Non-covalent binding of benzaldehyde to β-lactoglobulin: characterisation by spectrofluorimetry and electrospray ionisation mass-spectrometry

Abstract

β-Lactoglobulin (β-lg), which is found in the milk of several mammal species, is the most abundant protein in bovine whey (McKenzie, 1971). The interaction properties of β-lg with a large variety of small hydrophobic ligands have been extensively studied (Sawyer et al. 1998). β-Lg belongs to the super-family of hydrophobic molecule transporters called the lipocalins, which characteristically bind hydrophobic ligands inside a central calyx (Godovac-Zimmermann, 1988; Perez et al. 1989; Brownlow et al. 1997; Wu et al. 1997; Qin et al. 1998).

At present, whey protein concentrates containing a high percentage of β-lg are commercially available in a very large quantity, but most of the processes used in milk technology involve heat-treatments that are known to affect the initial conformational state of β-lg. Such conformational changes have consequences for both the physicochemical and functional properties in food systems, including a decrease in the availability of lysine, due to Maillard reactions (Léonil et al. 1997) and a decrease in the affinity constant for binding to retinol (Laligant et al. 1991) and to flavour compounds (O'Neil & Kinsella, 1988). In the field of protein-volatile compound interactions, we have recently reported a significant enhancement of foaming properties of β-lg solutions (50 mM-NaCl, pH 6) in the presence of aroma compounds such as isoamyl acetate (Marin & Relkin, 1999), benzaldehyde (BZA; Marin & Relkin, 2000) and vanillin (Relkin & Vermersh, 2000). The observed increase in foaming properties of β-lg was postulated to be due to formation of surface active complexes between β-lg and aroma compounds.

In the present work we have investigated the interaction between a β-lg concentrate (prepared by ultra-diafiltration on an industrial scale) and BZA; particularly, evidence was sought for the presence of covalently bound monomers, dimers and lactolated monomers.