Invertase storage stability and sucrose hydrolysis in solids as affected by water activity and glass transition

Effect of the glass transition temperature on alpha-amylase activity in a starch matrix

This study optimises a protocol for the estimation of α-amylase activity in a condensed starch matrix in the vicinity of the glass transition region. Enzymatic activity on the vitrified starch system was compared with that of a reference substrate, maltodextrin. The activity was assayed as the rate of release of reducing sugar using a dinitrosalicylic acid procedure. The condensed carbohydrate matrices served the dual purpose of acting as a substrate as well as producing a pronounced effect on the ability to enzymatic hydrolysis. Activation energies were estimated throughout the glass transition region of condensed carbohydrate preparations based on the concept of the spectroscopic shift factor. Results were used to demonstrate a considerable moderation by the mechanical glass transition temperature, beyond the expected linear effect of the temperature dependence, on the reaction rate of starch hydrolysis by α-amylase in comparison with the low-molecular weight chain of maltodextrin.

Inactivation of Bacillus subtilis spores by combining high-pressure thermal sterilization and ethanol

High-pressure thermal sterilization (HPTS) is a new and promising sterilization technology of foods. Effects of combining HPTS and ethanol treatment on inactivation of Bacillus subtilis spores were investigated. An interesting phenomenon was observed. The inactivation effect of HPTS treatment on the spores was enhanced significantly with the increase in ethanol concentration from 0 to 15%. However, the inactivation effect was decreased with further increase in ethanol concentration up to 70%. In addition, the release of DPA and leakages of OD(260) and OD(280) material from the spores increased continuously with the increase in ethanol concentration. Moreover, flow cytometry analysis suggested that although the inner membrane of the spores was damaged, PI could not bind with the spore DNA immediately after HPTS treatment. In conclusion, the mechanism of this special phenomenon could be attributed to the germination of spores under HPTS treatment and effects of ethanol on the protein or water activity. HPTS caused other lethal damages to the spores besides its damage to the inner membrane. Ethanol of low concentrations could significantly enhance the sterilization effects of HPTS, which was good for keeping the qualities of foods.

Combined use of thermomechanics and UV spectroscopy to rationalize the kinetics of bioactive compound (caffeine) mobility in a high solids matrix

An investigation of the diffusional mobility of a bioactive compound (caffeine) within a carbohydrate matrix (glucose syrup) at a glassy consistency is reported. The experimental temperature range was from 30 to - 70 degrees C, and the techniques of modulated differential scanning calorimetry, small-deformation dynamic oscillation on shear, and UV spectrometry were employed. It is not a straightforward matter to identify the relaxation dynamics of such a glassy matrix. This makes suggestions of the relationship between the structural properties of the matrix and the diffusional mobility of bioactive compounds reported earlier in the literature rather tenuous. To address this issue, we recorded mechanical spectra over the aforementioned temperature range and utilized the combined framework of the Williams, Landel, and Ferry (WLF) equation with the time-temperature superposition principle to rationalize results. The protocol produced a fundamental definition of the glass transition temperature and free volume parameters of the glucose syrup sample within the glass transition region. Results were related to the kinetic rates of caffeine diffusion derived by UV spectroscopy leading to the conclusion that the diffusional mobility of the chemical substance is independent of the carbohydrate matrix. This conclusion was further supported by the high level of fractional free volume of caffeine, which is congruent with the predictions of the reaction rate theory (modified Arrhenius equation), as compared to the collapsing levels of free volume in the glucose-syrup matrix that make appropriate WLF considerations.

Role of deliquescence lowering in enhancing chemical reactivity in physical mixtures

Mixtures of deliquescent solids are susceptible to deliquescence lowering, where water vapor condensation occurs in mixtures at a lower critical relative humidity (RH(0mix)) than individual component critical relative humidities (RH(0)s). The purpose of this study was to evaluate the effect of deliquescence lowering on chemical reactivity. Sucrose, citric acid and their physical mixtures were characterized using vapor sorption analysis to determine RH(0) and RH(0mix). Acid-catalyzed sucrose hydrolysis kinetics was determined using polarimetric analysis. Physical mixtures of sucrose and citric acid crystals were prepared and stored at various relative humidities at 22 degrees C. For these physical mixtures, sucrose hydrolysis was found to occur only when the environmental RH exceeded RH(0mix). Degradation kinetics correlated with the storage RH, being fastest at higher RH. In addition, a lag period was initially observed, which was most prominent for samples stored close to RH(0mix). With exposure to RHs below RH(0mix), no sucrose degradation was detected over the experimental time period. In conclusion, mixtures of deliquescent solids showed increased water sorption at lower RHs, which caused solid dissolution and subsequently led to an increase in the chemical reactivity.

Correlations between molecular mobility and chemical stability during storage of amorphous pharmaceuticals

Recent studies have demonstrated that molecular mobility is an important factor affecting the chemical stability of amorphous pharmaceuticals, including small-molecular-weight drugs, peptides and proteins. However, quantitative correlations between molecular mobility and chemical stability have not yet been elucidated. The purpose of this article is to review literature describing the effect of molecular mobility on chemical stability during storage of amorphous pharmaceuticals, and to seek a better understanding of the relative significance of molecular mobility and other factors for chemical reactivity. We first consider the feature of chemical stability often observed for amorphous pharmaceuticals; changes in temperature dependence of chemical stability around matrix glass transition temperature (Tg), and greater stability associated with higher Tg. Secondly, we review papers which quantitatively studied the effects of the global mobility (often referred to as structural relaxation or -relaxation) of amorphous pharmaceuticals on chemical stability, and discuss correlations between chemical stability and global mobility using various equations that have thus far been proposed. Thirdly, the significance of local mobility of drug and excipient molecules in chemical reactivity is discussed in comparison with that of global mobility. Furthermore, we review literature reports which show no relationship between chemical stability and molecular mobility. The lack of apparent relationship is discussed in terms of the effects of the contribution of excipient molecules as reactants, the specific effects of water molecules, the heterogeneity of the matrix, and so on. The following summary has been obtained; the chemical stability of amorphous pharmaceuticals is affected by global mobility and/or local mobility, depending on the length scale of molecular mobility responsible for the chemical reactivity. In some cases, when activation energy for degradation processes is high and when other factors such as the specific effects of water and/or excipients contribute the degradation rate, stability seems to be largely independent of molecular mobility.

β-Relaxation of Insulin Molecule in Lyophilized Formulations Containing Trehalose or Dextran as a Determinant of Chemical Reactivity

PURPOSE

The purpose of this study was to elucidate whether the degradation rate of insulin in lyophilized formulations is determined by matrix mobility, as reflected in glass transition temperature (Tg), or by beta-relaxation, as reflected in rotating-frame spin-lattice relaxation time (T1rho).

METHODS

The storage stability of insulin lyophilized with dextran was investigated at various relative humidities (RH; 12-60%) and temperatures (40-90 degrees C) and was compared with previously reported data for insulin lyophilized with trehalose. Insulin degradation was monitored by reverse-phase high-performance liquid chromatography. Furthermore, the T1rho of the insulin carbonyl carbon in the lyophilized insulin-dextran and insulin-trehalose systems was measured at 25 degrees C by 13C solid-state NMR, and the effect of trehalose and dextran on T1rho was compared at various humidities.

RESULTS

The degradation rate of insulin lyophilized with dextran was not significantly affected by the Tg of the matrix, even at low humidity (12% RH), in contrast to that of insulin lyophilized with trehalose. The insulin-dextran system exhibited a substantially greater degradation rate than the insulin-trehalose system at a given temperature below the Tg. The difference in degradation rate between the insulin-dextran and insulin-trehalose systems observed at 12% RH was eliminated at 43% RH. In addition, the T1rho of the insulin carbonyl carbon at low humidity (12% RH) was prolonged by the addition of trehalose, but not by the addition of dextran. This difference was eliminated at 23% RH, at which point the solid remained in the glassy state. These findings suggest that the beta-relaxation of insulin is inhibited by trehalose at low humidity, presumably as a result of insulin-trehalose interaction, and thus becomes a rate determinant. In contrast, dextran, whose ability to interact with insulin is thought to be less than that of trehalose, did not inhibit the beta-relaxation of insulin, and thus, the chemical activational barrier (activation energy) rather than beta-relaxation becomes the major rate determinant.

CONCLUSIONS

Beta-relaxation rather than matrix mobility seems to be more important in determining the stability of insulin in the glassy state in lyophilized formulations containing trehalose and dextran.