Rheological, microstructural and gastric digestion characterization of acidified caprine and bovine milks and their infant formulas

Abstract

Goat infant formula has become increasingly popular in recent years. This study compared the buffering capacity, casein micelle size, fat globule size, and protein gastric digestion profile of goat and cow skim milk and infant formulas. The rheological properties of the acid-induced gels of goat and cow skim milks and infant formulas were also investigated by small and large deformation oscillatory measurements. The microstructure of the gels was explored via a combination of ultra-small angle neutron scattering (USANS), small angle neutron scattering (SANS), and confocal laser scanning microscopy (CLSM). In addition, the internal structure of goat and cow casein micelles was characterized by both small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The result shows that the buffering capacity of goat milk and infant formula is higher than that of cow milk and infant formula. Goat casein micelles are approximately double the size of cow casein micelles. The internal structure of goat and cow casein micelles is different, which could be because of the differing distances between protein packing dense regions. Colloid calcium phosphate (CCP) nanoclusters are revealed in the casein micelles by both SAXS and TEM but are solubilized during acidification. The rheological experiments illustrated that the acid-induced gels of goat skim milk and infant formula are weaker and easier to be broken than those of the cow. Besides, infant formula with higher casein content exhibited a stronger gel network. This has also been certified by CLSM observation and USANS and SANS investigation, which detected a looser protein network and thinner protein strands in goat skim milk and infant formula. Moreover, the average diameter of the fat globule in infant formula is around 500-600 nm, and fat globules in goat infant formula are slightly larger than that in cow infant formula. Finally, the protein profile of goat and cow infant formula during in vitro gastric digestion was investigated by SDS-PAGE. The casein hydration rate of goat and cow infant formulas is similar, but the digestion rate of β-lactoglobulin (β-Lg) and α-lactalbumin (α-La) is faster in goat infant formulas. However, this result is unreliable because SDS-PAGE can only separate peptides according to molecular weight, which cannot distinguish macromolecular hydrolysis products and small molecule proteins.