The increase in temperature of a reaction mixture usually results in an increase in the reaction rate. This is mainly due to the increase in rate constants with temperature and partly due to the reduction in viscosity and mass transfer resistances. However, in enzymatic catalyzed reaction, this increase in reaction rate with temperature persists up to a certain optimum temperature, after which the rate decreases sharply. This sharp drop takes place at the onset of the denaturation of the enzyme that occurs at elevated temperatures. In addition to the deactivation of the enzyme, the presence of the inactive enzyme at the interface blocks the active enzyme from penetrating the interface, which would further decrease the reaction rate. This trend has been consistently observed in all studies that investigated the effect of temperature on the production of biodiesel by lipase. The critical temperature, at which the enzyme starts to deactivate, was different as shown in Table 6.1. Generally, lipases from bacterial sources, such as those from Pseudomonas species, have relatively higher thermo-stability than lipases from yeast source, such as those from Candida species that include Novozym 435. For example, the optimum operating temperature of lipase from P. fluorescens has been reported to be 65°C (Fukuda et al., 2001), whereas that of lipase from Novozym 435 has been reported to be 35-40°C (Chang et al., 2005). Immobilization provides a more rigid external backbone for lipase molecule, allowing it to maintain its activity at higher temperatures than if it is in free-form. Hence, the reaction optimum temperature is expected to increase, which results in faster rate of reaction.