Cross-polarization/magic angle spinning NMR to study glucose mobility in a model intermediate-moisture food system

Theories for the chemical stability of foods cite the role of moisture content or water activity in reactant mobility, though mobility has been variously defined. One theory, based on plasticization by moisture, is limited by a lack of research directly linking the mobility of a matrix to the mobility and reactivity of small solute molecules in foods. A cross-polarization/magic angle spinning technique was developed to study glucose rotational mobility in the solid state over a range of water activities and in matrixes with different glass transition temperatures. Data analysis stressed the significance of separating molecular mobility from relaxation time. Results showed that, in a caseinate matrix, compared to a control, adding glycerol yielded the highest glucose mobility and lowest glass transition temperature (T(g)), while adding sorbitol also increased mobility and lowered T(g). Consequently, plasticization by either moisture or these humectants increases the mobility of small solute molecules such as glucose.     

Nonenzymatic browning in food models in the vicinity of the glass transition: effects of fructose,glucose, and xylose as reducing sugar

Effects of a reducing sugar, fructose, glucose, or xylose, and glass transition on the nonenzymatic browning (NEB) rate in maltodextrin (MD), poly(vinylpyrrolidone) (PVP), and water systems were studied. Glass transition temperatures (T(g)) were determined using DSC. Water contents were determined gravimetrically, and NEB rates were followed at several temperatures spectrophotometrically at 280 and 420 nm. Reducing sugar did not affect water contents, but xylose reduced the T(g) of the solid models. Sugars showed decreasing NEB reactivity in the order xylose > fructose > glucose in every matrix material. The NEB reactivity and temperature dependence of the single sugars varied in different matrices. The NEB rates of the solid models increased at temperatures 10-20 degrees C above the T(g), and nonlinearity was observed in Arrhenius plots in the vicinity of T(g). The temperature dependence of nonenzymatic browning could also be modeled using the WLF equation.     

Physical aging of starch, maltodextrin, and maltose

The physical aging of low water content, amorphous starch/water, maltodextrin/water, and maltose/water mixtures in the glassy state was examined using mechanical testing and calorimetry. Stress relaxation measurements showed that upon storage of the glassy materials there was a time-dependent increase in both flexural modulus and mechanical relaxation time. The mechanical relaxation time increased with depth of quench below the calorimetric glass transition temperature and with aging time at the quench temperature. Calorimetry of the aged materials showed an overshoot in heat capacity in the vicinity of the glass transition. The logarithm of the mechanical relaxation time showed a simple linear relationship with the size of the overshoot expressed as an enthalpy change. The calorimetric behavior could be modeled using the Tool-Narayanaswamy-Moynihan method.     

Differential scanning calorimetry study of glass transition in frozen starch gels

The effects of initial water content, maximum heating temperature, amylopectin crystallinity type, and annealing on the glass transition of starch gels were studied by differential scanning calorimetry (DSC). The glass transition temperatures of the frozen gels measured as the onset (T(g,onset)) or midpoint temperature (T(g,midpoint)), heat capacity change during the glass transition (deltaC(p)), unfrozen water of starch gels, and additional unfrozen water (AUW) arising from gelatinization were reported. The results show that T(g,onset) and T(g,midpoint) of the partially gelatinized gels are independent of the initial water content, while both of the T(g) values of the fully gelatinized gel increase as the initial water content increases. These observations might result from the difference in the level of structural disruption associated with different heating conditions, resulting in different gel structures as well as different concentrations of the sub-T(g) unfrozen matrix. The amylopectin crystallinity type does not greatly affect T(g,onset) and T(g,midpoint) of the gels. Annealing at a temperature near T(g,onset) increases both T(g,onset) and T(g,midpoint) of the gels, possibly due to an increase in the extent of the freeze concentration as evidenced by a decrease in AUW. Annealing results in an increase in the deltaC(p) value of the gels, presumably due to structural relaxation. A devitrification exotherm may be related to AUW. The annealing process decreases AUW, thus also decreasing the size of the exotherm.     

Impact of melting conditions of sucrose on its glass transition temperature.

The impact of the melting conditions of sucrose crystals on the glass transition temperature (T(g)) of the sucrose melt was studied. Final temperature, heating rate, and the residence time at the final temperature were the experimental conditions considered. The glass transition temperature of the different glasses was measured by differential scanning calorimetry, and the degradation of sucrose during the thermal treatments was studied by high-performance liquid chromatography. The results showed that the T(g) is sensitive to the degradation of sucrose: T(g) decreases with the appearance of small molecules and then increases with the appearance of polymerization products. Thus, the choice of thermal treatment is of the utmost importance for the determination of the T(g) of pure sucrose.     

Definition of a mechanical glass transition temperature for dehydrated foods

A new concept of the mechanical glass transition temperature (T(g)) is presented with application in dehydrated high-sugar/gelatin mixtures, fish, and fruits. The macroscopic basis and manner of relaxation processes during vitrification of these foodstuffs are developed using small deformation dynamic oscillation, the master curve of viscoelasticity, and the time-temperature superposition principle. The quantitative features of the mechanical T(g) are based on the combined framework of free volume/WLF theory and the Andrade equation. It is proposed that the thermal profile of storage modulus on shear is a fundamental index of monitoring changes in a glassy structure, and several cases are presented in support of this concept.     

Effect of transglutaminase treatment on the glass transition of soy protein

The effect of microbial transglutaminase (MTG) treatment on the glass transition temperature (T(g)) of two fractions which were isolated from a soy protein sample was studied. The T(g) of each fraction measured by differential scanning calorimetry was lowered by the MTG treatment, which generated cross-links in the samples, and this result agreed with the result of dynamic mechanical analysis. From the (1)H NMR measurement, the line width of the (1)H signal of the MTG-treated sample was observed to be greater than that of the MTG-nontreated sample at similar water content, which implied that there was relatively more immobilized water in the MTG-treated sample. The MTG treatment seemed to cause the increment in immobilized water, which might affect the T(g) of the soy protein sample.     

Porosity and the effect of structural changes on the mechanical glass transition temperature

Continuing an investigation on the fundamentals and applications of a recently proposed concept, i.e., the mechanical or network glass transition temperature, we now report data on the macrostructural changes in dehydrated apple tissue in relation to apparent porosity. Care was taken to keep the moisture content of the matrix constant (approximately 81%) while the volume fraction of total pores ranged from 0.38 to 0.79. Reproducible mechanical profiles identified the first derivative of shear storage modulus as a function of temperature to be the appropriate indicator of the mechanical Tg at the conjunction of the William-Landel-Ferry/free volume theory and the modified Arrhenius equation. Information on the microstructural characteristics and morphology of porous apple preparations was also made available via modulated differential scanning calorimetry and scanning electron microscopy. The work reveals and discusses discrepancies in the Tg-porosity relationship obtained from calorimetry and mechanical analysis attributable to the different extent to which the two techniques respond to degrees of molecular mobility.     

Use of differential scanning calorimetry (DSC) as a new technique for detection of adulteration in honeys. 1. Study of adulteration effect on honey thermal behavior.

Differential scanning calorimetry (DSC) was used to study the thermal behavior of authentic honeys (Lavandula, Robinia, and Fir honeys) and industrial sugar syrups. Thermal or thermochemical parameters such as the glass transition temperature (Tg), enthalpies of fusion (DeltaH(fus)), and heat capacity variation (DeltaC(p)) were measured. The syrups and honeys showed significant differences in thermal phenomena, as well as in their amplitude and position on the temperature scale. Results showed good reproducibility of the method for all samples studied. The effect of adulteration of honey with different amounts of syrup (5, 10, 20, 40, and 60%) was investigated. A linear relationship was found between the percentage of added syrup and the glass transition temperature. A similar relationship was obtained from the enthalpy of fusion results in the temperature range of 40-90 degrees C. Under applied conditions, the effects of adulteration of honeys by industrial syrups appeared to be detectable from a level as low as 5%.     

The question of high- or low-temperature glass transition in frozen fish. Construction of the supplemented state diagram for tuna muscle by differential scanning calorimetry

The thermal behavior of fresh tuna muscle, rehydrated freeze-dried tuna muscle, and tuna sarcoplasmic protein fraction was studied by three types of differential scanning calorimetry (DSC): conventional DSC, alternating DSC, and sensitive micro-DSC. The relationship between glass transition temperature, T(g), and water content was established. Only a low-temperature glass transition was detected for fresh tuna and freeze-dried tuna rehydrated to high water contents, whereas for sarcoplasmic protein fraction both a low-temperature and an apparent high-temperature glass transition were detected for samples of high water content. Construction of the supplemented state diagrams for whole tuna muscle and for tuna sarcoplasmic protein fraction confirmed the low-temperature transition to be glass transition of the maximally freeze-dehydrated phase. The apparent upper transition of sarcoplasmic protein fraction was shown not to be a glass transition but rather to originate from the onset of melting of ice, and the temperature of this event should be denoted T(m)'. The glass transition temperature and the concentration of the maximally freeze dehydrated tuna muscle are -74 degrees C and 79% (w/w), respectively.     

Maillard reaction kinetics in model preservation systems in the vicinity of the glass transition: experiment and theory

Rates of reactant consumption for the Maillard reaction between lysine and glucose were measured for a noncrystallizing trehalose-sucrose-water matrix in the glass transition region. At temperatures above the glass transition temperature (T(g)), the consumption rates showed Arrhenius temperature dependence with activation energies of 135 and 140 kJ mol(-1) for lysine and glucose, respectively. Finite reaction rates were observed for glassy samples that were faster than that of one of the nonglassy samples. A comparison of experimental results with predicted diffusion-controlled reaction rate constants indicated that the reaction was reaction-controlled at temperatures above T(g) and approached the diffusion-influenced regime in the glassy state. The needs for further research on reactant diffusivity, the theory of the orientation dependence of reactivity, and a detailed understanding of the reaction mechanism and kinetics were identified.     

The effect of thermal history on the maillard reaction in a glassy matrix

To test whether the extent of physical aging affected the reaction rate, Maillard reaction kinetics were studied in glassy model preservation systems subjected to two different thermal histories. The glass transition temperature and physical aging of the matrix were determined using differential scanning calorimetry, and the normalized heat capacities were modeled using the Tool-Narayanaswamy-Moynihan approach. Samples prepared using the different thermal histories initially had different degrees of aging, but these were practically indistinguishable after 10 h under the reaction conditions (65 degrees C); the samples underwent rapid structural relaxation at that temperature. The reaction of glucose and lysine in an amorphous trehalose/sucrose matrix was followed using spectrophotometric and chromatographic analysis. A difference in reaction rate could only be distinguished in the rate of consumption of glucose, which was approximately 20% faster in the minimally aged matrix; no significant differences were seen in any other indicator of reaction.     

Glass transition and water effects on sucrose inversion by invertase in a lactose-sucrose system

Enzymatic changes are often detrimental to quality of low-moisture foods. In the present study, effects of glass transition and water on sucrose inversion in a lactose-sucrose food model were investigated. Amorphous samples were produced by freeze-drying lactose-sucrose (2:1)-invertase (20 mg invertase/49.4 g of carbohydrate) dissolved in distilled water. Sorption isotherms were determined gravimetrically at 24 degrees C. Sucrose hydrolysis was determined by monitoring glucose content using a test kit and the amounts of fructose, glucose, and sucrose using HPLC. The glass transition temperatures, T(g), at various water contents were measured using differential scanning calorimetry (DSC). The BET and the GAB sorption models were fitted to experimental data up to a(w) 0.444 and 0.538, respectively. Water sorption and DSC results suggested time-dependent crystallization of sugars at a(w) 0.444 and above. Significant sucrose hydrolysis occurred only above T(g), concomitantly with crystallization. Sucrose hydrolysis and crystallization were not likely in glassy materials.     

Multi-level approach for the analysis of water effects in corn flakes

The purpose of this work was to analyze the effect of water on thermal transitions, mechanical properties, and molecular mobility in corn flakes (CF), and their relationships. Commercial common (CCF) and sugar-frosted (SCF) corn flakes were studied in a water content (wc) range from 5 to 20 (% dry basis). The slope of (1)H NMR spin-spin relaxation time T 2* (determined by FID) versus temperature plot changed close to T g. Compression force showed a maximum at wc of ca. 12 and 16% (db) for SCF and CCF, respectively. (1)H NMR complemented DSC data in determining the temperature dependence of water and solid mobility, in order to assess quality of laminated corn products. The results of the present work indicate that while the compression force showed a maximum value as a function of water content, T g values determined by DSC or by spin-spin relaxation decreased progressively with increasing water content.     

Impact of caramelization on the glass transition temperature of several caramelized sugars. Part II: Mathematical modeling

Further to part I of this study, this paper discusses mathematical modeling of the relationship between caramelization of several sugars including fructose, glucose, and sucrose and their glass transition temperatures ( T g). Differential scanning calorimetry (DSC) was used for creating caramelized sugar samples and determining their glass transition temperatures ( T g). UV-vis absorbance measurement and high-performance liquid chromatography (HPLC) analysis were used for quantifying the extent of caramelization. Specifically, absorbances at 284 and 420 nm were obtained from UV-vis measurement, and the contents of sucrose, glucose, fructose, and 5-hydroxymethyl-furfural (HMF) in the caramelized sugars were obtained from HPLC measurements. Results from the UV and HPLC measurements were correlated with the Tg values measured by DSC. By using both linear and nonlinear regressions, two sets of mathematical models were developed for the prediction of Tg values of sugar caramels. The first set utilized information obtained from both UV-vis measurement and HPLC analysis, while the second set utilized only information from the UV-vis measurement, which is much easier to perform in practice. As a caramelization process is typically characterized by two stages, separate models were developed for each of the stages within a set. Furthermore, a third set of nonlinear equations were developed, serving as criteria to decide at which stage a caramelized sample is. The models were evaluated through a validation process.     

Corn protein-based thermoplastic resins: effect of some polar and amphiphilic plasticizers.

Homogeneous blends of corn gluten meal (CGM) and "polar" plasticizers (water, glycerol) or "amphiphilic" plasticizers [octanoic and palmitic acids, dibutyl tartrate and phthalate, and diacetyl tartaric acid ester of mono-diglycerides (DATEM)] were obtained by a hot-mixing procedure. The glass transition temperature (T(g)) of the blends was measured by modulated differential scanning calorimetry and dynamic mechanical thermal analysis, as a function of plasticizer type and content (0-30%, dwb). The plasticizing efficiency (i.e., decrease of T(g)) at equal molar content was found to be proportional to the molecular weight and inversely proportional to the percent of hydrophilic groups of the plasticizer. The migration rate of the plasticizers in the polymer was related to their physicochemical characteristics. It was assumed that polar substances interacted with readily accessible polar amino acids, whereas amphiphilic ones interacted with nonpolar zones, which are buried and accessible with difficulty. The temperature at which a thermoplastic resin of plasticized CGM could be formed was closely connected to the T(g) of the blend.     

Relationship between the glass transition of soy protein and molecular structure

The change in molecular structure of the soy protein samples as a result of the microbial transglutaminase treatment was studied using solid-state (13)C NMR spectroscopy and circular dichroism (CD), and the relation to the glass transition temperature (T(g)) was examined. From NMR measurements, the structure of the local region of the C(alpha) methine was observed to change, and the region had relatively high mobility. From CD measurements, the structural change seemed to be caused by the change in the secondary structure (disintegration of the beta-structure). By comparison with the T(g) of another protein, the state of the secondary structure of a protein was suggested to be a key in determining its T(g).     

Effects of heating conditions on the glass transition parameters of amorphous sucrose produced by melt-quenching

This research investigates the effects of heating conditions used to produce amorphous sucrose on its glass transition (T(g)) parameters, because the loss of crystalline structure in sucrose is caused by the kinetic process of thermal decomposition. Amorphous sucrose samples were prepared by heating at three different scan rates (1, 10, and 25 °C/min) using a standard differential scanning calorimetry (SDSC) method and by holding at three different isothermal temperatures (120, 132, and 138 °C) using a quasi-isothermal modulated DSC (MDSC) method. In general, the quasi-isothermal MDSC method (lower temperatures for longer times) exhibited lower T(g) values, larger ΔC(p) values, and broader glass transition ranges (i.e., T(g end) minus T(g onset)) than the SDSC method (higher temperatures for shorter times), except at a heating rate of 1 °C/min, which exhibited the lowest T(g) values, the highest ΔC(p), and the broadest glass transition range. This research showed that, depending on the heating conditions employed, a different amount and variety of sucrose thermal decomposition components may be formed, giving rise to wide variation in the amorphous sucrose T(g) values. Thus, the variation observed in the literature T(g) values for amorphous sucrose produced by thermal methods is, in part, due to differences in the heating conditions employed.     

Impact of caramelization on the glass transition temperature of several caramelized sugars. Part I: Chemical analyses

This study aims to investigate the relationship between caramelization of several sugars including fructose, glucose, and sucrose and their glass transition temperature (Tg). Differential scanning calorimetry (DSC) was used for creating caramelized sugar samples as well as determining their glass transition temperature, which was found to decrease first and then increase as the holding time at the highest temperature increased. The extent of caramelization was quantified by UV-vis absorbance measurement and high-performance liquid chromatography analysis. Results showed that the amount of small molecules from the degradation of sugar increased very fast at the beginning of heating, and this increase slowed down in the later stage of caramelization. On the other hand, there was a lag phase in the formation of large molecules from the degradation of sugar at the beginning of heating, followed by a fast increase in the later stage of caramelization. The obtained results clearly indicate the impact of melting condition on the T g of sugars through formation of intermediates and end products of caramelization. Generally, when the heating condition is relatively mild, small molecules are formed first by decomposition of the sugar, which leads to a decrease of the overall Tg, and as the heating time becomes longer and/or the heating condition becomes more severe, polymerization takes over and more large molecules are formed, which results in an increase of the overall Tg. Mathematical modeling of the relationship will be presented as part II of the study in a separate paper.