Formation of conjugates between β-lactoglobulin and allyl isothiocyanate: Effect on protein heat aggregation, foaming and emulsifying properties

Whey proteins are widely used food ingredients due to their nutritional and functional properties (gelling, emulsifying, foaming). Owning to their structure (free thiol group, lysine residues, hydrophobic pocket), they can also be used as carriers for bioactives. In this study, conjugates between β-lactoglobulin (β-lg), and a bioactive metabolite from Brassicaceae vegetables, allyl isothiocyanate (AITC) were formed. Heat aggregation behavior (85 °C, 15 min), foaming and emulsifying properties of conjugates, at pH 4.0 and 7.1, were evaluated. Conjugates were formed by incubating β-lg (0.5 mM) with AITC (0.05–20 mM) in water at pH 8.5 and room temperature. AITC primarily reacted with β-lg’s free thiol group (KD = 0.2 ± 0.1 mM) and thereafter with its amino groups (KD = 10.8 ± 3.4 mM). AITC binding destabilized secondary and tertiary structure of β-lg at pH 7.1, whereas induced molten globule conformation at pH 4.0. Conjugation reduced the heat aggregation of β-lg at pH 7.1, while promoting it at pH 4.0. Conjugates adsorbed faster to air/water and oil/water interfaces at pH 4.0 than at pH 7.1. After 30 min, air/water surface tension was lower at pH 4.0 (47 mN m−1) than at pH 7.1 (57 mN m−1), while the surface tension of the oil/water interface was 8 mN m−1 at both pHs. Foams produced with β-lg–AITC conjugates at pH 4.0 exhibited higher volume and liquid stabilities compared to foams obtained at pH 7.1. Emulsions formed with conjugates at both pHs were destabilized by creaming due to flocculation, but coalescence was prevented. This study revealed that whey protein could potentially be used for the delivery of isothiocyanates in the form of foam or emulsion-based products.