## 1 Introduction  
The use of lipases as catalysts for esterification reactions of short carboxylic acids has great interest because the esters thus obtained enzymatically may be labeled as \"natural\" [1]. These esters are used as flavor or fragrance ingredients by a variety of industries (food, pharmaceutical, cosmetics, _etc._) [2]. Esterification is a simple reaction from a chemical point of view, consisting on the reaction between unmodified substrates, being the yields determined by the thermodynamic constant of the reaction.  
Lipase-catalyzed esterifications have been performed in monophasic, low water activity systems, aiming at shifting the equilibrium towards the desired synthetic direction (water is a product of the esterification), usually using organic solvents [3, 4], but also ionic liquids or supercritical fluids [5, 6, 7]. It has been shown that the water produced may be accumulated in the enzyme environment, producing its inhibition or even its inactivation. This problem may be solved by washing with solvents every few reaction cycles [8], or by using molecular sieves to remove the water from the medium [9]. Carboxylic acids and alcohols are also compounds capable of inactivating the enzyme. It has been suggested that the accumulation of all these substances is the main cause of observed lipase activity reduction during esterification reactions [8]. Therefore, an improved mixing of all reaction components might prevent the formation of an aqueous phase around the enzyme caused by a better dispersion of the carboxylic acids in the solvents, thus improving the enzyme performance. This improved mixing may be achieved by using ultrasound [10, 11]. Ultrasound technology increases the interaction between phases by cavitation caused by the collapse of bubbles, and because of improved mixing, shearing, and mass transfer in aqueous solutions or suspensions [12, 13]. Ultrasound technology has recently been used as an efficient way of mixing lipase-catalyzed reactions, such as transesterifications [14, 15], or esterifications for the synthesis of isoascorbyl ester [16], sugar esters [17], and flavor esters [18, 19]. However, few studies are found in the literature and scarce information is available on the mechanism(s) of ultrasound activation and its interaction with the enzymes and the immobilization supports.  
The present work is part of a continued research project in our group in the field of flavor ester production, specifically of butyl acetate, an ester with apple notes, catalyzed by lipase B from _Candida antartica_ (CALB) [8, 19, 20]. This lipase, in its free and immobilized forms, is the most used lipase, showing very good activity and stability properties [21]. In previous studies, Novozym 435 was used for butyl acetate synthesis obtaining high conversions (over 90%) in relatively short reaction times (2.5 h) [8]. Using ultrasonic energy, the process productivity was improved in the presence of higher amounts of substrate and the biocatalyst could be recycled for more reaction batches [19].  
Additionally, CALB was immobilized via interfacial activation on very hydrophobic styrene-divinylbenzene beads (MCI-CALB) [22]. This preparation was also evaluated for the butyl acetate synthesis, where it displayed better performance than Novozym 435 under mechanical stirring [20].  
In this context, the objective of this work was to analyze the performance of MCI-CALB as a catalyst for the esterification of acetic acid and butanol using ultrasound technology, comparing the effects with those previously found for Novozym 435 and with the same MCI-CALB, but using mechanical stirring. Therefore, reactions parameters were determined by an experimental design. Moreover, the effects of different carrier materials on the operational stability of the enzyme were investigated.