Food Biochemistry and Food Processing (2 edition)

BLBS102-c25 BLBS102-Simpson March 21, 2012 13:23 Trim: 276mm X 219mm Printer Name: Yet to Come

478 Part 4: Milk

extraction, selective enzymatic hydrolysis and ion-exchange

resins have been described. On a commercial scale, the pro-

duction of products containing purified but unfractionated whey

proteins remains most common, although ion-exchange meth-

ods for the recovery of biologically significant (and hence high

value) proteins from whey, such as lactoperoxidase and lacto-

ferrin have been scaled up and are in commercial operation.

Lactose Processing

Whey or whey permeate, produced as a by-product of UF pro-

cessing, contains a high level of lactose. This can be recovered

by crystallisation from a concentrated (supersaturated, 60–62%

TS) preparation of either whey or whey permeate (for review see

Muir 2002, Peterson 2009). The crystalline lactose is usually re-

covered using a decanter centrifuge, dried in a fluidised bed dryer

and ground to a fine powder, which is used in food applications,

for example confectionery products and infant formulae. Lac-

tose is also used widely in pharmaceutical applications (e.g., as

a diluent in drug tablets); for such applications, the lactose is

generally further refined by re-dissolving in hot water and mix-

ing with activated carbon, followed by filtration, crystallisation

and drying.

Many derivatives of lactose can be produced, some of which

are more valuable and useful than lactose (Fig. 25.4, Playne

and Crittenden 2009). For example, lactulose (β-d-galactosyl-d-

fructose) and lactitol (β-d-galactosyl-sorbitol) are used in treat-

ment of patients with chronic hepatic encephalopathy, and both

have prebiotic effects. In addition, lactose in whey can be fer-

mented to alcohol for industrial use in beverages.

CASEINS: ISOLATION, FRACTIONATION

AND APPLICATIONS

In recent years, it has been increasingly recognised that milk

proteins have functionalities that can be exploited in food sys-

tems other than conventional dairy products; these proteins have

Lactitol

Lactulose

Lactose

Glucose, galactose, oligosaccharides

Enzymatic or acid

hydrolysis

Lactobionic

acid

Ammonium

lactate

Lactosyl urea

(animal feed)

Biomass, ethanol or

propionic lactic or acetic

acids

Fermentation

Oxidation

Fermentation with

neutralisation

Concentration and

acidic reaction with

urea

Biosynthesis

Chemical

modification

Figure 25.4.Food-grade derivatives of lactose.

been increasingly recognised as desirable ingredients for a range

of food products (Mulvihill and Ennis 2003). The very different

properties of the two classes of milk proteins, the caseins and

whey proteins, present different technological challenges for

recovery, and are suitable for quite different applications. The

whey proteins have been discussed previously, and the caseins

will be discussed in the following section.

Recovery and Application of Caseins

Technologies for the recovery of caseins from milk are based

on the fact that relatively simple perturbations of the milk sys-

tem can destabilise the caseins selectively, resulting in their

precipitation and facilitating their recovery from milk (Mulvi-

hill and Fox 1994, Mulvihill and Ennis 2003). As discussed for

rennet-coagulated cheese, the stability of the caseins in a col-

loidal micellar form is possible due to the amphiphilic nature of

κ-casein. To overcome this stability, and precipitate the caseins,

merely requires that this stabilising effect be overcome.

The two key principles used to destabilise the micelles are (1)

acidification to the isoelectric point of the caseins (pH 4.6) or (2)

limited proteolysis, for example by chymosin, which hydrolyses

κ-casein, removing the stabilising glycomacropeptide. In both

cases, the starting material for casein production is skim milk.

For acid casein, acidification can be achieved either by addition

of a mineral acid, usually HCl, or fermentation of lactose to lactic

acid by a culture of LAB. When the isoelectric point is reached,

a precipitate (rapid acidification) or a gel (slower acidification)

is formed; the latter is cut/broken to initiate syneresis and expel

whey. The mixture of casein and whey is stirred and cooked

to enhance syneresis, and the casein separated from the whey

(either centrifugally or by sieving). To improve the purity of the

casein, the curds are washed repeatedly with water to remove

residual salts and lactose, and the final casein is dried, typically

in specialised dryers, such as attrition or ring dryers. For rennet

casein, the milk gel is formed by adding a suitable coagulant

to skim milk, but subsequent stages are similar to those for

acid casein.

Both acid and rennet casein are relatively insoluble in wa-

ter, and are used in the production of cheese analogues (rennet

casein) and convenience food products, as well as non-food ap-

plications, such as the manufacture of glues, plastics and paper

glazing (acid casein).

To extend the range of applications of acid casein, and im-

prove its functionality, it may be converted to a metal salt (e.g.,

Na, K, Ca caseinates), by mixing a suspension of acid casein

with the appropriate hydroxide and heating, followed by drying

(typically with a low, that is approximately 20%, total solids con-

centration in the feed, due to high product viscosity, in a spray-

dryer). Caseinates, particularly sodium caseinate, have a range of

useful functional properties, including emulsification and thick-

ening; calcium caseinate has a micellar structure (Mulvihill and

Ennis 2003).

Microfiltration (MF) technology may be used to produce pow-

ders enriched in micellar casein (Pouliot et al. 1996, Maubois

1997, Kelly et al. 2000, Garem et al. 2000). Casein recovered

by MF has very different functionality and properties compared