Here at Wine Republic we focus on sustainable, organic and biodynamic wines. We also look for the best people to surround ourselves with and our intern is no exception, she's a killer. Today she schools us on wine chemistry.
(Bio)Chemistry 101: Winemaking Style!
Chances are most people on the street with at least a high school education are familiar with the basic concepts of general chemistry; chemical reactions have inputs and outputs, where the inputs, known as the reactants, come together to form an entirely different chemical compound(s), known as the product. As most people know, winemaking is no exception.
The fermentation process itself is probably most familiar to any person with a surface-level knowledge of wine and spirits. However before we can get to that step, the real science begins with knowing when to harvest the grapes. Winemakers must have quite the green thumb, at least when it comes to grapevines. In order to create the best product possible, the grapes must be harvested when the balance of sugar, phenolics (such as tannins that give wine its dryness and color), acid, and aroma compounds are just right for the style of wine which is to be produced. Some wines, such as many white varietals, will require higher levels of acid and lower sugar, whereas other varietals may need a higher sugar content and minimal acid. As a general rule-of-thumb, sugar levels will go up as the grape spends more time on the vine. This comes from the storage of carbohydrates that builds up in the roots and trunk of the grapevines as more and more photosynthesis occurs due to continued sun exposure. In a similar fashion, as these sugar levels rise, acidity tends to diminish due to the use of this acid in cellular respiration which coincides with a rise in potassium levels; all this leads to a rise in the pH within the fruit. Sugar levels are not only important in determining the sweetness of the wine that is to be produced, but also the overall alcohol content. More sugar available means more alcohol can be produced from that sugar.
Directly before fermentation, the grapes must be removed from the vine and crushed. This process is time-sensitive, as once a grape is plucked from the vine, it is cut off from continued nutrient supply and begins to turn to vinegar. During this step, sulfur dioxide is added to the crushed grapes to prevent oxidation and maintain purity of the product by preventing growth of bacteria (ick!). At this point, enzymes can also be added to help break down the skins of the grapes and release more juice.
Now comes the fun part: fermentation! This step, as most know, is what produces the alcohol that makes wine exclusive to us folks that are lucky enough to be 21+. In order to jump-start the process, live yeast is added to the mixture. This yeast facilitates the fermentation process by feeding on the sugars in the mixture. For those science nerds out there, here is the overall equation:
C6H12O6 —> 2C2H5OH + 2CO2
For those who do not consider themselves science nerds, here’s the breakdown: Glucose sugar (the most common sugar used in metabolism of living cells, including us) is fermented by the yeast to produce alcohol and carbon dioxide. This process doesn’t happen all at once, but rather in a series of complicated steps. This process can vary in length, but usually takes about 2-3 weeks in total. This process must be actively stopped, however. This can be done in a variety of ways such as via chemical additives (like potassium sorbate), rapid change in temperature to “stun-kill” the yeast, or fortification by adding a higher alcohol.
Once fermentation is completed, professional winemakers will purify the product. This can be thought of as filtering the wine, but often times to a much higher and complex level that allows for the removal of microorganisms, unwanted salts, and unwanted solids. Although much of this is filtered out, many minerals are still retained in the wine, which become a characteristic aspect of its taste. In general, 0.4% of the weight of the grapes can be attributed to mineral compounds such as magnesium, phosphate, and potassium, which are picked up from the soil during growth on the vine. These compounds not only contribute to flavor, but also are important key-players in the intermediate steps of the fermentation process as well.
So this fermentation process gives wine its taste, but what about the aroma? As any wine-lover knows, not all wines are equal, especially when it comes to the nose. Chemically speaking, the scent of a wine comes from a group of compounds called monoterpenes. These chemicals are responsible for many of the floral and fruity scents that we so love. However, these monoterpenes are not the only player in the scent game. Another important chemical reaction occurs in oak aging. When the skins of the fruit hit the oak of a barrel, one of two important reactions can occur depending on the type of grape. In white wines, contact of the skin to the oak causes the formation of a compound known as diacetyl, which gives the characteristic buttery nature that is found in many wines such as oaked Chardonnay. In red wines, this same contact triggers the formation of vanillin, which, just as it sounds, is responsible for the vanilla hint that many oaked reds carry.
In short, there is indeed a complex and extensive chemical science behind the flavors, sensations, and smells that make up a good quality wine. The beauty in this process is how all of these chemicals interact dynamically, and how winemakers can vary their processes to “tweak” the overall composition of the wine just by letting chemistry take its course.