Mathematical Modelling of Water Activity and Moisture Sorption Isotherms for Nigerian Dried Fish Products

Water activity is a primary determinant of the microbial, chemical, and physical stability of dried fish products, and moisture sorption isotherms provide the thermodynamic data essential for designing packaging systems and storage conditions that maintain product safety and quality. Despite the economic importance of dried fish to Nigerian fishing communities, particularly in the South South and South East zones, sorption isotherm data for locally produced dried fish under Nigerian climatic conditions are limited. This study determines the moisture sorption isotherms of three commercially significant dried fish products, namely smoked catfish from Bayelsa State, sun-dried stockfish from Anambra State, and smoked sardines from Cross River State, at temperatures of 25, 35, and 45 degrees Celsius and relative humidity from 11 to 97 percent. The Brunauer-Emmett-Teller, Guggenheim-Anderson-De Boer, Halsey, Oswin, and Smith models were fitted to the experimental data. The Guggenheim-Anderson-De Boer model provided the best fit for all three products at all temperatures, with R-squared values above 0.98 and root mean square errors below 0.009. Monolayer moisture content values decreased with increasing temperature, consistent with thermodynamic expectations. Isosteric heats of sorption were calculated using the Clausius-Clapeyron equation. The study contributes isotherm data specific to Nigerian dried fish products and provides design parameters for the selection of packaging materials and storage conditions. Findings are relevant for fish processors, packaging engineers, and food safety researchers. Keywords: moisture sorption isotherm, water activity, dried fish, mathematical model, Nigeria