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Effect of milk protein composition on in vivo gastric digestion of a model infant formula
The objective of this work was to determine the effect of protein composition, and in particular, the presence of whey proteins or b-casein, on the digestion behaviour of a model infant formula. An in vivo piglet model was employed, as this is an established model for human infants’ digestion. Three formulas optimized for piglets were prepared with the same concentration of protein, and same caloric content. One formula contained only whey proteins (WP) and two others contained a casein to WP ratio of 40:60, but differing in the amount of β-cas. To obtain modified protein ratios, microfiltration (using polyethersulfone membrane with 80 KDa of molecular weight cut-off) was conducted on skimmed milk either at 7 or at 22 °C. Retentates and permeates were combined with additional whey protein and other ingredients and after heating, the formulas were used to feed 24 piglets. The piglets were housed and fed from age 3 to 21 days, and animal behaviour and health conditions were investigated. The study was carried out in two blocks, sacrificing the animals after 60 and 120 minutes from the last meal, respectively. Gastric digesta samples were collected and studied for physicochemical properties. The tests were performed on fresh samples in less than 10 minutes after euthanizing. There were no differences in the properties of the curd, within a treatment, between the two blocks. All curds showed a shear-thinning behaviour, with a significantly higher viscosity and a higher modulus for curds obtained from casein/WP formula, compared to the curds from WP formula. Confocal microscopy showed structures with larger voids in WP digestates, compared to those from cas/WP formula, which showed a higher density throughout the matrix. Despite differences in physicochemical properties, a pH range of 4.4 to 5.8 was measured for the gastric contents, with no significant difference observed between diets, nor with time of digestion. There were also clear differences in animal growth between treatments. Casein/WP formulas were shown to improve growth performance, with approximately 50% higher average daily growth, and increased feed efficiency, compared with the WP formula. The results bring significant advances to our understanding of the importance of different protein ratios on the digestion of dairy matrices.
Keywords: Digestion, infant formula, milk proteins