In vitro gastric digestion of interfacial protein structures: visualisation by AFM

Abstract

The rational design of healthier foods to control fat uptake in the diet is an emerging area of research. In order to address these dietary issues effectively one requires an improved understanding of the influence of food structure on the digestion of lipids. Despite the crucial roles played by interfaces in determining the stability of emulsions and the breakdown of emulsion structure on digestion, there are few systematic basic studies dealing directly with the effects of digestion conditions on interfacial structures. A major concern in the attempted manipulation of protein-based interfacial structures is the potential enzymatic breakdown of these structures during digestion. This study has been designed to investigate the effects of the proteolytic enzyme pepsin on β-lactoglobulin surface layers under in vitro gastric digestion. Interfacial dilatational rheology has been used to measure the evolution of the viscoelastic properties of the pre-formed interfacial protein networks. Furthermore, Atomic Force Microscopy (AFM) has been used to visualise directly the effects of degradation on model interfacial protein layers (β-lactoglobulin). The results obtained show that pepsin partially hydrolyses the surface-adsorbed β-lactoglobulin molecules under gastric conditions, but does not break the interconnected surface network. In order to investigate the influence of the presence of surfactants on the enzymatic digestion of emulsions, the effect of addition of the model surfactant Tween 20 on the pepsin-induced changes in interfacial structure under simulated gastric conditions was studied. Modification of the surface conformation of the proteins by surfactant, during ‘orogenic’ displacement, revealed an unexpected synergism which leads to enhanced hydrolysis of the proteins; hence, weakening the network and allowing rupture to occur at lower surface pressures. Such observations are important new generic features of the digestion process which could be potentially manipulated to control the enzyme hydrolysis, allowing rational design of food structures and manipulation of properties such as fat digestion.