White wine is far more than a crisp, refreshing drink-it's an intricate concoction of organic compounds and chemical reactions, delivering complex flavours and aromas that delight the senses. Whether it's a Sauvignon Blanc from New Zealand or a buttery Chardonnay from California, each glass of white wine tells a fascinating story of chemistry. From the vineyard to your glass, the science behind white wine influences every aspect of its taste, aroma, and texture. Let's dive into the chemical intricacies that make white wine a drink to savor.

The grapes: The source of flavour

The story of white wine begins in the vineyard, with the grapes themselves. White wine can be made from both white and black grapes, but the key is in how the grapes are processed. Unlike red wine, the skins are typically removed before fermentation, which means the colour compounds (anthocyanins) present in the grape skins do not end up in the final product. This is what gives white wine its lighter, more translucent colour.

The primary chemical components of grapes are sugars (mainly glucose and fructose), acids (like tartaric and malic acid), water, and trace amounts of other organic compounds. Each of these plays a role in defining the final character of the wine. Grapes that have been allowed to ripen longer contain more sugars, which means they can produce wines with higher alcohol content if fully fermented, while acidity contributes to the sharpness and freshness of the wine.

Fermentation: The heart of the winemaking process

The magic of winemaking truly happens during fermentation, a biochemical process in which yeast, specifically Saccharomyces cerevisiae, converts the sugars present in grape juice into ethanol (alcohol) and carbon dioxide.

The yeast cells also produce a range of by-products, including esters, higher alcohols, aldehydes, and acids, which together contribute to the complex aroma and flavour of the wine. The specific yeast strain used can significantly impact the resulting wine's character, as different yeasts yield different by-products.

The temperature at which fermentation takes place also plays a critical role in white wine's chemical profile. Cooler temperatures (10-15°C or 50-59°F) are typically used for white wines to retain volatile aromatic compounds. This careful control of temperature helps preserve the fresh, fruity, and floral notes that characterize many white wines.

Acids: The Backbone of Flavour

Acidity is one of the key components that define the structure and flavour of white wine. The principal acids in white wine are tartaric, malic, and citric acids, though acetic acid can also be present in small quantities. The acidity provides a crisp, refreshing quality that balances the sweetness from the alcohol and any residual sugars.

Tartaric acid is unique to grapes and is one of the main contributors to the acidity of wine. Its stability under fermentation conditions means that it remains largely unchanged, giving wine its characteristic sharpness.

Malic acid, often associated with the tartness of green apples, can be converted to lactic acid through malolactic fermentation (MLF). This secondary fermentation, often employed in winemaking, softens the acidity of the wine, leading to a creamier texture and rounder flavour. Malolactic fermentation is commonly used in producing Chardonnay to create that signature buttery quality.

Citric acid is present in small amounts, contributing to the overall complexity of acidity. Though it's not the dominant acid, it can influence the perception of freshness in a wine.

Aromas: Esters, Terpenes, and Thiols

One of the joys of drinking white wine is its rich bouquet of aromas, which ranges from citrus and stone fruits to floral and herbaceous notes. These aromas are largely due to three classes of compounds: esters, terpenes, and thiols.

Esters are formed by the reaction between acids and alcohols during fermentation. They are responsible for many of the fruity aromas in white wine. For instance, ethyl acetate, which forms from ethanol and acetic acid, gives notes of pear and pineapple. The concentration of esters is influenced by fermentation conditions, particularly temperature, as well as the strain of yeast used.

Terpenes are volatile compounds found in the grape skins and are responsible for the floral notes in wines. These compounds are more common in aromatic white wines like Riesling and Gewürztraminer. Some common terpenes in white wine include linalool, which gives a lavender or citrus blossom aroma, and geraniol, which adds hints of rose.

Thiols are sulphur-containing compounds that can impart both pleasant and unpleasant aromas. In small concentrations, thiols contribute desirable notes like grapefruit, passion fruit, and guava, especially in Sauvignon Blanc. The presence and perception of thiols are highly influenced by the vineyard practices, grape variety, and fermentation conditions.

Phenolic Compounds: Adding Complexity

White wines contain phenolic compounds, although in much smaller quantities compared to red wines. Phenolics in white wine come primarily from the pulp and seeds, rather than the skin, since skin contact is limited. These compounds include flavonoids and non-flavonoid phenols like hydroxycinnamic acids.

The flavonoids in white wine include catechins and epicatechins, which contribute to the wine's bitterness and astringency. Though not as pronounced as in red wine, these compounds add subtle complexity to the flavour profile of white wines.

Non-flavonoids: These include hydroxycinnamic acids, such as caffeic acid and p-coumaric acid. These compounds can undergo oxidation, leading to the formation of browning pigments. Winemakers take care to minimize oxidation in white wine production to maintain the bright colour and freshness of the wine.

Aging: The Evolution of Flavour and Texture

White wines are typically aged for a shorter time compared to reds, but the aging process still plays an important role in shaping their final characteristics. White wine can be aged in stainless steel tanks, which preserve freshness and fruitiness, or in oak barrels, which impart additional flavours and increase complexity.

Stainless Steel Aging: Stainless steel tanks are inert, meaning they do not impart any flavours to the wine. Wines aged in these tanks, like Sauvignon Blanc and Pinot Grigio, maintain crisp, fresh notes with high acidity.

Oak Barrel Aging: Oak barrels contribute several compounds to the wine, such as lactones, which provide vanilla and coconut flavours, and tannins, which add structure. The process of aging in oak also allows for controlled oxidation, which can soften the acidity and add a creamy texture to the wine. This technique is commonly used in Chardonnay production, adding notes of butter, vanilla, and toast to the wine.

Sulphur Dioxide: A Protector of Quality

Sulphur dioxide (SO₂) is an essential additive in winemaking, used as an antioxidant and antimicrobial agent. It helps to protect the wine from unwanted oxidation, which can lead to browning and spoilage, and prevents the growth of harmful bacteria. Sulfur dioxide binds with aldehydes and other oxidation products, reducing the likelihood of off-flavours developing. However, it's important for winemakers to carefully regulate the levels of SO₂, as excessive amounts can impart an unpleasant burnt match aroma.

The Role of Dissolved Gases

Dissolved gases, such as carbon dioxide and oxygen, also have an impact on the chemistry of white wine. Carbon dioxide, produced during fermentation, can provide a slight effervescence that enhances freshness. In some white wines, a deliberate addition of carbon dioxide creates a spritz, which adds a crisp texture.

Oxygen, on the other hand, can be both beneficial and detrimental, depending on the stage of winemaking. While excessive exposure to oxygen can lead to oxidative spoilage, small amounts during certain phases of aging can add complexity to the wine. Winemakers must manage oxygen carefully to balance the development of desirable flavours with the risk of spoilage.

Residual Sugars and Sweetness

Residual sugar (RS) refers to the unfermented sugars that remain in the wine after fermentation is complete. In dry white wines, almost all the sugar is converted into alcohol, leaving only trace amounts. In contrast, sweeter white wines, such as Riesling or Moscato, have higher residual sugar levels, which contribute to their luscious sweetness. The amount of RS also affects the perception of acidity and body in the wine, with higher RS levels generally leading to a fuller mouthfeel.

The sweetness in white wine is not only determined by residual sugars but also by alcohol, which can provide a perception of sweetness even in dry wines. The interaction between sugars, alcohol, and acidity creates the balance that defines the wine's overall flavour profile.

The Chemistry Behind the Sip

The chemistry of white wine is a complex interplay of numerous factors, from the vineyard conditions to the yeast selected for fermentation, and the aging methods used. Every step involves carefully controlled chemical processes that ultimately determine the flavors, aromas, and textures that define a wine's character.

The balance between acidity, alcohol, phenolics, and aromatic compounds is what gives white wine its unique charm. Understanding the chemistry behind white wine not only deepens our appreciation for this popular beverage but also highlights the artistry.

Renald Blundell is a biochemist and biotechnologist with a special interest in Natural and Alternative Medicine. He is a professor at the Faculty of Medicine and Surgery, University of Malta.

Photo: AI-generated image created by Prof. Blundell