Using Tribology to Analyze the Mouthfeel of Wine

Investigations are underway at the University of California, Davis, using a Surface Forces Apparatus (SFA), where researchers are exploring whether an analytical technique to measure friction between two surfaces can provide enhanced understanding of wine sensory perception related to astringency and mouthfeel.

Researchers Dr. Tonya Kuhl and Dr. Aude Watrelot from the University of California, Davis, are using this Surface Force Apparatus to develop a tribology (physical analytical) method to assess mouthfeel perception of red wines. Photo: Ted Rieger

Researchers Dr. Tonya Kuhl and Dr. Aude Watrelot from the University of California, Davis, are using this Surface Force Apparatus to develop a tribology (physical analytical) method to assess mouthfeel perception of red wines. Photo: Ted Rieger

Especially with red wines, mouthfeel is an essential sensory characteristic in consumer perception regarding the quality of wine and overall enjoyment. Astringency, which is a drying or tactile sensation in the mouth, is a key mouthfeel characteristic in red wines. Chemical analysis has been used to comprehensively study astringency perception.

However, the physical aspect of wine perception on the palate and human oral physiology have not been studied to the same extent. UC Davis researchers are working to develop a physical analytical technique called “tribology” to evaluate the mouthfeel perceptions of red wines, and this research project has been recently funded with $80,500 for the 2017-18 fiscal year by the American Vineyard Foundation.This includes interactions between salivary proteins and wine tannins, along with sensory analysis with panels of human tasters.

This tribology and mouthfeel research project is directed by principal investigator Dr. Andy Waterhouse, a 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.

Dr. Andy Waterhouse, Professor, Department of Viticulture and Enology, UC Davis

Coordinating the laboratory analysis is Dr. Aude Watrelot, a post-doctoral researcher at UC Davis, who also gained a Ph.D. in biochemistry at the National Institute for Agricultural Research in Avignon, France.

Before joining UC Davis. Watrelot was a post-doc researcher at California State University, Fresno, working in Dr. Jim Kennedy’s lab who was a 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

Coined in the 1960s, tribology is a term with its roots in the Greek word for “rubbing.” It is defined in the Webster’s New Collegiate Dictionary as, “a science that deals with the design, friction, wear and lubrication of interacting surfaces in relative motion (as in bearings or gears).”

Tribology has frequently been used in mechanical engineering fields to study the interaction of surfaces in moving machine parts. It has also found use in materials science, engineering, and chemical engineering, as well as in biomedical research. In biomedical research, it is used to analyze the 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 carried out in the UC Davis Surface and Interfacial Science Laboratory in cooperation with Dr. Tonya Kuhl, who is a professor associated with the departments of biomedical engineering, chemical engineering and engineering and materials science.

The lab equipment, called a “Surface Force Apparatus” (SFA), developed by Jacob Israelachvili, is designed to measure the force between two surfaces as a function of their separation, according to Kuhl.

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.

Dr. Tonya Kuhl, Professor, Biomedical Engineering, Chemical Engineering, and Engineering and Materials Science, UC Davis

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

SFA Wine Analysis

To adequately understand and assess mouthfeel perception, Watrelot believes it is essential to consider tannins analytically from three different aspects: chemistry, tribology and sensory.

It’s thought that the sensation of astringency is associated with the lubrication and saliva layer between the tongue and palate. The astringency perception is thought to be influenced by change in lubrication.

The key objectives of the research project are to develop a tribology method that demonstrates an effect of wine tannin on saliva lubrication; to compare this effect with sensory analysis of astringency using the same wines; and to use this method to examine relationships between tannin structure and astringency.

During SFA wine analysis, the lab room temperature is maintained at 30 °C (86 °F), a higher temperature than usual. This is to closely mimic the temperature of the human mouth, while remaining lower than the real temperature of 37 °C (98.6 °F), which would be too hot for researchers working in the lab.

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Mucin, taken from bovine saliva, is used in conjunction with wine samples to imitate the conditions of the human palate. The same real-life ratio of human saliva to wine is also used, which is at a rate of 1 milliliter of saliva to 15 milliliters of wine. Individual wine samples are put in the SFA equipment between two surface layers.

The interactive surfaces move in two directions, which is invisible to the naked eye. Two red wine varietals are used, one Cabernet Sauvignon, and the other Pinot Noir. To monitor the effects of different tannin levels on the friction between surfaces, purified tannins from wine samples are added to the wine for SFA analysis.

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.

Dr. Aude Watrelot, Post-doctoral Researcher, UC Davis

This information has been sourced, reviewed and adapted from materials provided by SurForce, LLC.

For more information on this source, please visit SurForce, LLC.

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