Food Biochemistry and Food Processing (2 edition)

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25 Biochemistry of Milk Processing 477

further slow acidification by starter bacteria. To prevent the lat-

ter, and thereby extend the shelf life of yoghurt, the final product

may be pasteurised after acidification; to prevent damage to the

gel structure at this point, the role of hydrocolloid or protein

stabilisers is critical (Walstra et al. 1999).

Acid-Heat Coagulated Cheeses

These cheeses were produced initially in Southern European

countries from whey as a means of recovering nutritionally valu-

able whey proteins; their production involves heating whey from

rennet-coagulated cheese to approximately 90◦C to denature and

coagulate the whey proteins. Well-known examples are Ricotta

and Manouri. Today, such cheeses are made from blends of milk

and whey, and in this case, it is necessary to adjust the pH of

the blend to approximately 5.2 using vinegar, citrus juice or

fermented milk.

Acid-heat coagulated cheese may also be produced from

whole milk by acidifying to pH 5.2 and heating to 90◦C, for

example US-style Queso Blanco, which does not melt on heat-

ing and hence has interesting functional properties for certain

applications.

WHEY PROCESSING

Range of Whey Products

For many centuries, whey was often viewed as an unwanted

and valueless by-product of cheese manufacture; however, since

around 1970, whey has been repositioned as a valuable source

of a range of food ingredients, rather than a disposal problem.

While part of the impetus for this strategic re-evaluation of the

potential of whey was driven by environmental and other con-

cerns about the disposal of a product of such a high Biological

Oxygen Demand, it has resulted in a previously unsuspected

richness and diversity of products, and a significant new pro-

cessing sector of the dairy industry (Sienkiewicz and Riedel

1990, Jelen 2002).

The first stage in the production of any whey product is pre-

liminary purification of the crude whey, recovered after cheese

or casein manufacture. Fresh whey is classified as either sweet,

with a pH>5.6 and a low calcium content, or acid whey, with

a pH of around 4.6 and high calcium content; the difference

in calcium content arises from the solubilisation of colloidal

calcium phosphate as the pH of the milk decreases. Common

pre-treatments include centrifugal clarification to remove curd

particles (fines), and pasteurisation (to inactivate starter LAB).

For many whey protein-based products, removal of all lipids

is desirable; this can be achieved by centrifugal separation to

produce whey cream, which may be churned into whey butter.

To obtain a very low level of lipids, calcium chloride may be

added to the whey, with adjustment of the pH to more alka-

line values, followed by heating and cold storage to precipitate

and remove lipoprotein complexes (thermocalcic aggregation;

Karleskind et al. 1995).

The next level of processing technology applied to the pre-

treated whey depends on the end product to be manufactured.

The simplest whey products are probably whey beverages, con-

sisting of clarified whey, typically blended with natural or con-

centrated fruit juices. Whey beverages have a nutritionally ben-

eficial amino acid profile; although not widely commercialised,

such drinks have been successful in some European countries

(e.g., Rivella in Switzerland).

Whey may be concentrated by evaporation and spray-dried

to whey powders. The key consideration in spray-drying whey

is the constraints imposed by the fact that the major constituent

of whey is lactose (typically,>75% of whey solids). Thus, pro-

cesses for drying whey must include controlled crystallisation

of lactose to yield small crystals (30–50μm). This is typically

achieved by controlled cooling of concentrated (i.e., supersatu-

rated with respect to lactose) whey under a programmed temper-

ature regime, with careful stirring and addition of seed crystals,

typically ofα-lactose monohydrate, followed by holding un-

der conditions sufficient to allow crystallisation to proceed. In

general, only approximately 70% of the lactose present will crys-

tallise at this stage, and processes often include a post-drying

crystallisation stage, for example on a belt attached to the spray-

dryer, where the remainder crystallises.

The simplest dried whey product is whey powder, which is

produced in a single-stage spray-dryer. Additional care must be

taken in drying acid whey products, which may be sticky; the

corrosive nature of the acids (e.g., HCl) used in their manufacture

may also present processing difficulties and such products may

be neutralised prior to drying.

Whey powders may be unsuitable for use in certain food appli-

cations (e.g., infant formulae) due to their high mineral content;

in such cases, whey is demineralised by ion exchange or elec-

trodialysis before concentration and dialysis (for a discussion of

the technologies involved, see Burling 2002). Demineralisation

of whey is also desirable for use in ice cream production, to

reduce the salty taste of normal whey powder.

Whey-Protein-Rich Products

Whey powder has a low (approximately 12–15%) protein con-

tent and a high (approximately 75%) lactose content; many whey

products have been at least partially purified to increase the level

of a particular constituent, usually protein (Matthews 1984).

There is a family of protein-enriched whey-derived powders,

which are differentiated based on the level of protein; whey pro-

tein concentrates (WPCs) with a protein content in the range

35–80%, and whey protein isolates (WPIs) with a protein con-

tent>88%. Typically, WPCs are produced by UF, with protein

being progressively concentrated in the retentate, while lactose,

salts and water are removed in the permeate (Ji and Haque 2003).

Higher protein levels can be achieved by diafiltration, that is, di-

lution of the retentate followed by UF. The use of ion exchangers

to adsorb the proteins from whey, followed by selective release

into suitable buffer solutions, is required to achieve the high

degree of purity required of WPIs.

Methods for the purification of individual whey proteins have

been developed, usually exploiting differences in the stability

of individual proteins under specific conditions of pH, temper-

ature and ionic salts. In addition, methods involving organic