Protein digestibility and bioactive peptide release in liquid infant formulas, infant formula ingredients, and in bovine milk: modulation by UHT treatment and storage

Research output: Book/ReportPh.D. thesisResearch

  • Yuhui Ye
Infant formula (IF) is a high-value dairy product, provided to mothers who cannot breast feed or have limited breast milk. Ultra-high temperature (UHT) treatment, either indirect (ID) or direct UHT (D), combined with aseptic packaging is widely applied to milk or IF, in order to extend the shelf life outside the cooling chain. After UHT treatment, milk or IF are often transported overseas in containers at temperatures ranging up to 40 °C. The UHT treatment required for sterilisation and the storage during transportation can lead to structural changes and chemical modifications of proteins, which modulates the digestibility and the biological function of proteins. Structural changes result from heat-induced protein unfolding followed by protein aggregation, where protein cross-linkings may occur, e.g., disulfide, dityrosine cross-linkings. The chemical modifications are mostly induced by the Maillard reaction and protein oxidation. Owing to these thermal and storage effects, the immunogenicity of bioactive proteins such as lactoferrin (Lf), immunoglobulin G (IgG), insulin-like growth factor 1 (IGF-1) are affected. In addition, the release of bioactive peptides is also influenced. It is important to know how these heat treated proteins are digested in the gastrointestinal tract. Thus, simulated infant gastric and intestinal digestion was conducted in the first study, where IF treated with either low pasteurization, direct and indirect UHT treatments and storage were investigated. The study showed that the formation of protein aggregates correlated with thermal treatment and these aggregates were mainly disulphide-linked, but dityrosine linkages were detected after 120 days of storage. The digestibility of unaggregated beta-lactoglobulin (β-Lg) and alpha-lactalbumin (α-La) was enhanced with the severity of heat treatments and storage. In general, UHT treatments facilitated the release of bioactive peptides, whereas storage inhibited it. The detection of peptides with inhibitory activity against dipeptidyl peptidase-IV (DPP-IV) and angiotensin-converting enzyme (ACE) indicates that these released peptides may have anti-inflammatory activity. Peptides from Lf, lactoperoxidase (Lp), polymeric immunoglobulin receptor (pIgR) and cluster of differentiation 36 (CD36) were detected in all digested IFs. Storage of D & ID for 60 days significantly reduced peptide amounts from these known bioactive proteins. Regarding the digestibility of milk proteins, it is interesting to compare the digestion of IF with the digestion of whole milk, as they are distinct dairy products supplemented to infants and adults, respectively. Study 2 conducted a further investigation on the digestibility of milk under conditions of adult digestion. Results indicated that the bioactive proteins, IgG and Lf are sensitive to ID and storage. Protein aggregate formation and the digestion of unaggregated α-La and β-Lg both positively correlated with thermal treatment. The protein aggregation induced by heat treatments appears to be the most important parameter affecting the release of bioactive peptides. Modifications occurring on the bioactive peptides affect their digestibility and bioactivity. In order to produce IF with a superior quality, not only the processing method, but also the protein ingredients are important. Serum protein concentrate (SPC) is a relatively novel IF ingredient produced via microfiltration (MF), which contains more native proteins than whey protein concentrate (WPC). The third study investigated the digestibility of four protein ingredients: spray-dried SPC (SD-SPC), heat treated and spray-dried SPC (HTSD-SPC), heat treated, spray-dried and stored SPC (HTSDS-SPC), spray-dried WPC (SD-WPC). It found that MF is efficient in preventing aggregates in SD-SPC. However, small amount of caseins were present in SD-SPC after MF. SD-WPC was more denatured than SD-SPC due to the heat load applied during processing. The digestion of α-La and β-Lg correlated with thermal treatment: SD-SPC<HTSD-SPC<HTSDS-SPC<SD-WPC. Further, the peptides released from β-Lg and α-La indicated the potential sites which were sensitive towards enzymes. Protein lactosylation and oxidation were detected which reflected the different heat loads applied to the ingredients. Numbers of unique bioactive peptides indicated that SD-WPC may have a less nutritional value than SD-SPC. In addition to in vitro digestion, a further digestion of IF was conducted in preterm piglets. It indicated that IF with ID treatment significantly reduced protein bioactivity, which may have contributed to the subclinical gut dysfunction and inflammation observed in the preterm piglets. Following storage at elevated temperature at 40 ℃, the Maillard reaction was accelerated, and this promoted the formation and accumulation of in vivo advanced glycation end products (AGEs) in the gut and a more severe inflammation.
Original languageEnglish
PublisherDepartment of Food Science, Faculty of Science, University of Copenhagen
Number of pages231
Publication statusPublished - 2022

ID: 310502313