Stability of vitamin C in onion during drying and storage-a kinetic approach

Abstract

The study was conducted to determine vitamin-C stability in mechanical and solar dried summer onion during storage at different temperatures. The effect was studied in terms of drying time (15, 30, 45, 60 and 75 minutes) and temperature (52⁰C, 60⁰C, 68⁰C) for mechanical drying (MD) and 45-50⁰C for solar drying (SD). Increased loss of vitamin C was observed with the increase in drying time and temperature. It was found that the reaction rate constant, K value (/min) was the highest (0.0302/min) for solar drying (45-50⁰C) and was followed by that at 68⁰C (0.029/min) and 60⁰C (0.0276/min), while the lowest k-value (0.0236/min) was given by the sample dried at 52⁰C. Vitamin C losses in onion was the highest during solar drying followed by mechanical drying at 68°C and 60⁰C, while the lowest losses of vitamin C was found for mechanical drying at 52°C. It was also seen that rate constant has an exponential type relationship with inverse absolute temperature and activation energy; E_a for degradation of vitamin C was found to be 3.20 kcal/g-mole for onion. The highest rate constant (0.0046/day) was given by dried osmosed onion stored at RT which was successively followed by dried sulphited onion stored at RT (0.004 /day), and dried sulphited onion stored at RFT (0.0033/day). On the other hand, dried osmosed onion stored at RFT gave the lowest (0.0029/day) rate constant during the period of storage (one year).