Dry-heating pH modifies the structure and functional properties of whey proteins
Résumé
Heating in dry state has gained a lot of interest in pharmaceutical and food industries for viral and microbial decontamination of thermo sensible products. In addition controlled dry heating is a common industrial process for improving the functional properties of food proteins (egg white 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 pH conditions for dry heating affects the chemical modifications and denaturation/aggregation reactions of whey proteins. Model whey protein powders obtained from freeze drying of a 15% w/w protein concentrates adjusted at three different pH‘s (2.5, 4.5 & 6.5) were adjusted to a fixed water activity (Aw 0.2). All the samples were dry heated at 100°C up to 24 hours and structural modifications induced during dry heating were followed. We showed that whatever the pH value for dry heating, proteins were characterized by irreversible mass losses of 18 Da. Such mass losses were increased at lower pH value and with extending time for dry-heating. Strikingly, at low pH value (2.5) intermolecular disulfide bonds were the predominant crosslinks between proteins while covalent crosslinks other than disulfide bonds were prevalent at higher pH’s (4.5 & 6.5). Protein hydrolysis was also detected at low pH. Whatever the pH, alpha-lactalbumin was more susceptible to dry heat induced chemical modifications as compared to beta-lactoglobulin. 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 control the protein denaturation/aggregation process and having reproducible functionality in products where such modified proteins are incorporated as ingredient.