Formation of a Protein Aggregate Layer at a Milk/Acidified Gel Interface

Abstract In bilayer systems consisting of acidified sweetened agar gels and milk, mass transfer between the two layers led to the formation of a milk protein aggregate layer at the boundary, so called ‘hard layer’. The aim of this work was to understand main mechanisms involved in the formation of that ‘hard layer’. The kinetics of growth of the hard layer was followed using an experimetal set up described here, and in situ pH profile was determined by a microelectrode. The boundary of the hard layer with milk was found to coincide with pH 4.9. Increased acid concentration in the agar gel and preacidication of the milk increased the rate of formation of the ‘hard layer’. Increased sucrose concentration in the agar gel increased the rate of formation at longer contact times. This effect of sucrose was explained by a modification of water activity in the milk phase brought by mutual sucrose–water migration. It was concluded that the hard layer was induced by a pH drop of milk in contact of acidifed agar gels, below the gelation pH of milk in the boundary region. That pH drop was caused by acid and sugar migration from the agar gel into the milk phase.