Development of tannins and methoxypyrazines in Pinot noir grapes and management of their extraction into wine : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Pinot noir is the most important red grape variety grown in New Zealand. Managing extraction of tannins and methoxypyrazines (MPs) in winemaking is crucial for producing high quality Pinot noir wine. This study aimed to investigate the development of tannins and MPs in Pinot noir grape skin, seed, and stem tissues during grape ripening and the management of their extraction into wine via common viticultural (leaf removal) and winemaking (whole bunch fermentation) practices. In addition, this study examined how tannin extractability of different grape tissues is influenced by initial tannin concentration and composition that were affected by leaf removal, rootstock, and clone. Analysis of tannin and MP development in skins, seeds, and stems of two Pinot noir clones (AM10/5 and UCD5) showed that on a per berry basis, the highest concentrations of tannins were observed in skins 2-3 weeks after véraison, in seeds at véraison, and in stems 4 weeks before véraison. Stem MPs increased toward véraison and decreased towards harvest, while skin MPs remained consistently low or undetectable levels. Two clones showed similar tannin and MP development trends, but AM10/5 exhibited significantly higher seed and stem tannin levels (per berry) and a lower 3-Isopropyl-2-methoxypyrazine (IPMP) and 3-Isobutyl-2-methoxypyrazine (IBMP) levels than UCD5 at harvest. Stem tannins had a comparable tannin concentration (1.09-1.34 mg/berry) to skin tannins (1.04-1.15 mg/berry) at harvest and a similar mean degree of polymerisation (mDP) to seed tannins but lower than skin tannins. Leaf removal, rootstock, and clone selection were used to manipulate initial berry composition to study the consequent impact on skin, seed, and stem tannin extractability. Leaf removal trial was conducted at 7 days (LR7), 30 days (LR30), and 60 days (LR60) after flowering, comparing to no leaf removal control (LRC). The Swartzman rootstock versus Pinot noir vines on their own roots, and the two clones, AM10/5 and UCD5, were included to assess the impact of rootstock and clone on berry composition. Leaf removal significantly increased anthocyanin concentration, stem, and seed tannin concentration, and mDP of skin tannins. Compared to the own roots, Schwarzman rootstock showed lower seed tannin centration but higher mDP of skin and seed tannins. UCD5 clone showed higher mDP of skin tannins than AM10/5 clone. For the first time, this study reported that stems showed the highest tannin extractability (64%-78%) compared to skins (37-52%) and seeds (26-34%). The consequent impact of different leaf removal timing on Pinot noir wines was also investigated. Results showed that leaf removal treatments are effective in increasing anthocyanin concentration in grapes and reducing MPs in stems. LR7 and LR30 notably enhanced colour density, polymeric pigments, tannin concentration, and reduced IBMP in resultant wines. In addition, LR7 showed significantly higher skin-originated tannin proportion in wines than LRC. These results indicate the extraction management of colour, tannin, and MPs in Pinot noir wines could be effectively achieved through different timing of leaf removal. Whole bunch fermentation, a commonly used Pinot noir winemaking practice, was conducted by comparing two clones (AM10/5 and UCD5) with three treatments, 100% destemmed (DS), 30% whole bunch (WB30), and 60% whole bunch (WB60). Higher proportion of whole bunch addition increased skin and stem originated tannin proportion and MP levels in wines. Clone selection is also important for managing tannin and MP extraction, with AM10/5 grapes producing wines having higher concentrations of tannin, polymeric pigments, and darker colour than UCD5 grapes. AM10/5 wines also had higher concentrations of phenylethyl alcohol, but lower concentrations of IBMP and ethyl esters, indicating more floral but less fruity and green notes in wines. These results indicate the clone and whole bunch, or grape stems could be a useful tool to manipulate tannin and MPs in wines. This research provides winemakers with valuable insights about tannin and MP distribution and relative concentrations in different grape tissues during berry development, as well as implications of leaf removal timing in the vineyard, proportion of whole bunch addition, and clone selection in winemaking to manage and manipulate tannin and MP extraction into wines. These research findings could help winemakers to produce high-quality Pinot noir wines through informed decisions regarding viticultural and winemaking practices.