Tribology to analyze wine mouthfeel

Researchers at the University of California, Davis, are investigating whether an analytical technique to measure friction between two surfaces can provide better understanding of wine sensory perception related to astringency and mouthfeel.

Mouthfeel is an important sensory characteristic in consumer perception related to wine quality and enjoyment, particularly with red wines. Astringency—a drying or tactile sensation in the mouth—is a primary mouthfeel characteristic associated with red wines. The perception of astringency has been extensively studied through chemical analysis, including interactions between salivary proteins and wine tannins, and through sensory analysis with panels of human tasters. The physical aspect of wine perception on the palate and human oral physiology have not been as intensively studied. UC Davis researchers are working to develop a “tribology” method (a physical analytical technique) to assess mouthfeel perceptions of red wines.

This tribology and mouthfeel research project was recently funded by the American Vineyard Foundation in the amount of $80,500 for the 2017-18 fiscal year. It is directed by principal investigator Dr. Andy Waterhouse, professor in the UC Davis Department of Viticulture and Enology. “A goal of the project is to measure the lubricating effect of saliva and determine if wine and wine tannins can reduce that lubrication,” Waterhouse said.

Dr. Aude Watrelot, a post-doctoral researcher at UC Davis, is coordinating laboratory analysis for the project. Watrelot earned a Ph.D. in biochemistry at the National Institute for Agricultural Research in Avignon, France. Prior to joining UC Davis, she was a post-doc researcher at California State University, Fresno, in the lab of Dr. Jim Kennedy, former chair of the CSU Fresno Department of Viticulture and Enology. Watrelot has extensive experience in wine tannin chemistry and sensory perception research.

Tribology defined and applied

Tribology is a term based on the Greek word for “rubbing” that was coined and introduced in the 1960s. The Webster’s New Collegiate Dictionary definition of tribology is: “a science that deals with the design, friction, wear and lubrication of interacting surfaces in relative motion (as in bearings or gears).” Tribology has commonly been applied in the field of mechanical engineering to study interaction of surfaces in moving machine parts, and applied in the fields of materials science and engineering as well as chemical engineering. It is also used in biomedical research to study interaction of surfaces in human body joints and the movement of fluids across and between biological membranes.

Lab analysis and equipment

The tribology and wine analytical research is being conducted in the UC Davis Surface and Interfacial Science Laboratory in cooperation with Dr. Tonya Kuhl, a professor affiliated with the departments of biomedical engineering, chemical engineering and engineering and materials science. The lab equipment is called a “Surface Force Apparatus” (SFA) that Kuhl said is designed to measure force between two surfaces as a function of their separation. Kuhl explained: “This device can measure strain, or friction, very precisely, with 30 square-microns being a normal area of contact—or, put another way, it can measure at the level of a taste bud.”

Kuhl said development of the SFA technology dates to the late 1970s. “This technique is considered the gold standard for measuring force between surfaces, and there are only about 20 labs in the world that do these measurements well,” she said. Kuhl’s lab has used the SFA to study surface interactions of polymers and fluid materials for construction applications, and to measure interactions between biological membrane surfaces for biomedical engineering applications.

SFA wine analysis
To fully understand and assess mouthfeel perception, Watrelot believes it is important to look at tannins from three analytical aspects: chemistry, tribology and sensory. The sensation of astringency is believed to be related to lubrication and the saliva layer between the tongue and the palate. A change in lubrication is believed to impact the astringency perception. The main objectives of the research project are to: develop a tribology method that shows an effect of wine tannin on saliva lubrication, compare this with sensory analysis of astringency using the same wines, and use this method to investigate relationships between tannin structure and astringency.

The lab room temperature during SFA wine analysis is kept at a warmer temperature than normal—30° C (86° F) to approach the temperature of the human mouth, but less than the actual temperature—37° C (98.6° F) that would be too hot for researchers in the lab.

Mucin (from bovine saliva) is used with wine samples to mimic conditions on the human palate, and in the same real-life ratio as the typical amount of human saliva to a sip of wine—1 milliliter of saliva: 15 milliliters of wine. Individual wine samples for analysis are placed in the SFA equipment between two surface layers. The interactive surfaces in the analytical equipment move in two directions, although this movement is not visible to the naked eye.

The researchers are working with two red wine varietals: Cabernet Sauvignon and Pinot Noir. Purified tannins from wine samples are added to the wine for SFA analysis to monitor the possible effects of different tannin levels on friction between the surfaces. Watrelot said, “We’re looking at wine alone with natural levels of tannins, and then adding tannins in measured amounts to see if it is possible to measure astringency. It may then be possible to use these measurements in a predictive manner.”

Material provided by Wine&Vines.