Periodic punchdowns improve color by increasing contact between juice and grape skins.

Article from Il Corriere Vinicolo

Editor’s note: This article is published as part of Wines & Vines cooperative editorial effort with Il Corriere Vinicolo, the leading Italian wine industry publication. Il Corriere Vinicolo is edited by Unione Italiana Vini, the largest Italian wine trade association. On a regular basis, our two publications will share key articles in order to give readers a broader view of important wine industry topics in Italy and North America.

The sensory qualities of wine are the result of a complex balance between wine components. The polyphenolic component, in particular, distinguishes and influences the quality of red wines. This group of substances determines color, sensory sensations of bitterness and astringency, and forms the structure and body of the wine, with an impact on its longevity.

The polyphenolic substances of wines are basically derived from the grapes, where they are contained in the solid parts of the bunch (skins, seeds and stems), as shown in the berry diagram on page 104. For many winemakers it is common practice to only exploit the solid parts of the berries, removing the stems that could add unpleasant bitter and herbaceous flavors.

The term polyphenols indicates various classes of compounds that differ for their chemical structure, properties and concentration. The main classes of polyphenolic compounds of red wines and the spectrophotometric indices used to assess them are summarized in the table at the bottom of page 104.

The passage of the components from the solid parts (particularly polyphenols) to the liquid phase is not as simple or immediate as that of the solutes in the grape’s pulp, necessitating an extraction process between different phases as well as contact between the liquid phase and the solid parts.

This process is generally referred to as “maceration.”

To complicate everything, there is also the varied location and the chemical and physical state of polyphenolic substances present in skins or seeds, with substantial differences for the same class of substances. The anthocyanins are located in the vacuoles of cells in the hypodermis of skins (rarely in the pulp). The proanthocyanidic tannins in skins, contained in the same tissues, represent a highly complex situation, as can be seen in the black and white diagram at the top of page 104. The tannins in seeds are contained in the various layers of cells that make up the external coat. In the seeds, monomer flavanols and oligomer forms are predominant, whereas in the skins we mainly find the polymerized forms. The flavanols in seeds have higher values of the FRV/PC ratio than those in skins.

The presence and quality of phenolic substances in red wines inevitably requires suitable work in the vineyard and monitoring in order for the raw material to reach optimal phenolic ripeness. During the vinification process, maceration is the fundamental stage for the properties of the wine. It lays the foundations for the quantity and quality of the complex development processes that characterize the aging of both great red wines and those meant to be consumed young.

Factors of extraction
The importance of this problem on a winemaking level has prompted ample scientific literature in the past 50 years, with increasingly more detailed contributions, and it does not appear to have exhausted itself yet.

Since Ribereau-Gayon’s classic work in the 1960s and 70s, we have known how polyphenol extraction occurs with different dynamics between anthocyanins and tannins. The former reach a maximum in the initial stage of maceration-fermentation and then tend to decrease; the tannins, on the other hand, increase with skin and seed contact time.

Various authors have shown that, on average, the fraction of polyphenols extracted is lower than 50% of the grape content, depending on the type of cultivar, the state of ripening and health reached by the grapes and the variables that control the process.

The degree of grape skin ripeness influences the maceration process mainly at the initial stage (i.e., the degradation reached by the cell walls regulates the release of easily extractable phenolic substances). Higher levels of seed ripeness, linked to oxidation processes, make it more difficult to extract flavanols.

The extraction process is undoubtedly influenced by the solubility of phenolic compounds: It is higher in aqueous solution for anthocyanins than flavanols, especially polymers, but in both cases it increases with the alcohol content that increases during fermentation. This, however, is not a limiting factor in itself. During maceration between different phases, the diffusive processes are clearly linked to the separation surfaces between phases. The release from the solid phase to the liquid one is controlled by the state of degradation of the cell walls and membranes (porosity), as well as the diffusion process in the liquid phase and also the establishing of chemical balances that involve the phenolic substances.

The phenomena described are widely influenced by the chemical and physical conditions of the system, by the type of equipment used and by the operations applied. Among the chemical and physical variables are temperature and duration of maceration, alcohol by volume and the use of sulfites. The different factors work in s ynergy among themselves, and it is not always easy to extrapolate the effect of one single parameter.

The temperature at which maceration occurs in consequence of the fermentation phenomenon is considered a strategic factor of the extraction process, since higher temperatures correspond to a clear increase in polyphenolic substances. Values between 25° and 30° C (77°-86° F) mainly favor tannins that increase their solubility, while the anthocyanins do not show any significant improvement. The temperature allows the extraction of flavanols to be regulated and, for young red wines with a limited structure, the temperature should not exceed 25° C. In addition, high temperatures favor the formation of polymeric pigments (various types of tannin-anthocyanin copolymers) with a consequent increase in the color intensity of the wine.

The maceration time generally determines an increase in the polyphenolic substances extracted, especially tannins. In fact, as has already been observed, the concentration of anthocyanins reaches a maximum a few days after the start of fermentation, whereas longer contact with the pomace stimulates the formation of polymeric pigments. Some authors also have reported that the kinetic diffusion of the less-stable disubstituted anthocyanins is faster than that of the trisubstituted ones.

Maceration that lasts more than two weeks favors an accumulation of polymers with a larger molar mass, which is naturally more difficult to dissolve. Although it depends on the type of grape, it has been observed that the extraction phenomena tend to peter out during the fifth to sixth week of maceration.

The formation of ethanol during fermentation alters the solvent capacity of the liquid phase; this increases the solubility of tannins with a larger molar mass. Furthermore the ethanol, disrupting the external lipid layer that protects the seeds, favors more extraction of flavanols from them. Consequently, the contribution of tannins and flavanol monomers requires longer maceration times. A recent study by Spanish researchers (2012) has shown, however, that the dispersion of flavanols in seeds also occurs in the aqueous phase, although the presence of ethanol intensifies and speeds up extraction in the first stage of maceration (six days).

Under ordinary conditions with healthy grapes, the use of sulfites does not generally show effects on the extraction of polyphenols, with other parameters (temperature, time, mechanical action, etc.) clearly having more effect. The extractive contribution of sulfur dioxide, however, turns out not to be negligible in the preparation of rosé wines. Finally, the indirect role of sulfites in protecting and preserving the anthocyanins from possible oxidative degradation should be remembered.

Technological operations
Finally, the winemaking practices used are particularly important, and it’s worth mentioning the classic operations of punching down and pumping over, alongside more recent practices such as pre-fermentation cold maceration, the use of maceration enzymes, partial freezing of the must and practices that fall outside the classic scheme of red wine vinification such as thermomaceration.

The start of fermentation in the must, with the formation of large quantities of CO2, causes the “cap” to rise, which minimizes contact between pomace and the liquid phase and creates a slight warming of the mass. The first aspect is very limiting for extraction in the mixed phase. The periodical punching down (manual and mechanical) and pumping over solve the problem by remixing the mass.

In the case of pumping over, the efficiency of the spraying process is fundamental for avoiding the formation of preferential routes in the cap, which would frustrate the efficiency of the operation. Numerous experimental works show that the performance of the two techniques strongly depends on the cultivar. In general, results are linked to the frequency and duration of the operations as well as the type of equipment used.

Undoubtedly, the shape and size of the vinification containers play a decisive role in managing maceration, just like other conditions. In order to exploit space in the cellar, fermentation vats generally tend to be taller rather than wider, with the consequent difficulty in managing maceration, especially for larger quantities, and making the extraction process less efficient, all other conditions being equal. An alternative to classic maceration is the use of rotary fermentors or the practice of délestage. The extraction of polyphenols is particularly fast in the case of rotary fermentors, which enable higher levels to be achieved (see tables above).

It has often been noticed that punching down is less effective in terms of quantity when compared to pumping over, but we would like to underline the value of less haziness from lees in the wine.

The use of maceration enzymes (essentially pectinase) contributes to the degradation of the cell walls (through partial hydrolysis of the polysaccharides of the pecto-cellulosic structure of the wall), which favors the diffusion of the components present. Different authors do not always agree on the real efficacy with regard to the effect on anthocyanins. These inconsistencies can be explained by the different polyphenolic composition of the grape, the different extraction speed and the reactions in which the anthocyanins are involved. The presence of a secondary and not entirely negligible β-glycosidasic activity in commercial preparations could also explain the loss of anthocyanins due to the formation of the less soluble aglycone. The possible influence on the anthocyanic profile has also been observed, with an increase in more stable trisubstituted anthocyanins such as malvin. Moreover, is has been ascertained that many of the differences initially observed tend to lessen as the wine ages. The extraction of tannins from the skin is regulated by the type of interaction established with the polysaccharides of the vacuolar membrane or wall. With the use of maceration enzymes, a general increase in tannins was observed, along with an increase in polymeric pigments.

Pre-fermentation cold maceration (preservation at 10°-15° C [50°-59° F] for a few days) does not appear to produce striking results in the case of anthocyanins, especially for varieties like Pinot Noir and Sangiovese. There is, however, improved extraction in the presence of high levels of SO2.

The influence of this treatment on the extraction of proanthocyanidins is linked to the grape variety. In the case of Cabernet Sauvignon there is a rise in the level of tannins but a decrease in their average degree of polymerization, which could be explained by an increase in the extraction from seeds; in Syrah, meanwhile, the treatment did not show significant effects.

The use of dry ice with partial freezing of the must can produce important effects as it causes the collapse of the cell structures of the berry. Moreover, the carbon dioxide that sublimates creates an atmosphere that protects the must from oxygen. The anthocyanins undergo increases of around 50%, with considerable variability between cultivars. A 2014 study conducted at the University of Turin Department of Agricultural, Forestry and Food Sciences showed similar results for the Barbera and Nebbiolo varieties, with an enrichment in anthocyanins and a rise in the fraction of malvin derivatives at the expense of less stable anthocyanins.

The effect on tannins shows important increases also due to the breaking of the seed cells caused by freezing.

Sensory properties
The sensations of bitterness (taste) and astringency (tactile) that influence the quality of red wines are associated with polyphenolic substances, especially flavanols. Astringency is a complex of oral sensations that follow the precipitation of salivary proteins, which cause a reduction in viscosity and an increased sensation of friction. Both of these sensations have been correlated to the concentration and structure of flavanols: Proanthocyanidins are the polymer forms of flavan-3-ols; the number of structural units present defines the degree of polymerization (DP), whereas the degree of galloylation (DG) identifies the number of units present in the form of gallic acid esters.

Bitterness generally diminishes with the increase in the degree of polymerization, therefore the monomers and proanthocyanidin dimers and trimers appear more bitter. Astringency increases with the degree of polymerization (higher for PC in skins than in seeds), with the presence of units with the lateral B-ring trihydroxylate (prodelphinidic units typical of skin flavanols) and with the degree of galloylation (higher in the PC in seeds than in skins). Consequently, skin tannins are considered more astringent (higher average DP), but the DG also should be remembered, as a study conducted in France on Cabernet Franc grapes showed that the fractions of PC extracted from skins and seeds were quite similar in astringency, though with a much higher average DP in skins than seeds, but a high DG of tannins in seeds. Average levels of polymerization and galloylation depend on grape variety, and the former is much affected by the state of ripening of the raw material. A 2013 work by German researchers with reference to the subquality of astringency (velvety, puckering, drying) concludes that average DP and DG are less
important than they are with astringency intended as a single descriptor. These latest results confirm that there is still a lot to learn about relations between polyphenols and sensory effects.

However, it should be remembered that polyphenols are particularly reactive, and they establish fairly complex reactions starting during the first stages of fermentation that alter molecular structure and also sensory properties: For example, the formation of anthocyan-tannin polymeric pigments influences color but also reduces the sensation of astringency in the wine.

Preserving color
Color, as we have mentioned, is a fundamental characteristic of red wines, and it’s the one the taster perceives immediately, often influencing his whole assessment.

The technological problem, then, is working to maximize the coloring component during the extraction stage, but this is not enough because it is necessary to favor the chemical transformations that induce a stabilization of the coloring component, which otherwise is easily subject to degradation or precipitation phenomena. For example, we can remember the influence that a yeast strain carries out on the phenolic profile of red wines due to the adsorption of anthocyanins on the wall of dead cells. The choice of a suitable selected strain could help reduce the phenomenon. Huge research work and application on a production level have made a lot of operational intervention available to winemaking companies, but that is another story.

Enzo Cagnasso is a member of the Group on Wine Research at the University of Turin Department of Agricultural, Forestry and Food Sciences. The school is located in Alba, Italy, an important winegrowing zone of southern Piedmont. The main work of the enological group is focused on indigenous grapevine varieties including Nebbiolo, Barbera, Moscato Bianco and others.