Glass transition and enthalpy relaxation of amorphous lactose glass

The glass transition temperature, T(g), and enthalpy relaxation of amorphous lactose glass were investigated by differential scanning calorimetry (DSC) for isothermal aging periods at various temperatures (25, 60, 75, and 90 degrees C) below T(g). Both T(g) and enthalpy relaxation were found to increase with increasing aging time and temperature. The enthalpy relaxation increased approximately exponentially with aging time at a temperature (90 degrees C) close to T(g) (102 degrees C). There was no significant change observed in the enthalpy relaxation around room temperature (25 degrees C) over an aging period of 1month. The Kohlrausch-Williams-Watts (KWW) model was able to fit the experimental enthalpy relaxation data well. The relaxation distribution parameter (beta) was determined to be in the range 0.81-0.89. The enthalpy relaxation time constant (tau) increased with decreasing aging temperature. The observed enthalpy relaxation data showed that molecular mobility in amorphous lactose glass was higher at temperatures closer to T(g). Lactose glass was stable for a long time at 25 degrees C. These findings should be helpful for improving the processing and storage stability of amorphous lactose and lactose containing food and pharmaceutical products.     

Glass transition and time-dependent crystallization behavior of dehydration bioprotectant sugars

It has been suggested that the crystallization of a sugar hydrate can provide additional desiccation by removing water from the amorphous phase, thereby increasing the glass transition temperature (T_g). However, present experiments demonstrated that in single sugar systems, if relative humidity is enough for sugar crystallization, the amorphous phase will have a short life. In the conditions of the present experiments, more than 75% of amorphous phase crystallized in less than one month. The good performance of sugars that form hydrated crystals (trehalose and raffinose) as bioprotectants in dehydrated systems is related to the high amount of water needed to form crystals, but not to the decreased water content or increased T_g of the amorphous phase. The latter effect is only temporary, and presumably shorter than the expected shelf life of pharmaceuticals or food ingredients, and is related to thermodynamic reasons: if there is enough water for the crystal to form, it will readily form.     

Protein inactivation in amorphous sucrose and trehalose matrices: effects of phase separation and crystallization

Trehalose is the most effective carbohydrate in preserving the structure and function of biological systems during dehydration and subsequent storage. We have studied the kinetics of protein inactivation in amorphous glucose/sucrose (1:10, w/w) and glucose/trehalose (1:10, w/w) systems, and examined the relationship between protein preservation, phase separation and crystallization during dry storage. The glucose/trehalose system preserved glucose-6-phosphate dehydrogenase better than did the glucose/sucrose system with the same glass transition temperature (T_g). The Williams-Landel-Ferry kinetic analysis indicated that the superiority of the glucose/trehalose system over the glucose/sucrose system was possibly associated with a low free volume and a low free volume expansion at temperatures above the T_g. Phase separation and crystallization during storage were studied using differential scanning calorimetry, and three separate domains were identified in stored samples (i.e., sugar crystals, glucose-rich and disaccharide-rich amorphous domains). Phase separation and crystallization were significantly retarded in the glucose/trehalose system. Our data suggest that the superior stability of the trehalose system is associated with several properties of the trehalose glass, including low free volume, restricted molecular mobility and the ability to resist phase separation and crystallization during storage.     

Trehalose–Water–Salt Interactions Related to the Stability of β-Galactosidase in Supercooled Media

The conservation of desirable properties in foods and ingredients is often based on the maintenance of the amorphous metastable properties of the systems. Enzymes may be stabilized by drying in saccharide matrices, but a second excipient is generally required to improve sugar protective effects. The effect of electrolytes on the thermophysical properties of sugar systems is of special interest because of their major influence on water structure and their possible interactions with biomolecules. Salts affect the kinetics of very important changes in sugar systems such as crystallization. The purpose of the present work was to analyze fungal β-galactosidase stability in supercooled systems of trehalose-containing electrolytes (water soluble acetates, citrates, and chlorides of magnesium and potassium). The degree of sugar crystallization was also related to enzyme stability. Potassium citrate and acetate improved enzyme stability during freeze-drying and thermal treatment of samples at water activity (a _w) of 0.22. However, trehalose crystallization inhibition at a _w = 0.43 (which was about 50–60%, related to the system without salt) impaired enzyme protection. Certain salts may act retarding sugar crystallization, but in the presence of salts, trehalose crystallization is even more critical because the enzyme is confined in a highly salt-concentrated region. Thus, crystallization inhibition by sugar–salt combinations should be carefully conducted. Santagapita, Research Fellow, CONICET, Argentina. Buera, Member of CONICET, Argentina. An erratum to this article can be found at     

Influence of low-temperature nucleation on the crystallization process of poly(L-lactide)

The crystallization kinetics of poly(l-lactide), PLLA, is slow enough to allow a quasi-amorphous polymer to be obtained at low temperature simply by quenching from the melt. The PLLA crystallization process was followed by differential scanning calorimetry and optical microscopy after nucleation isothermal treatments at temperatures just below (53 degrees C) and just above (73 degrees C) the glass transition temperature. The crystallization exotherm shown in the heating thermograms shifts toward lower temperatures as the annealing time at 73 degrees C increases. The same effect is shown to a lesser extent when the sample nucleates at 53 degrees C, showing the ability to nucleate in the glassy state, already shown in other polymers. The shape of the DSC thermograms is modeled by using Avrami's theory and allows an estimation of the number of crystallization germs formed. The results of optical microscopy are converted to thermograms by evaluating the average gray level of the image recorded in transmission mode as a function of temperature and calculating its temperature derivative. The shape of such optical thermograms is quite similar to that of the DSC traces but shows some peculiarities after long nucleation treatments. Atomic force microscopy was used to analyze the crystal morphology and is an additional proof of the effect of nucleation in the glassy state. The crystalline morphology observed in samples crystallized after nucleation in the glassy state is qualitatively different from that of samples nucleated above the glass transition temperature, and the number of crystals seems to be much greater than what could be expected from the crystallization kinetics.     

FT-NIR Microspectroscopy: A Method for Quantitatively Mapping One-Dimensional Moisture Penetration Into Sugar Glasses

In this methods paper, one-dimensional moisture penetration into thin films of sugar glass exposed to different relative humidity was measured by use of FT-near infrared microspectroscopy working in a spectrum range of wavelength from 1.35 to 2.50 μm. The peak height ratio of characteristic peaks for H—OH (primarily water) and O—H (primarily sugar) bands, respectively, were highly correlated to moisture content for both sucrose and lactose systems and were used for calibration of moisture content. Moisture content along one-dimensional locations was accurately measured using microscopic mapping. Sorption penetration profiles of moisture content vs. penetration depth were obtained. Depending on the sorption time and storage conditions, moisture penetration profiles typically showed a fairly sharp boundary between intact glass in the interior and a high moisture content surface layer, which slowly moved into the sample interior over time. Crystallization was observed to occur at the surface under certain conditions when insufficient inhibitors were present. In these cases, a moisture peak was seen to penetrate into the glass. Microspectroscopic mapping proved to be an excellent technique for studying moisture penetration behavior and kinetics, providing quantitative data for future diffusion modeling. Support source: NIRCGP grant from USDA.     

The State of Aggregation of Casein Affects the Storage Stability of Amorphous Sucrose, Lactose, and Their Mixtures

The effect of the state of aggregation of casein (micellar or non-micellar, as milk protein concentrate [MPC] or sodium caseinate [Na-caseinate], respectively) on water sorption, plasticization, and crystallization of freeze-dried matrices containing sucrose, lactose or their blends were studied. The Guggenheim–Anderson–de Boer (GAB) equation satisfactorily fitted to the water sorption data. In most cases, sugar crystallization—studied by water sorption behavior, x-ray diffraction, and non-isothermal calorimetry—occurred significantly slower in systems containing Na-caseinate compared to MPC. The type of casein did not affect the temperature range where the glass transition (T _g) was observed. Sugar/Na-caseinate mixtures showed higher instant crystallization temperatures (up to 20°C) than sugar/MPC mixtures. X-ray diffraction showed that: (a) crystallinity increased with increasing relative vapor pressure (RVP) > 44%; (b) lactose crystallized mainly as α-lactose monohydrate regardless of casein type; and (c) that sucrose crystals predominated the patterns of the sucrose/lactose mixtures. Results suggested that the way proteins organize in space (i.e., aggregation state) affected their interactions with neighboring sugar and water molecules, which led to differences in sugar crystallization behavior. Poster presented at the 4th International Workshop on Water in Food in Brussels March, 2006. Funded by CONACyT (Mexico) and Dippin’ Dots Inc., KY, USA.     

Crystallization and X-ray diffraction of spray-dried and freeze-dried amorphous lactose

Crystallization of spray-dried and freeze-dried amorphous lactose over different relative vapor pressures (RVP) and storage times was studied. Crystallization was observed from increasing peak intensities in X-ray diffraction patterns. Lactose was crystallized in the samples stored at RVP of 44.1% and above in both types of dehydrated powders. The rate of crystallization increased with increasing RVP and storage time. Similar crystallization behavior of both spray-dried and freeze-dried lactose was observed. Lactose crystallized as alpha-lactose monohydrate, anhydrous beta-lactose, and the anhydrous form of alpha- and beta-lactose in a molar ratio of 5:3 and 4:1 in both spray-dried and freeze-dried forms. Peak intensities of X-ray diffraction patterns for anhydrous beta-lactose were decreased, and for alpha-lactose monohydrate increased with increasing storage RVP and time. The crystallization data were successfully modeled using Avrami equation at RVP of 54.5% and above. The crystallization data obtained is helpful in understanding and predicting storage stability of lactose-containing food and pharmaceutical products.     

The glass transition temperature of mixtures of trehalose and hydroxyethyl starch

Although mixtures of HES and sugars are used to preserve cells during freezing or drying, little is known about the glass transition of HES, or how mixtures of HES and sugars vitrify. These difficulties may be due to the polydispersity between HES samples or differences in preparation techniques, as well as problems in measuring the glass transition temperature (T(g)) using differential scanning calorimetry (DSC). In this report, we examine the T(g) of mixtures of HES and trehalose sugar with <1% moisture content using DSC measurements. By extrapolating these measurements to pure HES using the Gordon-Taylor and Fox equations, we were able to estimate the T(g) of our HES sample at 44 degrees C. These results were additionally confirmed by using mixtures of glucose-HES which yielded a similar extrapolated T(g) value. Our approach to estimating the glass transition temperature of HES may be useful in other cases where glass transitions are not easily identified.     

Gelatinization and freeze-concentration effects on recrystallization in corn and potato starch gels

Freeze-concentration of starch gels was controlled by temperature and gelatinization with glucose and lactose. The aim of the study was to evaluate the effects of freezing temperature and gel composition on starch recrystallization behaviour of corn and potato starch gels (water content 70%, w/w) in water or glucose or lactose (10%, w/w) solutions. Starch gels were obtained by heating in differential scanning calorimetry (DSC). Samples of starch gels were frozen at -10 degrees C, -20 degrees C and -30 degrees C for 24h and, after thawing, stored at +2 degrees C for 0, 1, 2, 4 and 8 days. The extent of starch recrystallization was taken from the enthalpy of melting of the recrystallized starch by DSC. Freezing temperatures, glucose, lactose and the origin of the starch affected the recrystallization behaviour greatly. The recrystallization of amorphous starch during storage was enhanced by freeze-concentration of gels at temperatures above T'(m). Molecular mobility was enhanced by unfrozen water and consequently molecular rearrangements for nucleation could take place. Further storage at a higher temperature enhanced the growth and the maturation of crystals. In particular, glucose decreased the T'(m) of the gels and consequently lower freezing temperatures were needed to reduce enhanced recrystallization during storage. Freeze-concentration temperatures also showed a significant effect on the size and the perfection of crystals formed in starch recrystallization.     

Slow molecular mobility in the crystalline and amorphous solid states of pentitols: a study by thermally stimulated depolarisation currents and by differential scanning calorimetry

The molecular mobility of the pentitol isomers (xylitol, adonitol, D-arabitol and L-arabitol) was studied by thermally stimulated depolarisation currents (TSDC) in the crystalline and in the amorphous solid states. Differential scanning calorimetry (DSC) was used to characterise the phase transformations, to detect polymorphism and to analyse the dynamics of the structural relaxation in the glassy state (from the heating rate dependence of the onset temperature of the glass transition signal). The mobility in crystalline xylitol and adonitol displays features that are different compared with crystalline arabitols. No difference of the dynamic behaviour seems to emerge from our results on the primary and secondary relaxations in the amorphous isomeric pentitols. The values of the steepness index or fragility obtained in this work by TSDC and DSC are compared with the values reported in the literature obtained from other experimental techniques, and with values predicted by empirical formulae.     

Crystal growth and solid-state structure of poly(lactide) Stereocopolymers

Solid-state structure and melting behavior for random stereocopolymers of L-lactide with meso-lactide (P(L-LA-co-meso-LA)) with different meso-LA compositions of 0, 2, 4, and 10 mol % were investigated under various isothermal crystallization conditions. The crystalline morphology of P(L-LA-co-meso-LA) samples changed from the spherulitic aggregates to hexagonal lamellae stacking with a rise in crystallization temperature. Under each crystallization condition, P(L-LA-co-meso-LA) samples formed alpha-crystal modifications for homopolymer of L-LA. By using the atomic force microscopy and small-angle X-ray scattering, the stacking structure of lamellar crystals was examined for the isothermally crystallized P(L-LA-co-meso-LA) thin films. The lamellar thickness of P(L-LA-co-meso-LA) ranged from 6.2 to 15.5 nm, and the values increased with crystallization temperature. Melting profiles of crystalline regions were examined by the differential scanning calorimetry (DSC) for the P(L-LA-co-meso-LA) samples. Distinct two melting peaks were detected in the DSC thermograms of several samples. Investigations on the time-dependent changes in lamellar structure and melting temperature of the P(L-LA-co-meso-LA) samples under isothermal crystallization conditions provided the evidence that a small amount of D-lactyl units was trapped in the crystalline regions during early stage of crystallization process under the certain crystallization condition. In addition, it was found that the D-lactyl units trapped in crystalline regions were excluded from crystalline lamellae to form the thermally stable crystals without changes in crystal thickness during further isothermal storage at a crystallization temperature. The equilibrium melting temperature (T(m)0) of P(L-LA-co-meso-LA) samples was estimated by using modified Hoffman-Weeks methods, and the obtained values decreased from 215 to 184 degrees C as the meso-LA composition was increased from 0 to 10 mol %. Furthermore, the crystal growth kinetics of the P(L-LA-co-meso-LA) samples was analyzed by using the secondary nucleation theory. Transitions of crystalline regime both from regime III to regime II and from regime II to regime I were detected for each sample. The transition temperature from regime II to regime I of each of the P(L-LA-co-meso-LA) samples was very close to the temperature region revealed the morphological changes in the crystalline aggregates from the spherulitic aggregates to hexagonal lamellae stacking.     

Effect of polyols on the conformational stability and biological activity of a model protein lysozyme

The purpose of this study was to investigate the stabilizing action of polyols against various protein degradation mechanisms (eg, aggregation, deamidation, oxidation), using a model protein lysozyme. Differential scanning calorimeter (DSC) was used to measure the thermodynamic parameters, mid point transition temperature and calorimetric enthalpy, in order to evaluate conformational stability. Enzyme activity assay was used to corroborate the DSC results. Mannitol, sucrose, lactose, glycerol, and propylene glycol were used as polyols to stabilize lysozyme against aggregation, deamidation, and oxidation. Mannitol was found to stabilize lysozyme against aggregation, sucrose against deamidation both at neutral pH and at acidic pH, and lactose against oxidation. Stabilizers that provided greater conformational stability of lysozyme against various degradation mechanisms also protected specific enzyme activity to a greater extent. It was concluded that DSC and bioassay could be valuable tools for screening stabilizers in protein formulations.     

Protein stability in the amorphous carbohydrate matrix: relevance to anhydrobiosis

The formation of intracellular glass is proposed to be relevant to protein stabilization and survival of anhydrobiotic organisms in the dry state. The stability of proteins in the amorphous carbohydrate matrix and its relevance to seed survival have been investigated in the present study. Glucose-6-phosphate dehydrogenase (G6PDH) was preserved in the amorphous glucose/sucrose (1:10, w/w) matrix by freeze-drying. The stability of freeze-dried G6PDH was examined at temperatures above and below the glass transition temperature (T_g). The rate of G6PDH inactivation in the amorphous carbohydrate matrix deviated significantly from the Arrhenius kinetics, and conformed to the Williams-Landel-Ferry (WLF) relationship. The temperature dependence of G6PDH inactivation in two sets of samples with different T_g values was compared. Identical temperature dependence of G6PDH inactivation was observed after temperature normalization by (T?T_g). Seed survival of Vigna radiata Wilczek (mung bean) showed a similar WLF kinetics at storage temperatures T≥T_g. In situ protein stability in mung bean embryonic axes was studied using differential scanning calorimetry (DSC). Thermal stability of seed proteins exhibited a strong dependence on the T_g of intracellular glass. These results indicate an important role of the glassy state in protein stabilization. Our data suggest an association between protein stability in intracellular glass and seed survival during storage.     

Mechanically Induced Amorphization of Drugs: A Study of the Thermal Behavior of Cryomilled Compounds

The purpose of this work was to determine what aspect of the milled compound influences its thermal profile. For this, six different compounds with different properties were chosen and cryomilled for different times to get an amorphous solid. Differential scanning calorimetry (DSC) and X-ray powder diffraction were used to characterize the material and look at the thermal behavior. Melt-quenched samples were also prepared, and the thermal profile upon milling was determined and correlated with the thermal behavior of the cryomilled samples. Growth rates were determined by hot-stage microscopy. Ketoconazole, when cryomilled, showed only one crystallization exotherm in the DSC profile. Ursodiol, and to some extent indomethacin, initially showed a double exotherm which eventually become a single exotherm on further milling. Griseofulvin, carbamazepine, and piroxicam exhibited a double exotherm in the DSC profile upon cryomilling to the amorphous state. Surface crystal growth rates around T (g) were found to be highest for compounds showing the double exotherm in the DSC. Thus, it was seen that compounds which have high surface crystallization tendency will exhibit the double exotherm during heating.     

Preparation and crystallization kinetics of new structurally well-defined star-shaped biodegradable poly(L-lactide)s initiated with diverse natural sugar alcohols

This study presents syntheses, structural characterization, and crystallization kinetic investigation of new structurally well-defined star-shaped poly(l-lactide)s (PLLAs). First, a series of new 3- to 6-arm star-shaped PLLAs were synthesized through SnOct(2) catalyzed ring-opening polymerization of (l)-lactide with natural sugar alcohols of glycerol, erythritol, xylitol, and sorbitol as the favorable initiators. Subsequently, their chemical structures were characterized by means of GPC, NMR, and viscometer with respect to the star-shaped structures, demonstrating the well-defined arm structures as evidenced on the g(1/2)/g' values, where g and g' denote the ratios of mean-square radius of gyration and intrinsic viscosity of a star-shaped polymer to those of a linear structural reference with similar absolute molecular weight. Furthermore, spherulite morphologies and growth rates were studied by a polarized microscopy (POM) for the synthesized star-shaped PLLAs with different molecular weights, and it was found that the more arms of a star-shaped PLLA finally resulted in a lower spherulite growth rate. With regard to the crystallization kinetics of these star-shaped PLLAs, isothermal and nonisothermal crystallization were examined by differential scanning calorimeter (DSC). It was found that Avrami exponent n values of isothermal crystallization were almost independent of the isothermal crystallization temperature T(c) for different series of star-shaped PLLAs. In contrast, the values of Avrami exponent n were observed to strongly depend on the star-shaped structures with different arms, implying their distinct nucleation mechanisms, and the more arms of a star-shaped PLLA led to a slower isothermal crystallization rate. On the basis of a modified Avrami equation, new light was shed on the nonisothermal crystallization kinetics for the star-shaped PLLAs, and the activation energies were found to vary from 146.86 kJ/mol for the linear PLLA EG-3 to 221.23 kJ/mol of the star-shaped S-3, demonstrating much decreased crystallizabilities of star-shaped PLLAs with more arms.     

Trehalose amorphization and recrystallization

The stability of the amorphous trehalose prepared by using several procedures is presented and discussed. Amorphization is shown to occur by melting (T(m)=215 degrees C) or milling (room temperature) the crystalline anhydrous form TRE-beta. Fast dehydration of the di-hydrate crystalline polymorph, TRE-h, also produces an amorphous phase. Other dehydration procedures of TRE-h, such as microwave treatment, supercritical extraction or gentle heating at low scan rates, give variable fractions of the polymorph TRE-alpha, that undergo amorphization upon melting (at lower temperature, T(m)=130 degrees C). Additional procedures for amorphization, such as freeze-drying, spray-drying or evaporation of trehalose solutions, are discussed. All these procedures are classified depending on the capability of the undercooled liquid phase to undergo cold crystallization upon heating the glassy state at temperatures above the glass transition temperature (T(g)=120 degrees C). The recrystallizable amorphous phase is invariably obtained by the melt of the polymorph TRE-alpha, while other procedures always give an amorphous phase that is unable to crystallize above T(g). The existence of two different categories is analyzed in terms of the transformation paths and the hypothesis that the systems may exhibit different molecular mobilities.     

Manifestation of the size effect during crystallization and melting of dispersed water in native and amorphous starches with various degrees of hydration

Differential scanning calorimetry (DSC) was used to study the melting and crystallization of frozen water dispersed in humid potato starch. Melting and crystallization temperatures and heats as functions of the degree of hydration of the starch were obtained for native and amorphous starch states. Manifestations of the size effect were observed in the dependences of heat for the processes in both starch states. Crystallization and melting heats of frozen water were found to change nonlinearly with the increasing degree of hydration in all cases. In contrast, a size effect in the dependences of melting and crystallization temperatures of frozen water was detected only for native starch. Reasons responsible for the absence of a size effect in the amorphous state were considered. Hysteresis, which is characteristic of small particles, was observed upon melting and crystallization of frozen water and its manifestation strongly differed in the native and amorphous states of potato starch.     

Neat DODAB vesicles: Effect of sonication time on the phase transition thermodynamic parameters and its relation with incomplete chain freezing

Vesicle solutions formed by the cationic lipid dioctadecyldimethylammonium bromide have been extensively used in the past 20 years as model membrane systems. Often, discrepancies are found in the literature for the thermodynamic parameters of the gel-to-liquid crystal (gel-lc) phase transition of these vesicles. In this work we present a systematic DSC investigation on the influence of the sonication method on the main temperature and enthalpy of the transition for DODAB vesicles, prepared both in normal and deuterated water. It is shown that as the sonication time increases, the T(m) and enthalpy values decrease until a plateau value is reached. This effect is associated with incomplete chain crystallization in the highly curved vesicles (produced by sonication) when they are cooled below the transition temperature. The effect of aging on the sonicated solutions has also been monitored by DSC.     

Optimal design for CSTR's in series performing enzymatic lactose hydrolysis

Analytical expressions are derived for the optimal design (based on minimum overall reactors volume) of a series of N CSTR's performing enzymatic lactose hydrolysis. It is assumed that lactose hydrolysis obeys Michaelis-Menten kinetics with competitive product (galactose) inhibition and no enzyme deactivation occurs. The optimum design of a cascade of ideally mixed reactors are compared with equal size reactors and with plug flow reactor required for a given overall degree of lactose conversion. The effect of operating parameters such as temperature, lactose initial (feed) concentration and conversion, enzyme and product initial concentration on the optimal overall holding time are also investigated. Optimization results for a series of N CSTR's up to five are obtained and compared with plug flow reactor.