Molecular self-assembly of partially hydrolysed α-lactalbumin resulting in strong gels with a novel microstructure

Abstract

Gelation of α-lactalbumin (α-la) incubated with a protease from Bacillus licheniformis (BLP) at 50 °C for 4 h was monitored using small oscillatory shear and the large deformation properties of final gels were characterized by uniaxial compression. Transmission electron microscopy was used to visualize the microstructure. Gels made from α-la (10 g/l) using BLP were almost transparent, although somewhat whitish, and they were more than 20 times stiffer (measured as complex modulus) than equivalent gels made from β-lactoglobulin (β-lg) at the same concentration. The microstructure of the gels consisted of non-branching, apparently hollow strands with a uniform diameter close to 20 nm, similar in overall structure to microtubules. Adding Ca2+ in amounts of 50 or 100 mM changed the spatial distribution of the strands and resulted in a reduction in the failure stress recorded in uniaxial compression. Apart from affecting the microstructure, Ca2+ was shown to be essential for the formation of the gels. It is proposed, that the mechanism behind the self-assembly of the partially hydrolysed α-la into long tubes is a spatially restricted creation of ionic bonds between Ca2+ and carboxyl acid groups on peptide fragments resulting from the action of BLP on α-la. Proteolysis of α-la with BLP in the presence of Ca2+ thus results in formation of a strong gel with a microstructure not previously observed in food protein systems.

Thermal aggregation of β-lactoglobulin in presence of metal ions

“…Moreover, in mixtures of whey proteins, fibril-like aggregates have been found [37,38] as in production of gels or aggregates in presence of calcium or zinc, respectively [39][40][41]. In fact, metal ions are also able to induce cold gelation and the gel texture formation depends on different parameters as pH conditions and protein and/or iron concentration [42,43].…”

Pilot-scale purification of α-lactalbumin from enriched whey protein concentrate by anion-exchange chromatography and ultrafiltration

“…Furthermore, it represents the best characterized model of the intermediate folding structure, the molten globule state, formed upon removal of Ca 2+ by lowering the pH to below 4, or by using chelating agent such as EDTA at neutral pH (Kuwajima 1996). In addition, α-La has been shown to form nanotubular structures upon limited proteolysis (Ipsen et al 2001a), which in turn can result in formation of strong transparent gels and provide the possibility for α-La to be used as a thickening agent or gelling agent in food systems or for encapsulation of bioactive compounds (Ipsen and Otte 2007). However, this nanostructure only forms when α-La occurs in high purity without the presence of β-Lg (Ipsen et al 2001b).…”

Rheological and Structural Characteristics of Nanometre-Scale Food Protein Particle Dispersions and Gels

“…Partial hydrolysis of α-la with a serine proteinase from B. licheniformis resulted in fibrils that are tubes approximately 20 nm in diameter (Ipsen et al 2001;Ipsen and Otte 2007). Such a fibril gel from a C and 80 C (Gosal et al 2004b, c) 10.0 % w/v α-la solution had a higher modulus than a heat-set gel from a 10 % w/w β-lg solution, pH 2.5; further, the gelation times of α-la fibrils were longer than those of β-lg fibrils.…”

“…Partial hydrolysis of α-la with a serine proteinase from Bacillus licheniformis was shown to form hollow tubes approximately 20 nm in diameter and several micrometres in length (Ipsen et al 2001;Ipsen and Otte 2007). The minimum concentration of α-la necessary to form nanometre-diameter tubes was 3.0 % w/v at 50 C, 75 mM Tris buffer, pH 7.5, 2 mol Ca 2+ /mol α-la.…”