New insights into red wine astringency and the salivary precipitation index

Study of the salivary precipitation index


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The quest to unravel the complex relationship between the chemical composition of red wine and its sensory impact on consumers has led to a significant development: the study of the Salivary Precipitation Index (SPI). This investigative report delves into the intricate world of wine astringency, a key factor that influences consumer preference, and the efforts to predict and control it through a better understanding of wine's chemical makeup.

Astringency in red wine, described as a complex blend of dryness, roughness, and tingling sensation in the mouth, primarily arises from the interaction between wine tannins and salivary proteins. The molecular binding model, developed by Jöbstl et al. in 2004, explains this interaction as a three-stage process involving simultaneous binding, dimerization, and eventual precipitation of large aggregated particles. While the precipitation of proline-rich salivary proteins by wine tannins is a widely accepted mechanism for explaining astringency, other mechanisms have also been proposed, including activation of mechanoreceptors and direct interactions between tannins and salivary proteins.

Tannins, pivotal in red wine's polyphenolic composition, fall into two main categories: condensed (proanthocyanidins) and hydrolyzable tannins. Research indicates that hydrolyzable tannins, particularly ellagitannins, impart a stronger astringent sensation than condensed tannins at equivalent molar concentrations. However, their overall impact on perceived astringency is limited due to their lower concentration in wine. The interaction of condensed tannins with proteins is influenced by various factors such as concentration, molecular weight, type of interflavan bond, and hydroxylation patterns.

Beyond flavan-3-ols, anthocyanins, another group of flavonoids, also contribute to protein precipitation and astringency perception. Studies have shown that anthocyanins react more strongly with salivary proteins and can modify astringency intensity and its subqualities.

Apart from tannins, other wine constituents like ethanol, organic acids, pH, cations, and polysaccharides play a role in modulating astringency. The correlation between these physicochemical parameters and perceived astringency remains an ongoing challenge. Various studies have attempted to correlate wine's physical-chemical properties with its astringency using assays like the Bovine Serum Albumin (BSA) precipitation test, cloroglucinolysis, and gel permeation chromatography.

The Salivary Precipitation Index (SPI), an advanced assay using SDS-PAGE electroforesis, measures the binding and precipitation of salivary proteins with wine polyphenols to predict wine astringency. Qi et al. in 2023 proposed an Artificial Salivary Precipitation Index (ASPI) using a response surface methodology to develop an artificial saliva for predicting astringency with a higher coefficient of determination.

To further understand the relationship between chemical parameters and astringency, multivariate analysis is seen as a valuable approach. Preliminary studies have focused on multiple linear regression and nonlinear PLS regression. This multivariate approach could aid winemakers in achieving a holistic understanding of factors contributing to wine quality and developing precision practices during the winemaking process.

This study aimed to analyze the feasibility of predicting SPI using a multivariate approach to identify the most significant variables for astringency prediction in red wine. A data matrix of 52 physicochemical parameters was used to predict the SPI in 110 monovarietal Italian red wines. The final PLSr model had an R2test of 0.630 and an RMSEtest of 0.994, with 19 significant independent variables.

The results represent a pioneering effort to predict SPI based on chemical composition, enhancing understanding of the physicochemical variables associated with astringency. This knowledge could improve enologists' and researchers' ability to produce more consumer-pleasing wines through a comprehensive understanding of astringency-related issues.

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