Extent of Inhibition of Malonic Acid on the Fermentation of Soursop Juice: A Thermodynamic Evaluation

The fermentation of soursop juice as substrate by Saccharomyces cerevisiae (yeast) was investigated to obtain certain useful thermodynamic parameters. This was achieved by the determination of the effects of temperature, substrate and inhibitor on the rate of carbon (IV) oxide (CO₂) production. The extent of inhibition was examined by the addition of malonic acid inhibitor. The results indicate that the rate of fermentation of soursop juice increased in proportion with temperature (optimum 36^oC), substrate (optimum 50%v/v) and malonic acid (optimum 30%v/v) up to a limit and decreased. This suggests that the reaction takes place in two steps (equilibrium and decomposition steps). The thermodynamic parameters determined are: enthalpy, H* 151.14 kJmol^-1; entropy of activation, S* 275.359 Jmol^-1k^-1; Gibb’s free energy of activation, G* -752.21 kJmol^-1 and Equilibrium constant K_eq, 1.33 respectively. The results show that the enthalpy value is positive indicating that the fermentation process is endothermic while the activation energy is same as the enthalpy. The entropy value is positive corroborating the positive enthalpy value. The fermentation process is spontaneous as shown by the negative change in free energy. These values were subsequently used to obtain by calculation Arrhenius constant A, 1.51×10²³; orientation factor P, 1.41×10^-34 and the collision frequency, Z, 1.07×10¹¹ respectively. The calculated A and the P values obtained on the basis of collision theory and absolute reaction rate theory (or transition state theory), respectively show that the collision factor is of the order of 10¹¹ min^-1, about half that of gaseous molecules that can only be attained by an induced reduction in activation energy by a catalyst. The equilibrium constant value K suggests that the intermediate complex is rather interacting more with the substrate and also showing that only a limited amount of the substrate is converted to product. The equilibrium constant K is observed to be 1.33dm³mol^-1 and not significantly far from unity implying that G* can be conveniently equated with the change in free energy at standard conditions. Finally, the type of inhibition was found to be uncompetitive from the extrapolated Lineweaver-Burk plots and did not bring about marked decrease in the rate of fermentation as expected.

Keywords: Fermentation, Inhibition, malonic acid, soursop juice, thermodynamic evaluation