Composition, applications, fractionation, technological and nutritional significance of milk fat globule membrane material

Abstract

Among the components of mammalian milks, the milk fat globule membrane (MFGM) is the least close to being fully understood. As a result of the synthesis and secretion process, the fat globules in milk are composed of a non-polar lipid core, surrounded by a layer of phospholipids and proteins. This structure is then bounded by a membrane bilayer that is, in turn, derived from the apical surface of the mammary epithelial cell. Substantial biochemical investigations have been conducted to elucidate the details involved in the synthesis, transport and secretion of milk fat globules, yet there has been little discussion on the uniqueness of this system, and the evolutionary forces that have led to its development. On one hand, the complex structure of the globules may have arisen as a result of the physiological constraints on the secretion process, and would therefore not be expected to contribute a signiWcant beneWt to the health of the oVspring. It should be recognized, however, that the simple process of lipoprotein secretion, which does not require the addition of extra cell bilayer surface, had evolved well before the appearance of mammals and would presumably have been a simple and easy alternative for fat secretion. Thus, it is also possible that there is a nutritional or physiological beneWt that may be conferred to the consumer of milk by the structure of the fat globules, which would have led to the Darwinian selection of this process during mammalian evolution. According to evolutionary theory, individuals of a species tend to be selected over time as a function of survival and reproductive success, and hence pass on their genetic material. Parturition is a tenuous time in the mammalian life cycle, as the constant threat of microbial pathogens is compounded by developmental delays in mucosal and systemic immunity (Goldman, 2002). However, the biological role of milk is to provide a support system that promotes metabolic regulation, assists in rapid physical development and complements the functionally immature immune system. Consequently, infants who consume breast milk experience phenotypic beneWts, which we cannot explain with our current understanding of nutrition, food composition and structure. Although alternate formulae match the macronutrient, vitamin and mineral composition of breast milk, it must be the composite, non-essential structures in milk, not present in formulae, which are responsible for this extent of diVerential beneWt. The milk fat globule membrane is composed of an interesting mixture of proteins, lipids and carbohydrates. With its similarity to epithelial cell surfaces, it may well have functions and activities above the simple delivery of the nutrients it contains. In food and nutrition research, there is growing awareness of dietary components, which, while not essential, nonetheless provide tangible beneWts to health when consumed.Many of the components of theMFGMhave been associated positively with health, whether or not the mechanism of action is understood. Importantly, the identiWcation and subsequent characterization of the constituents responsible for these eVects will not be as straightforward as was the scientiWc research on the essential nutrients. If a nutrient is essential, removing it from the diet will always result in speciWc, reproducible insults to health. Yet, food science and nutrition research has not yet been as successful in developing research protocols to probe the importance and function of conditionally essential nutrients. As milk contributes to the survival and Wtness of mammalian young, it is not simply a participant in evolution, but rather a driver, with simultaneous selection for eYcacy and eYciency. Therefore, it is hard to imagine that there is anything in milk, including theMFGM, which is not interesting from a nutritional perspective.