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 journalArticle

20 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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