The evaluation of tannin activity in South African red wines

Date
2022-04
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Proanthocyanidins are complex and important phenolic compounds, which play a significant role in defining overall red wine quality. In fact, some of the beneficial bioactive components associated with moderate red wine consumption can be attributed to these non-volatile, polar compounds. Additionally, they are involved in multiple chemical reactions with other compounds from the onset of fermentation throughout periods of long-term ageing, which ultimately influence the organoleptic properties of red wine. Therefore, it is beneficial to quantify proanthocyanidin content and composition in red wine. Several chemical methods exist for the analytical quantification of tannin content and composition in red wines and/or their grape skin-, seed- or tannin extracts. These methods are based primarily on spectrophotometry and chromatography. Many methods have been used in chemosensory studies to understand factors affecting red wine mouthfeel. In recent years, the use of tannin activity has gained traction as a useful chemical measurement to understand red wine mouthfeel, with astringency in particular focus. However, from a South African context, such studies are lacking. To address this, an explorative study was performed on experimental red wine extracts, separated into various mass fractions by Sephadex LH-20 by flash column-chromatography. Following chemical characterisation of these fractions based on tannin concentration and composition, tannin activity values were shown to be strongly influenced with proanthocyanidin molecular mass. Moreover, tannin activity-concentration relationships were identified using HPLC-tannin estimation methods, while MCP tannin concentrations were poorly correlated with activity values. Although tannin estimation methods were well correlated with one another, the study showed that other factors were also involved, setting the basis for the second part of the study. The effect of wine vintage on tannin composition and activity values was investigated across multiple vintages. Sixteen Pinotage wine samples (2003-2018) isolated by an optimised FCC method rendered four fractions (F1-F4) differing in degree of polymerisation and subunit composition, increasing in size from F1 to F3, with F4 pooled from these three samples. Similar findings were reported as in the explorative study. The extent to which MCP and RPLC- based tannin estimation methods were correlated with one another, was strongly dependent on wine age and tannin size. Activity values increased with tannin size but decreased in older wines, as a function of DP. This was indicative of reactions related to age, such as oxidation and precipitation. Finally, chemosensory analysis illustrated that low positive correlations were observed between fractions of high molecular weight, and bitterness and astringency, suggesting that oligomeric and polymeric proanthocyanidins play a greater role in modifying tannin activity values. More research is needed to understand how purified tannin fractions of various sizes affect tannin activity and mouthfeel attributes, by a chemosensory approach. Additionally, this may also be integrated with other experiments to evaluate how activity values are modified by tannin origin or maturity, and other processes such as tannin oxidation or precipitation.
AFRIKAANSE OPSOMMING: Proantosianidiene (asook gekondenseerde tanniene, of ‘condensed tannins’, in Engels) is komplekse en belangrike fenoliese verbindings wat ‘n betekenisvolle rol in algehele rooiwyn kwaliteit speel. Trouens, ‘n paar van die voordelige, bioaktiewe komponente – wat met matige rooiwyn verbruik geassosieer word – kan aan hierdie nie-vlugtige, polêre verbindings toegeskryf word. Daarbenewens is hulle by veelvuldige ander chemiese reaksies – met ander verbindings – betrokke. Hierdie reaksies kan vanaf die aanvang van alkoholiese gisting plaasvind, en kan deur periodes van langtermyn wynveroudering voortduur. Hierdie reaksies kan die finale organoleptiese eienskappe van rooiwyn beinvloed. Daarom is dit voordelig om die proantosianidien-inhoud en samestelling in rooiwyn deur chemiese metodes te kan kwantifiseer. Verskeie chemiese metodes bestaan vir die analitiese kwantifisering van tannien-inhoud en die samestellings daarin, in rooi wyndruiwe, wyne, of tannien-ekstrakte hiervan. Hierdie metodes is hoofsaaklik op spektrofotometriese en chromatografiese tegnieke gebaseer. Baie metodes is in chemosensoriese studies gebruik – om faktore wat rooiwyn mondgevoel-eienskappe (of ‘red wine mouthfeel properties’) beïnvloed – te verstaan. In die afgelope paar jare het die gebruik van tannien-aktiwiteits- of tannien entalpie-waardes aangryping gekry as ‘n nuttige chemiese meting, wat moontlik gebruik kan word om rooiwyn-mondgevoel beter te verstaan. Dit wil voorkom asof hierdie waardes meer gebruik word, om die persepsie óf relatiewe intensiteit van wyn frankheid (in Engels, ‘red wine astringency’), chemies te meet/bepaal. Dit blyk egter asof daar nie Suid-Afrikaanse studies was wat hierdie waardes probeer verstaan het nie. Om hierdie gaping aan te spreek, is ‘n verkennende (Engels: ‘explorative’) studie op verskillende eksperimentele rooiwyn-ekstrakte – wat deur Sephadex LH-20-flitskolom- chromatografie in verskillende fraksies geskei is – uitgevoer. Nadat chemiese karakterisering (gebaseer om tannien- konsentrasie en samestelling) op hierdie fraksies uitgevoer is, is daar bevind dat tannien aktiwiteit-waardes sterk deur die molekulêre massa van die proantosianidiene beïnvloed was. Verhoudings tussen aktiwiteit-waardes – en tannien- konsentrasie (deur MCP en HPLC bepaal) – was geïdentifiseer, veral met die HPLC-metode. Aktiwiteit-waardes en konsentrasies vanaf die MCP-metode was swak met mekaar gekorreleer. Alhoewel albei tannien-skattingsmetodes (ten opsigte van konsentrasie) relatief goed met mekaar gekorreleer is, blyk dit steeds dat ander chemiese faktore ook betrokke was. Dit het die basis vir die tweede deel van hierdie studie gevorm. In hierdie geval is die effek van wyn-oesjaar op tannien-samestelling en aktiwiteit-waardes in verskeie opeenvolgende oesjare ondersoek. Sestien Pinotage-wyne (2003-2018) is deur ‘n geoptimiseerde Sephadex LH-20-flitskolom-chromatografie-metode geïsoleer. Elke wyn het vier saamgevoegde fraksies (van verskillende molekulêre massas, naamlik: fraksies F1 tot F4) elk gelewer. Hierdie vier fraksies het ook in hul polimerisasie-graad (Engels: ‘mean degree of polymerisation’, hierna as ‘mDP’) en subeenheid-samestelling (Engels: ‘subunit composition’) verskil. Vanaf F1 tot F3, het mDP waardes toegeneem, met F4 wat uit ál drie fraksies bestaan het. Soortgelyke bevindinge is soos in die verkennende studie gerapporteer. Die mate waartoe MCP en RPLC-gebaseerde tannien-skattingsmetodes met mekaar gekorreleer was, was sterk afhanklik van wyn ouderdom, asook tannien-grootte (Engels: ‘tannin size’). Aktiwiteit-waardes het toegeneem met tanniengrootte, maar het afgeneem in ouer wyne, as ‘n funksie van mDP- waarde. Dit was ‘n aanduiding van moontlike chemiese reaksies wat verband hou met wyn ouderdom, soos tannien oksidasie, en presipitasie (Engels: ‘tannin precipitation’). Laastens, chemo-sensoriese ontleding het geïllustreer dat lae positiewe korrelasies – tussen fraksies met hoër molekulêre massas, en die persepsie van bitterheid en frankheid – bestaan. Dit dui oók aan dat oligomeriese en polimeriese proantosianidiene ‘n groter rol in die wysiging/verandering (Engels: ‘modification’) van tannien aktiwiteit-waardes speel. Meer navorsing word benodig om te verstaan hoe gesuiwerde tannien-fraksies (van verskillende groottes óf massas) aktiwiteit- waardes en mondgevoel-eienskappe beïnvloed. Daarbenewens kan dit ook met eksperimente geïntegreer word om te evalueer hoe aktiwiteit-waardes deur tannien-oorsprong of volwassenheid, of ander prosesse soos tannien oksidasie of presipitasie, gewysig word.
Description
Thesis (MScAgric)--Stellenbosch University, 2022.
Keywords
Tannins, Red wines -- South Africa, Proanthocyanidins, Wine and wine making -- Chemistry, Phenolic compounds, Wine -- Composition, UCTD
Citation