In order to get where we’re going, to the intriguing world of wine science, we have to start before the wine. We have to start before the grapes make their way into the cellar. We have to start even before the harvest. We have to start by understanding the vine.
Origins are revelatory. Trying to understand wine science without understanding vine science is like trying to fix your car without understanding how cars are put together. Only once we have understood several truths about the vineyard can we start to understand and appreciate the fuller picture of wine.
With that, here are the four most important takeaways of vine science that will guide our understanding of wine science — even wine ratings — down the road.
The vine is a liana. This means that it is rooted in the ground, and its woody limbs climb to the sky, clinging to whatever it can on the way, in search of the most unencumbered sunlight it can find.
Undomesticated vines do not seek to make perfect berries by winemaking standards. They seek to spread their scraggly arms as far as they can reach, and to photosynthesize. This shows us that what a vine wants to do in its natural state is to privilege its vegetative growth. Yes, it seeks to make berries in order to propagate its seeds. But an undomesticated vine does not produce the high quality fruit that we see on trained grapevines.
Thus, when humans domesticate vines in order to produce high quality fruit, they are putting it out of its natural state of existence. Because the vine privileges its vegetative growth, this necessarily comes at the expense of its reproductive system. We have to manipulate the vine to give less energy to its vegetative cycle and more energy to produce the fruit that we want to harvest.
This brings us to point number two.
The vegetative cycle refers to the processes of growing roots and shoots. The reproductive cycle refers to the processes whereby fruit is grown and ripened.
A vine, just like a human, spends the juvenile portion of its life with an inactive reproductive system. It will spend its precious energy in its first years of life trying to grow vertically, from its roots growing down into the soil to its apex growing tall to the sky. Only around the third year will we see the vine start to produce some fruit.
How are the vegetative and reproductive cycles linked?
In the winter, the temperatures drop and the vine enters a period of dormancy whereby abscisic acid (ABA) inhibits cell growth. The vine still maintains a baseline level of metabolism to survive. In the spring, as the temperature reaches and maintains 10°C (50°F) for roughly three days, the plant hormones cytokinin and auxin are reactivated and ABA decreases. This initiates metabolic activity in the roots, which causes an increase in root pressure, and xylem sap is forced up the plant until the buds “bleed” i.e. the sap oozes out of the buds. Bud burst occurs in the spring, generally in March or April, and the vine resumes its vegetative growth.
After bud burst, clusters of little baby flowers, called “inflorescences,” are developed. These inflorescences shed their caps to reveal flowers, and those flowers are then fertilized to give fruit. What’s important to know is that buds that are developed in year 1 produce shoots that contain fruit in year 2. In other words, fruit production takes two years. First, the inflorescence is pre-formed in the dormant bud. It is only in the following year that the shoot containing the inflorescence will break from the bud, now the primary bud, and grow externally, giving rise to the fruit.
So, why does this matter?
First, we can see that the reproductive cycle is initiated by the vegetative cycle. We need the plant to grow because this provides the hormonal signals necessary to induce flowering and the bearing of fruit.
Second, if inflorescences are pre-formed in year 1 and we see them flower and yield fruit in year 2, impacts to the vine in year 1 can influence what we see during the harvest in year 2. This means that if the vine is stressed during the vegetative growth portion of the spring (say, due to not enough nutrients or water or because there is a hail storm), this could negatively impact the primordial inflorescence, and subsequently, the fruit that will set in the following year.
Now that we’ve covered vine cycles, let’s look at how humans can manipulate the vine to get high quality fruit.
Crop load is the ratio of fruit to leaves. There is an ideal crop load for each vine; this is called vine balance. Let’s break that down.
The vine has a limited amount of nutrients at its disposal. The amount to which it can photosynthesize and create life-sustaining energy is limited by the number of leaves it has. This, in turn, limits its capacity to support fruit. Of course, there are many other factors that will influence how much fruit a vine can carry, such as soil water holding capacity, organic matter content of the soil, temperature, etc. But the most fundamental idea is that the vine has a bank of energy, and that energy is divided into different bins: the vegetative bin or the reproductive bin. Finding the right balance of energy in each bin is the central idea behind vine balance.
So how can we achieve the right vine balance?
The first place to start is with pruning. In the winter, vines are pruned in order to set the stage for the crop load the following summer. The central idea to pruning is that you are determining for the vine how many buds it will maintain and in what arrangement it will maintain them. In other words, there are many different pruning methods, but the reason we prune vines is to tell the vine how much fruit we want it to carry.
Over-pruning (i.e. leaving the vine with less fruit than it should carry) means the vine will favor its vegetative growth, leading to a large leaf canopy. Under-pruning (i.e. leaving the vine with more fruit than it should carry) means the vine will struggle to provide enough nutrients to the fruit, leading to a reduction in fruit quality. Plus, it might need to dip into its carbohydrate reserves to “feed” all that fruit, meaning there are fewer reserves left for winter dormancy and the development of dormant buds.
Remember, what you do in year 1 will impact the harvest in year 2. If the vine is pushed to its limit to just support the fruit that it has to ripen in year 1, being forced to dip into its energy savings, it will have less energy to give to the developing inflorescences of year 2. This can lead to either lower bud fruitfulness or lower fruit quality in year 2.
How do you know what the ideal vine balance is?
There are two basic schools of thought here: trial by error or technical. The trial by error route assesses visually whether the vine seems too vigorous (i.e. lots of leaf area) or too tired (i.e. poor quality fruit, thin branches) and adjusting the number of buds retained accordingly. The technical route compares vegetative biomass to reproductive biomass. Concretely, this means measuring the weight of all the pruned branches from the vine in the winter and comparing it to the crop load in the summer. The vine is then assigned a score on the Razav Index which helps you to know if you should leave more or fewer buds.
So, now that we have our ideal crop load, we have to consider what happens to the berries during ripening that impacts the finished wine.
It’s late summer now, and the berries begin to change from green to their final color of deep red or golden yellow. This is the stage, called véraison, when the fruit starts to look edible. The flesh softens and the pulp gets juicier as the berry ripens. Things like aroma compounds and color pigments develop. The tannins mellow out. In short, the berry becomes more of a pleasure to consume. What’s critical to understand is that nature is not whimsical; there is an evolutionary reason for these berry traits to develop.
Before ripening, the grape berries are firm, green, and bitter. Acid levels are higher than sugar levels. There is little juice in the berries at this stage, and aroma compounds are predominantly pyrazines (the family of aromas that give Cabernet Franc their green pepper smell). All of these unpleasantries mean that the berry seed is protected as it forms, and eating the berries at this point would be a downright disappointment. Humans surely know better — and so does the animal kingdom. If nature is all about survival of the fittest, then grape vines need to figure out how to make their fruit more attractive to seed dispersers (i.e. birds and mammals).
How do they do this? The key lies in a metabolic pathway that is switched on at véraison.
You can think of the vine as having two metabolisms: the primary and the secondary. The primary metabolism is when the grape consumes sugars, acids, and nitrogen in order to give itself energy. The primary metabolism is vital; without it, the plant dies. The secondary metabolism is when the grape creates products (metabolites) that are not necessary for the growth and function of the plant, but rather are in response to some sort of stress or interaction with its environment.
So, after véraison, this secondary metabolite pathway gets activated. In red grapes, things like anthocyanins start to form, the pigment molecules with antioxidant properties. In the white grapes Muscat or Riesling, for example, their signature floral aromas (part of the terpene family) start to develop in the skin and the pulp. In other grape varieties, something called “ glycosidic aroma precursors” develop. These odorless molecules are bound to sugars, and it is during the process of fermentation when they will break free and volatilize so that we are able to smell them.
So, during ripening, we have these secondary metabolites that start to form. At the same time, we have sugar accumulating and acid levels dropping. What’s the raison d’être? Now, the berry is attractive to seed dispersers. The secondary metabolism is switched on at véraison so that the vine may pass on its genes to offspring.
It is this biological impulse for things in nature to spread and reproduce that gives us wines with all of their color, texture, and aromas.
Wine, then, is fundamentally a wonder of nature.