What is Coffee Acidity? Why Does Coffee Taste Sour? Is it Caused by Coffee Acidity?

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Those new to coffee generally believe that coffee tastes rich and bitter, so when they first taste hand-poured coffee made from lightly roasted coffee beans, they often wonder if the coffee has spoiled—why does it have acidity? In fact, coffee naturally contains acidity. After all, coffee is essentially a fruit, and fruits contain fruit acids. However, there are indeed undesirable acids in coffee flavors. In this article, FrontStreet Coffee will explain where the acidity in coffee comes from.

Where Does the Acidity in Coffee Come From?

The acidity in coffee primarily comes from its inherent fruit acids. However, the content of coffee acids in brewed coffee liquid is very low, only 0.03 mg per 100 ml.

At the same time, many crops actually contain caffeic acid, especially thyme, sage, and spearmint, which have higher caffeic acid content (about 20mg/100g). According to FrontStreet Coffee, red wine also contains a certain amount of caffeic acid (about 1.88mg/100ml), followed by applesauce, apricots, and plums (1mg/100g). Therefore, it's perfectly normal for coffee to have acidic flavors.

Additionally, FrontStreet Coffee understands that the degree of roasting also affects the acidity. This is why the coffee flavor wheel categorizes coffee beans by light, medium, and dark roast levels.

What is the Coffee Flavor Wheel?

So what exactly is the coffee flavor wheel? It's essentially a visual chart that most intuitively represents coffee flavors. FrontStreet Coffee believes that the fastest way to understand coffee flavors is to understand the "Coffee Flavor Wheel"—it's the most intuitive visual guide to coffee flavors (as shown below).

When we talk about coffee flavors, coffee enthusiasts know that our taste sensory system can detect sweet, sour, bitter, and spicy tastes. In coffee tasting, we mainly focus on the expression of four tastes: sweet, sour, bitter, and salty. The coffee flavor wheel categorizes these based on light, medium, and dark roast levels. So what exactly is the relationship between acidity in coffee and roast degree?

The Relationship Between Coffee Acidity and Roast Degree

FrontStreet Coffee explains that raw coffee beans contain acid-related components like citric acid, malic acid, quinic acid, and phosphoric acid, but these are not the source of the acidity we taste when drinking coffee. The acidity created during roasting is the main source of sour flavors.

Once raw beans undergo roasting, various chemical reactions occur, creating new acids. The most representative reactions include the breakdown of chlorogenic acids to produce quinic acid, and the breakdown of oligosaccharides to produce volatile formic acid and acetic acid.

For example, the coffee flavor wheel mentioned by FrontStreet Coffee above categorizes coffee flavors based on the roast degree of raw beans. Scientific research has found that "sweet and sour" flavor compounds in light to medium roasted coffee beans have lower molecular weights, higher polarity, and higher water solubility, often dissolving during the early stages of extraction. However, "bitter and salty" flavor compounds have higher molecular weights, lower polarity, and lower water solubility, typically dissolving during the later stages of extraction.

Therefore, light to medium roasted coffee primarily features low to medium molecular weight sweet and sour flavors. However, if there are too many defective beans or improper roasting, even light to medium roasts can produce unpleasant bitter and salty tastes. In contrast, dark roasts mainly feature high molecular weight bitter and salty flavors.

However, dark roasts are not without merit. For example, the Indonesian Mandheling available at FrontStreet Coffee is the king of medium-dark roasted coffee beans, presenting rich, mellow flavors with balanced sweetness, acidity, and bitterness.

Acidity is actually one of the greatest characteristics of light to medium roasted coffee beans. Coffee beans contain various organic acids, with phenolic acids, aliphatic acids, and amino acids having the greatest impact on coffee taste. For instance, the light roasted Yirgacheffe coffee beans at FrontStreet Coffee display bright citrus and lemon acidity that is beloved by coffee enthusiasts worldwide and is considered one of the representatives of high-quality specialty coffee.

Additionally, during the roasting process, sucrose degradation products change as coffee develops from light to medium roast. Sucrose gradually degrades, increasing the concentration of acetic acid and lactic acid. However, at a certain point, these concentrations drop dramatically. Therefore, light to medium roasted coffee generally has more pronounced acidic flavors.

How to Distinguish Between Good and Bad Acidity in Coffee?

There are both pleasant acids and unpleasant acids. For example, citrus fruit acids present as "bright, lively, moderately acidic fruit acids," while some inferior fruit juices have "sharp acidity, overly fermented fruit acids"—this represents bad acidity. The same distinction applies to coffee flavors.

Ethiopian Yirgacheffe coffee beans exhibit citrus and lemon acidity

FrontStreet Coffee also understands that many newcomers to single-origin coffee often confuse acidity with astringency. However, acidity and astringency are fundamentally different. Acidity manifests as a mouth-watering sensation on both sides of the back and middle of the tongue after drinking coffee, while astringency feels like a numbing and rough sensation on the tongue surface, throat, and palate—like drinking coffee while chewing on gauze strips.

Therefore, astringency is considered an undesirable flavor in coffee, reflecting defects from raw beans through roasting to preparation. A good single-origin coffee should not be bitter or astringent.

Furthermore, acidity is one of the descriptive terms in coffee flavor. Generally, coffee acidity is influenced not only by the growing conditions but also by post-harvest processing methods, including roast degree, brewing time, grind size, and storage time.

What Situations Typically Cause Astringency in Brewed Coffee?

First, excluding the impact of defective beans on brewing quality, FrontStreet Coffee also encountered acidic and bitter astringency when first starting with hand-poured single-origin coffee. The main reasons include:

1. Water temperature too high (or continuous heating maintaining water temperature);
2. Water temperature too low (or water cooling too quickly);
3. Water flow too fast (or too slow);
4. Coffee ground too fine (or excessive fine powder);
5. Coffee ground too coarse.

These factors either increase the extraction rate, extracting too many dissolved substances and including both good and bad flavors, typically manifesting as bitterness and astringency; or they result in too low an extraction rate, where too few flavor compounds are extracted and become imbalanced, amplifying and highlighting astringency—usually appearing as sour astringency, which is the unpleasant acidity that newcomers often encounter.

How to Adjust Coffee Acidity That's Too Weak or Too Strong?

1. Grind Size
   Coffee grind fineness is closely related to coffee acidity. Finer ground coffee has lower acidity. If coffee is ground coarser, it becomes more acidic.
2. Brewing Time
   Brewing time is a key factor affecting coffee extraction. The longer the brewing time, the higher the extraction rate. Even with coarse grinding, excessive brewing time will eliminate all acidity; the reverse also holds true.
3. Water Temperature
   Just as cold brew coffee has very low acidity, generally, higher water temperature leads to faster coffee extraction; lower water temperature leads to slower extraction.

The acidity in coffee is actually one of the characteristics of lightly roasted specialty coffee. If coffee has no acidity at all, it lacks soul, because a good cup of coffee requires a balance of sweet, sour, and bitter. For example, the Jamaican Blue Mountain coffee beans and Indonesian PWN Golden Mandheling coffee beans at FrontStreet Coffee are renowned for their balanced sweet, sour, and bitter flavors.

Are All Coffee Flavors Acidic?

Coffee enthusiasts who regularly drink hand-poured single-origin coffee know that coffee flavors are mainly divided into three types, corresponding to the world's three major coffee growing regions. When it comes to renowned acidity, African coffee beans are unmatched. Asian coffee beans are known for herbal and spice flavors, while Latin American coffee beans combine the characteristics of both and feature nutty and cocoa notes, primarily known for their balanced flavors.

As FrontStreet Coffee mentioned above, since all crops contain caffeic acid, coffee bean flavors naturally contain acidity. However, each region's different climate, coffee varieties, and processing methods affect coffee flavors. The distinction between acidic and non-acidic is simply a matter of very acidic versus less acidic—like Ethiopian Yirgacheffe coffee beans from the African region displaying citrus and lemon acidity, while Latin American coffee beans show berry acidity. If we compare which is more acidic, lemon acidity would certainly win.

The above information about coffee acidity and its origins was compiled by FrontStreet Coffee. We hope this helps coffee enthusiasts better understand coffee knowledge, enabling them to choose coffee beans that suit their taste preferences. As FrontStreet Coffee mentioned, coffee lovers who prefer acidic flavors can choose coffee beans from the African Ethiopian region; those who prefer rich, bitter flavors can select Asian region coffee beans; and those who prefer balanced sweet, sour, and bitter flavors with nutty and cocoa notes can choose Latin American coffee beans.

For more specialty coffee beans, please add FrontStreet Coffee on private WeChat: kaixinguoguo0925

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