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Adsorption and desorption isotherms of two commercial enzyme preparations of papain and bromelain were determined with a Dynamic Vapor System. The Guggenheim-Anderson-deBoer (GAB) modeling of the obtained sorption isotherms allowed the definition of different levels of hydration of those samples. Afterward, these enzyme preparations were used as biocatalysts in water and solvent-free esterification and alcoholysis reactions. The evolution of the obtained fatty acid ester level as a function of the initial hydration level of the biocatalyst, i.e., thermodynamic water activity (a(w)) and water content, was studied. The results show an important correlation between the initial hydration level of the biocatalyst and its catalytic activity during the lipase-catalyzed synthesis reactions. Thus, the Carica papaya lipase (crude papain preparation) catalytic activity is highly dependent on the biocatalyst hydration state. The optimized synthesis reaction yield is obtained when the a(w) value of the enzyme preparation is stabilized at 0.22, which corresponds to 2% water content. This optimal level of hydration occurs on the linear part of the biocatalyst's sorption isotherm, where the water molecules can form a mono- or multiple layer with the protein network. The synthesis reaction yield decreases when the a(w) of the preparation is higher than 0.22, because the excess water molecules modify the system equilibrium leading to the reverse and competitive reaction, i.e., hydrolysis. These results show also that an optimal storage condition for the highly hydrophilic crude papain preparation is a relative humidity strictly lower than 70% to avoid an irreversible structural transition leading to a useless biocatalyst. Concerning the bromelain preparation, no effect of the hydration level on the catalytic activity during esterification reactions was observed. This biocatalyst has too weak a catalytic activity which makes it difficult to observe any differences. Furthermore, the bromelain preparation is far more hydrophobic as it adsorbs only 18 g of water per 100 g of dry material at a(w) around 0.90. No deliquescence of this enzymatic preparation is observed at this a(w) value.

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Lipase fatty acid typoselectivities of Euphorbia characias latex and commercially available crude preparation of bromelain were determined in the hydrolysis of homogeneous triacylglycerols (TAG) and natural TAG mixtures. Their activities were compared to a commercially available crude preparation of papain. Under optimal lipolysis conditions at pH 8.0 and 10 min of incubation time, maximal activities were observed at 45, 55, and 50°C, respectively, for E. characias latex, crude bromelain, and crude papain. Commercially available crude preparations of bromelain exhibited very poor hydrolysis activity. Latex from E. characias, which contained 340 mg of dried material per milliliter of fresh latex, exhibited a high lipase activity and a short-chain fatty acid preference in the hydrolysis of homogeneous TAG. For all substrates, it showed a better activity than crude papain. Lipase fatty acid typoselectivities of crude bromelain and crude papain also were studied in interesterification reactions of tributyrin with a series of homogeneous TAG. Experiments showed that crude bromelain [water activity (A w ): 0.21] had no activity in interesterification. Regarding reactions with crude papain (A w : 0.55), yields of newly formed TAG decreased with increasing chain length of TAG, except for the reaction with trimargarin. For interesterification of tributyrin with unsaturated TAG, triolein reacted faster than polyunsaturated TAG. During these interesterification reactions, the proportion of new TAG with two butyroyl residues was higher than new TAG with only one butyroyl residue. This phenomenon was more pronounced for reactions with long-chain TAG.

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Short-chain fatty acids are usually located at position sn-3 in natural triglycerides, particulary in dairy fats. As a result, it is extremely difficult to differentiate between sn-3 stereospecificity and short-chain typoselectivity in many lipases and acyltransferases that perform in this way. This ambiguity can be removed through successive use of a chiral triglyceride with a short fatty acid in position sn-1 and of its racemic in controlled hydrolysis reactions. After checking that the proposed method effectively confirmed the type of activity of control biocatalysts (Candida cylindracea nonspecific lipase and Mucor miehei 1-3 regiospecific lipase), we confirmed that Carica papaya latex has a strict sn-3 stereospecificity. JAOCS 72, 7.53-755 (1995).