Physico-chemical stability of casein micelles cross-linked by transglutaminase as a function of acidic pH

Abstract

Abstract Casein micelles (CMs) can be used as nanocarriers in food and pharmaceutical applications, but can be destructured by changes in the physicochemical environment. Enzymatic crosslinking with transglutaminase (Tgase) is a possibility to stabilize CMs. The objective of this paper was to evaluate the stability of native CMs and CMs cross-linked by Tgase (CMs-Tgase) as a function of pH, in the presence of following destabilizing agents: urea, sodium citrate, high temperature and ethanol. Suspensions of native and CMs-Tgase were prepared at 27.5 g/L in 25 mM HEPES buffer with 2 mM CaCl2 at pH 7.10 The CMs’ size and ζ-potential were determined by dynamic light scattering. Native CMs precipitated below pH of 5.5. CMs-Tgase precipitated between pH 4.5 and pH 3.5 but was stable in the pH range of 7.0–4.5 and between pH 2.0 and 3.0. Isoelectric points of CMs and CMs-Tgase were determined as been 4.6 and 4.1, respectively. CMs-Tgase were stable at pH 2.0, in the presence of 100 mM of sodium citrate, 8 M urea, 99.5% (v/v) of ethanol and presented an heat coagulation time higher than 500 s at 140 °C. These results were interpreted as a consequence of the formation of internal covalent bounds between casein chains, and to a covalent attachment of the κ-caseins onto the surface of the CMs. In light of these results, it is possible to investigate CMs-Tgase as stable food-grade nanocarriers, whose characteristics can be modulated according to the chemical nature of the target biomolecule.