Studies of casein micelle structure: the past and the present

Abstract

At the heart of the milk system are the colloidal casein-calcium-transport complexes termed the casein micelles. The application of physical chemical techniques such as light, neutron, and X-ray scattering, and Electron Microscopy (EM) has yielded a wealth of experimental detail concerning the structure of the casein micelle. From these experimental data bases have arisen two conflicting models for the internal structure of the casein micelle. One model emphasizes protein submicellar structures as the dominant feature, while the other proposes that inorganic calcium phosphate nanoclusters serve this function. In this study, these models are critically examined in light of the two current primary dogmas of structural biology which are: protein structure gives rise to function and that competent and productive protein-protein interactions (associations) will lead to efficient transit through the mammary secretory apparatus. In this light an overwhelming argument can be made for the formation of proteinaceous complexes (submicelles) as the formative agents in the synthesis of casein micelles in mammary tissue. Whether these submicelles persist in milk has been questioned. Recently we have carried out studies on casein micelles and submicelles using Atomic Force Microscopy (AFM) and high resolution Transmission Electron Microscopy (TEM) to gain insights on the nature of protein-protein interactions in submicelles and micelles from a structural biology perspective. The results provide experimental evidence that protein-protein interactions are important in the formation and stabilization of casein micelles. milk / micelle / casein / structure / protein-protein interaction

Insolubility of milk powder products – A minireview

“…Some models have proposed arrangements of the proteins within the micelle as clusters or sub-micelles [16,25,45], but the physical arrangement is still subject to much debate [12,40]. One of the models of casein in milk is as a cluster of several hundred casein molecules, with the proteins arranged with a dense hydrophobic core, in which most of the hydrophobic parts of the casein molecules are buried, and a more loosely packed hydrophilic outer layer, containing most of the acidic (carboxylic and phosphoric) groups and some of the basic groups [47].…”

Rheology and structural arrest of casein suspensions

“…This behavior is not unexpected as, unlike model hard spheres, caseins are loose self-assembled structures. Despite the numerous structural investigations (by small-angle X-ray and neutron scattering), the exact structure of the casein is not yet established and still debated [4,[8][9][10]. This loose structure of the caseins apparently does not give rise to deviations from hardsphere behavior up to volume fractions of about 0.25.…”

Modifications of structures and functions of caseins: a scientific and technological challenge

“…Lot of researches focused on the understanding of the organization of casein micelles via different approaches and methods (from molecular to colloidal scale) and with special attention paid to protein-protein and protein-mineral interactions (Walstra 1990(Walstra , 2002Schmidt 1982;Horne 1986;Holt 1992;Holt and Horne 1996;Fox and McSweeney 1998;Swaisgood 1992Swaisgood , 2003Tuinier and de Kruif 2002;De Kruif and Holt 2003;Dalgleish et al 2004;Walstra et al 2006;Qi 2007;Fox and Brodkorb 2008;Dalgleish 2011;Dalgleish and Corredig 2012;De Kruif et al 2012;Holt et al 2013). The casein micelles contain αs 1 -, αs 2 -, β-, and κ-caseins in proportions 40, 10, 35, and 15% (w/w), respectively.…”