Enzymatic hydrolysis of [beta]-casein and [beta]-lactoglobulin : foam and emulsion properties of peptides in relation to their molecular structure

Peptides derived fromβ-casein (βCN) andβ-lactoglobulin (βLg) were analysed for their foam- and emulsion-forming and -stabilising properties (further denoted functional properties) and for their structural characteristics in order to elucidate structure-function relationships.βCN was hydrolysed by plasmin and subsequent fractionation of the hydrolysate resulted in various hydrophilic, amphipathic and hydrophobic peptide fractions with clear differences in functional properties. The highly-charged N-terminal part of the amphipathic peptides appeared to be important for the emulsion-stabilising properties ofβCN peptides. The main secondary structure element ofβCN(-peptides) in solution was the unordered random coil, but upon adsorption onto an hydrophobic interfaceα-helix was induced. The hydrophobic C-terminal part ofβCN accounted for the high maximum surface load on the interface, while the N-terminal part ofβCN seemed to be responsible for theα-helix induction upon adsorption. No clear relation between the secondary structure and the functionality was observed in this system but a relation between a high surface load and good stabilising properties seemed to exist.BovineβLg was hydrolysed by the action of trypsin, plasmin and <em>Staphylococcus aureus</em> V8 protease. Overall, the plasmin hydrolysate had the best functional properties at pH 6.7, compared to the other hydrolysates and was investigated further. DuringβLg/plasmin hydrolysis significant SH/SS-exchange has taken place yielding a large number of different peptides. The peptides present were (1) peptides composed of a single amino acid chain lacking a cysteine residue, (2) peptides composed of a single amino acid chain containing intramolecular disulphide bonds and (3) peptides composed of 2 amino acid chains linked by an intermolecular disulphide bond. The occurrence of the SH/SS exchange and the homogeneous distribution of charge and hydrophobicity hinder an efficient fractionation of the hydrolysate.In conclusion, the production of specific peptides and peptide fractions is more complicated forβLg than forβCN, mainly because of the differences in primary structure (such as the distribution of charge and hydrophobicity) between the proteins. The foam- and emulsion-forming properties of peptides can be superior to those of intact proteins, as long as they have both charged and hydrophobic areas. The foam- and emulsion-stabilising properties of peptides depend highly on the amount of repulsion they can produce (either by a strong amphipathicity or by a high surface load).