Have you ever found a wine to taste "bitter"? How about sake—have you ever experienced bitterness when drinking it?
When drinking alcoholic beverages, we occasionally encounter bitterness. Have you ever wondered why this happens? What would you think if told that "wines made from more mature grapes might actually be more likely to taste bitter"?
The sensation we call "bitterness" actually encompasses many different types. The "bitterness" you taste in coffee and the "bitterness" you experience in beer are both bitter sensations, yet they are fundamentally different types. Coffee's bitterness comes from caffeine, while beer's bitterness primarily derives from humulones, a type of α-acid, particularly a compound called isohumulone.
So what about wine's bitterness?
The bitterness we perceive in wine is sometimes linked to the ripeness of the grapes used as raw material. The common assumption is that using unripe grapes makes wine more bitter. However, when we trace the true nature of wine's bitterness, we find almost no correlation with low grape ripeness. At the very least, using fully ripe grapes does not guarantee a wine free from bitterness. In fact, it might even increase the likelihood of perceiving bitterness.
The Impact of Bitterness on Wine
Bitterness is generally classified as an undesirable flavor in wine. This is particularly true for white wines, where bitter wines are often strongly rejected by consumers. Naturally, winemakers want to avoid including this flavor in their wines as much as possible.
How can we prevent bitterness in wine? When we taste bitterness in wine, how should we evaluate it? What is bitterness, fundamentally? Let us examine the nature of bitterness in wine.
What is Bitterness?
What kind of taste is bitterness, exactly? This appears to be a simple question, but do you realize it's actually quite difficult to answer?
Try to recall when you've experienced bitterness and how it felt.
Bitterness as a taste is often associated with what we call astringency or puckering sensations. Astringency and puckering are sensations, not tastes. Bitterness, on the other hand, is a taste. While it might seem surprising that these two are connected, weak bitterness is often perceived as astringency. Consider coffee's bitterness—when you really focus on the taste, doesn't it feel somewhat astringent as well?
This means that compounds that produce bitterness simultaneously possess astringency, and conversely, astringent compounds are also bitter compounds. Take astringent persimmons, for instance—while clearly astringent, they can also be perceived as bitter under certain circumstances.
The Complexity of Bitter Taste Receptors
Humans are said to possess as many as 26 different bitter taste receptors. This means we can perceive 26 different types of bitterness. Moreover, human sensitivity to bitterness is reportedly 10,000 times greater than our sensitivity to sweetness. As they say, we become more sensitive to things we dislike—and humans are indeed extremely sensitive to bitterness. This is also why we can classify and consider bitter compounds by their different types.
Many substances contain multiple bitter compounds simultaneously. For example, when sake tastes "bitter," it contains at least eight different types of substances, including multiple amino acids such as kynurenic acid and compounds called bitter peptides.
Bitterness is, in reality, a "highly complex taste."
The Origin of Bitterness
When we ask where bitterness comes from, the answer in most cases is the raw materials. Looking specifically at alcoholic beverages, beer's bitterness derives from hops, sake's from rice, and wine's from grapes.
While beer uses multiple raw materials, both sake and wine principally use only a single raw material. When we compare the bitterness in sake and wine, we can see that their characteristics differ somewhat. Sake's bitterness centers on amino acid-derived compounds, while wine's bitterness is primarily based on phenolic compounds. However, wine also contains amino acid-derived bitterness, whereas sake basically contains no phenol-based bitterness. As a collection of bitter compounds, wine is more complex.
This difference stems from the raw materials used. While sake uses no ingredients rich in phenolic compounds, wine uses grapes that contain large amounts of phenolic compounds and small amounts of protein, plus proteins derived from the yeast used in fermentation and the amino acids that result from them. Humans perceive different types of bitterness from each of these sources.
Wine's Primary Bitter Components: Flavonoids
The bitterness we perceive in wine primarily comes from a specific type of phenolic compound called flavonoids. Wine certainly contains amino acids as well, which contribute their own bitter flavors, but because flavonoids are present in overwhelming quantities, most of the bitterness we taste derives from these compounds.
What are flavonoids? They include anthocyanins, which give red wine its color, and tannins. In other words, these are compounds found in much higher concentrations in red wines than in white wines.
Remember this: bitterness can also manifest as astringency and puckering sensations. The tannins in wine are primarily known as the cause of astringency. But tannins are also bitter compounds. This should help you understand that bitter components are also astringent components.
How Flavonoid Structure Affects Taste
Whether phenolic compounds taste bitter or astringent depends on their type and state. Generally speaking, in what are called monomeric flavonoids—where flavonoids remain as small units not bound to other flavonoids—we strongly perceive bitterness. Conversely, with polymeric flavonoids—where flavonoids have bound with other flavonoids to become larger—we perceive astringency more than bitterness.
Tannins refer to phenolic compounds like flavonoids that have bound together to become larger. This is why we perceive tannins as astringent rather than bitter.
We know that oxidatively polymerized phenols show reduced bitterness. The fact that even similarly polymerized phenolic compounds affect taste differently depending on their bonding state is a very interesting point.
Anthocyanins are also classified as flavonoids. However, we know that these compounds have almost no effect on either bitterness or astringency. This means that deeply colored red wines are not necessarily bitter or astringent.
How Alcohol Enhances Bitterness
There is another reason why alcoholic beverages tend to taste bitter: the presence of alcohol, or more precisely, ethanol.
The reason alcoholic beverages are alcoholic is that they contain alcohol. Alcohol is a component that can never be completely removed from alcoholic drinks. However, as long as alcohol is present, we know that humans become more likely to perceive bitterness in these beverages.
When wine has a high alcohol content, the alcohol itself can sometimes be perceived as "sweet," though some literature describes alcohol itself as "bitter." There are still few studies verifying alcohol's inherent taste, so whether alcohol is sweet or bitter remains unclear.
However, it has been proven that when alcohol is added to beverages containing bitter compounds, the bitterness clearly increases. The key points here are:
- Alcohol's effect is greater than that of the bitter compounds themselves
- The degree of effect is proportional to alcohol concentration
The effect of adding alcohol to beverages containing bitter compounds is far greater than directly adding more bitter compounds instead of alcohol. Conversely, when the same amount of bitter compounds is added to water versus alcohol, the alcohol solution will taste more bitter. This effect is proportional to alcohol concentration. When the same amount of bitter compounds is present, higher alcohol content results in stronger and longer-lasting bitter perception.
Conclusion: Do Riper Grapes Make Wine More Bitter?
The primary cause of bitterness when drinking wine, especially red wine, is phenolic compounds called flavonoids. In white wines, which contain fewer phenolic compounds, amino acids and some inorganic compounds are thought to contribute more strongly to bitterness, similar to sake. And what enhances all these bitter compounds is alcohol.
When we taste bitterness in wine, we want to think it's because the grapes were unripe. However, upon reflection, this doesn't seem necessarily correct. Looking at conditions alone, higher grape ripeness might actually increase the likelihood of perceiving bitterness.
What significantly affects wine's bitterness is not so much the bitter compounds themselves, but rather the alcohol concentration. Alcohol concentration is usually higher in wines made from riper grapes. Following this logic, wines made from riper grapes are more likely to taste bitter.
Even regarding the content of phenolic compounds, which are bitter substances, lower ripeness doesn't necessarily mean higher content. It's true that when grapes become riper, the ratio of phenolic compounds in the fruit tends to shift toward more tannins. In this sense, we can say that riper grapes tend to produce stronger astringency rather than bitterness. However, this doesn't mean that wine becomes free of bitterness.
The same applies to white wines.
In white wines, where grapes don't contain abundant phenolic compounds, bitterness is thought to derive mainly from amino acids. In this case, what becomes important is the yeast used for alcoholic fermentation, as yeast serves as a major source of amino acids in wine.
The amount of amino acids supplied by yeast depends on the quantity of yeast and the yeast's autolysis tendency—that is, the rate at which yeast breaks down. This is where alcohol concentration becomes relevant. Higher alcohol concentrations generally lead to higher rates of yeast autolysis.
That wines made from riper, sweeter grapes might be more likely to taste bitter is a surprising result when considered normally. However, when we consider specific winemaking techniques alongside this, we might realize this isn't so unexpected after all.
We will examine such more specific points in circle or magazine articles. If you're interested, please consider joining our circle or subscribing to our magazine.
