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How are the microcapsules formed and what holds them together? Clearly, emulsification must occur during the microscopic dispersion of the nonaqueous phase into the aqueous protein solution. Ultrasonic emulsification is a well-known process14 and does occur in this biphasic system. Emulsification is necessary for microcapsule formation. However, if vortex mixing emulsification is used instead, microcapsules are not formed. Consequently, emulsification by itself is not sufficient for microcapsule formation. Denaturation of the protein by thermal or hydrophobic processes might be invoked to hold the microcapsules together after initial emulsification. High concentrations of microcapsules are observed when the mixture is sparged with air or 02. If the reaction is run under an inert atmosphere (He, Ar, or N2), however, microcapsules are not formed. Thus, thermal or solvent denaturation (for which 02, N2, and Ar should give similar results) cannot explain the microcapsule permanence.Another, chemical process must be involved. There is a wide range of high-energy chemistry associated with ultrasonic irradiation of liquids, arising from acoustic cavitation (the implosive collapse of bubbles).6 Aqueous sonochemistry produces15 OH* and H*. The radicals so produced by ultrasound16 form H2, H202, and in the presence of 02, superoxide17 (H02). Hydroxyl, superoxide, and peroxide are all potential protein cross-linking agents.

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