Anthocyanins

[originally appeared in the 21st September 2019 newsletter]

This time of year is filled with prevarication about the traits of a particular vintage climate. If has been super hot and dry leading up to vintage everyone talks of reduced canopy leading to light exposure, if it has been cool and wet you generally hear of attenuated development leading to synchronised flavour and sugar development. The beauty of modern science is we can predict a lot about a vintage before it even happens and can ignore the white noise. The best starting place for me is colour. The red colour is due to the pigment anthocyanin, a thoroughly bloody amazing molecule that produce a light red through to deep blues depending on slight tweaks to their structure and pH. Incidentally, the change is brightness of anthocyanins with pH (the lower the pH the brighter they are) is due to the chemical mechanism of resonance which is a beautiful word for an incredible phenomenon. There isn’t one type of anthocyanin but upwards of 37 different variants which all have slightly different colour and stability divided into 6 types: delphinidin, malvidin, cyanidin, peonidin, pelargonidin and petunidin. Their relative composition in grapes leads to wines of differing colour intensity, hue and aging potential. So how do we work it all out? It is actually surprisingly easy. Anthocyanins and tannins are produced by the same pathway, the Flavonoid Biosynthetic Pathway. It’s a personal favourite.

It looks pretty complicated but it isn’t really. The whole thing starts with phenylalanine, a single amino acid. From that plants produce tannins (proanthocyanins; PAs), accessory pigments involved in stabilising and brightening colour (flavonols), the red pigment (anthocyanins) and even lignin (essentially wood!) 

From the key point in the pathway is a branch point mediated by two enzymes: Flavonone 3’5’ hydroxylase (F35H) and Flavonone 3’ Hydroxylase. If the plant use F35H then we get the tannins and anthocyanins of the left side of the pathway and vice versa for F3H. We can largely ignore the middle branch as Pelargonidin is only found in very small quantities and as yet not believed to be influential to wine quality.

The F35H leads to anthocyanins that are deeper in hue, more on the blue end of the spectrum, and the tannins have been associated with softness. The F3H leads to the redder, lighter anthocyanins and more astringent tannins. So if we want a  classic deep red wine, we want more of the F35H side, right? How do we get this? Well since we know the chemistry now we can just find out what makes F35H tick. All these enzymes are driven by a little molecular ‘on’ button called transcription factors, in this case transcription factors called MYBs. F3H and F35H are turned on by MYBs but only at particular times, F3H before veraison, F35H after. So if we get stressful conditions at those times then we will get MYBs and thus these enzymes turned on! A good example is from drought stress. Drought stress  before veraison leads to F3H and after leads to F35H. So if we get post-veraison rain then we may get a lighter anthocyanin profile. One last enzyme worth mentioning is the bottom one, UFGT. This has a large bearing on the overall level of anthocyanins. It is also turned on by MYBs. So if we get stressful conditions overall we will get more anthocyanins produced. This we know, but it is confirmed by the pathway. I’ve talked about drought but MYBs get turned on in the same way by most stresses, drought, high light, and low temperature. Why does shoot thinning before veraison help colour, well it increases light stress on grapes. Easy!

So what about this year? Well we have a look at this soil moisture profile leading up to January and we can see creeping drought stress coming into play. Just in time for F3H and F35H! If we don’t get substantial rain through February we can expect good, deep colour with nice balanced tannins, but low yields, as long as the vines can keep enough canopy to get the berries to ripeness.