Minerals and tannins. Can anyone truly claim they have never used these terms? That's how established minerals and tannins have become as tasting expressions in wine. However, few people can properly explain the specific meanings these expressions refer to.
Tannin is an indispensable expression in red wine tasting. To understand its specific content, a thorough understanding of phenols is absolutely essential. Even those in the position of making wine are often vague about phenols and tannins. Therefore, this article will provide a clear explanation of the knowledge necessary to understand phenols and tannins.
Tannins are Phenols
Before proceeding with the discussion of phenols and tannins in wine, there is a fundamental premise that must be understood first: tannins are phenols.
Phenols are a general term for organic compounds with a certain chemical structure. Tannins are compounds formed when several of these phenols bond together. In other words, when multiple phenols connect, they become tannins, and when tannins are broken down into smaller parts, they become phenols.
Some may wonder whether multiple phenols bonded together should be called polyphenols. That's correct. Organic compounds formed by multiple phenols bonding together are called polyphenols.
"Wait, didn't you just say that multiple phenols bonded together are tannins?" you might think. This is also correct. In other words, tannins are polyphenols. There are chocolates that contain polyphenols, and in some cases, these could be described as chocolates containing tannins.
The Difference Between Polyphenols and Tannins
However, tannins are not necessarily the same as polyphenols.
Polyphenols are compounds formed by multiple phenols bonding together. The key point here is what kinds of phenols are bonded together.
Chemically speaking, phenols are a general term for organic compounds consisting of a benzene ring with hydroxyl groups (-OH groups) attached. This may sound complex, but in simple terms, there are an enormous variety of phenols. Therefore, phenols are generally called phenolic compounds. They are not simple enough to be grouped under a singular term, but rather form one large collective group.
Each phenol has a different molecular size and different reactivity with other compounds. Therefore, the size of the resulting polyphenol varies completely depending on which phenols are bonded together. Incidentally, reactivity differs according to the number of hydroxyl groups attached to phenolic compounds—those with two OH groups are called dihydric phenols, and those with three OH groups are called trihydric phenols.
Tannins, on the other hand, refer to compounds with a certain minimum size, specifically those with molecular sizes equal to or larger than flavonoid dimers called procyanidin B. In rough terms, there is a large category called phenols, within which there is a category called polyphenols, and within that, there is a category called tannins.
Classification and Relationships of Polyphenols
Now we somewhat understand the differences between phenols, polyphenols, and tannins. It might seem like we've grasped everything about phenols. Unfortunately, the truly complex part starts here. Let's first focus on polyphenols.
We've already explained that polyphenols refer to compounds formed by multiple phenol molecules bonding together. What we need to know next is the classification within polyphenols.
First, polyphenols are classified into two types: flavonoids and non-flavonoids. This is determined by whether the molecular structure contains a chemical bonding form called the flavan skeleton. Those with this bond are flavonoids, and those without are non-flavonoids. Incidentally, both exist in grapes.
Flavonoids Beyond Anthocyanins
Flavonoids constitute one category, positioned as a medium-level classification. This means there are further subdivisions below this level. Among flavonoids related to wine, anthocyanins, the pigments that make red wine red, are well-known. However, other compounds are also included in flavonoids: flavanols and flavan-3-ols. Catechin, which is commonly heard of because it's found in green tea, along with epicatechin and epigallocatechin, are specific compounds belonging to this flavan-3-ol group.
When multiple flavan-3-ols such as catechins bond together, they are specifically called proanthocyanidins. These are also polyphenols, flavonoids, and tannins, but despite having names similar to anthocyanins, they are not pigments in the usual sense. Proanthocyanidins are named "pro"anthocyanidins because they produce compounds called anthocyanidins when their bonds are broken.
Incidentally, proanthocyanidin is also a classification name, so specific compounds belong within this category. This includes procyanidins that bond around catechin, but particular attention should be paid to prodelphinidins, which have epigallocatechin or gallocatechin as their structural units. Prodelphinidins are tannins found in grape skins but not in seeds.
Anthocyanins and Anthocyanidins
Wine, particularly red wine, is red because it contains large amounts of pigments called anthocyanins. Along with anthocyanins, it's useful to remember anthocyanidins.
As already mentioned, anthocyanidins are substances produced when proanthocyanidin bonds are broken. At the same time, they can be understood as the sugar-free state of compounds contained in the structure of anthocyanins. In this sense, anthocyanidins might be considered the precursors to anthocyanins.
Anthocyanins are compounds in which organic acids or sugars are bonded to anthocyanidins. Since most of them contain sugars in their structure, it wouldn't be a major mistake to think of anthocyanins as glycosides of anthocyanidins. Incidentally, since anthocyanidins have no sugar bonds, many compounds showing strong antioxidant activity are also characteristic. Cyanidin and malvidin are representative anthocyanidins.
Sugar Numbers and Grape Varieties
While anthocyanins are formed by sugars attaching to anthocyanidins, the number of bonded sugars is not limited to one. They are classified according to the number of bonded sugars: those with only one sugar bond are called monoglucoside anthocyanins, and those with two bonded sugars are called diglucoside anthocyanins.
Monoglucoside anthocyanins are detected only in grapes belonging to Vitis vinifera species, i.e., European grape varieties. In contrast, diglucoside anthocyanins may also be detected in vinifera species, but are particularly abundant in Vitis riparia and Vitis rupestris species, i.e., American grape varieties.
When European and American grapes are crossed to develop new disease-resistant varieties, the first generation shows both monoglucoside and diglucoside anthocyanins. However, in varieties that have undergone repeated crossings enough times to be newly registered as Piwi varieties, only monoglucoside anthocyanins are detected, while diglucoside anthocyanins are not detected. This is thought to be because genes producing diglucoside anthocyanins are recessive genes that are eliminated during multiple crossings.
Non-flavonoids and Types of Tannins
We've seen that within flavonoids, multiple flavan-3-ols bond to form proanthocyanidins, these proanthocyanidins break apart to produce anthocyanidins, and when sugars or acids bond to these anthocyanidins, they become anthocyanins. So what kind of process occurs in the other wing of polyphenols, non-flavonoids?
Non-flavonoids include substances that are probably not generally recognized as phenolic compounds: acids.
Cinnamic acid, caffeic acid, ellagic acid, and gallic acid are acids with structures classified as phenols, and they are polyphenols and also tannins. These are called phenolic acids. You might think this makes no sense, but you'll have to accept it. Of course, not all organic acids are phenolic compounds. For example, tartaric acid and malic acid are not included in phenolic compounds.
Among non-flavonoids, gallic acid holds an important position. Gallic acid is the basic structural component of non-flavonoid tannins and is released when non-flavonoid tannins decompose, while also being contained in grapes itself.
Compounds formed by multiple gallic acid or ellagic acid molecules bonding together are called ellagitannins, and those formed by multiple gallic acid molecules bonding with additional glucose are called gallotannins. While flavonoid tannins were called by names like proanthocyanidins and anthocyanidins, non-flavonoid compounds have the term "tannin" in their names themselves.
Condensed Tannins and Hydrolyzable Tannins
Since both flavonoids and non-flavonoids contain substances corresponding to tannins, different names have been assigned to distinguish between these two types: condensed tannins and hydrolyzable tannins.
Tannins belonging to flavonoids are condensed tannins. This is the name for compounds formed by multiple flavonoid compounds bonding together, but generally refers to proanthocyanidins. Therefore, procyanidins, prodelphinidins, and profisetinidins classified as proanthocyanidins are all condensed tannins. These are also tannins derived from grapes.
On the other hand, tannins belonging to non-flavonoids are called hydrolyzable tannins. As the name suggests, these are tannins that are easily hydrolyzed. While derived from oak, chestnut, and tara, they are tannins not contained in grapes. It's interesting that while gallic acid itself, the basic structural component of hydrolyzable tannins, is contained in grapes, its polymers—hydrolyzable tannins—are not derived from grapes.
Incidentally, since hydrolyzable tannins can be supplied to wine through oak, they tend to be detected in large quantities in wine, particularly red wine.
The Relationship Between Phenolic Compounds and Grapes
Now that we understand how phenolic compounds are classified, let's proceed to discuss the relationship between these phenolic compounds and grapes, and subsequently wine.
In grapes, phenolic compounds accumulate in three main locations: skins, seeds, and stems. Of these, the phenols or tannins contained in skins and seeds are most likely to enter wine.
Even within the same grape, the phenols contained in skins and those contained in seeds differ somewhat in character. Incidentally, both contain condensed tannins, as you should already understand.
The most notable difference between skins and seeds is the presence or absence of anthocyanins. Anthocyanins, being pigments, accumulate mainly in grape skins. While you might think anthocyanins are most abundant because color is prominent in skins, both skins and seeds primarily contain proanthocyanidins. Let's first establish that proanthocyanidins—that is, condensed tannins—are more abundant than anthocyanins.
Grape Skins Have More Complex Tannin Compositions
While both skins and seeds contain large amounts of condensed tannins, what distinguishes these two is the difference in types of condensed tannins.
As mentioned repeatedly, proanthocyanidins are merely a general term for compounds formed by multiple flavan-3-ols such as catechins bonding together. Since flavan-3-ols include multiple substances like catechin, epicatechin, and epigallocatechin, the resulting compound varies depending on which substances within flavan-3-ols are bonded.
For example, those with catechin as the structural unit are called procyanidins, and those with epigallocatechin or gallocatechin as structural units are called prodelphinidins. Incidentally, to describe prodelphinidins more chemically, they are proanthocyanidins containing pyrogallol groups in their structure. The point is that while they're all proanthocyanidins, their names and properties change slightly based on differences in what they contain.
Regarding the crucial relationship with grapes: phenols contained in seeds consist of tannins other than those containing epigallocatechin-type structures, while phenols contained in skins include tannins containing epigallocatechin-type structures as well. To put this very simply, seeds contain only procyanidins, while skins contain both procyanidins and prodelphinidins. In essence, skins contain a greater variety of accumulated phenols.
Astringency, Tannins, and Their Origins
Looking more closely at the differences between the two, phenols contained in skins have high mean degrees of polymerization (called mDP) and low proportions of galloylated subunits, while seeds conversely have low mDP and high proportions of galloylated subunits. Additionally, total content is higher in seeds than in skins for both white and black grapes, with approximately twice the difference.
Incidentally, an unavoidable topic when discussing tannins is the astringent sensation felt in wine—what is called astringency.
The astringency felt in wine is caused by tannins binding with proteins contained in saliva and removing them from the saliva. The key point is the reactivity between proteins and tannins. Higher reactivity produces stronger astringency, while lower reactivity reduces the sensation of astringency. What's involved here is something called the mean degree of polymerization.
While this explanation may be somewhat questionable from a strictly chemical standpoint, to put it very roughly, tannins with lower mean degrees of polymerization tend to have higher reactivity with proteins. There is a difference in the mean degree of polymerization of tannins contained in skins and seeds, with seeds having relatively lower values. In other words, tannins contained in seeds have higher reactivity with proteins in saliva and are more likely to produce stronger astringency.
Since mean degrees of polymerization and galloylated subunits are not particularly necessary to know at the level of this article, which aims to understand phenols, I won't explain them in detail. Here, it's sufficient if the following points are clear:
- Both skins and seeds primarily contain condensed tannins
- Even among the same condensed tannins, skins have more variety, but seeds have higher content
- The difference in phenolic composition between skins and seeds is mainly the presence or absence of anthocyanins and prodelphinidins
- Skins and seeds have different effects on wine taste and other characteristics
Tannins in Grapes vs. Tannins in Wine
Now that we understand the types of phenols contained in grapes, let's consider the phenols contained in wine. Since wine is made from grapes, one might think that phenols contained in grapes and those contained in wine are the same. However, this is not necessarily the case, as those who have read this far should understand.
First, wine often contains tannins not found in grapes: hydrolyzable tannins.
Hydrolyzable tannins—that is, non-flavonoid polyphenols. While within the same non-flavonoid polyphenol framework, substances like gallic acid may be contained in grapes, hydrolyzable tannins are not contained in grapes. However, they are contained in wine. In some cases, they may even constitute a dominant amount among the phenols contained in wine. The reason is the use of wooden barrels and similar materials.
While hydrolyzable tannins are not contained in grapes, they are abundantly contained in certain plants like oak. When wine comes into contact with substances rich in such hydrolyzable tannins, the tannins dissolve into the wine. Hydrolyzable tannins are particularly soluble in alcohol and acid, so wine, which contains both, is an ideal liquid for extracting hydrolyzable tannins from oak barrels.
Additionally, past studies have reported interesting results. There was a study examining the relationship between phenol content in grapes and phenol content in wines made from those grapes.
One would normally expect that grapes containing more phenols would produce wines with more phenols. However, in that case, the wine made from grapes with the lowest phenol content contained the most phenols. Naturally, the ratio of types of phenolic compounds contained in grapes differed from that contained in wine. This result clearly shows that the state of phenols contained in grapes does not transfer directly to wine. Conversely, it means that the state of tannins contained in grapes cannot be accurately predicted based on the tannins contained in wine.
Summary: Phenols Can Be Understood by Knowing Terminological Relationships
To truly understand phenols, it's necessary to delve further into aspects not covered in this article, including their chemical effects and significance on wine. The same applies to the biosynthesis of phenolic compounds in grapes, tannin extractability, and the existence of oenological tannins. However, such knowledge is not particularly important for wine appreciation. What's far more important is first understanding what phenols are and what tannins—often used carelessly in tasting comments—actually are.
This article has explained what substances are actually grouped under phenols or tannins and how they relate to each other.
To summarize simply: tannins and polyphenols are essentially the same thing. Both are names for compounds formed by several components called phenols bonding together. Both tannins and polyphenols are broadly classified into two types—for tannins, condensed tannins and hydrolyzable tannins; for polyphenols, flavonoid and non-flavonoid types, with names further divided accordingly. Condensed tannins are flavonoid polyphenols, but more specifically, they refer to proanthocyanidins. Hydrolyzable tannins are various non-flavonoid polyphenols bonded together.
Regarding wine relationships, grape-derived tannins are condensed tannins. They're contained in both skins and seeds. Skins have more variety than seeds, but seeds have higher quantities. Since skins and seeds contain different types of phenolic compounds, their effects on wine taste and other characteristics also differ. To give a clear example, those derived from seeds tend to produce stronger astringency. However, the state of phenols contained in grapes doesn't directly become the state of phenols contained in wine. Furthermore, wine may contain tannins not found in grapes through winemaking processes such as using wooden barrels, chips, or oenological tannins.
Once you firmly understand up to this point, you shouldn't get lost when people talk about phenols, tannins, or astringency while drinking wine. The question "What exactly are tannins?" should also be thoroughly resolved. The expression "tannin" is actually an extremely rough expression—if phenols among living things are like primates, then tannins are like humans within that group; that's about the level of meaning it has.
More detailed explanations of phenols and tannins in winemaking will be covered in separate articles. First, use this article to understand the basics of phenols and tannins, and if you become more interested, please read those additional articles.
For detailed information on grape physiology and its relationship with phenols and tannins, see: https://note.com/nagiswine/n/ne5ac6fbeaf3d
