Tribo-rheology as an investigative tool for the elucidation of mouthfeel properties in alcoholic and non-alcoholic beer

Holt, Thomas ORCID: 0000-0002-8367-3774 (2024). Tribo-rheology as an investigative tool for the elucidation of mouthfeel properties in alcoholic and non-alcoholic beer. University of Birmingham. Ph.D.

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Abstract

Interest in low and no-alcohol beers has markedly increased in recent years, both from consumers and scientifically. Despite the increasing breadth of published work on the area little has been directed towards mouthfeel properties. This attribute is well known to be challenging to assess, normally requiring tasting panels with significant training and experience to produce useable data. While tasting panels are used in other food and beverage areas, tribology and tribo-rheology have been utilized for other examples, offering an instrumental mechanism to assess lubrication properties of a product or system. This work aims to investigate the feasibility of utilizing tribometry based techniques in the assessment of properties of both experimentally relevant systems and commercially relevant products, to validate the methodology for this system as well as investigate the molecular causes of the effects observed.

Initial work was focused around selecting a system that was reproducible and could provide the sensitivity required for analysis of closely related samples are low axial forces and speeds relevant to oral processing. It was found that Tribo-rheology offered such a solution and proved able to differentiate between closely related commercial beer products as well as demonstrate differences between aqueous solutions with very similar compositions. Using this methodology behavioural differences were identified between several commercial beers both non-alcoholic and alcoholic, produced by the same brewery, conversely some products were shown to closely match their alcohol containing versions. The method was also used to investigate potential causes for differences, in ethanol itself, sugar (maltose), polysaccharide (maltodextrin) and sodium chloride in water. This identified some unexpected patterns, whereby low concentrations of molecules would cause higher friction than pure water, indicating that products low in specific molecules may face worse lubricity outcomes than those where the component is entirely absent. This was of particular note for ethanol, where concentrations vary commercially and previously any amount of alcohol was largely considered to be positive for friction and thus mouthfeel.

Having demonstrated the techniques capability to differentiate between finished products, a microstructure approach was undertaken to attempt to elucidate the possible causes of the effects previously measured. This approach is to the authors knowledge novel, particularly in the scope of different molecules examined as well as their mixtures with others. This data indicated that even similar classes of molecules, e.g., inorganic chloride salts, can exhibit markedly different behaviour individually but more importantly when mixed with other molecules. These interactions were not easily predictable based on any property of the molecule, indicating that the interactions in lubrication are complex and varied. It was also found that many molecules exhibit unexpectedly strong effects despite concentrations below 100 parts per million, this included strong disruptive capabilities for acetoin and isoamyl alcohol. Volatile organic molecules are not classically considered to be of importance in tribology and when examined alone in water the effects were very minimal, once mixed with others however, there was a significant change in the tribological properties when compared to either molecule alone. This presented the possibility that molecules below taste thresholds could be causing changes in mouthfeel properties of products without presenting a clear taste profile, which would cause their identification to be challenging.

Following the microstructure-based study, the idea of manipulating the content of a beer post fermentation to achieve a desired goal was examined, this used proteins and amino acids, which had been demonstrated to have strong effects on friction in water previously. The analysis was conducted in water, a defined model beer analogue, a commercial 0% ABV beer and a commercial 4% ABV beer. Due to the exogenous nature of the nitrogen sources added here “real” concentrations were not used, a fixed molar concentration, allowed for more stoichiometrically fair comparison between all molecules, especially given the greatly different molecular masses. This methodology produced results demonstrating that while in water most amino acids provide positive lubricity this was not easily applicable to even a minimal defined beer analogue, while in commercial beer the results indicated that additions of amino acids or proteins could actively disrupt the existing lubricity, increasing friction as was seen with low concentrations of molecule in chapter 3. It was however seen that the proteins especially were able to entirely replace the previous systems lubrication properties, with 1.5 mM BSA in low alcohol beer showing no significant differences from 1.5 mM BSA in an alcoholic beer, despite there being clear difference between the two products before any additions were made.

Overall, it was found that tribo-rheology represents a useful tool in the investigation of lubricity in beer and beer related studies, able to demonstrate small differences between low concentration solutions and assist with elucidating the interactions potentially causing the observed effects. This method was then able to investigate the effect of additions of protein and amino acids to commercial and experimental beer products in terms of the potential effects on mouthfeel.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):