Interfacial shear rheology of β-lactoglobulin—Bovine submaxillary mucin layers adsorbed at air/water interface

Abstract

The interfacial rheological properties of solutions of β-lactoglobulin (BLG), as a model food compound, mixed with bovine submaxillary mucin (BSM), a major salivary protein, have been investigated. Time, frequency, stress sweep and flow measurements have been performed at different pHs (7.4, 5.0 and 3.0), to investigate the air/water interfacial properties. All protein layers (BLG, BSM, and BLG-BSM mixtures) formed an elastic network at the air/water interface with low frequency dependence of the interfacial modulus. The results indicated that BLG moves faster as smaller molecule than mucin, and dominate the surface adsorption and the network formation for the BLG-BSM mixtures. Moreover, BLG-BSM protein mixtures exhibited interfacial properties with lower elastic and viscous moduli than BLG, as a result of competitive displacement of BLG proteins with BSMs from the interface. It is suggested that hydrophobic patches of BSM can be imbedded into the BLG monolayer as driven by a strong hydrophobic interaction with air and disrupt the cohesive assembly of BLG, whereas the hydrophilic (negatively charged) parts of the BSM chain are protruding from the interface towards the bulk water.

β-Lactoglobulin Adsorption Layers at the Water/Air Surface: 4. Impact on the Stability of Foam Films and Foams

“…The whey globular protein β-lactoglobulin (BLG) is frequently used as a model protein in foam and interfacial studies. Adsorption dynamics and surface rheology of native BLG layers at static water/air interfaces [8,25,[35][36][37][38][56][57][58][59][60], as well as foam film thickness [25,61], have been previously studied as affected by pH. It was proven by vibrational sum-frequency generation (SFG) spectroscopy that the pH-dependent net charge of BLG in solution determines the magnitude of the electric field at the solution's interface and thereof, a charge reversal can occur when the isoelectric point of BLG moieties (pI ≈ 5.1) is crossed [35].…”

Protein–saliva interactions: a systematic review

“…Zeta-potential analysis again indicated that whey protein, β-lg, gelatine, and lactoferrin electrostatically interact with mucin. [51][52][53][54] However, gelatine and WPI have also been shown to interact with mucin via non-electrostatic mechanisms (Table 1). 55 Evidence supported entropically and enthalpically driven aggregation with formation of hydrogen bonds or hydrophobic interactions even at neutral pH where both whey protein, gelatine and mucins are negatively charged.…”

“…As previously discussed with WPI, this is likely due to repulsion between β-lg and mucin due both having net negative charges. 43,53 Further, frictional behaviour was dominated by salivary proteins (bovine submaxillary mucin, BSM) 51 rather than by β-lg. With regards to emulsions, at neutral pH, β-lg showed reversible flocculation with…”

“…3d), which overshadowed any β-lgmucin interaction. 53 Tribology showed that at pH 5.0, a reduction in lubrication in β-lg was observed compared to that at pH 3.5 and pH 7.4 51 and an increase in friction 36 compared to pH 3.5 (Table 1). This is expected as β-lg aggregates were particulate in nature and were incapable of forming a continuous load bearing film at the tribo-contact surface as opposed to β-lg films or β-lg + mucin films.…”

“…Noticeably, the mixing ratio is directly related to the earlier discussed parameter of protein concentration however this section will also detail saliva's contribution. The majority of studies of bulk solutions used a 1 : 1 w/w ratio 43,46,48,[51][52][53][54]61 (Table 1) and for emulsion studies 1,37,41,[57][58][59]66,73,74 (Table 2), irrespective of the type of saliva employed. However, using this ratio has been suggested not to fully simulate real oral conditions, studies revealed that saliva mixes with food in more of a 1 : 4 w/w ratio.…”