Crafting Ciders

Cider is a popular fermented beverage made from apple. It has a distinctive taste, ranging from sweet to dry and still to sparkling. But, apple juice and cider are distinctively different. It is all the result of a complex interplay of chemistry during fermentation and aging.

Cider starts with apples, and the chemistry of the fruit itself plays a crucial role in the final product. Apples are primarily made up of sugars, acids, tannins and aromatics. They contain natural sugars like fructose, glucose, pectin and sucrose which are used up by yeast during fermentation. They also contain organic acids, predominantly malic acid, that gives apples its tartness. Some cider varieties may also contain smaller amounts of citric and acetic acids. Apples, especially those used in traditional cider making, contain tannins, which are polyphenolic compounds that contribute bitterness, astringency, and mouthfeel. Tannins give cider a dry, puckering sensation similar to that in red wine or tea. They also contain aromatic compounds, such as esters and aldehydes, which contribute to the fruity and floral notes in cider. Minerals, such as potassium, magnesium, and calcium, are also present in cider.

After picking the apples, they are left to mature for some time. Then, the apples ground down into a pulp. The pulp produced by this process is known as pomace. This pomace is then pressed to squeeze out all of the juice, which is collected into either vats or casks. Then, the pomace undergoes the process of fermentation.

Fermentation is the key process that transforms apple juice into cider. The primary reaction in fermentation is the conversion of sugars into ethanol and carbon dioxide by yeast. These yeasts can be the natural yeasts present in the apples, or yeasts that are added specifically for fermentation. There are two key phases in cider fermentation: primary fermentation and malolactic fermentation.

In primary fermentation, yeast, typically Saccharomyces cerevisiae, are added to the apple juice to begin fermentation. The yeast metabolizes the sugars in the juice, producing ethanol and carbon dioxide, along with a variety of other compounds that influence flavor and aroma. During primary fermentation, the ratio of fructose to glucose in the apple juice plays a significant role. Fructose is more fermentable than glucose, and cider yeast tends to favor fructose. It often leaves glucose as a residue, which can affect the cider’s sweetness and final alcohol content.

In some cider styles, a secondary fermentation process called malolactic fermentation occurs. This process is carried out by Lactobacillus bacteria, which converts the sharp-tasting malic acid in apples into the softer, creamier lactic acid. Malolactic fermentation reduces the acidity of the cider, making it smoother and rounder in taste. This process is more common in traditional and craft ciders, especially those that are aged for a longer period. It is particularly important in balancing high-acid apple varieties.

Fermentation plays a key role is giving cider its signature flavor and aroma, this is due to the formation of a variety of volatile compounds during fermentation. Mainly, esters and phenols create the cider’s flavor profile.

Esters are formed when organic acids react with alcohols during fermentation. They are responsible for the fruity and floral aromas in cider. The most common ester in cider is ethyl acetate that gives cider a light, fruity aroma. Other esters, such as isoamyl acetate, ethyl-2-methyl butanoate, 2-methyl-4-pentyl-1,3-dioxane and 1,3-octanediol also add fruity notes. The production of esters is influenced by factors such as yeast strain, fermentation temperature, and nutrient availability. Lower fermentation temperatures tend to enhance ester formation, while higher temperatures can suppress it.

Likewise, phenolic compounds in cider can come from both the apple and yeast. These compounds contribute spicy, smoky, or medicinal notes to the cider’s flavor. In particular, polyphenols like tannins from the apple skins and seeds give cider a bitter, astringent character. Phenols like 4-Vinyl guaiacol that are produced by yeast, impart a clove-like aroma. In certain traditional ciders, phenols may also lend smoky or earthy flavors, adding complexity. The concentration of phenols can vary based on the apple variety, fermentation process, and even the yeast strain used.

Tannins are a group of polyphenolic compounds that are particularly abundant in traditional cider apple varieties. Tannins are responsible for astringency in cider, as well as its bitter taste. These compounds are extracted from the apple skins, seeds, and stems during pressing. Tannins bind with proteins in saliva, reducing lubrication and causing the drying sensation, also described as astringency. They also help stabilize cider against oxidation, preserving flavor and color during storage and aging. In tannin-rich cider varieties, the astringency can be softened over time through maturation, allowing more complex flavors to develop.

Additionally, carbonation is an essential aspect of cider, especially in sparkling varieties. There are two primary ways to achieve carbonation in cider: natural fermentation and forced carbonation.

In naturally carbonated cider, carbonation occurs when residual sugars continue to ferment after bottling. The yeast consumes these remaining sugars, producing carbon dioxide, which becomes trapped in the sealed bottle, creating bubbles. In bottle conditioning, additional sugar or unfermented juice, also called priming sugar, may be added to the cider before bottling to ensure proper carbonation.

In commercial ciders, carbonation is often achieved by forced carbonation. In this process, carbon dioxide is injected directly into the cider under pressure, creating the desired level of bubbles without additional fermentation.

After fermentation, the cider is matured through the aging process. This process can significantly impact the cider’s final flavor. During aging, a variety of chemical reactions occur, including oxidation, ester formation, and the breakdown of complex sugars and acids. Low levels of oxygen introduced during aging can enhance flavor complexity, but too much oxidation can lead to off-flavors, such as those resembling sherry or cardboard. Over time, the tannins in cider may polymerize, softening the astringency and adding depth to the flavor. Aging in wooden barrels, similar to wine, can also impart additional flavors to the cider, such as vanilla, caramel, or smokiness, due to compounds in the wood. In commercial ciders, non-fermentable artificial sweeteners are also used. Artificial sweeteners like saccharin and sucralose are commonly used for ciders.

Stopping the fermentation prematurely, a process known as ‘keeving’, is less common in ciders. Instead, sugars can be added, but it is difficult to stop fermentation as sugar is still present. In commercial ciders, after aging, the cider is filtered and pasteurised to remove any remaining yeast, preventing further fermentation from occurring. On small scales, manufacturers can either wait several months after fermentation, to ensure the yeast is dead before adding sugar, or resort to other methods.

The chemistry behind cider is a fascinating interplay of various compounds in different environments. The transformation of apple juice into cider is not just magic, but a precise and intricate chemical process. Whether sweet or dry, still or sparkling, the chemistry of cider-making creates a beverage that is both delicious and deeply rooted in tradition.

References:

• Chemistry LibreTexts: Cider

• Chem Europe (2016): The Chemistry of Cider

• Compound Interest (2015): Compound Interest: The Chemistry of Cider