Anthocyanins in grapes and wine

Anthocyanins are the red pigments in red/black grapes, located mainly in the skin and also in the flesh of ‘teinturier’ grape varieties such as Rubired. Anthocyanins are water-soluble vacuolar pigments which may appear red, purple, or blue depending on the pH. They are odourless and nearly flavourless, contributing to taste as a moderately astringent sensation. Anthocyanins occur in all tissues of higher plants, including leaves, stems, roots, flowers, and fruits.

Figure 1. Typical chemical structure of an anthocyanin.

Figure 2. Image of Cabernet Sauvignon berry with broken berry skin and clear pulp. This image demonstrates that all the anthocyanin pigments are located in the berry skin tissues.

In flowers, bright reds, blues and purples are adaptive for attracting pollinators. In grapes, the colourful skins also attract the attention of animals, which may eat the fruits and disperse the seeds. In photosynthetic tissues, such as leaves, anthocyanins have been shown to act as a ’sunscreen’, protecting cells from high light damage by absorbing blue-green and ultraviolet light, thereby protecting the tissues from photoinhibition, or high light stress.

Anthocyanin synthesis in grapes occurs via the same biosynthetic pathway that makes tannin, flavonols (e.g. quercetin, kaempferol – natural sunscreen products) and phenolic acids (e.g. caffeic acid, resveratrol). During berry development the main phase of anthocyanin biosynthesis occurs between veraison and harvest. Some viticultural management practices known to influence anthocyanin/colour levels in grapes and wine are discussed in a later section.

Figure 3. The anthocyanin and tannin biosynthetic pathway in grapes. Image taken from Dr Simon Robinson (CSIRO) - http://www.awri.com.au/wp-content/uploads/2014/06/Robinson_Shiraz_Symposium.pdf

The accumulation of anthocyanins in the skin of red varieties begins at veraison, and is one of the most commonly recognised features of berry ripening. Of the 15 monomeric anthocyanins found in Shiraz, malvidin-3-glucoside and its acylated derivatives make up the majority of anthocyanins found in this variety and other common wine-grape varieties found in Australia. The pattern of anthocyanin accumulation in grapes tends to follow that observed for sugar accumulation, but can differ drastically depending on variety, site and cultural management practice.

Figure 4. Accumulation of anthocyanin during grape berry development. Note that the major period of anthocyanin synthesis occurs between veraison and harvest.

Anecdotal evidence suggests that anthocyanin levels do vary substantially between different grape varieties, with some varieties considered high in anthocyanins, e.g. Shiraz, Petit Verdot and Lagrein, and others comparatively low in anthocyanins, e.g. Pinot Noir, Sangiovese and Nebbiolo. Many varieties do not have all 15 anthocyanins, e.g. Pinot Noir lacks the acylated forms of anthocyanins, and certain table grape varieties lack the malvidin forms of anthocyanin. These differences in both concentration and types of anthocyanin can lead to differences in colour and intensity overall.

Figure 5. Varietal differences in anthocyanin concentrations (mg/g). These varieties were grown in the Murray Darling region of Victoria on the same vineyard.

As in most other biological processes, temperature plays a significant role in the synthesis of anthocyanins in grapes. Early in berry development (between fruit set and veraison) higher temperatures have been associated with a decrease in anthocyanin levels (mg/g) in berries, indicating that these warmer temperatures decrease biosynthesis and stimulate the degradation of anthocyanins. Previous research indicates that maintenance of berry temperatures between 15^oC and 35^oC appears to be optimal for enzymes involved in the flavonoid pathway responsible for anthocyanin, flavonol and tannin synthesis. The problem is in black wine-grape varieties, such as Shiraz, berries directly exposed to sunlight can be up to 15^oC above the ambient air temperature. This can lead to berry temperatures on a 35^oC day being at, or near 50^oC, which can result in lower anthocyanin levels in grapes if exposed to these temperatures post-veraison.

Large seasonal differences are often observed in grape and wine quality and composition from within the same vineyard. In many cases, the variation observed between seasons can often be larger than that observed effects between viticultural treatments within a single season. These seasonal differences can be attributed to the differences in weather patterns observed during a given growing season. Many of these seasonal differences are often attributed to differences in temperature and plant water availability.

Modifying bunch exposure

Canopy management and bunch exposure management techniques have long been touted as tools for managing anthocyanin/colour levels in wine-grapes. However, the results from increasing bunch exposure to direct sunlight or shading appear to be variable and depend on the specific variety, season and site. In some varieties, increasing exposure to direct light has proven effective in increasing anthocyanin, flavonol and tannin (polymeric flavan– 3-ols) levels in grapes. However, at other sites, no effect of bunch exposure has been observed.

Impacts of bunch exposure have been well investigated over the past 35 years. The general consensus is that exposure per se (light influence) does not have a major effect on the anthocyanin levels in common Australian red wine-grape varietals such as Shiraz. However, there may be more of a light influence in varieties such as Cabernet Sauvignon. The effects of light exposure on anthocyanin formation in grapes appear to be more dominant prior to veraison rather than after veraison.

Figure 6. Comparison of anthocyanin levels (mg/berry) of exposed and shaded Shiraz in McLaren Vale. There were no significant differences in anthocyanin levels at harvest between both treatments

Influence of yield and berry size

Yield has often been implicated as being important and inversely related to quality parameters such as anthocyanin levels in grapes. The evidence for such a relationship is weak. The influence of yield is only seen at the extremes, when sugar accumulation is also affected. A more important relationship appears to exist with berry size. Smaller berries have been related to higher concentrations (mg/g) of anthocyanins. This has been thought to be related to an increase in skin surface area per unit volume. However, it appears that this is not necessarily the case; how the grapes have been managed to produce the smaller berry size seems to be more important than small berries per se.

Figure 7. The relationship between berry weight (g) and anthocyanin levels (mg/g) for a range of commercial Cabernet Sauvignon vineyards in the Murray Valley in 2000.

Using irrigation to modify anthocyanin levels

There have been many studies looking at the effect of irrigation on anthocyanin levels in grapes and wine. A number of these have demonstrated that a strategic application of a water deficit, through irrigation strategies such as Regulated Deficit Irrigation (RDI) between berry set and veraison can reduce final berry size and thus improve the concentration of anthocyanins (mg/g) at harvest through a hydraulic effect.

Irrigation research into Partial Rootzone Drying (PRD) has demonstrated that roots growing in a drying soil may result in an abscisic acid (ABA) mediated chemical response, which reduces transpiration, increases water use efficiency and may improve anthocyanin levels (mg/g) in grapes via this hormonal effect. The application of synthetic ABA to grape bunches pre-veraison stimulates anthocyanin and skin tannin accumulation post-veraison. It is important to realise that seasonal effects often outweigh any treatment effects. Therefore the temperature conditions during berry development are critical in determining the level of anthocyanins at harvest.

Figure 8. Partial Rootzone Drying (PRD) irrigation in a Shiraz vineyard in the Murray Darling region.

The influence of vine vigour

In a precision viticulture study of three Shiraz sites across south-eastern Australia, where berry composition and many other viticultural parameters were studied, an inverse relationship between anthocyanin concentration in the grapes and vine vigour was observed. High vigour vines tended to have lower anthocyanin levels while low vigour vines had higher anthocyanin levels. Also, there appears to be a good relationship between anthocyanin and tannin levels in these vineyards as well, both negatively correlated to vine vigour.

Figure 9. Tannin (mg/g) and anthocyanin (mg/g) variability across a single Shiraz vineyard located in the Murray Darling region in the 2006/07 growing season.

Anthocyanin contents of red wine-grapes has been used as one objective measure of the potential value of a parcel of grapes. In the major production regions of Australia (Riverland, Murray Darling and Riverina), the warmer climatic conditions can restrict the production of anthocyanins in grapes relative to cooler production regions at a similar level of ripeness. Also, work conducted in the mid-1990s demonstrated a good relationship between grape colour and wine colour, and a subsequent relationship between wine colour and wine quality grades.

Figure 10. Relationship between grape colour (anthocyanins mg/g) and wine colour density in Riverland Shiraz (1995).

Figure 11. Relationships between grape glycosyl-glucose(GG), grape colour (anthocyanin mg/g) and wine score. Data collected from the AWRI’s 1995 Shiraz trial conducted in the Riverland.

Anthocyanins in red grape berries are typically measured based on the methods described by Iland et al. (1996, 2000). This involves extraction of these compounds from a homogenised grape sample, expression of their colour at low pH and quantification based on their absorbance in the visible region of the light spectrum. Further details about this method can be found in this fact sheet - https://www.awri.com.au/wp-content/uploads/anthocyanins_fact_sheet.pdf

Research over the past 10-15 years has provided many valuable insights into the flavanoid biosynthetic pathway and the genes/regulators responsible for tannin, anthocyanin and flavonol synthesis in grapes – Vitis vinifera L. Understanding how anthocyanins in the grape translate to wine colour density and wine hue in the wine is complex. While anthocyanins are integral to wine colour, the formation of stable polymeric pigments is critical for stabilising wine colour.

Anthocyanins can be managed at every stage of production, from the vineyard right through to the winery. Variety, site selection, climate, season, canopy management, bunch exposure, vine vigour management/irrigation and fruit maturity all play an important role in defining the level of anthocyanin achieved at harvest. Winemakers can then use their extensive toolkit to optimise extraction of both anthocyanins and tannins to help achieve stabilised polymeric pigments in wines.

This webpage includes some of the research undertaken by the anthocyanin and tannin research team located at Agriculture Victoria/Department of Economic Development, Jobs, Transport and Resources (DEDJTR) in Mildura, Victoria over the past decade, contributing to the understanding of anthocyanins and tannins in winegrapes. The author also wishes to acknowledge Dr Keren Bindon (AWRI) for all her guidance and assistance in assembling this article.

Downey, M.O.; Dokoozlian, N.E.; and Krstic, M.P. 2006. Cultural practice and environmental impacts on the flavonoid composition of grapes and wine: a review of recent research. Am. J. Enol.Vitic. 57(3): 257-268.

Iland, P., Ewart, A., Sitters, J., Markides, A. and Bruer, N. 2000. Techniques for chemical analysis and quality monitoring during winemaking. Patrick Iland Wine Promotions, Campbelltown, SA.

Kilmister, R. 2015. Identifying vineyard and winery management practices that impact on tannin extraction. Final report to Australian Grape and Wine Authority, project DPI1402 June 2015, 63 pages. http://research.wineaustralia.com/wp-content/uploads/2016/03/DPI-1402-Final-Report.pdf

https://www.awri.com.au/wp-content/uploads/anthocyanins_fact_sheet.pdf

http://www.wineanorak.com/polyphenols_in_wine.htm

http://www.wineanorak.com/extraction.htm