Effect of ethanol on the solubilization of hydrophobic molecules by sodium caseinate

Many food and pharmaceutical ingredients are lipophilic small molecules, insoluble in water, and therefore difficult to disperse in aqueous oral formulations. While some can be solubilized with surfactants or with ethanol, there is a demand for alternative approaches including binding by proteins. This study examines the use of ethanol to enhance the loading of sodium caseinate with pyrene and piperine. A large excess of pyrene or piperine (i.e., many times the aqueous solubility limit) was mixed with sodium caseinate in buffer and varying amounts of ethanol as a co-solvent, then lyophilized to remove ethanol and water. The resultant powder was rehydrated in buffer, the remaining insoluble pyrene or piperine was removed by gentle centrifugation and the concentration of solubilized fraction was determined. Pyrene is effectively insoluble in water however, in the presence of the caseinate, it was solubilized and showed a characterized fluorescence spectrum in buffer, the magnitude of which increased linearly with the concentration of protein present at the loading step. The fluorescence was lost following in vitro treatment with acid or proteolytic enzymes. The solubility of piperine in water was only 49 mg/L but increased to 130 mg/L in the presence of 2% sodium caseinate and 521 mg/L in the presence of 4% sodium caseinate if 40% ethanol was present at the loading step (i.e., 50 or 175 mol of piperine per mole of caseinate). Two mechanisms are studied to explain the role of ethanol in enhancing the loading capacity of caseinate. First, ethanol increases the solubility of piperine in water, which could facilitate better mass transport from undissolved piperine powder to the protein during the loading step. Second, ethanol changes the structure of the protein or protein aggregates which may allow improved loading.