Liposomal stabilization of ascorbic acid in model systems and in food matrices

Ascorbic acid (AA) was encapsulated into dipalmitoylphosphatidylcholine (DPPC) and dipalmitoyl-phosphatidylcholine/cholesterol (DPPC/chol) liposomes. We have shown that co-encapsulation of citric and ascorbic acids into liposomes results in considerable stabilization of the latter in model systems containing catalytic concentrations of copper ions. The rate of AA oxidation was decreased by up to 300-fold that of free AA. Incorporation of cholesterol into membranes resulted in lower stabilization efficacy at room temperature and better thermal stability at higher temperatures than pure DPPC liposomes. Total AA, extra-liposomal AA and leakage of AA out of liposomes during thermal treatment were evaluated using the free radical 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS'). A modification of the ABTS based assay for liposome permeability makes it applicable to reducing agents and antioxidants as well as AA. Encapsulated AA was also stabilized in real food matrices. The rate of AA oxidation in apple juice, relative to that of free AA, was decreased by two orders of magnitude when encapsulated in DPPC liposomes and by more than one order of magnitude in DPPC/chol liposomes. Liposomal stabilization of AA in a fermented milk product was less pronounced. (C) 2011 Elsevier Ltd. All rights reserved.