The evaluation of tannin activity in south african red wines

Abstract

Astringency is an important red wine quality attribute, which can be measured both chemically and sensorially. The use of tannin activity shows potential as a valuable chemical measurement in understanding red wine mouthfeel properties such as astringency and bitterness, which is also affected by tannin structural factors, in addition to matrix effects. Tannin activity is defined as the enthalpy of interaction between tannins and a hydrophobic surface. Studies involving tannin activity have been performed since the early 2010’s, but chemosensory studies used to evaluate how structure-activity relationships change across multiple, consecutive vintages are limited. The aim of this study is to investigate how tannin activity may be linked to red wine mouthfeel, and how all these variables may change according to wine age.

The effect of wine vintage on tannin activity was investigated in red wine extracts isolated from 16 Pinotage wines from a well-known producer (2003-2018) using Sephadex LH-20 chromatography. Approximately 17-18 polymeric fractions were obtained per wine, and furtherly grouped into four sub-fractions of various classes: low, medium, high, and bulk, giving 64 unique extract samples. Bulk samples represent a combination of the three other obtained fractions. Pooled extracts were grouped to obtain samples of variable but increasing molecular mass, which may each reflect differences in total phenolic and tannin content, and degree of polymerization, parameters previously shown to affect tannin activity. Retention thermodynamics were used to calculate activity values by utilising reverse-phase liquid chromatography (RPLC) on a polystyrene divinylbenzene column. Other complementary tannin-based techniques – to investigate structure-activity-concentration relationships – were also performed by obtaining chemical information based on subunit size and composition (by phloroglucinolysis), and total tannin content (by MCP and RPLC). Sensory analysis was performed by an expert panel to evaluate sweetness, acidity, body, complexity, astringency and bitterness. Younger wines’ polymeric profiles may be characterized by lower tannin activities and content, smaller degrees of polymerization and therefore an increased perception of bitterness, with higher activities and tannin concentrations in aged wine due to polymerization reactions – this may also explain why aged wines are perceived as more astringent than younger wines (Barak & Kennedy, 2013; Watrelot et al., 2016; Yacco et al., 2016). Furthermore, tannin activity values may plateau and show a decrease in older wines as tannin structural changes may lead to decreased tannin-protein interactions. Wine vintage may show a similar effect across all weight classes except in bulk wine fractions. These measurements could serve to establish how chemical measurements are linked to the sensory outcomes of this study and ultimately how mouthfeel perception is modulated according to wine vintage.