**2. Structure of MFGM**

Understanding the origin and mechanism of membrane formation is crucial for the better knowledge in case of MFGM, since its complex structure has effect on stabilizing and sensory application on dairy products. The origin of MFGM was found during lipid secretion along with formation of fat globules in the mammary gland. MFGM has three different origins, primarily from apical plasma membrane, endoplasmic reticulum (ER) and certain post- golgi apparatus of mammary gland cells. Fat globules of diameter < 0.5 μm accumulates and reaches ER at centre and gets trapped between the outer and inner lipid bilayer of ER, which is later expelled into cytosol as cytoplasmic covered lipid droplets. The monolayer protein present in ER is responsible for the growth and fusion of lipid droplets before reaching the apical plasma membrane.

In budding stages of lipid droplets, there occurs a static distance of 10–20 nm between lipid globules and apical plasma membrane, that gap gets covered with electron clouded inner face of apical plasma membrane and phospholipids (PLs) that forms as primary lipid bilayer of MFGM (pathway A in **Figure 1**). Few micro lipids resist the

#### **Figure 1.**

*A = cytoplasmic lipid droplet pathway; B = microlipid droplet pathway; C = secretory pathway for milk plasma constituents.*

*Milk Fat Globular Membrane: Composition, Structure, Isolation, Technological Significance and… DOI: http://dx.doi.org/10.5772/intechopen.106926*

change in the size that constitutes microlipid pathway (pathway B in **Figure 1**). Milk plasma components like casein, whey protein, lactose and other substances condense in the golgi apparatus and get compacted within the secretory vesicles membrane through a process called exocytosis (pathway C in **Figure 1**). Similar patterns of micro lipid fusion have been reported in cell free systems with the aid of gangliosides in the presence of calcium [2, 3].

Understanding the fusion mechanism in the growth of micro lipids and sealing into MFGM would be useful in separation of milk, based on the size of fat globules that help to avoid the use of centrifugation. Manipulation of expression level of fat from the mammary cell by means of genetic engineering would be able to produce low fat milk naturally from udder. Since MFGM anchors several lipolytic enzymes this manipulation will be useful in the elongation of storage stability of the fat rich dairy products. Historical review of MFGM with their physico-chemical properties was reviewed briefly by Leroy S. Palmer, refer [4].

### **2.1 Composition of MFGM**

### *2.1.1 Protein fraction of MFGM*

On isolation and characterization of MFGM from fresh cream of jersey cow, it was concluded that the membrane is mostly protein in nature [5]. The composition of MFGM itself can be divided into lipid rich MFGM and protein rich MFGM [6] since, protein and lipid constitute 90% of MFGM. Protein content of MFGM ranges from 26 to 60% as its concentration is greatly affected by the method of isolation. The highly sialylated part of MFGM is mucins, it can be further classified into MUC 1 and MUC 15 with molecular weight of 160–200. MFGM also consists of Butyrophilin (BTN), Adipophilin (ADPH), lactadherin, Proteose peptone 3, some fatty acid binding protein [7] and RNA [3]. Molecular weight of native proteins present in MFGM was tabulated in **Table 1**.


#### **Table 1.**

*Protein fractions of naturally extracted MFGM isolates, from [7].*

#### *2.1.2 Lipid fraction of MFGM*

MFGM contains 35% of high melting point unsaturated fatty acids constituting 3% of total triglyceride composition [5], but these fatty acids was not originated from MFGM, rather it comes from fat globules attached to the membrane during processing operation [6]. The major lipid present in the MFGM was found to be PLs (26–31% of total lipids) in the form of protein –phospholipid complex [8] that exhibits emulsion stabilizing property along with other phosphatides proteins lecithin, cephalin and sphingomyelin. Among these phosphatides lecithin was notably prominent in creaming stability and emulsion of cow milk [9]. Triacylglycerols constitutes 62% of total lipids and other minor constitutes are mono, di-acylglycerols (responsible for the lipolysis in dairy products), sterols and their esters, non-esterified fatty acids and hydrocarbons [5, 10]. Next to protein and lipids, enzymes are highly concentrated in MFGM that are significantly crucible for lipolytic activity in dairy products. About 28 enzymes have been found in MFGM, but their physiological activities are undiscovered. The major enzyme is xanthine oxidase (XDH), responsible for the development of fat globules in plasma membrane and purine metabolism. The origins of these enzymes are predominantly from plasma membrane [11, 12] and cytosol. Composition and their proportions are detailed by Keenan and Mather [3].
