjected to microwave processing were established and the effect of microwave radiation on the physico-chemical properties and structure of potato and tapioca starches was studied. The experimental starch samples were examined by the Brabender rheological method, light microscopy, scanning electron microscopy and X-ray diffractometry. Microwave radiation was evidenced to affect the temperature and moisture contents of the experimental starches, with a strong correlation between the moisture content and the rate of temperature rise. An isothermal transformation was revealed with the samples of moisture contents over 20%, causing a rise in the gelatinisation temperature of the starch and a drop in its solubility in water. The most pronounced change was observed in the case of potato starch: its crystal structure changed from type B to type A. The tapioca starch underwent similar but less marked changes.     

Calorimetric investigations of the structural stability and interactions of colicin B domains in aqueous solution and in the presence of phospholipid bilayers

The effects of pH and temperature on the stability of interdomain interactions of colicin B have been studied by differential-scanning calorimetry, circular dichroism, and fluorescence spectroscopy. The calorimetric properties were compared with those of the isolated pore-forming fragment. The unfolding profile of the full-length toxin is consistent with two endothermic transitions. Whereas peak A (T(m) = 55 degrees C) most likely corresponds to the receptor/translocation domain, peak B (T(m) = 59 degrees C) is associated with the pore-forming domain. By lowering the pH from 7 to 3.5, the transition temperature of peaks A and B are reduced by 25 and 18 degrees C, respectively, due to proton exchange upon denaturation. The isolated pore-forming fragment unfolds at much higher temperatures (T(m) = 65 degrees C) and is stable throughout a wide pH range, indicating that intramolecular interactions between the different colicin B domains result in a less stable protein conformation. In aqueous solution circular dichroism spectra have been used to estimate the content of helical secondary structure of colicin B ( approximately 40%) or its pore-forming fragment ( approximately 80%). Upon heating, the ellipticities at 222 nm strongly decrease at the transition temperature. In the presence of lipid vesicles the differential-scanning calorimetry profiles of the pore-forming fragment exhibit a low heat of transition multicomponent structure. The heat of transition of membrane-associated colicin B (T(m) = 54 degrees C at pH 3.5) is reduced and its secondary structure is conserved even at intermediate temperatures indicating incomplete unfolding due to strong protein-lipid interactions.     

Extrusion of pectin/starch blends plasticized with glycerol

The microstructural and thermal dynamic mechanical properties of extruded pectin/starch/glycerol (PSG) edible and biodegradable films were measured by scanning electron microscopy (SEM) and thermal dynamic mechanical analysis (TDMA). SEM revealed that the temperature profile (TP) in the extruder and the amount of water present during extrusion could be used to control the degree to which the starch was gelatinized. TDMA revealed that moisture and TP during extrusion and by inference the amount of starch gelatinization had little effect on the mechanical properties of PSG films. Furthermore, TDMA revealed that PSG films underwent a glass transition commencing at about −50°C and two other thermal transitions above room temperature. Finally, it was concluded that the properties of extruded PSG films were comparable to those cast from solution.     

Legume-Fortified Pasta. Impact of Drying and Precooking Treatments on Pasta Structure and Inherent In Vitro Starch Digestibility

The low glycaemic index of pasta can be attributed to its specific structure. A change in pasta structure can therefore lead to a change in its starch digestibility. The use of drastic drying conditions or the addition of non-traditional ingredients to durum wheat pasta was already demonstrated to affect its structure, leading to a modification of its starch digestibility. However, the combining effect of using different technological treatments and different raw materials on pasta structure and consequences on its starch digestibility pasta are still unknown. The objective of this work was therefore to determine the impact of different technological treatments on the structure and the in vitro starch digestibility (i.e. rapidly available glucose value) of legume-fortified pasta. Legume-fortified pasta was prepared from 65% of durum wheat semolina and 35% of split pea or faba bean flour. Four different technological treatments were applied: drying at low temperature of 55 °C (LT), drying with the application of a very high temperature of 90 °C at low moisture content (VHT.LM), lyophilisation, and precooking followed by LT drying. Legume-fortified pasta dried at LT served as a reference. Lyophilisation induced a higher starch digestibility that could be attributed to the high porosity of pasta and the weakness of its protein network. In contrast, VHT.LM drying and precooking treatment led to a lower in vitro starch digestibility, probably as a result of the strengthening of the protein network at a macromolecular level, protecting starch from enzymatic attack.     

Alkaliviscogram and Other Properties of Starch of Tropical Rice

In the alkaliviscogram of starch of 26 nonwaxy rices grown in the tropics, gelatinization normality correlated positively with final gelatinization temperature (BEPT) of starch (r=0.969^**) and negatively with alkali spreading value of milled rice (r= ?0.931^**). Peak viscosity was not linearly related to amylose content. Among samples of rice starch having a high amylose (>28%) content, peak viscosity was correlated with the gel consistency of starch (r=?0.690^**) and of milled rice (r=?0.644^**) (n = 18). These high-amylose starches showed the widest variation in peak viscosity. Amylose content, and gel consistency were inherited from the same parent in all nine varieties and lines studied, whereas peak viscosity, gelatinization normality and the final BEPT were inherited from either parent. The starch of five waxy rices showed higher peak viscosities even at a concentration of 1.8% as compared with a 2.0% nonwaxy rice starch.     

差示扫描量热法直接测定沙冬青抗冻蛋白的热滞效应

用差示扫描量热法直接测定了从沙冬青中提取的一种抗冻蛋白（ＡＦＰ）组分的低温热行为。结果表明,该组分的低温热行为远较文献报道的各种抗冻蛋白复杂。在降、升温过程中,在低和高温侧都给出两个放或吸热峰,两个峰表现出相互独立而又相互依存的热滞行为。低温峰的热滞活性远高于高温岭。我们认为,这种ＡＦＰ分子对水及冰晶很可能有两种不同的相互作用和影响。