Crystallization inhibitor properties of polymers and effects on the chemical and physical stability of L-ascorbic acid during preparation and storage
Crystalline L-ascorbic acid (VitC) is an essential nutrient solid that is widely used in food applications for fortification, antioxidant, and label claim purposes. VitC is known as one of the most unstable vitamins, and its content must be declared on food labels. Interest has been increasing in creating amorphous solid structures for improving the solubility and/or dissolution rates of crystalline compounds. One of the most promising methods studied to prevent the crystallization of an amorphous solid is the use of polymers in dispersions, although the specific polymer properties that lead to the best physical stability for a particular type of compound have not yet been identified. Although crystalline ascorbic acid is hydrophilic and therefore there is likely little nutritional advantage for forming an amorphous structure, there is the possibility that food formulation and processing treatments render the vitamin amorphous. There is little published information on manipulating the physical structure of VitC. Thus, there is an opportunity to not only improve the understanding of crystallization inhibitor properties of polymers applied to VitC dispersions, but also generate valuable information about how amorphous forms of VitC differ in their stability from the crystalline structure. The objectives of this study were to create amorphous solid dispersions of VitC and determine the effects of polymer types on inhibiting the crystallization of these dispersions, and to characterize the physical and chemical stability of the VitC amorphous solid dispersions. VitC amorphous solid dispersions were formed with different types of polymer (pectin, polyacrylic acid, and polyvinylpyrrolidone) using lyophilization. Samples were stored at different temperature (25, 40°C) and relative humidity (0, 23, 54, 75%RH) conditions. Physical stability was monitored with powder X-ray diffraction, polarized light microscope, Fourier transform infrared spectroscopy, differential scanning calorimetry, and SPS moisture sorption isotherm techniques. Chemical stability was monitored using a microplate reader assay and brown color development measured with a Hunter colorimeter. Amorphous solid dispersions formed with pectin and polyvinylpyrrolidone (PVP) (50% w/w) were stable for up to four months when stored at low %RHs conditions (below 23%RH), but not above 54%RH. The dispersions formed with polyacrylic acid (PAA) crystallized immediately upon formation. The ability of the polymers to interact with VitC via hydrogen bonding was found to be the most important element for forming a stable VitC amorphous dispersion, correlating better with the dispersion stability than the Tg and the hygrosocicity of the polymers. The order of VitC chemical stability from the highest to the lowest was the pure VitC = freeze dried VitC (both crystalline) > crystalline VitC - amorphous polymer physical mixtures > amorphous VitC - polymer solid dispersions. VitC amorphous dispersions were successfully formed with polymers that were able to form strong hydrogen bond interactions with VitC, and these dispersions were stable when stored at low RH conditions (≤ 23 %RH). When the VitC in a dispersion recrystallized, it was less chemically stable than its initially crystalline counterpart. The decreased stability of Vit C upon recrystallization could account for some of its degradation in intermediate moisture food products wherein such occurrences are possible.
Mauer, Purdue University.
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