Surface adsorption and lubrication properties of plant and dairy proteins: A comparative study

Abstract

The aim of this work was to compare the surface adsorption and lubrication properties of plant and dairy proteins. Whey protein isolate (WPI) and pea protein isolate (PPI) were chosen as model animal and plant proteins, respectively, and various protein concentrations (0.1-100 mg/mL) were studied with/without heat treatment (90 °C/60 min). Quartz crystal microbalance with dissipation monitoring (QCM-D) experiments were performed on hydrophilic (gold) and hydrophobic polydimethylsiloxane (PDMS) sensors, with or without a mucin coating, latter was used to mimic the oral surface. Soft tribology using PDMS tribopairs in addition to wettability measurements, physicochemical characterization (size, charge, solubility) and gel electrophoresis were performed. Soluble fractions of PPI adsorbed to significantly larger extent on PDMS surfaces, forming more viscous films as compared to WPI regardless of heat treatment. Introducing a mucin coating on a PDMS surface led to a decrease in binding of the subsequent dietary protein layers, with PPI still adsorbing to a larger extent than WPI. Such large hydrated mass of PPI resulted in superior lubrication performance at lower protein concentration (≤10 mg/mL) as compared to WPI. However, at 100 mg/mL, WPI was a better lubricant than PPI, with the former showing the onset of elastohydrodynamic lubrication. Enhanced lubricity upon heat treatment was attributed to the increase in apparent viscosity. Fundamental insights from this study reveal that pea protein at higher concentrations demonstrates inferior lubricity than whey protein and could result in unpleasant mouthfeel, and thus may inform future replacement strategies when designing sustainable food products.

Keywords: Lubrication; Mucin; Pea protein; QCM-D; Tribology; Whey protein.

Graphical abstract

Fig. 1

Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) of untreated and soluble fractions of WPI, HT WPI, PPI and HT PPI. Protein concentration was 1 mg/mL. Lane 1–6 represents: (1) WPI (untreated fraction), (2) WPI (soluble fraction), (3) HT WPI, (4) PPI (untreated fraction), (5) PPI (soluble fraction) and (6) HT PPI. Lane (M) represents the molecular weight marker of 10–250 kDa molecular weight range.

Fig. 2

Frequency shift (a), dissipation shift (b) and –ΔD/Δf (c) obtained (5th overtone shown), as a function of time, of 0.1 mg/mL WPI (■) and PPI (□) on PDMS-coated surface without (i) or with (ii) heat treatment at 90 °C. B and P indicate the point of addition of buffer and protein, respectively. Error bars represent standard deviations.

Fig. 3

Frequency shift (a), dissipation shift (b) and –ΔD/Δf (c) obtained (5th overtone shown), as a function of time, of 1.0 mg/mL BSM followed by addition of 0.1 mg/mL WPI (■) and PPI (□) on PDMS-coated surface without (i) or with (ii) heat treatment at 90 °C. B, BSM and P indicate addition of buffer, BSM and protein, respectively. Error bars represent standard deviations.

Fig. 4

Mean static water contact angle of 0.1 mg/mL of WPI, HT WPI, PPI and HT PPI with and without the presence of BSM on the PDMS-coated surface. Error bars represent standard deviations. Samples with the same alphabet do not differ significantly (p > 0.05) according to Tukey's test.

Fig. 5

Friction coefficients of soluble fraction and heat-treated versions of (a) WPI and (b) PPI solutions as a function of entrainment speeds. Friction coefficients of buffer is also presented for comparison purposes.

Fig. 6

Shear viscosity (a) and friction coefficient (b) curves as a function of ηU of soluble fraction and heat-treated versions of WPI and PPI solutions at protein content of 100 mg/mL. The black continuous line represents the fitting using equation (3) to the elastohydrodynamic regime on the curve for the heat-treated WPI solution.

Fig. 7

Schematic representation (not to scale) of the adsorption and lubrication behavior of WPI and PPI on PDMS surface, illustrating the effect of BSM on the adsorption behavior and hydrated mass (HM) of proteins and the impact of protein concentration on their lubrication performance.