Why Some Coffee is Acidic: Understanding Coffee Acidity and Its Origins

Why Is Coffee Acidic?

Beginners exploring single-origin coffee are often amazed by its acidity. Most people perceive coffee as rich and bitter, so where does this acidity come from? This time, FrontStreet Coffee will explain why coffee is acidic.

Coffee is actually a fruit, and what we commonly call coffee beans are precisely the seeds (pits) of this fruit. When the fruit ripens, there's a layer of pectin around the seed that's quite sweet. Coffee beans continuously absorb substances from this pectin, which is an important source of the fruity sweet and sour flavors in coffee. There are many acidic compounds in coffee beans. Scientists have currently extracted over a hundred different acidic compounds from coffee, with chlorogenic acid, quinic acid, malic acid, citric acid, acetic acid, and lactic acid being the most common.

Chlorogenic Acid

Its astringent and unpleasant taste helps with insect resistance. In his book "Coffee Science," Huaizong Han describes how Robusta, growing at low altitudes with severe pest problems, contains much more chlorogenic acid than Arabica beans. This is one reason why Robusta coffee beans are inferior in flavor to Arabica beans.

Quinic Acid

Formed from the degradation of chlorogenic acid, it's a type of phenolic acid. Being non-volatile, it cannot be detected by smell and tastes bitter. After roasting, chlorogenic acid continues to degrade into quinic acid. However, opinions about quinic acid remain controversial. Supporters believe that quinic acid dissolves in water, increasing coffee's body, complexity, and brightness. Critics argue that quinic acid is the culprit behind the bitter taste of dark roast coffee.

Citric Acid and Malic Acid

Both are products of the coffee tree's metabolism. Neither is volatile, so they cannot be detected by smell but require the sense of taste to perceive their acidity.

Acetic Acid and Lactic Acid

These are not metabolic products of the coffee beans themselves. Raw beans contain almost no trace of these substances, which mainly come from two sources: derivatives from the washed fermentation process and products of sucrose during roasting. Excessive levels of acetic acid and lactic acid can cause coffee to develop unpleasant sour and rotten flavors.

The origin of these acids is largely related to the soil where the coffee is grown or the humidity of the climate.

The Relationship with Soil

Volcanic rock regions tend to produce coffee with excellent body and fruity acidity. Coffee from volcanic rock soil has the most balanced flavors of acidity, aroma, sweetness, purity, and bitterness. FrontStreet Coffee believes that Guatemalan Huehuetenango coffee best exemplifies the flavor characteristics of volcanic rock regions.

Kenyan soil contains high concentrations of phosphoric acid, giving coffee beans from this region rich fruit acidity and a berry juice-like mouthfeel.

The Relationship with Climate and Humidity

Research has found that coffee from high-humidity regions tends to produce coffee with intense fruit flavors. High humidity can increase the concentration of malic acid in coffee pulp, allowing fruit aroma and acidity to be fully absorbed by the coffee beans. FrontStreet Coffee believes that coffee beans from tropical rainforest climate regions have significantly more prominent fruit aroma and sweet-sour flavors than other regions, with relatively lower body.

After coffee beans are harvested, the processing method of the raw beans also affects the coffee's acidity, especially the washed processing method.

The washed processing method involves an 18-36 hour fermentation process, which makes the acidity of coffee beans more prominent, particularly in citric and malic acid. FrontStreet Coffee believes that washed processing results in elevated acidity, high complexity, high cleanliness (no negative flavors), and a crisp mouthfeel.

Roast Level Also Affects Coffee's Acidity Expression

During the coffee roasting process, sugars and organic acids in the beans begin to degrade or decompose into acids such as formic acid, acetic acid, lactic acid, fumaric acid, and quinic acid. However, as the roast level deepens, these acids continue to degrade or evaporate. The most dramatic changes in acidity and acid concentration occur during the development stage after the first crack begins. This suggests that the acidity in coffee reaches its peak at some point after the first crack and then decreases due to the thermal decomposition or evaporation of acids.

The Impact of Brewing Parameters on Acidity Expression

1. Grind Size

Coarser grinding reduces extraction rate because the coffee grounds have a smaller surface area (note that extraction rate differs from brewing time - extraction rate refers to the extraction of flavor and aromatic compounds, while brewing time refers to the duration of contact between water and coffee grounds).

This means coarser grinding brings out bright acidity, but brewing for too long can lead to woody flavors. If you want more acidity, use a coarser grind. If you want less acidity, you can grind a bit finer.

2. Brewing Time

Another major factor affecting extraction is brewing time. The longer the brewing time, the more substances will be extracted. You can use your favorite beans, quality water, and appropriate grind size to brew.

3. Water Temperature

Water temperature is also an important factor affecting extraction rate. The higher the water temperature, the faster substances are extracted. The lower the water temperature, the slower the aroma and flavors are extracted. Simply put, low-temperature extraction makes coffee's acidity more apparent, but can easily lead to under-extraction and flat flavors. High-temperature extraction brings out coffee's body, but can easily lead to over-extraction with impurity flavors.

FrontStreet Coffee recommends using water at 90-91°C for medium-light roast coffee beans and 88-89°C for medium-dark roast coffee beans. Appropriate water temperature with good extraction yields positive and comfortable aromas and flavors - for example, rich acidity like plum or stone fruit sweet-sour sensations, overall cleanliness, clarity, and transparency that allow flavors to be clearly distinguished, delicate and rich prominent acidity that can evoke associations with certain fruits or even wine, and a persistent aftertaste that leaves a memorable impression.

Finally, FrontStreet Coffee will brew Ethiopian Washed Gedeb, which has prominent acidity, to demonstrate how to brew high-acidity coffee beans.

Coffee Information:

• Region: Yirgacheffe, Gedeo Zone
• Altitude: 1900-2300m
• Variety: Heirloom
• Processing: Washed
• Grade: G1

To highlight the acidity of washed Yirgacheffe Gedeb, FrontStreet Coffee will use V60 brewing. The V60 conical dripper has a wider opening, and its unique spiral ribs allow air to escape more easily, improving extraction quality. The mouthfeel might not be as full-bodied, but its high concentration brings out sweet-sour balance and prominent aroma as its distinctive characteristics.

FrontStreet Coffee Brewing Parameters

• Water Temperature: 90-91°C
• Grind Size: BG#6m (fine sugar size)
• Coffee-to-Water Ratio: 1:15
• Coffee Amount: 15 grams

FrontStreet Coffee Brewing Method: First pour 30g of water for a 30-second bloom, then pour 95g (scale shows around 125g), finishing in about 1 minute. When the water level drops to 2/3 of the coffee bed, pour the remaining 100g (scale shows around 225g), finishing in about 1 minute and 40 seconds. The drip completes between 1 minute 50 seconds to 2 minutes. Remove the dripper to complete extraction.

Brewing Flavor: Bright citrus acidity on entry, full-bodied berry juice sensation, with distinct oolong tea notes.

Important Notice :

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FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473