Knowing and understanding grapes is absolutely essential to making good wine. After all these little berries are what it all starts with right?

By their very nature grapes are the perfect winemaking fruit. No other fruit contains the perfect amounts of sugar, acidity, and phenolic compounds to create such an amazing beverage. Any other fruit requires additional sugar or other ingredients to even produce alcohol.

Let’s get to know our little friend a little bit better. Shall we?

Physical Components of the Grape

The Skin

Diagram by Mariana Ruiz Villarreal

At only six to ten cells thick you wouldn’t think there’s much to the skin of a grape. However, this membrane contains many key elements for red wines. Less so for white wines as the juice spends little time in contact with the skins.

The outer surface of the skin is the cuticle, a wax like covering that waterproofs the berry. Protecting it from outside influences. Within the thin skin are a ton of components including aromatic substances, potassium, and phenolic compounds.

Phenolic compounds refer to a group of compounds, however, there are two very important ones that need to be explored.

The first are anthocyanins. These are pigment compounds that give the grape its color and in turn gives red wine its color. As wine ages these anthocyanins combine with other phenolic compounds which serves to stabilize the color of the wine.

The second phenolic compound of interest are tannins. Also present in the seeds tannins give wine an astringent and bitter taste. Tannins also combine over time and alter the taste and mouth feel of a wine.

Red wines in particular get most of their flavor and spunk from the skins. Merely pressing red grapes and fermenting the juice results in what the French call “Blanc de Noir” meaning white wine from red grapes, or literally white from black.

The Pulp

The bulk of the grape is made up of the pulp beneath the skin. This is where the grape juice comes from. Vacuoles contain the juice and when broken release the “free run” juice.

As you can see in the diagram the pulp contains many compounds of its own including:

  • sugar
  • water
  • aromas
  • potassium
  • tartaric acid
  • malic acid

In white wine making the pulp provides the bulk of the flavor and acidity. Red wines get their flavor first from the skins but also from the pulp.

Seeds

Moving inward we come to the seeds. These are large caches of tannins. So much so that as winemakers we must be careful not to crush the seeds during the pressing of the grapes.

By crushing the seeds, and stray stems, the tannins are overdone. If this happens your wine will need much more time in the bottle to become palatable.

Chemicals Within The Grape

The chemical makeup of a grape is quite diverse and complex. We’ll just hit the major components here.

Sugars

The sugars within the grape are what the yeast consume to produce the alcohol in wine, as you already know. What we refer to as “sugar” in a grape is actually a combination of several different kinds of sugar.

Primarily there is fructose and glucose. To a lesser degree there is also sucrose followed by several others in lesser quantities.

The interesting thing here is that these different types of sugars vary in sweetness. Fructose is sweetest and sucrose is less sweet. As a grape ages on the vine the levels of these different sugars vary in relation to each other.

Ripe grapes contain a more equal ratio of glucose and fructose. As the grape becomes over ripe fructose becomes the predominant sugar.

Not all sugars in a grape are fermentable by yeast. This is why some varietals when fermented to “dryness” will still have a sweet overtone to them. While alcohol can also be perceived as sweetness part of that sweetness can be these left over sugars.

Acids

The most prevalent and important acids contained in grapes are tartaric acid and malic acid. The concentration of these acids vary with varietal, climate, and the ripeness of the grape.

Tartaric acid has a tendency to precipitate out in cooler storage conditions. It is this acid that is responsible for creating the “crystals” you’ll sometimes see in a bottle. Contrary to popular belief these crystals are not a sign of quality. They are a sign of a wine that was not cold stabilized and racked off of the crystals.

Interestingly enough tartaric acid is the same chemical in cream of tartar.

Malic acid is a liability. With the proper bacteria present malic acid can be converted to lactic acid through what’s called malolactic fermentation.

While this is an optional fermentation you do run the risk of a malolactic fermentation occurring in the bottle if you don’t do purposefully do it sooner. If this does happen the gasses released during this fermentation will build up and you can have corks blowing out or bubbly wine if the gasses stay suspended.

Lactic acid is the same acid found in milk and other dairy products. It gives wine a smoother almost creamy taste.

Aroma Compounds

These compounds are largely contained in the skin of the grape however, there are two other sources of aroma when making wine.

First is fermentation. Different yeasts produce different aromas and flavors. Some yeasts are known for being more fruity than others.

Second is aging and maturation. Aging in oak of course can change the aroma of a wine. Beyond that aromatic compounds can combine over time which can alter them.

Of course there are many more chemicals and compounds found in grapes but this is a summary of the most important ones. All of these compounds can vary with any number of variables including climate, varietal, age of the fruit, age of the vine, and how much the vine had to struggle to produce the fruit.