The Refractive Index of Human Milk Serum: Natural Variations and Dependency on Macronutrient Concentrations

The refractive index (RI) of human milk serum (also known as whey, milk soluble fraction or milk plasma) depends on the individual molecular species dissolved in the serum and their concentrations. Although the human milk serum RI is known to influence milk analysis methods based on light scattering, the RI dependency on human milk serum composition is currently unknown. Therefore, we systematically evaluate how the RI depends on natural variations in macronutrient concentrations in the soluble fraction. We measure RI variations in serum simulating samples with controlled macronutrient concentrations, as well as skimmed and whole fore-, bulk, and hindmilk from 19 donors. For both types of samples, we relate the measured RI to the macronutrient composition. From the serum simulating samples, we observe that the RI depends more on variations in whey protein, than carbohydrate concentrations, while minerals have negligible influence. For all donated samples, the average RI was 1.3470 (range 1.3466-1.3474). Per donor, no significant differences were observed in RI between fore-, bulk, and hindmilk. We conclude that protein and solids-not-fat (i.e., the total contribution of carbohydrates, proteins and minerals present in milk) concentrations are most predictive for human milk serum RI.