Wine is a complex mixture, not a pure substance with a fixed chemical identity. This classification helps in understanding its physical properties and the intricate chemistry that gives this ancient beverage its unique character. The complexity of wine stems from the natural processes of fermentation and aging, combining numerous elements into a single liquid.
The Chemical Definition of Purity
In chemistry, matter is broadly divided into two main categories: pure substances and mixtures. A pure substance is defined as a material composed of only one type of particle, which can be either a single element or a compound. Elements, such as pure gold, cannot be broken down into simpler substances, while compounds like distilled water (H₂O) consist of two or more elements chemically bonded in fixed proportions. Both elements and compounds share the trait of having a uniform and definite composition, with consistent properties like a sharp boiling point.
A mixture, by contrast, is a physical combination of two or more pure substances that are not chemically bonded together. The components in a mixture retain their individual chemical properties and can often be separated by physical means, such as distillation or filtration. Mixtures are further classified as either heterogeneous, where the components are visibly separate, like in beef stew, or homogeneous. A homogeneous mixture, also known as a solution, has a composition that is uniform throughout, meaning only one phase of matter is visible, such as in saltwater.
Why Wine is Classified as a Mixture
Wine is definitively classified as a mixture because it contains multiple chemically distinct components that are not joined by chemical bonds. Its primary components, water and ethanol, can be physically separated through a process like distillation. Since wine does not possess a single, fixed chemical formula, it fails the fundamental test of being a pure substance.
The liquid exhibits a uniform appearance, meaning a sample taken from the top of a glass is chemically similar to a sample taken from the bottom. This characteristic places wine specifically into the category of a homogeneous mixture, or a solution. Wine is essentially a hydro-alcoholic solution, where the various compounds are dissolved evenly throughout the liquid phase.
The Essential Ingredients of Wine
Wine’s classification as a mixture is supported by its diverse composition, which includes hundreds of different molecules. Water constitutes the majority, typically 82% to 86% of the volume, acting as the solvent for all other components. Ethanol, the main alcohol, is the second largest component (12% to 15% by volume), and is responsible for the wine’s body and volatility.
Organic acids are present in concentrations of around 0.4%, with tartaric acid and malic acid being the most prevalent. Tartaric acid is unique to grapes and is the strongest acid, contributing to the wine’s structure and stability. Malic acid provides a sharp, green-apple taste, which often decreases as grapes ripen. Glycerol, a byproduct of fermentation, makes up approximately 1% of the volume and contributes to the wine’s viscous texture and mouthfeel.
Phenolic compounds, though present in small amounts (about 0.1%), are important to the wine’s characteristics. These include anthocyanins, which provide red wine its color, and tannins, which are responsible for astringency and bitterness. Tannins undergo polymerization over time, forming larger molecules that soften the wine’s texture during aging.
How Processing Affects Wine’s Composition
The winemaking process involves several dynamic steps that continuously alter the mixture’s composition. Fermentation is a biological conversion where yeast consumes grape sugars, producing ethanol and carbon dioxide. This process also generates volatile compounds, such as esters and higher alcohols, which contribute significantly to the wine’s primary aromas.
Following fermentation, malolactic fermentation can occur, where bacteria convert malic acid into the softer lactic acid, changing the wine’s acid profile and mouthfeel. Aging, whether in stainless steel or oak barrels, introduces further chemical changes. Aging in oak allows for micro-oxygenation, promoting the polymerization of phenolic compounds. This process softens tannins and stabilizes color, leading to the development of complex, tertiary aromas like vanilla, spice, and nutty notes.