An australian researcher team led by Cooley have compared different levels of water deficit severity to analyze their effect on berry and wine composition (Cabernet Sauvignon). Interestingly their results show that the more limited irrigation regime, (ie. the “prolonged” deficit irrigation) can improve the consistency of wine quality.
By connecting contrasted water regimes with ripening and wine profiles, authors have shed new light on the effects of vine water deficit on berry color accumulation and color extraction into the wine. Their results may sound counter intuitive but they contribute to refresh common knowledge. We succinctly review some of them.
METHOD AND TREATMENTS
As part of their study, authors compared 2 treatments.
- Regulated deficit irrigation (RDI) before veraison is a classical watering regime, typically imposed when winegrower wish to induce more qualitative berry traits before harvest. Irrigation strategy for the RDI is reported in green in Figure 1. For comparison purposes, a more challenging irrigation strategy is also imposed.
- Prolonged drought strategy is reported in orange in Figure 1. At first, such irrigation treatment may sound bad. It is only after leaf loss or paler green leaves become visible that researchers stopped completely irrigation. Irrigation was only resumed after veraison. Needless to say: risk averse winegrowers would not like the look of the vineyard under such treatment!
Figure 1: The green treatment imposes a moderate water deficit pre-veraison (Regulated Deficit Irrigation). The orange treatment imposes a more severe water deficit pre-veraison (Prolonged Drought = no water for 2 to 5 weeks) – adapted from Cooley et al, 2017
Berry skin mechanical properties
Why is that important from a winemaker standpoint ?
Skin mechanical properties depend upon skin structure, which in turn affect the process of berry crushing before fermentation as well grape skin quality (ie. phenolic substances, cell wall stiffening, cell wall pectin composition, etc…). Consequently, changes in berry mechanical properties reflect textural and structural changes directly impacting polyphenols extraction. This is important to keep in mind for winemaking applications. Interestingly, authors found that under “Prolonged drought” irrigation regime, Cabernet sauvignon berries acquire stronger berry skin (ie. offer more mechanical resistance to deformation). These berry traits probably lead to less berry damage during harvesting and differences in anthocyanins extraction during winemaking.
Wine color vs. berry skin anthocyanins
Authors reported that total anthocyanins concentration from whole berry are unaffected by irrigation treatments. However – and this may sound counter intuitive – wine color density and polymeric anthocyanin concentration is higher under prolonged drought.
#1: Prolonged drought not only induces paler leaves and leaf losses. It also induces changes in the way polymeric pigments interact. These changes will impact anthocyanins extraction differently at a whole berry level vs. at a wine level.
#2: Water deficit treatments have no effect over the time profile of whole berry total anthocyanin as measured in the experiment. However prolonged vine water deficit has a direct impact on wine properties. With more vine water deficit pre-veraison, wine colour density is greater and the concentration of anthocyanin condensated with other substances is higher.
Is it really a surprise?
In fact not really. Similarly, Bindon and her team showed higher wine color density and higher polymeric anthocyanins in wines from water deficit treatments. Sadras and his colleagues have indicated that the rate of anthocyanins accumulation may be a more useful information to estimate potential wine color density than the total anthocyanins content as discussed in our previous blog.
- The timing of plant response to environmental stresses seems to be key in driving wine attributes, particularly when they relate to polyphenols.
- “Chewing berry skin is not only for chewing”. Empirically, many winemakers chew the skin of the fruit sampled in the field to support the timing of harvest. When walking the vineyard before harvest and dilacerating the skin in the mouth, winemakers can gain insight in skin mechanical properties. The degree of resistance to deformation can be used as a proxy for the quality of anthocyanins extraction into the wine. Thus, for a given varietal, easier skin chewing may be the consequence of high water supply earlier… and lead to less wine color and a lower polyphenol concentration.