Correlating Digestion-Driven Self-Assembly in Milk and Infant Formulas with Changes in Lipid Composition.

Ben J Boyd,Gisela Ramirez,Andrew J Clulow,Anna C Pham,Adrian Hawley,Kang-Yu Peng,Malinda Salim

doi:10.1021/acsabm.0c00131

Abstract

Lipids in mammalian milks such as bovine milk and human breast milk have been shown to self-assemble into various liquid crystalline materials during digestion. In this study, the direct correlation between the composition of the lipids from three types of mammalian milk, three brands of infant formulas (IFs), and soy milk and the liquid crystalline structures that form during their digestion was investigated to link the material properties to the composition. The self-assembly behavior was assessed using in vitro digestion coupled with in situ small-angle X-ray scattering (SAXS). Lipid composition was determined during in vitro digestion using ex situ liquid chromatography–mass spectrometry. All tested milks self-assembled into ordered structures during digestion, with the majority of milks displaying nonlamellar phases. Milks that released mostly long-chain fatty acids (>95 mol % of the top 10 fatty acids released) with more than 47 mol % unsaturation predominantly formed a micellar cubic phase during digestion. Other milks released relatively more medium-chain fatty acids and medium-chain monoglycerides and produced a range of ordered liquid crystalline structures including the micellar cubic phase, the hexagonal phase, and the bicontinuous cubic phase. One infant formula did not form liquid crystalline structures at all as a consequence of differences in fatty acid distributions. The self-assembly phenomenon provides a powerful discriminator between different classes of nutrition and a roadmap for the design of human milklike systems and is anticipated to have important implications for nutrient transport and the delivery of bioactives.

Highlights

Milk is a naturally occurring complex emulsion consisting of proteins, fats, salts, sugars, and vitamins, which are essential for the growth and development of infants during the first months and years of life
The persistent V2 phase observed at the end of the digestion of the fat droplets in bovine milk was not seen in the case of goat milk
Human milk (Figure 2C) and soy milk (Figure 2G) both formed micellar cubic (I2) phases, with the Fd3m space group in the fat droplets, and were digested more slowly than bovine/goat milk, which indicated that the selfassembled liquid crystalline phases had a greater negative curvature at the oil−water interface than the other milks

Summary

Introduction

Milk is a naturally occurring complex emulsion consisting of proteins (whey and casein), fats, salts, sugars, and vitamins, which are essential for the growth and development of infants during the first months and years of life. Bovine milk is the most consumed mammalian milk primarily due to its early domestication.[2] Goat milk has grown popular as an alternative to bovine milk.[3,4] Commercially available full-cream bovine milk contains approximately 3.0−4.5 wt % fat, of which 98 wt % is in the form of triglycerides (TGs).[5] The triglycerides exist in water as stabilized fat droplets, ranging in diameter from 3 to 20 μm in raw milk and 0.2 to 2.0 μm in commercial homogenized and pasteurized milk.[6] Native fat droplets are enveloped by milk fat globular membranes, consisting of a trilayer of phospholipids intercalated with a wide range of glycoproteins and other compounds.[7]

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