The meat industry's high environmental impact necessitates a shift towards plant-based proteins. However, the astringency of plant proteins, stemming from high friction and poor lubrication, limits their widespread adoption. Oral lubrication serves as an in vitro measure of friction-related mouthfeel. Studies have shown that plant proteins, unlike dairy proteins, increase oral friction due to aggregation and interaction with saliva. This "delubrication" significantly impacts consumer acceptability. While proteins are considered ideal fat mimetics due to their lower caloric density and viscosity-enhancing properties, their poor lubrication performance hinders their application. Protein microgels, cross-linked, swollen networks sheared into micron-sized particles, have shown promise in improving texture and lubrication. However, research has primarily focused on dairy proteins, neglecting the potential of plant proteins. This study investigates the lubrication properties of plant protein microgels to address the astringency challenge and pave the way for more palatable and sustainable food products. The central question this study aims to answer is whether microgelation of plant proteins can significantly enhance their lubrication performance, thus improving consumer acceptance.

The literature extensively documents the environmental impact of the food industry, particularly the meat industry's contribution to greenhouse gas emissions. Several studies highlight the negative sensory attributes of plant proteins, specifically their astringency, which is directly linked to their poor lubrication properties in the oral cavity. Existing research on protein microgels demonstrates their potential in improving texture and lubrication, primarily using dairy proteins. However, there's a gap in understanding the lubrication behavior of plant protein microgels, which is crucial for their wider application in food products. This lack of research on plant protein microgels necessitates investigation into their potential for improving the mouthfeel and overall acceptability of plant-based foods.

This study used pea protein concentrate (PPC) and potato protein isolate (PoPI) as model plant proteins. Four types of microgels were fabricated using a top-down approach: thermal gelation-induced physical crosslinking of proteins followed by controlled shearing. Different protein concentrations were used to achieve varying microgel elasticities. The size, morphology, and stability of the microgels were characterized using dynamic light scattering (DLS) and atomic force microscopy (AFM). Rheological properties, including storage modulus (G') and viscosity, were determined using oscillatory shear rheology. Tribological performance was evaluated using a steel ball-on-PDMS (hard-soft) contact and a biomimetic 3D tongue-like surface, measuring friction coefficients under different conditions. A mathematical model was developed to describe the lubrication performance of the microgels. Adsorption measurements using quartz crystal microbalance with dissipation monitoring (QCM-D) were employed to investigate the interaction between microgels and a hydrophobic surface. Statistical analysis (ANOVA with Tukey post hoc test) was used to compare the results. The details of material preparation, microgel fabrication, characterization techniques, tribology setup, adsorption analysis, and statistical analysis are thoroughly explained in the methods section.

DLS and AFM revealed that the fabricated plant protein microgels were sub-micron sized (50-200 nm) with low polydispersity, indicating improved size control and stability compared to native proteins. Rheological analysis showed that most microgels exhibited Newtonian behavior, unlike the shear-thinning behavior of native proteins. Tribological experiments using both PDMS and the biomimetic tongue-like surface showed that the microgels significantly reduced friction compared to native proteins. In many cases, the friction coefficient was reduced by more than an order of magnitude. Remarkably, several microgels demonstrated lubrication performance comparable to or exceeding that of a 20:80 oil/water emulsion. Mathematical modeling supported the experimental findings, suggesting that the microgels' ability to swell and release water, creating a hydrated lubricating layer, contributes significantly to their superior lubrication properties. Adsorption measurements showed correlations between the hydrated mass of the microgels and their lubricity. In some cases, notably in the biomimetic tongue tests, the topography of the test surface revealed additional challenges. Specifically, PoPM5, PoPM10, and PPM7.5:PoPM5 exhibited reduced lubricity at both low and high volume fractions due to interactions with the 3D-printed tongue's papillae. This points to the importance of using biomimetic surfaces in oral tribology research.

The findings demonstrate that microgelation significantly improves the lubrication properties of plant proteins, addressing a key limitation in their application as food ingredients. The observed lubrication enhancement is attributed to the microgels' unique structural and rheological properties, including their ability to swell, retain water, and act as effective viscosity modifiers. The similarity in lubrication performance between certain microgels and oil/water emulsions is particularly significant, highlighting the potential of these microgels as fat mimetics. The development of a biomimetic tongue-like surface for tribological testing provides a more physiologically relevant model for assessing mouthfeel, enhancing the accuracy of in vitro studies. The results suggest that the effective height of the microgel layer is crucial for lubrication, with swelling and water release contributing to hydration lubrication. The superior load-bearing capacity of microgels is another essential factor in their excellent lubrication performance. This work bridges the gap between material science and food science, demonstrating the practical application of microgel technology to improve the functionality and consumer acceptability of plant-based food products.

This study provides strong evidence that microgelation is a viable technique for enhancing the lubricity of plant proteins. The resulting microgels exhibit significantly improved lubrication properties compared to native proteins, achieving performance comparable to oil/water emulsions. The study's findings have significant implications for the development of healthier, more sustainable, and palatable plant-based foods, addressing a major bottleneck in the transition towards plant-based diets. Future research could focus on sensory evaluation to further correlate the in vitro tribological findings with actual mouthfeel perception and on exploring the application of these microgels in a wider range of food products.

The study primarily focused on two model plant proteins (pea and potato) and a limited range of microgel formulations. While the biomimetic tongue-like surface improved the physiological relevance of the tribological tests, direct sensory evaluation was not performed, limiting the ability to directly connect the in vitro measurements with the subjective experience of mouthfeel. The production of microgels at the laboratory scale also limits the direct extrapolation of the environmental benefits of the process to industrial settings. Further research is required to investigate the effect of microgelation on other plant proteins and to conduct life cycle assessments on larger scales of production.