Physico-chemical properties of human breast milk were compared to four Similac ™ infant formulas, and correlated with in vitro free fatty acid bioaccessibility using a simulated gastrointestinal system (TIM-1). Viscoelastic measurements, as a function of pH (pH 6.5 to 3.0) and shear rate, showed lower viscosities in breast milk compared to infant formulas. Droplet size and distribution measurements showed distinct differences between the tested formulas and breast milk. During lipid digestion, a lag period was observed for only breast milk. The rate of lipolysis was found to be higher in breast milk compared to Similac ™ formulas. The total bioaccessible free fatty acids for Advance infant formula and breast milk were not statistically different for the in vitro TIM-1 model and the shifted-logistical model using one-way ANOVA (p<0.05) with a Tukey's Multiple Comparison Test. All other infant formulas had significantly lower free fatty acid bioaccessibilities at the end of the simulated digestion. A positive correlation between rate of lipolysis and droplet surface area per gram for the Similac ™ infant formulas was found. However, breast milk did not follow that trend, suggesting the possible involvement of other factors in rate of lipolysis for breast milk.

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