Effect of pH and Temperature on the Stability of Vitamin C in Freshly Squeezed Orange Juice During Storage in Osun State

Orange juice is a commercially important product in Osun State, South West Nigeria, where widespread citrus cultivation supports a growing juice processing industry centred on Osogbo and Ife-Jesa. This study investigates the degradation kinetics of ascorbic acid in freshly squeezed Navel orange juice under varying pH and temperature conditions during a 7-day storage period. Juice was extracted from oranges harvested from farms in Ife North Local Government Area, adjusted to pH values of 3.0, 3.5, 4.0, and 4.5 using food-grade citric acid and sodium hydroxide, and stored at 4, 25, and 37 degrees Celsius in amber glass containers. Vitamin C concentration was determined at 24-hour intervals using the iodometric titration method verified against HPLC measurements at selected time points. Vitamin C degradation followed first-order kinetics at all pH and temperature combinations tested. The highest degradation rate constant was recorded at pH 4.5 and 37 degrees Celsius (k = 0.087 per day), while the lowest was at pH 3.0 and 4 degrees Celsius (k = 0.009 per day). Activation energy calculated from Arrhenius plots ranged from 34.2 to 48.7 kJ/mol depending on pH. Results confirm that refrigeration at pH 3.0 to 3.5 is optimal for preserving Vitamin C in orange juice for up to 7 days without artificial stabilisers. The findings provide practical guidance for small-scale juice processors in Osun State. Keywords: ascorbic acid, orange juice, degradation kinetics, storage, Osun State