Beta-lactoglobulin

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β-lactoglobulin is the major whey protein of cow's milk (~3 g/l), and is also present in many other mammalian species; a notable exception being humans. Its structure, properties and biological role have been reviewed many times ^[1][2][3].

Structure and role

Unlike the other main whey protein, α-lactalbumin, no clear function has been identified for β-lactoglobulin, although it binds to several hydrophobic molecules, suggesting its role in their transport. Several genetic variants have been identified, the main ones in the cow being labelled A and B. Because of its abundance and ease of purification, it has been subjected to a wide range of biophysical studies. Its structure has been determined several times by X-ray crystallography and NMR. One such structure is shown on the right (from http://www.pdb.org entry 3BLG). β-lactoglobulin is of direct interest to the food industry since its properties can variously be advantageous or disadvantageous in dairy products and processing ^[4].

Bovine β-lactoglobulin is a relatively small protein of 162 residues, with an 18.4 kDa molecular weight (1 Dalton being defined as 1 molecular weight unit). In physiological conditions it is predominantly dimeric, but dissociates to a monomer below about pH 3. Nevertheless, its native state remains fairly intact at lower pH values, as determined using NMR ^[5].

β-lactoglobulin solutions form gels in various conditions, when the native structure is sufficiently destabilised to allow aggregation ^[6]. Under prolonged heating at low pH and low ionic strength, a transparent `fine-stranded' gel is formed, in which the protein molecules assemble into long stiff fibres.

As milk is a known allergen (as listed in Annex IIIa of Directive 2000/13/EC), manufacturers need to prove the presence or absence of β-lactoglobulin to ensure their labelling satisfies the requirements of the aforementioned directive. Food testing laboratories such as Genon Laboratories Ltd ([7]) can use ELISA (enzyme linked immunoassay) methods to identify and quantify β-lactoglobulin in food products.