Investigating the effect of amorphization method on the physicochemical properties and heat- and moisture-induced crystallization behaviors of amorphous sucrose

Amorphous;sucrose;amorphization methods;characterization

Morrow, Elizabeth Anne

关键词: Amorphous;    sucrose;    amorphization methods;    characterization;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/101143/MORROW-DISSERTATION-2018.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Amorphous sugars, including sucrose, are essential ingredients in food and pharmaceutical products due to their encapsulation abilities, desirable textural characteristics, and enhanced dissolution rate and solubility. However, amorphous sugars are prone to unwanted heat and/or moisture-induced physical changes, including stickiness, caking, and recrystallization, which decrease product stability and alter product quality attributes, such as texture, taste, aroma retention, and drug efficacy. As a result, the physical stability of amorphous sugars has stimulated substantial research; however, comparatively few studies have examined the effect of different amorphization methods on the physicochemical properties and stability of amorphous sugars created from the same crystalline starting material. In general, amorphous materials can be produced via several different preparation routes, including supercooling from the melt (e.g., melt-quenching, spin-melt-quenching), precipitation from solution (e.g., freeze-drying, spray-drying), and mechanical disruption of the crystalline lattice (e.g., grinding, milling, compaction). It has been demonstrated for a number of materials, mainly active pharmaceutical ingredients, that different amorphization methods generate amorphous products with dissimilarities in morphology, local structural order, thermal behavior, and vapor sorption characteristics. Given the particular importance of amorphous sucrose in the food and pharmaceutical industries, and the relatively limited amount of research investigating the implications of its different methods of preparation, the main goal of this research was to identify key dissimilarities in the temperature and relative humidity conditions at which amorphous sucrose prepared by industrial processing methods, including freeze-drying, spray-drying, ball milling, melt-quenching, and spin-melt-quenching, lose their desirable physical properties. Various analytical techniques, including scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), total scattering pair distribution function (TSPDF), high-performance liquid chromatography (HPLC), and differential scanning calorimetry (DSC), were employed to characterize the “as is” physicochemical properties of the differently amorphized sucrose samples. Freeze-dried (FreD), spray-dried (SprayD), ball milled (BallM), melt-quenched (MeltQ), and spin-melt-quenched (SpinMeltQ) amorphous sucrose significantly differed in the majority of physicochemical properties studied, including morphology, local structure, moisture content, chemical composition, and thermal behavior. These general differences were attributed to dissimilarities in the amorphization methods used to produce the amorphous sucrose samples (e.g., solution-based versus melt-based amorphization route). A particularly noteworthy observation was that FreD, SprayD, BallM, and SpinMeltQ exhibited an exothermic crystallization peak upon heating, termed cold crystallization, whereas MeltQ did not. This finding has been previously reported in the literature, and three main explanations have been proposed for the anomalous cold crystallization behavior of MeltQ: absence of heterogeneous nuclei, lower sample moisture content, and structural dissimilarity to the crystalline state. While these explanations for the lack of cold crystallization of MeltQ seem plausible, chemical analysis results determined herein suggested that thermal decomposition may also be involved in the lack of cold crystallization of MeltQ, as thermal decomposition indicator compounds were present in MeltQ, but not in FreD and SprayD. Thus, the next phase of this research aimed to determine if thermal decomposition was an additional factor that contributed to the observed lack of cold crystallization of MeltQ. To investigate the effects of thermal decomposition on the cold crystallization behavior of MeltQ, an amorphous sucrose sample produced by melt-quenching was subsequently freeze-dried (FreD-MeltQ), and its physicochemical characteristics and thermal behavior were compared to those of MeltQ and FreD. Although FreD-MeltQ was similar to FreD in morphology, % amorphous/crystalline content, moisture content, and glass transition temperature, like MeltQ it did not cold crystallize upon heating. However, local structure appeared to play an important role in cold crystallization behavior, as the combined PXRD and TSPDF findings demonstrated that MeltQ and FreD-MeltQ had a more similar local structure to one another than to FreD. The underlying cause of these structural differences seems to be related to the presence of thermal decomposition compounds, as similar amounts of thermal decomposition indicator compounds were measured in both MeltQ and FreD-MeltQ, but not in FreD. Based on this evidence, it was concluded that thermal decomposition is an additional factor that contributes to the lack of cold crystallization of MeltQ.Given the four explanations currently suggested for the lack of cold crystallization of MeltQ—absence of heterogeneous nuclei, lower sample moisture content, structural dissimilarity to the crystalline state, and thermal decomposition—the next aim of this research was to examine their relative influence on the cold crystallization behavior of MeltQ. Several different conditions based on the aforementioned explanations (i.e., seeding, partial-melting, moisture content, grinding, and heating rate) were employed in an attempt to induce cold crystallization in MeltQ. Seeding with ground analytical grade crystalline sucrose, partial-melting, and grinding induced cold crystallization in MeltQ, however, the average cold crystallization onset temperature was significantly higher and average enthalpy significantly smaller than those measured for FreD. The increase in cold crystallization onset temperature and decrease in enthalpy were linked to thermal decomposition diminishing the effectiveness of the various conditions employed to induce cold crystallization, since thermal decomposition indicator compounds, along with several other unidentified decomposition products, were detected in the MeltQ substrate upon which attempts to induce cold crystallization were made. Since thermal decomposition compounds are inherently present in MeltQ sucrose, future investigations could compare the behavior of a wide range of low molecular weight amorphous carbohydrates to further explore the factors affecting cold crystallization behavior.The final phase of this research aimed to compare the moisture-induced crystallization behavior of FreD, SprayD, BallM, MeltQ, and SpinMeltQ, and to assess the thermal behavior of the resultant crystals. Moisture sorption profiles of differently amorphized sucrose were collected at 25°C over a wide range of relative humidity conditions using a Dynamic Vapor Sorption (DVS) instrument and saturated salt slurries in miniature desiccators. For amorphous sucrose samples studied using DVS (i.e., FreD, SprayD, BallM, and SpinMeltQ), moisture-induced crystallization was observed in FreD, SprayD, and BallM at 30%RH, and in all amorphous sucrose samples at 40, 50, 60, 70, and 80%RH. Although the moisture sorption profiles of FreD, SprayD, BallM, and SpinMeltQ within this range of %RH values followed the same trends in moisture-induced crystallization onset time and maximum equilibrium moisture content, the actual values differed. Namely, BallM exhibited the shortest crystallization onset times and lowest maximum equilibrium moisture contents, whereas SpinMeltQ exhibited the longest crystallization onset times. These differences were attributed to unique physicochemical characteristics (e.g., significant % crystalline content in BallM, thermal decomposition compounds in SpinMeltQ) resulting from inherent differences in amorphization route (e.g., disruption of the crystalline lattice versus supercooling of the melt). Compared to the other amorphous sucrose samples, MeltQ exhibited the least inclination to moisture-induced crystallization, as samples held at 43 and 53%RH only partially recrystallized after 24 weeks of storage. Interestingly, under both moisture and heat conditions, BallM had the greatest tendency to crystallize followed by SprayD, FreD, SpinMeltQ, and MeltQ, which suggests a potential connection between the cold crystallization and moisture-induced crystallization behaviors of differently amorphized sucrose. Finally, sucrose recrystallized from FreD, SprayD, BallM, and SpinMeltQ at different %RH values generally exhibited two endothermic peaks. However, for a given %RH condition, the onset temperature associated with the initial endothermic peak (Tm onset) and total enthalpy of both peaks (ΔHm) varied as a function of amorphization method, with BallM and SpinMeltQ samples generally exhibiting lower Tm onset and ΔHm values than FreD and SprayD. On the other hand, MeltQ remained semi-amorphous after extended storage at 43, 53, and 64%RH, as indicated by the presence of two glass transitions and a broad endothermic peak.