Probing the interaction between salivary proteins and wine tannins using surface analytical tools

Author: Shyamsundar Muthuramalingam

Muthuramalingam, Shyamsundar, 2017 Probing the interaction between salivary proteins and wine tannins using surface analytical tools, Flinders University, School of Chemical and Physical Sciences

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Abstract

Taste and mouth-feel are very complex phenomena, influenced by a wide range of processes. Astringency plays a crucial part in evaluating the flavour profile of wine, especially red wine. Astringency is described as the drying, roughing and puckering of the epithelium felt in the mouth as a result of interactions between polyphenolic compounds from wine (tannins) and salivary proteins. Tannins are phenolic polymers found in plant–derived food and beverages. In wine, tannins are extracted from grape skin and seeds. Tannins from grapes constitute one of the most important quality parameters of wines, since they contribute to the organoleptic characteristics such as bitterness and astringency. Astringency has been studied largely during the last decades, since it represents an important quality attribute of red wine. One of the major mechanism behind astringency is the loss of lubrication as a result of binding between tannins and salivary proteins. Mucins are large (0.5-20MDa), highly glycosylated salivary protein, which are important components for oral lubrication and wear protection. Studies of the binding mechanisms between polyphenols and salivary mucins are rare. This is surprising given the pre-eminent role of mucins in creating the viscoelasticity of saliva. In order to fill the void, we used Human Mucin as our protein of interest along with Bovine Submaxillary Mucin and Human Whole Saliva. There are two main drivers behind the binding of wine tannin with salivary proteins and mucin: hydrogen bonding and the hydrophobic effect. These drivers of binding are tested in our research by varying pH and ethanol concentration of model wine. The probing of the binding between salivary proteins and tannins are investigated by surface analytical techniques and rheology. Surface Plasmon Resonance (SPR) spectroscopy is a surface sensitive technique used to monitor the kinetics of binding in real time and the results from SPR highlighted the effect of ethanol and pH of wine on binding between mucin and tannins. SPR results showed the influence of hydrogen and hydrophobic interaction on astringency. Further, Quartz Crystal Microbalance with Dissipation (QCM-D) experiments of Human Whole Saliva (HWS) and its interaction with tannins in model wine highlighted the change in viscoelasticity/ rigidity of HWS film as a result of the interaction. Using Atomic Force Microscopy (AFM) we were able to visualize the topography of Human whole saliva and the impact of tannins to the salivary network. Rheological studies on HWS and Mucin protein revealed the change in viscosity when they bind to tannins. The results from these experiments validate our hypothesis that “The primary function of MG1, mucin protein is oral lubrication. When the Human whole saliva reacts with wine polyphenols, the mucin protein forms insoluble complexes, which lead to the decrease in oral lubrication. Thus an increased astringency will be perceived by the wine drinker”. Polysaccharides in wine originate from grape berry fruit cellulose and cell wall of yeast. These macromolecular polysaccharides not only affect the sensory properties by altering the mouthfeel of the wine but also plays a vital role in stability and viscosity of wine during wine production and storage. Polysaccharides have the ability to affect the interaction between tannins and salivary protein. This ability of polysaccharides to prevent aggregation and flocculation of tannins and protein complexes can modify astringent response in wine. Two approaches have been proposed in the literature to understand the capacity of polysaccharides in inhibiting protein-tannin interactions. We are able to identify that the polysaccharides form soluble complexes with tannin and this prevents the interaction between tannin and protein to form insoluble ternary complexes which elicit astringency. This mechanism was identified through SPR, QCM-D and AFM studies on HWS, tannins and polysaccharides.

Keywords: Astringency, wine tannins, salivary mucins, SPR, QCM, AFM
Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2017
School: School of Chemical and Physical Sciences
Supervisor: Associate Professor Ingo Koeper