CHEMISTRY OF MILK

Module 3. Milk proteins

Lesson 8
CASEIN MICELLE: STRUCTURE, PROPERTIES AND ITS IMPORTANCE

8.1 Introduction

In order to provide proteins along with a considerable portion of its calcium and phosphate to it’s offspring, the casein in cow milk forms intricate particles which are recognized as casein micelles. Highly insoluble material (calcium phosphate) has to be carried without disturbing either stability or increase in its size. Apart from this , it must be able to form a clot once it enters the stomach of the calf. Considering these limitation nature s has formulated these casein micelles with several intricacies in it.

8.2 Structure

The whole casein will form aggregate when it is in solution at a concentration, pH, and ionic strength as in milk and low Ca²⁺ activity. The micelles contain about ten to one hundred casein molecules. The aggregates like globular proteins will have a dense hydrophobic core in which most hydrophobic parts of the casein molecules are buried and a more loosely packed, hydrophilic outer layer containing most of the acidic (carboxylic and phosphoric) and some of the basic groups. Each of these small aggregates of the whole casein,usually called submicelles contains different casein molecules. The relative proportion of α _s1: α _s2 : β and κ in casein micelle is 3: 0.8: 3:1 respectively.More over κ-casein probably exists in milk as an oligomer containing several molecules and held together by covalent bonds (S-S linkages). Consequently there may be essentially two types of submicelles with and without κ-casein.

Earlier casein micelle models described were

1) Core-coat model

2) Internal structure model

3) Sub-unit model

However, Encyclopaedia of Dairy Sciences (2004) states what those is no universally accepted model for the structure if the casein micelle. The major contenders are the sub –unit model, the Holt model and the dual-binding model introduced to overcome criticism levelled at the first two.

Dual binding model for micelle assembly and structure

According to this model micellar assembly and growth take place by a polymerization process involving two distinct forms of bonding:cross linking through hydrophobic regions of the casein or bridging across calcium phosphate nanoclusters. Central to the model is the concept that micellar integrity and hence stability is maintained by a localised excess of hyrophobic attractions over electro static repulsion. Each casein micelle effectively functions as a block co polymer, with each bloc possessing different different and possibly multiple functionality for the cross linking paths. κ-casein is the most important of the caseins in this model of micellar assembly and structure. It can link in to the growing chain through its hydrophobic N-terminal block but its C-terminal block has no phophoserine cluster and therefore cannot extend the polymer chain through an anocluster link. Therefore, chain and net work growth are determined here leaving the casein micelle net work with a surface layer of κ-casein molecules, a prime reason for any structural model.

Fig. 8.1 Dual binding model of casein micelle

(Source: Horne, Opinion Colloidal Interface, 2006)

There will be considerable increase in the association of the casein micelle with increase in the Ca²⁺ activity. Under conditions as in milk which means in particular that besides calcium phosphate is present submicelles aggregate in to micelles. The calcium phosphate acts as cementing agent. The aggregation will go on until a gel or a precipitate was formed if there were noκ -casein, which thus acts as a “protective colloid”. The C-terminal part of κ -casein is very hydrophilic particularly those molecules that contain carbohydrate and it also has a considerable negative charge. This part of the molecule sticks partly out in to the surrounding medium as a flexible “hair” more or less behaving as a random coil polymer chain. There will be some areas which having steric repulsion causing hinderance for aggregation of the sub micelles. In this way aggregation of sub micelles would go on until the surface of the micelle was more or less covered with κ-casein. The κ- casein will be inters paced with other caseins, instead of covering the entire surface of the micelles.

Most of the k- casein is at the outside and the protruding chains of its C- terminal end give the micelles a hairy surface which are flexible and show perpetual Brownian motion. The effective thickness of the hairy layer is at least 5nm. A small part of κ- casein is in the interior. The casein micelle is fairly voluminous; electron micrographs indicate that the voluminosity ‘V’ is about 2 ml per gram of casein. About half of the voluminosity of casein micelle is due to the sub micelles while remaining volume is serum between sub micelles.The serum inside the micelles however is depleted of part of the large solutemolecules. The pore width in the micelles is probably a few nanometers, Permitting easy access by many solute molecules but restricting the access of globular proteins. Taking the hairy outer layer in to consideration micelles voluminosity is higher on average about 4 ml per gram of casein. The hydrodynamic voluminosity increases with the decreasing micelles size. The casein micelles are not static and have three dynamic equilibrium with its surroundings. The equilibrium is between the free casein molecules and sub micelles(a tiny part of the casein is in true solution) the equilibrium strongly depends on temperature, between the dissolved and colloidal Ca and phosphate,between free sub-micelles and micelles. The casein micelles are rather variable particularly in milk of individual cows. Casein composition varies though not very much. The proportion of κ-casein varies by about 9-15% which will be reflected in the average casein micelle size. The voluminosity varies by a factor of 2 variations in the size and voluminosity usually from 6-9g per 100gdry casein. There will also be a variation in the composition of inorganic matter.

The quantity of calcium phosphate in the micelles increases on heating and very much decreasing when lowering the pH. The casein micelles will be dispersed into smaller units by any treatment that dissolves a considerable proportion of the calcium phosphate at constant pH such as adding Na Cl.Micelles disperse in to submicelles when milk is subjected to high pressure(100MPa for 1 hr) this disintegration is irreversibly and it may be caused by the formation of hydroxyl apatite from the calcium phosphate. Casein micelles also dissolve when submicelles dissociate. In the sub micelles the molecules are held together mostly by hydrophobic interaction and by ‘H’ bonds in the hydrophobic interior of the submicelles. Consequently addition of large quantities of urea or guanidinium chloride and sodium dodocyl sulfate in small quantities will dissolves the micelles. Electro static interactions (i.e internal salt bridges) also participate in keeping the submicelles together, but they cannot be broken without dissolving the calcium phosphate. Reagents that break S-S linkage do not disintegrate the micelles fully. Lowering the temperature (eg: to 5 °C)considerably effects the casein micelles. Hydrophobic interactions become much weaker and part of the casein particularly of the beta casein dissociate from the micelles. The voluminosity of the micelles increases, probably in part from increased “hairiness” as β-casein chains may protrude from the micelle surface.A small part of the calcium phosphate dissolves. These changes may be the cause of the slight disintegration of the micelles despite the increase in voluminosity their average size decreases to some extent ( b y 15%).

8.3 Properties

Almost all casein in un-cooled milk is present in roughly spherical particles mostly 0.02 to 0.30 µ m in diameter, comprised of some 20 to 150000casein molecules. These micelles also contain inorganic matter mainly calcium phosphate,about 8 g per 100g casein. The casein micelles are voluminous and hold more water than pure casein. They also contain small quantities of other proteins such as the milk enzymes lipase and plasmin and part of proteose-peptone

8.4 Importance

The existence and properties of casein micelles have several consequences in the use of milk. The stability of milk products during heating, concentrating, and holding is largely determined by these micelles. The rheological properties of sour milk and concentrated milk are largely determined by the changes occurring in these micelles. The interaction of these micelles with air- water and oil- water interfaces is important in homogenization. The casein micelles also influence some of the properties of the homogenized products. As such a study of the properties and structure of casein micelles will help in proper understanding of various phenomena occurring in milk and milk products.