Transforming sustainable plant proteins into high performance lubricating microgels

Abstract

With the resource-intensive meat industry accounting for over 50% of food-linked emissions, plant protein consumption is an inevitable need of the hour. Despite its significance, the key barrier to adoption of plant proteins is their astringent off-sensation, typically associated with high friction and consequently poor lubrication performance. Herein, we demonstrate that by transforming plant proteins into physically cross-linked microgels, it is possible to improve their lubricity remarkably, dependent on their volume fractions, as evidenced by combining tribology using biomimetic tongue-like surface with atomic force microscopy, dynamic light scattering, rheology and adsorption measurements. Experimental findings which are fully supported by numerical modelling reveal that these non-lipidic microgels not only decrease boundary friction by an order of magnitude as compared to native protein but also replicate the lubrication performance of a 20:80 oil/water emulsion. These plant protein microgels offer a much-needed platform to design the next-generation of healthy, palatable and sustainable foods.

Fig. 1. Schematic illustration of microgelation of native plant proteins.

Visual representation of microgelation procedure. Native plant proteins are highly aggregated causing functional and sensory problems in food design. By hydrating them with water and thermally gelling using hydrophobic interactions, hydrogen bonding and disulphide-based covalent crosslinking occurring without any added crosslinking agents, the native plant proteins act as connecting particulate points in a highly percolating hydrogel network, which is then converted into gel-like particles via controlled homogenisation consisting of 5–15 wt% protein and 85–95 wt% water. These microgels remove functional issues associated with native protein allowing for improved functional application of plant proteins in food.

Fig. 2. Particle size of plant protein microgels.

Visual images (a) showing various degrees of opacity and size distribution obtained using dynamic light scattering (DLS) (b) using pea protein concentrate to form a 15.0 wt% total protein microgel, (PPM15), potato protein isolate to form a 5.0 wt% total protein microgel (PoPM5), potato protein isolate to form a 10.0 wt% total protein microgel, (PoPM10), and using a mixture of pea protein concentrate at 7.5 wt% total protein and potato protein isolate at 5.0 wt% total protein microgel (PPM7.5:PoPM5) at volume fractions (ϕ) 10–70 vol%, at 25.0 °C. Insets in (b) shows the mean hydrodynamic diameter (dH) and polydispersity index (PDI) values. Results are plotted as average of six measurements on triplicate samples (n = 6 × 3).

Fig. 3. Images of plant protein microgels on silicon under buffer.

Topographic images and respective histograms showing diameters of aqueous dispersions of protein microgels prepared using (a) pea protein concentrate to form a 15.0 wt% total protein microgel, (PPM15), (b) potato protein isolate to form a 5.0 wt% total protein microgel (PoPM5), (c) potato protein isolate to form a 10.0 wt% total protein microgel, (PoPM10), and (d) a mixture of pea protein concentrate at 7.5 wt% total protein and potato protein isolate at 5.0 wt% total protein to form microgel (PPM7.5:PoPM5).

Fig. 4. Rheological properties of the parent plant protein gels and volume fraction-dependent apparent viscosities of the microgels.

Storage modulus (a) and Young’s modulus (b) of parent plant protein gels with apparent viscosities (η) of microgels prepared using pea protein concentrate to form a 15.0 wt% total protein microgel, (PPM15), potato protein isolate to form a 5.0 wt% total protein microgel (PoPM5), potato protein isolate to form a 10.0 wt% total protein microgel, (PoPM10), and using a mixture of pea protein concentrate at 7.5 wt% total protein and potato protein isolate at 5.0 wt% total protein microgel (PPM7.5:PoPM5) with corresponding storage modulus (G’) of parent plant protein gels (a) as a function of volume fractions (ϕ) at pH 7.0 at shear rates of (c) 0.1 s–1 and (d) 50 s–1, the latter representing orally relevant shear rates performed at 37 °C. Data was recorded with shear rate increasing from 0.1 to 50 s–1, Figures a–b display means and standard deviations of 5 measurements on triplicate samples (n = 5 × 3) where statistical analysis was performed using two tailed unpaired Student’s t-test with differing lower-case letters in the same bar chart indicate a statistically significant difference (p < 0.05). Figures (c, d) shows the mean of 6 measurements on triplicate samples (n = 6 × 3) with error bars representing standard deviations. The original temperature ramp and frequency sweeps of the parent heat set gelled proteins are shown in Supplementary Figs. 3 and 4, respectively. The true stress-strain curves are shown in Supplementary Fig. 5 from which the Young’s moduli are computed. Original flow curves for the microgel dispersions at each volume fractions are shown in Supplementary Fig. 6.

Fig. 5. Stribeck curves in hard-soft contact surfaces in presence of plant protein microgels.

Tribological performance of steel ball on PDMS surfaces in the presence of plant protein microgels, native plant protein (matched protein content for Φ = 70 vol% with numbers displayed relating to total protein content) or oil-in-water emulsion. Friction coefficient (µ) as a function of entrainment speed (U) scaled with high rate viscosity (η ∞ = 1000 s − 1) in the presence of plant protein microgels prepared using (a1–a3) pea protein concentrate to form a 15.0 wt% total protein microgel, (PPM15), (b1–b3) potato protein isolate to form a 5.0 wt% total protein microgel (PoPM5), (c1–c3)), potato protein isolate to form a 10.0 wt% total protein microgel, (PoPM10), and (d1–d3) using a mixture of pea protein concentrate at 7.5 wt% total protein and potato protein isolate at 5.0 wt% total protein microgel (PPM7.5:PoPM5) with 1, 2 and 3 showing increased volume fractions from 10 to 70 vol%, respectively. Frictional responses of the plant proteins at the highest concentration and 20 wt% oil-in-water emulsion (O/W emulsion) and buffer are included in each graph (a-d) as controls. Results are plotted as average of six repeat measurements on triplicate samples (n = 6 × 3) with error bars representing standard deviations. Statistical comparison of mean at 0.1 Pa m is shown in Supplementary Table 3. Original friction coefficient versus entrainment speed curves for the microgel dispersions at each volume fractions are shown in Supplementary Fig. 7.

Fig. 6. Mechanism of lubrication performance of plant protein microgels in hard-soft contact surfaces.

Tribological performance of steel ball on PDMS contact surfaces showing (a) theoretical modelling of lubrication performance at a load of 2 N of exemplar plant protein microgels (pea, potato and mixed pea and potato microgel) showing close resemblance to the emulsions as opposed to the large friction coefficients obtained in presence of the native protein. Here the dashed lines show the best theoretical fit using Eq. 11 and (b) load dependency of microgels as compared to the native protein (matched protein content for Φ = 70 vol%) with 20:80 O/W emulsion as control with (c) schematic illustration of microgel performance as compared to native protein in hard-soft contacts. Friction coefficient (µ) is plotted as a function of entrainment speed (U). Results are plotted as average of three repeat measurements on triplicate measurements (n = 3 × 3) with error bars representing standard deviations.

Fig. 7. Tribological performance in 3D biomimetic tongue-like surfaces.

Tribological performance of 3D-printed biomimetic tongue-like polymeric surfaces in presence of plant protein microgels or oil-in-water emulsions. Friction coefficient (µ) as a function of linear speed (VR) in the presence of plant protein microgels prepared using (a1–a3) pea protein concentrate to form a 15.0 wt% total protein microgel, (PPM15), (b1–b3) potato protein isolate to form a 5.0 wt% total protein microgel (PoPM5), (c1–c3)), potato protein isolate to form a 10.0 wt% total protein microgel, (PoPM10), and (d1–d3) using a mixture of pea protein concentrate at 7.5 wt% total protein and potato protein isolate at 5.0 wt% total protein microgel (PPM7.5:PoPM5), respectively. Frictional responses of 20 wt% oil-in-water emulsion (O/W emulsion) and buffer are included in each graphs (a–d) as controls. Results are plotted as average of six measurements (n = 6 × 3) with error bars representing standard deviations. Statistical comparison of means at each lubrication regimes is shown in Supplementary Table 4.