Factors influencing the dynamics of emulsion structure during neonatal gastric digestion in an in vitro model

Abstract

We investigated the chemical, electrochemical and physical changes in the composition of a formula milk following digestion in an in vitro system that replicated the shear rates that would normally be encountered in the infant stomach. We examined these changes over a range of shear rates and pH levels that would be encountered in stomach lumen postprandially, and in the presence and absence of proteolytic and lipolytic enzymes. Digestion was conducted either with a fungal lipase that is commonly used in ex vivo systems (Rhizopus oryzae) or a mammalian gastric lipase (calf pregastric lipase).Prior to digestion the fat droplets in the formula milk were considerably smaller than those in human breast milk, and were stabilised by an adsorbed layer of milk protein. Droplet size increased progressively by coalescence during in vitro digestion at pHs between 3.5 and 4.5 when both lipase and protease were present, but not when either enzyme was omitted or when pH levels were outside this range. Coalescence was augmented by shear, notably at rates above the normal physiological range.Hence the stability and structural properties of formula emulsions appear to be influenced by disruption of the proteinaceous oil/water interface during digestion, being most susceptible to the concerted activity of pepsin and gastric lipase over a limited range of pH. Given that the onset of secretion of pepsin, lipase and hydrochloric acid does not occur synchronously in the developing infant stomach, inappropriately formulated milks may lower digestive efficiency. The fidelity of in vitro systems does not appear to be compromised by the use of fungal lipases but may be compromised by the use of inappropriately high stirring rates.