PhD Defence Johanna de Wolf | Optical and physical properties of human milk

Optical and physical properties of human milk

Johanna de Wolf is a PhD student in the department Biomedical Photonic Imaging. Promotors are prof. dr. ir. N. Bosschaart and prof. dr. ir. W. Steenbergen from the faculty of Science and Technology, University of Twente.

Milk is the primary source of nutrition for infants during the first months of their lives. Understanding the biological variation in human milk composition is crucial for understanding the synthesis and secretion of milk components and to deepen insights into the health benefits from each milk component on infant health. From a scientific perspective, it will be ideal to measure human milk composition while the infant is drinking. Optical milk composition analysis based on light scattering presents a promising fast and non-destructive solution for human milk macronutrient composition analysis. However, the current knowledge on the optical and physical properties of human milk is scarce. Bovine milk properties are often used for the optical analysis of human milk, which potentially introduces errors due to differences in milk composition between both species. This thesis focusses on quantifying the optical and physical properties of human milk, as well as their variation during a breastfeed and between mothers.

The first goal of this thesis was to development and validation dedicated sample handling protocols which preserves native particles. We designed a new dilution medium which mimics the chemical composition of human milk serum. We conclude that our new dilution medium most optimally preserves native particles, and that – ideally – freezing and thawing should be avoided. The second goal of this thesis was to perform a pilot study on the potential of human milk analysis using Raman spectroscopy (inelastic light scattering). While we found no significant changes in lipid composition, the lipid conformational state increased during a single breastfeed at room temperature. This new finding suggests that the process of lipid synthesis in the mammary gland changes during a single breastfeed.

The third goal was to measure the refractive index of human milk serum and human milk particles. This knowledge is required for Mie theory based calculations on the elastic light scattering by human milk. We concluded that the refractive index of human milk serum depends more on the whey protein concentration than the carbohydrate concentration, while the mineral concentration is of negligible influence. Furthermore, we demonstrate that the refractive index distribution of particles in human milk are significantly different than those of bovine milk. With these results, we highlight that human milk cannot be considered as a simple variation on bovine milk.

The last goal was to analyse whether the size distribution of human milk fat globules changes during a breastfeed, next to the documented change in milk fat globule number concentration from the literature. We conclude that the mean diameter of milk fat globules increases during a breastfeed. In addition, the mean diameter of milk fat globules is more predictive of milk fat concentration than the milk fat globule number concentration.

These findings provide a framework for the optical and physical properties of human milk. Hereby, we laid the groundwork for inline monitoring of human milk composition based on visible light.