UC Davis collaboration to determine factors that affect colour in Pinot noir wines

Grose C, Oberholster A, Martin D, Sherman E, Stuart L, Albright A

What is this new collaboration?

The New Zealand Winegrowers Incorporated (NZW)-funded Pinot noir Colour project is an international collaboration with University of California (UC Davis) Davis, USA, The New Zealand Institute for Plant & Food Research Limited (PFR)’s Viticulture and Enology team, and the New Zealand Winegrowers Research Centre Limited. Anita Oberholster, Extension Specialist in Enology from UC Davis, is travelling to New Zealand in April 2018 to visit Claire Grose, PFR Research Winemaker. This is not just a one way visit; New Zealand scientists will also travel to UC Davis to conduct research.

Anita Oberholster

We will investigate the respective influences of pomace and juice on the phenolic composition of Pinot noir wines (especially anthocyanins) and on the sensory perception of these wines, especially on poor colour and ‘greenness’ when grapes are harvested at lower than target soluble solids contents (usually 24-25 °Brix). The New Zealand industry has identified these features as major challenges to producing high quality Pinot noir wine.

This project establishes an important international collaboration with UC Davis, which will help to generate new knowledge to develop commercially suitable colour in Pinot noir wine, to support New Zealand’s growing Pinot noir export markets. The colour research undertaken in this project is aligned to and complements the New Zealand Winegrowers Research Centre Pinot noir programme, a five-year programme funded by the Ministry of Business, Innovation and Employment (MBIE), aimed at growing returns through the dissociation of quality from productivity in New Zealand Pinot noir production.

What is the challenge?

New Zealand Pinot wines can be lightly coloured but there is limited knowledge of factors that contribute to low colour in these wines. Previous research has indicated that monomeric anthocyanin concentrations reach a maximum in the Pinot noir berry at a relatively low soluble solids content (19-20 °Brix; well before optimal harvest maturity of 24 -25°Brix), but that other juice or skin compositional factors may prevent the development of commercially suitable colour in Pinot noir wine when grapes need to be harvested early. This indicates that anthocyanins are not the limiting factor in stable wine colour development in wines made from less-ripe grapes. Grape maturity at harvest not only affects the appearance of Pinot noir wines; ripeness plays an important role in wine flavour and mouthfeel as well.

While monomeric anthocyanins are primarily responsible for the red colour of grapes, red ferments and very young red wines, co-pigmentation and the development of polymeric pigments are critical for the formation of stable red wine colour. The formation of these pigments occurs as wine ages primarily from the reactions of anthocyanins with tannins, with the source, structure, and concentration of the tannins influencing stable colour development. The development of stable wine colour, while primarily important visually, may also play an important role for in-mouth sensory quality.

Additionally, some NZ winemakers are concerned about perceived ‘unripe’ or ‘green’ tannins in their Pinot noir wines. The untested hypotheses tend to revolve around the idea that highly coloured wines, and particularly wines that have high concentrations of anthocyanin-tannin polymers, are perceived to be less green. This project is designed to acquire new knowledge in this area also.

What research will be undertaken?

Our primary research question: Is grape berry pomace (skins and seeds) phenolic composition at various stages of berry ripening the determining factor in Pinot noir wine phenolic composition?

To control the composition of the liquid phase preferment, a single Pinot noir juice will be fermented with pomaces from grapes of varying maturities. Initial work will involve developing and refining this novel methodology to explore the respective impacts of juice and pomace manipulations on wine composition. Once optimised, the methodology will be available for use in the wider Pinot noir research programme.

The experiment will harvest grapes at multiple maturities (20, 22 and 24 °Brix, weather permitting). Grapes will be gently pressed to remove a maximum of their own juice and that juice will be substituted with a common single, low-phenolic juice sourced at an early stage of the harvest period. The experiment will also ferment the Pinot noir juices without pomace, and ferment the pomace with a synthetic juice solution, to further investigate the extractability of phenolics and anthocyanins in the resulting wine.

As well as Anita Oberholster’s visit to New Zealand, Claire Grose and Emma Sherman (PFR research associate) will each visit UC Davis. Claire will implement a replicate experiment for juice pomace manipulations during the US 2018 vintage, and Emma’s visit will utilise analytical chemistry techniques to evaluate relationships between wine composition and sensory properties. Anita Oberholster is expected to return to New Zealand in 2019 to present a summary of project findings to the industry at the Romeo Bragato Conference.

This article first appeared in the April/ May 2018 issue of the New Zealand Winegrower magazine.