Investigating the Degradation of Amorphous Vitamin C in Polymeric Model Systems
Vitamin C is an essential nutrient and a chemically labile compound found in natural foods and added as an ingredient to food products. A challenge for delivering vitamin C in foods is the tendency for the vitamin to degrade, which leads not only to a loss of nutritional quality but also to browning. Recent work has shown that vitamin C is likely in the amorphous solid state in many dry and intermediate moisture foods. Amorphous forms of an ingredient are inherently unstable and tend to crystallize over time. The study of vitamin C degradation in model amorphous polymeric matrices is limited and a comparison of simple polymer systems has not been used to systematically investigate the factors that affect vitamin C degradation. In this study, two forms of vitamin C (ascorbic acid and sodium ascorbate) were formulated with four polymers, polyvinylpyrrolidone, pectin, pectin (potassium esterified), polyacrylic acid, and sodium polyacrylate at different vitamin weight proportions. These polymers were selected for their different hydrogen bonding capabilities and to investigate the relationship between polymer vitamin C crystallization inhibition properties and vitamin degradation. Samples were solubilized in water, lyophilized in a freeze drier, and stored at different temperatures (20°C, 25°C, 30°C, 40°C, 50°C) and different relative humidities (RHs) (11%, 52%, 75%) for up to one month. High performance liquid chromatography with a photodiode array detector (HPLC-PDA) was used to quantify vitamin content remaining. Additional properties were analyzed using a pH meter, powder X-ray diffraction (PXRD) to characterize physical state, dynamic moisture analyzer to collect moisture sorption isotherms, and differential scanning calorimetry (DSC) to characterize glass transition temperatures (Tg). Generally, vitamin-polymer solid dispersions formulated with ascorbic acid experienced less degradation than did systems formulated with sodium ascorbate; however, the difference in vitamin loss was dependent on the polymeric matrix with some polymers exhibiting similar vitamin loss for either vitamin form. Vitamin degradation significantly increased with increases in moisture (storage at higher RH) but hygroscopicity of the polymer matrices did not explain the differences in degradation observed between different vitamin-polymer solid dispersions. The Tgs of the vitamin-polymer solid dispersions also did not correlate with degradation. The degradation of vitamin C in amorphous model systems depended most on the polymeric matrix in which the vitamin was dispersed. The greatest vitamin degradation occurred in polyvinylpyrrolidone matrices (70% ascorbic acid and 54% sodium ascorbate remaining after one month) while little to no vitamin degradation (>98% vitamin remaining) occurred in polyacrylic acid matrices in the same timeframe and conditions (11%RH and 30°C). The degree of vitamin-polymer interactions and the vitamin weight proportion determined the miscibility and, thus, crystallization tendency. The degree of vitamin-polymer interactions and the specific sites of hydrogen bonding explained the largest differences in vitamin degradation found among different vitamin-polymer solid dispersions. This study proposes that inter-matrix hydrogen bonding is a critical factor for chemical reactivity of small molecules in amorphous systems.
Mauer, Purdue University.
Food Science|Materials science
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