MOISTURE SORPTION AND SELECTED THERMODYNAMIC CHARACTERISTICS OF MORINGA (Moringa oleifera Lam.) SEED AND GRITS

Abstract

Moringa seed is a raw material that needs long period of storage. This necessitates drying the seed so as to extend its shelf life. An important factor in ensuring stability of moringa seed and grits in storage is its water activity (aw). However, there is dearth of information on the sorption characteristics of moringa seed and grits. This study was therefore, designed to investigate the sorption and some thermodynamic characteristics of moringa seed and grits during storage. Moringa seeds and grits (particle size 106-212 μm) were kept in two packaging materials (Polypropylene [Pp] and Low-Density Poly-Ethylene [LDPE]) and stored at ambient condition for 12 days, and shelf life was predicted using Heiss-Eichner model. Sorption Isotherms (SI) of moringa seed and grits were obtained by determining the Equilibrium Moisture Content (EMC) at a temperature range of 20-40℃ and nine aw (0.09-0.92) levels using thermostatic water bath and concentrated H2SO4. The sorption data were fitted into six selected models namely: Brunauer-Emmett-Teller (BET), Guggenheim-Anderson-De Boer (GAB), Hailwood-Horrobin (HH), Modified Hasley (MH), Modified Henderson (MHM) and Modified Hailwood-Horrobin (MHH) models. The performance of the models was tested using Root Mean Square Error (RMSE), co-efficient of determination (R2) and Residual Sum of Squares (RSS). The effect of temperature on the monolayer moisture content (Mo) of the seed and grits was determined using BET, GAB and Caurie models. Clausius-Clapeyron equation was used to determine the isosteric heat (ΔH) and entropy of sorption (ΔS) of the seed and grits. Data were analysed using ANOVA at α0.05. The predicted shelf life in Pp and LDPE were 1,852 and 1,446 days; 994 and 748 days for seed and grits, respectively. The SI curves generated were sigmoidal in shape indicating Type-II isotherm. The GAB and MHM were the best-fitted models and predicted the EMC at all temperatures with highest R2 (0.9994, 0.9999) in adsorption and (0.9994, 0.9999) in desorption processes for seed and grits, respectively. The BET and HH models did not fit well at all temperatures with R2 (0.6957, 0.6869); (0.8439, 0.8149) for seed and grits, respectively, while MHH fitted well with R2 (0.9111) for grits at a temperature of 20℃ only. The GAB gave the least RMSE and RSS and it ranged 0.0775-0.2124 and 0.0060-0.0431, respectively. The Mo for the seed and grits ranged 2.9–5.9% db and 2.1–4.4% db, respectively. Hysteresis did not have significant effect on Mo for both seed and grits. The ΔH decreased as the EMC increased and it ranged 9.90-40.86 kJmol-1 for seed and 4.75-38.71 kJmol-1 for grits in adsorption; 10.85-45.09 kJmol-1 for seed and 4.82-41.66 kJmol-1 for grits in desorption. The ΔS ranged 0.03-0.124 kJmol-1K-1 for seed and 0.014-0.113 kJmol-1K-1 for grits in adsorption; 0.033-0.137 kJmol-1K-1 for seed and 0.015-0.124 kJmol-1K-1 for grits in desorption. The ΔH and ΔS increased exponentially as the EMC decreased. Sorption isotherm of moringa seed and grits demonstrated Type-II behaviour. Moringa seed and grits stored in polypropylene had longer shelf life and less interaction with the micro-environment during storage than Low Density Polyethylene.