Oral lubrication of alternative proteins

Alternative proteins are gaining significant prominence in food design, driven by sustainability which is underscored by substantial greenhouse gas emissions associated with food production, particularly stemming from animal protein sources. However, the utilisation of alternative proteins remain limited, marked by subpar sensory performance, off mouthfeel and limited solubility in aqueous phase. Often such poor mouthfeel performance is hypothesised to be linked with astringency and lubrication failure. Although there has been substantial work in recent years in sensory performance of alternative proteins in various food applications, mechanistic understanding of the lubrication performance of alternative proteins remain elusive. A thorough literature search identified key gaps in this thesis which include limited characterisation of a range of alternative proteins, particularly plant proteins in terms of surface interactions and lubrication, all of which could play a pivotal role in improving their mouthfeel and acceptability in food products. Also, there is limited understanding on the origin of such lubrication failure in alternative proteins. Finally, how to limit such offmouthfeel also remain underexplored.

This thesis has combined in-vitro, ex-vivo and in vivo methodological approaches in elucidating these knowledge gaps on mechanism behind off-tactile perception of alternative proteins. In particular, this thesis has highlighted the usefulness of new characterisation techniques including quartz crystal microbalance with dissipation, tribology using biomimetic surfaces, functional near-infrared spectroscopy and cellular models with salivary coating, Rate-All-That-Apply and theoretical assessment of tribological mechanism. Alternative proteins, such as pea, potato, lupin, and insect proteins were selected which hold high regard for future application. However, it was shown these alternative proteins suffer from high friction with friction coefficients increasing with higher concentration, a behaviour contrasting with those of animal protein (whey protein) counterparts. Next, model plant proteins either as single proteins or in mixed state were comprehensively characterised in their sensory profiling (n=100) identifying their taste and texture characteristics. Lupin and potato proteins surpassed the more often used pea protein in sensory aspects signifying their greater suitability for use in food applications. However, astringency immediately or as an aftertaste underlined the sensory profile of all plant proteins. When subjected to neural analyses, this astringent quality elicited a marked prefrontal cortex response, akin to the response observed with well-known astringent compound i.e. tannic acid. Furthermore, the presence of salivary protein binding was observed in presence of plant proteins again similar to tannic acid in cell culture experiments, a characteristic notably absent even at high concentrations of whey protein. With such poor lubrication and astringency aspects identified, the next step was to explore a colloidal strategy “microgelation” to limit lubrication failure. This process, never before applied to plant protein in lubrication context was able to enhance the lubrication performance of plant proteins remarkably irrespective of the type of plant proteins used. Using in-vitro methodology of biomimetic tonguelike surface as well as mathematical modelling, the microgels of plant proteins were found to be ultra-lubricating, highly dispersible, stable and heat resistant irrespective of type or concentration. Strikingly, the plant protein microgels demonstrated lubricity resembling that of an oil-in-water emulsion despite having no lipidic additives in the microgels. In summary, this thesis provides a mechanistic understanding of lubrication failure of alternative proteins focusing primarily on plant proteins and provides a unique microgelation approach to address off-mouthfeel perception with the insights gathered paving the way forward for sustainable food development limiting “dryness” and “astringency” to improve consumer acceptability of alternative proteins.