Why Do Some Teas Taste Astringent? Exploring the Causes and Mechanisms of Astringency | HOJO Online Speciality Tea Shop

Tea can range from having no noticeable astringency to possessing a very strong one. What causes this astringency? This article explores the causes and mechanisms behind astringency in tea.

Causes of Astringency

Astringency arises from the binding of tea components to proteins in the oral cavity, creating a sensation of tightness or dryness. The tongue and mucous membranes contain proteins, which interact with certain tea-derived compounds, resulting in the perception of astringency.

There are two main causes of astringency in tea: tannins and metal ions. Both interact with oral proteins to produce astringency. The variation in astringency across different teas is due to the diverse range of compounds involved.

Tannins: A Group of Polyphenols with Protein-Binding Properties

When asked about the astringent components in tea or wine, many people immediately think of tannins. However, few fully understand what tannins are and how they are defined.

Tannins are often associated with the color brown. Yet, white wine and green tea are not brown. Does this mean that catechins and other polyphenols in tea are considered tannins? Similarly, are the polyphenols in white wine equivalent to tannins?

Contrary to common belief, tannins are not a specific substance but a collective term for a subset of polyphenols that bind to proteins. While some compounds may induce astringency through protein binding, they are not classified as tannins if they are not polyphenols. Likewise, polyphenols that do not bind to proteins, such as catechins, are not considered tannins.

Tannins in tea can be categorized into two types:

Catechin gallates: Derived from the tea leaves themselves.
Thearubigins: Formed during the tea production process through fermentation or oxidation.

Each type originates from different processes and conditions, contributing to the unique characteristics of astringency in tea.

Tannins Derived from Tea Leaves

Tea leaves harvested in spring are believed to contain fewer polyphenols with protein-binding properties. As a result, first flush spring tea is characterized by weaker astringency and, in other words, lower tannin content.

Catechin, one of the primary components of tea, binds minimally to proteins and, therefore, is not defined as a tannin. On the other hand, epicatechin gallate (ECG) and epigallocatechin gallate (EGCG) contain a gallate group (gallic acid), which enables them to bind strongly to proteins. This is why catechin gallates are defined as tannins, and teas rich in these compounds exhibit stronger astringency.

While ECG and EGCG are present in small amounts in spring tea, their levels are 1.5 to 2 times higher in summer-harvested teas. Consequently, summer teas and bancha tend to have a stronger astringency.

Tannins Arising from Improper Enzymatic Oxidation in Tea Production

During the production of teas such as white tea, black tea, and oolong tea, if the enzymatic oxidation process (known as fermentation in the tea industry) is not halted at the appropriate time, the oxidative enzyme polyphenol oxidase remains active, allowing oxidation to continue. As a result, polyphenols such as catechins in the tea leaves undergo excessive oxidation and polymerization, leading to the formation of complex high-molecular-weight polyphenols known as thearubigins. This condition is commonly referred to as “over-fermentation.”

In simple terms, it is the result of failed enzymatic control, causing an uncontrolled oxidation reaction and resulting in pronounced astringency. Thearubigins are a type of polyphenol that possesses protein-binding properties, and therefore, they are defined as tannins.

On the other hand, properly produced black tea contains theaflavins, which are formed during the enzymatic oxidation process. Theaflavins are yellow pigments that contribute to the vibrant color and balanced flavor of black tea. Although many books and online sources refer to theaflavins as tannins, they do not have protein-binding properties and, therefore, are not strictly classified as tannins.

Astringency Caused by Metal Ions

The astringency of tea is not only attributed to tannins but also to metal ions. Metal ions such as aluminum and magnesium bind to proteins in the oral cavity, creating a sensation of astringency (tightness). However, since these are not polyphenols, they are not classified as tannins.

Especially in oxidized teas, such as oxidized matcha or gyokuro, strong astringency can sometimes be perceived. This occurs because when chlorophyll in the tea is oxidatively degraded, the magnesium ions at the center of the chlorophyll molecule are released. These magnesium ions bind to proteins in the oral cavity, producing a sensation of astringency. Since matcha and gyokuro are shade-grown and contain higher levels of chlorophyll, oxidation releases more magnesium ions compared to other teas, resulting in a stronger astringency. For more details, please refer to the article below.
https://hojotea.com/en/posts-225/

In addition, drinking canned beer or beverages can cause astringency. This is due to aluminum ions leaching from the can and binding to proteins in the oral cavity, resulting in a sensation of tightness. Similarly, if teapots or kettles are made from materials containing specific metals (such as copper, aluminum, zinc, magnesium, or manganese), the tea can develop astringency. The belief that tea brewed in ceramic teapots is always milder is not necessarily accurate; the type of metals present in the ceramic material plays a significant role.