Casein Products

CASEIN PRODUCTS
Casein is the principal protein found in cow’s milk from which it has been extracted
commercially for most of the 20th century. It is responsible for the white, opaque
appearance of milk in which it is combined with calcium and phosphorus as clusters of
casein molecules, called “micelles”.
The major uses of casein until the 1960s were in technical, non-food applications suchas
adhesives for wood, in paper coating, leather finishing and in synthetic fibres, as well as
plastics for buttons, buckles etc. During the past 30 years, however, the principal use of
casein products has been as an ingredient in foods to enhance their physical (so-called
“functional”) properties, such as whipping and foaming, water binding and thickening,
emulsification and texture, and toimprove their nutrition.
In New Zealand, casein is precipitated from the skim milk that is produced after
centrifugal separation of whole milk. The skim milk may be acidified to produce acid
casein or treated with an enzyme, resulting in the so-called rennet casein. The precipitated
casein curd is separated from the whey, washed and dried. Water-soluble derivatives of
acid caseins, producedby reaction with alkalis, are called caseinates.

INTRODUCTION
The amount of casein in cow’s whole milk varies according to the breed of cow and stage of
lactation, but is generally in the range 24-29 g L-1. Casein contains 0.7-0.9% phosphorus,
covalently bound to the protein by a serine ester linkage. Casein is consequently known as a
phospho-protein. All the amino acids that are essentialto man are present in casein in high
proportions, with the possible exception of cysteine. Thus, casein may be considered as a
highly nutritious protein.
Casein exists in milk in complex groups of molecules (sometimes referred to as calcium
phospho-caseinate) that are called “micelles”. The micelles consist of casein molecules,
calcium, inorganic phosphate and citrate ions, and have a typicalmolecular weight of several
hundred million. In terms of physical chemistry, the casein micelles may be considered to
exist in milk as a very stable colloidal dispersion. The caseins, as proteins, are made up of
many hundreds of individual amino acids, each of which may have a positive or a negative
charge, depending on the pH of the [milk] system. At some pH value, all the positive chargesand all the negative charges on the [casein] protein will be in balance, so that the net charge
on the protein will be zero. That pH value is known as the isoelectric point (IEP) of the
protein and is generally the pH at which the protein is least soluble. For casein, the IEP is
approximately 4.6 and it is the pH value at which acid casein is precipitated. In milk, which
has a pH of about 6.6,the casein micelles have a net negative charge and are quite stable.
Although casein has been shown to consist of several individual casein components, referred
to as αs1-, αs2-, β- and κ-casein, each having slightly different properties (which are caused
by small variations in their amino acid content), only the commercial product, which contains
all of these components, is considered in thisarticle.

III-Dairy-E-Casein-1

EXTRACTION OF CASEIN FROM MILK
Separation
Casein is extracted from milk using the processes outlined in Figures 1 and 2. Whole cow’s
milk (with a typical fat content of 4.65% - see Figure 1 in the introductory article) is first
separated by means of centrifuges that produce cream (for the manufacture of butter or other
milkfat products - see the articleon milkfat products) and skim milk. Skim milk can thus be
considered as the raw material from which casein products are made.
Precipitation
The operations involved in the precipitation of casein in New Zealand are shown in Figure 1.
Precipitation by means of acidification can be considered in terms of simple chemistry as
follows, R being the casein protein:
H2N-R-COO- + H+
casein micelle...