Evaluating Stability of Vitamin C in Fortified Formula Using Water Activity and Glass Transition

Abstract

Stability of vitamin C in fortified formula food at constant temperature, as affected by the water activity (a_w ) and glass transition (T_g ), was investigated in an attempt to differentiate the effects of a_w and T_g on the degradation reaction. Fortified formula samples, equilibrated at different moisture contents/water activity levels, were stored for more than 10 weeks at the room temperature, 23°C. The water activity and glass transition temperatures as a function of moisture content were determined for fortified formula to compare the two distinct criteria of food stability. The adsorption experiments, which were carried out at controlled water activities, at 23°C using an isopiestic method, were modeled using the GAB equation. The glass transition temperatures, which were measured by modulated differential scanning calorimeter, were modeled using the Gordon-Taylor equation. The loss of vitamin C followed a zero-order kinetic model. The vitamin C retention after storage of 73 days at water activities of 0.11 and 0.87 was 40 and 7%, respectively. The rate constants ranged from 5.8 to 10 mgd⁻¹, depending on the water activities. The constants were correlated with water activities and glass transition temperatures. Results indicated that reactions rates correlated with both water activity and the glass transition temperatures. In the storage and processing of fortified food products, both the water activity and glass transition should be recognized as important criteria for gaining a better understanding of vitamin stability.

Keywords:

• Differential scanning calorimeter
• Neutraceuticals
• Reaction rate kinetics
• Water sorption isotherms