Enzymatic acylglycerol synthesis in membrane reactor systems

<p>Up till twenty years ago, only chemical modifications of agricultural oils for novel uses were studied. Because of the instability of various fatty acids, enzymatic biomodifications can have advantages above the chemical route. Nowadays, enzymatic catalysis can be used for the modification of oils and fats. One way of biomodification is the enzymatic esterification of glycerol with fatty acid for the synthesis of mono- and triacylglycerols. Monoesters (monoacylglycerols) are used as emulsifiers in food and in cosmetics, tailor made triesters (triacylglycerols) are used to adjust the melting range of foods and cosmetics. This thesis describes a number of membrane reactor systems for the enzymatic esterification of glycerol with decanoic acid in hexadecane as solvent. Description and modelling of the kinetics and thermodynamic equilibrium have resulted in reactor concepts to reach the objective of mono- and triester synthesis.<p>The basic reactor studied is a two-phase immobilized enzyme membrane reactor. In the membrane reactor, lipase from <em>Candida ragosa</em> is immobilized at the inner fibre side of a hydrophilic hollow fibre module. Decanoic acid in n-hexadecane is circulated at the same side, meanwhile a water-glycerol phase is circulated at the shell side. The glycerol diffuses through the membrane matrix allowing the synthesis to take place at the interface. The water produced diffuses backwards.<p>Chapter 2 describes the enzymatic esterification of decanoic acid with glycerol for an emulsion system and for a hydrophilic membrane system. In a two-phase system, the enzyme activity is related to the oil-phase volume, the interface area and the enzyme load. The rate per unit interface area of the membrane system approximates the rate measured in an emulsion system. This implies that the cellulose membrane does not affect the esterification. Another consequence is that the activity per oil-phase volume is only specific surface area related, therefore a hollow fibre device is desirable. The optimum enzyme load in the membrane system is half of that in the emulsion system.<p>The enzyme stability in glycerol-water mixtures is described in chapter 3. The activity of lipase from <em>Candida rugosa</em> with time can be described with a two-step model, assuming the native lipase reversibly altering its conformation to a form having no activity. The reversibility is experimentally verified. Both, the native and inactive form do inactivate irreversible at the same time to a completely inactive form. The inactivation is a function of the glycerol concentration. The activity of immobilized enzyme is reduced to the same level of activity as is found for free lipase.<p>Not only activity and stability of the enzymatic system are of importance, also the equilibrium ester concentrations must be known in the non-ideal two-phase system. Chapter 4 presents the program TREP (Two-phase Reaction Equilibrium Prediction). With the use of measured thermodynamic activity based equilibrium constants, mass balances and the UNIFAC group contribution method, TREP predicts the equilibrium product and substrate concentrations for given initial amounts. Equilibrium predictions show that an excess of triesters can be obtained only at low water activity conditions, in this case an one-phase system is predicted. Predictions show that pure monoesters cannot be obtained in a two-phase system of decanoic acid-hexadecane phase and a glycerol-water phase, even with a high glycerol to fatty acid ratio. This is experimentally verified.<p>From the knowledge gathered in these chapters, two membrane reactor systems are designed, one membrane reactor for the triester production and a second membrane reactor system equipped with an in-line adsorption column for the synthesis of monoesters.<p>Chapter 5 describes a pervaporation system in which an excess of triesters can be synthesized at low water activity conditions. Lipase is immobilized onto the lumen side of a cellulose membrane where the organic phase is present. At the shell side, air circulates and the water activity is controlled with the use of a condenser. The lipase catalyzed esterification of decanoic acid with partial glycerides is studied in this reactor. In agreement with the predictions made in chapter 4, an excess of triacylglycerols, is obtained at low water activity conditions only.<p>A second membrane reactor concept is described in chapter 6, the organic-phase is led over an adsorption column in order to adsorb the monoglycerides onto the adsorbate. When the column is saturated with monoesters, the column can be desorbed off-line in a continuous membrane/repeated batch column process. If a 5 % ethanol in hexane solution is used as desorption solvent, monoesters are desorbed selectively leading to a 90 % purity.<p>Finally, in chapter 7, the potentials and limitations of the enzymatic esterification are discussed. To predict the steady-state concentration of a continuous reactor, the enzyme kinetics must be described. The membrane reactor is reaction limited, this could be overcome by placing a column packed with immobilized enzyme in the organic phase recirculation loop. Not only esterification can be performed in the pervaporation system, this system could also be suitable for interesterification or transesterification. Then the program TREP should be extended for reactions with different types of fatty acids