Structural consequences of dry heating on Beta-Lactoglobulin under controlled pH

Abstract

Heating in dry state has gained a lot of interest in pharmaceutical and food industries for viral and microbial decontamination of thermo sensible products. Controlled dry heating has now become a common industrial process for improving the functional properties of food proteins. Besides this improvement, chemical modifications in protein structures involving degradation of amino acids, new intra or inter-molecular disulfide bonds, isopeptide bonds, and some other links may occur. These chemical modifications are favored by severe heating under neutral or alkaline conditions, and except disulfide interchanges, are usually not predominant during heat treatments in solution and their occurrence is not well understood. Understanding chemical modifications in protein structure that occur during dry heating of protein powders is a prerequisite for reproducible properties of final products at industrial scale. In the present work, we focused on how dry heating under acidic pH conditions affects the chemical modifications and denaturation/aggregation reactions of whey proteins. Model whey protein (β-lactoglobulin) powders obtained from freeze drying of protein concentrates adjusted to pHs 2.5 & 6.5 were adjusted to a fixed water activity (Aw 0.23). The samples were dry heated at 100°C for up to 24hours and structural modifications induced during dry heating were followed. We showed that whatever the pH value, proteins were characterized by irreversible mass losses of 18Da. Such mass losses were increased at lower pH value. Dry heating mainly generated small aggregates (dimers and oligomers). Strikingly, at pH 2.5 intermolecular disulfide bonds were the only crosslinks between proteins in the aggregated forms while covalent crosslinks other than disulfide bonds also participated at pH 6.5. β-Lactoglobulin hydrolysis was also detected at pH 2.5 and some of the peptides were incorporated in the oligomers. These results underline that chemical modifications in protein structures induced by dry heating are highly pH dependent. Hence, strict control of pH conditions for dry heating is indispensable to give reproducible functionality in products where such modified proteins are incorporated as ingredient.