Although mothers' milk is the ideal food for babies, infant formula has become an alternative when breastfeeding is not possible or inadequate for babies. To design a proper formula for babies, it is essential to understand the digestibility of macronutrients and their bio-accessibility in the infant gastrointestinal tract. Because in vivo gastrointestinal studies on human infants are restricted by ethical constraint, cost issues, and intensive resource, in vitro models could be a better replacement.In vitro models offer advantages with low cost, easy sampling accessibility and no ethical issues. This thesis aims to assess the digestibility of each ingredient proteins, lipids, and carbohydrates in infant formulation then compare with mothers' milk. A static bench-top in vitro model for infant digestion was set up with infant gastric pH (4.0-4.5) and the activity of simulated digestive enzymes suitable for human infants with 60 minutes of gastric phase and 120 minutes of intestinal phase.Popular protein sources of caseins, whey, and soy proteins were employed in infant formulations.The in vitro digestion of these proteins in infant formulations was studied in the presence of enzyme proteases only (without lipolytic enzymes). Obtained results showed around 20% of caseins and no components of whey were hydrolysed after 60 minutes in the simulated stomach. In the simulated intestinal phase, 8% of α-lactalbumin was hydrolysed while caseins and β-lactoglobulin were completely digested immediately and 30 minutes respectively after addition of intestinal digestive proteases. Overall, soy proteins indicated lower level of hydrolysis than dairy proteins during in vitro infant digestion as observed by SDS-PAGE. The soy protein fractions glycinin and β-conglycinin were partially hydrolysed during the gastrointestinal phase. The observed pH drop confirms that caseins are easily digested in the intestinal phase compared to whey and soy protein. Gastric digestion resulted in a decrease of the particle size of protein aggregates, but no fat coalescence was observed during both gastric and intestinal digestion in the given conditions.The in vitro digestion of hydrolysed and non-hydrolysed dairy (casein and whey proteins) was studied under conditions without lipolytic enzymes. Results show hydrolysed proteins were completely digested in the small intestine while non-hydrolysed proteins (caseins, α-lactalbumin, β-lactoglobulin, conglycinin, glycinin) were only partially digested in the simulated gastrointestinal tract. Although observed pH-drop for non-hydrolysed protein formulations was lower, significantly higher levels of ninhydrin-reactive amino nitrogen in hydrolysed proteins suggested higher digestibility of hydrolysed proteins than their non-hydrolysed counterparts. Only formulations containing caseins showed a decrease in particle size of protein aggregates during gastric digestion. No fat globule coalescence was observed during both gastric and intestinal digestions in the given conditions. Lipid digestion of infa...