Enzymatic cross-linking of α-lactalbumin to produce nanoparticles with increased foam stability

Abstract

Hard colloidal nanoparticles (e.g. partly hydrophobised silica), are known to make foams with very high foam-stability. Nanoparticles can also be produced from proteins by enzymatic cross-linking. Such protein based particles are more suitable for food applications, but it is not known if they provide Pickering foam stabilisation to the same extent as hard colloidal particles. α-Lactalbumin (α-LA) was cross-linked with either microbial transglutaminase (mTG) or horseradish peroxidase (HRP) to produce α-LA/mTG and α-LA/HRP nanoparticles. With both enzymes a range of nanoparticles were produced with hydrodynamic radii ranging from 20-100 nm. The adsorption of nanoparticles to the air-water interface was probed by increase in surface pressure (Π) with time. In the beginning of the Π versus time curves, there was a lag time of 10-200 s, for nanoparticles with Rh of 30-100 nm, respectively. A faster increase of Π with time was observed by increasing the ionic strength (I = 0-125 mM). The foam-ability of the nanoparticles was also found to increase with increasing ionic strength. At a fixed I, the foam-ability of the nanoparticles decreased with increasing size while their foam-stability increased. Foams produced by low-shear whipping were found to be 2 to 6 times more stable for nanoparticles than for monomeric α-LA (Rh≈ 2 nm). At an ionic strength of 125 mM ionic strength and protein concentration ≥ 10 g L(-1), the foam-stability of α-LA/mTG nanoparticles (Rh = 100 nm, ρapp = 21.6 kg m(-3)) was 2-4 times higher than α-LA/HRP nanoparticles (Rh = 90 nm, ρapp = 10.6 kg m(-3)). This indicated that foam-stablity of nanoparticles is determined not only by size but also by differences in mesoscale structure. So, indeed enzymatic cross-linking of proteins to make nanoparticles is moving a step towards particle like behavior e.g. slower adsorption and higher foam stability. However, the cross-link density should be further increased to obtain hard particle-like rigidity and foam-stability.

Correlation between Physico-Chemical Characteristics of Particulated β-Lactoglobulin and Its Behavior at Air/Water and Oil/Water Interfaces

“…Thus, it plays a role at longer time scales rather than in the initial phase of adsorption. Similar conclusions were drawn by [ 10 ] using enzymatic cross-linked α-lactalbumin particles. Hence, as expected, no specific effect of the degree of cross-linking during the initial phase ( t < 5 s) of adsorption was observed.…”

“…Concluding from a multitude of foam-related studies, it is generally recognized that protein diffusion to the interface and thus foamability decreases with increasing particle size [ 8 , 9 , 10 ]. However, the lowering of the protein particles’ surface charge (zeta potential) via pH adjustment or salt-mediated charge screening leading to a reduced electrostatic barrier of adsorption as well as an increase in surface hydrophobicity seems to be even more important in view of foamability than particle size [ 11 , 12 , 13 , 14 ].…”

“…By contrast, [ 15 ] reported contradicting findings regarding the influence of the zeta potential. Moreover, the degree of cross-linking (i.e., the structural flexibility) is known to influence foam stability [ 10 , 12 , 16 ]. Regarding the degree of cross-linking, however, the drawing of generalized conclusions is quite difficult.…”

Protein-Based Nanostructures for Food Applications

“…PPO is an enzyme that can be used to cross-link casein in the presence of a low molecular weight phenolic compound such as caffeic acid. Overall, food functional effects of enzymatic protein cross-linking also include improvements of gelation properties and emulsion stability as well as enhanced heat-stability [62,63,64].…”

“…As the moisture evaporates from the droplets, a solid product (powder) is formed and recovered from the drying air [63]. This has the advantage of forming a convenient dry powder product with extended shelf life and low storage or transport costs [64]. Recently, a new generation of laboratory scale spray dryers that is able to produce particles at nano scale, has been introduced.…”