Absolute Quantification of Human Milk Caseins and the Whey/Casein Ratio during the First Year of Lactation

Yalin Liao, Darren Weber, Wei Xu, Blythe P. Durbin-Johnson, Brett S. Phinney, Bo Lönnerdal

Research output: Contribution to journalArticlepeer-review

39 Scopus citations


Whey proteins and caseins in breast milk provide bioactivities and also have different amino acid composition. Accurate determination of these two major protein classes provides a better understanding of human milk composition and function, and further aids in developing improved infant formulas based on bovine whey proteins and caseins. In this study, we implemented a LC-MS/MS quantitative analysis based on iBAQ label-free quantitation, to estimate absolute concentrations of α-casein, β-casein, and κ-casein in human milk samples (n = 88) collected between day 1 and day 360 postpartum. Total protein concentration ranged from 2.03 to 17.52 with a mean of 9.37 ± 3.65 g/L. Casein subunits ranged from 0.04 to 1.68 g/L (α-), 0.04 to 4.42 g/L (β-), and 0.10 to 1.72 g/L (α-), with β-casein having the highest average concentration among the three subunits. Calculated whey/casein ratio ranged from 45:55 to 97:3. Linear regression analyses show significant decreases in total protein, β-casein, κ-casein, total casein, and a significant increase of whey/casein ratio during the course of lactation. Our study presents a novel and accurate quantitative analysis of human milk casein content, demonstrating a lower casein content than earlier believed, which has implications for improved infants formulas.

Original languageEnglish (US)
Pages (from-to)4113-4121
Number of pages9
JournalJournal of Proteome Research
Issue number11
StatePublished - Nov 3 2017


  • amino acid analysis
  • human milk
  • iBAQ
  • label-free LC-MS/MS
  • total protein
  • whey/casein ratio
  • α-casein
  • β-casein
  • κ-casein

ASJC Scopus subject areas

  • Biochemistry
  • Chemistry(all)


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