Transfers of small analytes in a multiphasic stirred fruit yoghurt model

The transfer of small analytes in a multiphasic stirred fruit yoghurt model, made of a pectin gel aimed to mimic fruit pieces and of a dairy gel done with milk acidified by glucono-δ-lactone hydrolysis, have been studied. The concentration gradients between the pectin gel and the dairy gel were the driving force for the migration of small analytes (i.e. water, protons and colorants). Water migrated from the dairy to the pectin gel, causing modifications in the water content of both gels and an equilibration of their water activity at 0.938±0.003. Inversely, protons migrated from the pectin to the dairy gel. These changes in composition of both gels being likely to have induced structure modifications. Migration of relatively small molar mass colorants did not depend on steric considerations, but rather on their charge and structure. In both gels, most of the colorants studied were charged at the equilibrium pH 4.1. This charge was linked to their diffusivity in the pectin gel. Negatively charged colorants were thought to be affected by the attractive electrostatic forces between the pectin carboxylic groups and Ca2+, causing a decrease in their diffusivity. Transfer of colorants in the dairy gel were affected by electrostatic interactions with milk proteins and ions or by hydrophobic interactions with milk proteins and fat.