Structural changes in casein aggregates under frozen conditions affect the entrapment of hydrophobic materials and the digestibility of aggregates

Abstract

Freeze-dried casein nanoparticles that could entrap β-carotene were produced after aging under frozen conditions. Structural changes that occurred during aging and simulated digestion were investigated. Freeze-dried specimens of casein particles prepared from sodium caseinate solutions containing dispersed β-carotene successfully entrapped this dispersion in the resulting freeze-dried powders. The entrapped β-carotenes were distributed between the surface (surface load) and interior (inner load) of these dried powders. Because of aging, the amount of the inner load decreased while the surface load simultaneously increased. The hydrophobicity of rehydrated casein particles indicated that a change, caused by the aging process, occurred in particle structure. These structural modifications increased the hydrophobicity in the dried specimens. When rehydrated, these hydrophobic surfaces reassociated with each other to cancel the net gain in hydrophobicity. SAXS measurements on freeze-thawed casein nanoparticles also suggested the formation of modified nano- and microstructures. These modified structures were formed by freezing and thawing, along with interactions between clusters or between clusters and ice. The kinetics of the proteolytic reactions of the freeze-thawed specimens in a simulated gastric fluid were measured. The degree of aggregation and processing conditions did not significantly affect the digestion kinetics of the casein clusters. The SAXS analyses, however, suggested that these conditions affected nano- and microstructure formation during digestion. When the aggregated casein clusters were exposed to the gastric conditions, the fractured clusters produced by the proteolytic reactions produced 2–3 times larger aggregates with hollow networks and rough surfaces created by the crosslinked casein clusters. Processing conditions, such as pH and aging time, likely affected these nano- and microstructure formations under gastric conditions. These results may provide exciting future research opportunities if the structural modifications can impact the bioavailability of entrapped materials.