Food Biochemistry and Food Processing (2 edition)

BLBS102-c24 BLBS102-Simpson March 21, 2012 13:47 Trim: 276mm X 219mm Printer Name: Yet to Come

450 Part 4: Milk

may not be possible to meet the upper limit for fat content in

some products, for example casein, during certain periods.

Stability of Milk Fat Globules

In milk, the emulsifier is the MFGM. On the inner side of the

MFGM is a layer of unstructured lipoproteins, acquired within

the secretory cells as the triglycerides move from the site of syn-

thesis in the rough endoplasmic reticulum (RER) in the basal

region of the cell towards the apical membrane. The fat glob-

ules are excreted from the cells by exocytosis, that is they are

pushed through and become surrounded by the apical cell mem-

brane. Milk proteins and lactose are excreted from the cell by

the reverse process: the proteins are synthesised in the RER and

are transported to the Golgi region, where the synthesis of lac-

tose occurs under the control ofα-La. The milk proteins and

lactose are encapsulated in Golgi membrane; the vesicles move

towards, and fuse with, the apical cell membrane, open and dis-

charge their contents into the alveolar lumen, leaving the vesicle

(Golgi) membrane as part of the apical membrane, thereby re-

placing the membrane lost on the excretion of fat globules. Thus,

the outer layer of the MFGM is composed of a trilaminar mem-

brane, consisting of phospholipids and proteins, with a fluid

mosaic structure.

Many of the proteins of the MFGM are strongly hydrophobic

and difficult to isolate and characterise. Modern proteomic meth-

ods have shown that the MFGM contains about 100 proteins, of

which the following are the principal and have been isolated

and characterised: butyrophilin (BTN), xanthine dehydrogenase

(XDH), acidophilin, PAS (periodic acid Schiff staining) 6/7, CD

(cluster of differentiation) 36, fatty acid-binding protein, mucins

1 and 15 (MUC). BTN is a trans-membrane protein that com-

plexes with XDH (located on the inner face of the membrane)

that initiates the blebbing of the fat globule through the api-

cal membrane of the cell (in this role, XDH does not act as

an enzyme). MUC1, MUC 15, CD 36 and PAS6/7 are heav-

ily glycosylated and are located mainly on the outer surface of

the membrane, increasing its hydrophilicity. Very considerable

progress has been made on the proteins during the past 10 years,

which has been reviewed by Mather (2000, 2011) and Keenan

and Mather (2006). In human and equine milk, the MUC form

long(upto50μm) filaments that are lost easily; the filaments

probably retard the passage of lipids through the small intes-

tine, thereby improving digestibility, and prevent the adhesion

of pathogens.

The MFGM contains many enzymes that originate mainly

from the Golgi apparatus: in fact, most of the indigenous en-

zymes in milk are concentrated in the MFGM, notable excep-

tions being plasmin and LPL that are associated with the casein

micelles. The trilaminar membrane is unstable and is shed dur-

ing storage, and especially during agitation, into the aqueous

phase, where it forms microsomes.

The stability of the MFGM is critical for many aspects of the

milk fat system:

 The existence of milk as an emulsion depends on the effec-

tiveness of the MFGM.

 Damage to the MFGM leads to the formation of non-

globular (free) fat, which may be evident as ‘oiling-off’

on tea or coffee, cream plug or age thickening. An elevated

level of free fat in whole milk powder reduces its wettabil-

ity. Problems related to, or arising from, free fat are more

serious in winter than in summer, probably due to the re-

duced stability of the MFGM. Homogenisation, which re-

places the natural MFGM by a layer of proteins from the

skim milk phase, principally caseins, eliminates problems

caused by free fat.


 The MFGM protects the lipids in the core of the globule

against lipolysis by LPL in the skim milk (adsorbed on the

casein micelles). The MFGM may be damaged by agitation,

foaming, freezing, for example on bulk tank walls, and es-

pecially by homogenization, allowing access for LPL to the

core lipids and leading to lipolysis and hydrolytic rancidity.

This is potentially a major problem in the dairy industry

unless milking machines, especially pipeline milking instal-

lations, are properly installed and serviced.


 The MFGM appears to be less stable in winter/late lactation

than in summer/mid lactation; therefore, hydrolytic rancid-

ity is more likely to be a problem in winter than in summer.

An aggravating factor is that less milk is usually produced

in winter than in summer, especially in seasonal milk pro-

duction systems, which leads to greater agitation and air

incorporation during milking and, consequently, a greater

risk of damage to the MFGM.

Creaming

Since the specific gravity of lipids and skim milk is 0.9 and

1.036, respectively, the fat globules in milk held under quiescent

conditions will rise to the surface under the influence of gravity,

a process referred to as creaming. The rate of creaming,V,of

fat globules is given by Stoke’s equation:

V=

2 r^2 (ρ^1 −ρ^2 )g

9 η

where

r=radius of the fat globules

ρ^1 =specific gravity of skim milk

ρ^2 =specific gravity of the fat globules

g=acceleration due to gravity

η=viscosity of milk

The typical values of r,ρ^1 ,ρ^2 andηsuggest that a cream

layer should form in milk after approximately 60 hours but milk

creams in approximately 30 minutes. The rapid rate of creaming

is due to the strong tendency of the fat globules to agglutinate

(stick together) due to the action of indigenous immunoglobulin

(Ig) M, which precipitates onto the fat globules when milk is

cooled (hence, they are called cryoglobulins). Considering the

effect of globule size (r) on the rate of creaming, large glob-

ules rise faster than smaller ones and collide with, and adhere

to, smaller globules, an effect promoted by cryoglobulins. Ow-

ing to the larger value of r, the clusters of globules rise faster

than individual globules, and therefore the creaming process