Kajian Isotermi Sorpsi Air Biji Jarak Pagar (Jatropha curcas L)

Abstract

Jatropha seeds storage is an important step that can affect the process of biodiesel production. At jatropha seeds storage, free fatty acid content can be increased so that the oil yield will be reduced. This situation is due to oxidation and hydrolysis reactions that have relationship with water. The existence of water on jatropha seeds is reflected in the water activity (aw). Bell and Labuza (2000) said that oxidation can increased at water activity above 0.3. The relationship between water content with aw is described by the moisture sorption isotherms curves. The objectives of this research is to study the equilibrium moisture content, the best fitted model of moisture sorption isotherms and the relationship between equilibrium moisture content with free fatty acid content of jatropha seeds. The moisture sorption isotherms of jatropha seeds were determined at two different temperatures (30 and 40°C) and ten different water activity (0.14, 0.15, 0.27, 0.37, 0.43, 0.54, 0.68, 0.71, 0.79 and 0.83). Eight models namely the BET, GAB, Halsey, Harkins-Jura, Henderson, Oswin, Smith and Iglesias-Chirife were fitted to the sorption data. The models were compared using the modulus deviation (P) and coefficient of determination (R2). The moisture sorption isotherms were sigmoidal in shape. On the two highest aw 0.79 and 0.83 there was a sharp increase of equilibrium moisture content. The hysteresis phenomena was not occurs between desorption and adsorption process. The moisture sorption isotherm of jatropha seeds decreased with increased in temperature at constant aw. The best fitted model in the range aw from 0.14 to 0.83 was GAB model. Modulus deviation value of GAB model at adsorption 30°C, desorption 30°C, adsorption 40°C and desorption 40°C is 11.3, 12.14, 15.37 and 12.48 respectively. Coefficient of determination value of GAB model at adsorption 30°C, desorption 30°C, adsorption 40°C and desorption 40°C is 0.80, 0.80, 0.70 and 0.85 respectively. Monolayer moisture content resulting from the BET and GAB model was 3.00 to 4.36 (% db). At monolayer moisture content, free fatty acids content were not increased. The free fatty acids content increased with increased in equilibrium moisture content although fluctuating at 30°C. Free fatty acid content in the two highest aw 0.729 and 0.825 tend to increase. This relationship can be more specifically described by polynomial equations.