1. What are citric acid and malic acid?

Citric acid: Associated with bright acidity

Citric acid is a tricarboxylic organic acid with the molecular formula C₆H₈O₇. This compound occurs naturally in many fruits and participates in energy metabolism pathways in plant cells.

In coffee, citric acid can contribute to a bright acidic sensation, evoking associations with oranges, lemons, or other citrus fruits. However, the citrus aroma is not produced by citric acid alone. The fruity perception arises from the combination of acidity, volatile aromatic compounds, and other components in the brew.

Malic acid: Associated with apple-like acidity

Malic acid, with the formula C₄H₆O₅, is a dicarboxylic organic acid involved in plant carbon metabolism. Malic acid is often associated with an acidic sensation reminiscent of green apples, pears, and some fruits with a clean tartness.

In coffee sensory evaluation, malic acid does not automatically create sweetness or mild acidity. The actual perception also depends on concentration, degree of ionization, buffering capacity of the brew, drinking temperature, and interaction with other compounds.

Therefore, when noting that a cup of coffee has green apple or citrus-like acidity, tasters are describing a sensory experience, not determining acid content solely by taste.

2. How do altitude and cultivation temperature affect acids?

During fruit development, ambient temperature and growth conditions influence ripening speed, carbohydrate metabolism, and the activity of organic metabolic pathways. These changes can alter the acid composition in the coffee fruit and beans.

Temperature and fruit ripening time

In some cooler climate growing regions, coffee cherries may ripen more slowly. Extended development time can alter the accumulation of sugars, organic acids, and flavor precursors. However, slow ripening does not mean all acids increase.

Acid levels depend on the balance between synthesis, metabolism, and degradation during fruit development. Thus, two lots of the same variety grown under different temperatures, rainfall, shade levels, or nutrition regimes may have different acid compositions.

Altitude is not the sole determining factor

Altitude is often associated with temperature, diurnal temperature variation, and ecological conditions. However, altitude also interacts with variety, slope aspect, light exposure, water availability, and processing methods.

A study on coffee grown at different altitudes in Pu'er, China, noted that organic acid composition did not show a consistent increasing or decreasing trend with altitude. Another study on fermented coffee in Brazil showed that citric and malic acid levels could vary between different altitudes and processing conditions.

This indicates that one should not use the simplistic rule that higher-altitude coffee always contains more citric and malic acid. Altitude is an influential ecological variable, but it must be considered alongside other factors and actual analytical data.

The role of ripeness and processing

Harvesting unevenly ripe cherries can increase raw material heterogeneity. During processing, enzyme and microbial activity can also alter acid composition, particularly when coffee undergoes fermentation.

Therefore, when evaluating a coffee lot with prominent acidity, it is necessary to simultaneously consider the variety, growing conditions, ripeness at harvest, and processing method. One should not attribute all acidic characteristics solely to temperature or altitude.

3. How do citric and malic acids change during the roasting process?

Coffee roasting is a complex thermal transformation process involving water loss, Maillard reactions, thermal decomposition, carbohydrate changes, and interactions among various compounds within the bean. The post-roast acid composition results from multiple simultaneous reactions, not just a single temperature threshold.

Malic acid and the limitations of the melting point concept

Pure malic acid is commonly described as having a melting point of approximately 130°C, depending on crystal form and measurement conditions. However, this figure does not mean that malic acid in coffee beans begins to decompose immediately when the bean temperature probe reads 130°C.

In coffee beans, acids exist within a complex material matrix containing water, carbohydrates, minerals, and numerous organic compounds. The temperature measured by the probe also does not fully represent the temperature at every location within each bean.

Therefore, the transformation of malic acid must be understood in terms of the roast batch's thermal history, exposure time, moisture state, and chemical environment. To accurately determine the level of degradation, chemical analysis of samples before and after roasting using appropriate methods is required.

Citric acid and changes according to roast level

Citric acid can also transform during the roasting process. However, the degree of reduction is not necessarily the same across all varieties, origins, and roast profiles. Thermal reactions may reduce one compound while simultaneously creating or relatively increasing other compounds within the same system.

Therefore, one should not assert that citric acid always decreases by exactly 50% at a medium roast level without specifying the study, coffee type, analytical method, and roasting conditions that produced that figure.

A study on acids in brewed coffee at different roast levels noted that the concentration of individual acids can vary with roast level, while the average total organic acid concentration does not necessarily decrease correspondingly. This emphasizes that changes must be considered at the level of individual compounds and within the entire chemical system.

4. Why do dark roasts often taste less acidic?

The sour sensation in a cup of coffee depends not only on the content of citric and malic acids. As the roast level increases, many other compounds also transform, including chlorogenic acids, their decomposition products, bitter-tasting compounds, Maillard reaction products, and aroma compounds.

Dark roasting significantly alters the chemical structure and sensory properties of coffee. Bitterness, roasted aromas, and body may become more prominent, making the perceived acidity less distinct in the overall cup.

However, it cannot be concluded that all acids decrease at the same rate. Some acids decrease, while others may form or increase relatively due to other chemical reactions. The final result also depends on the raw material and roasting method.

Distinguishing acid concentration from perceived acidity

Acid concentration is a chemical quantity that can be quantified by analytical methods. Acidity in sensory evaluation is the drinking experience, including the intensity of sourness, the quality of the sour taste, and how it harmonizes with sweetness, bitterness, and body.

pH reflects hydrogen ion activity in a solution, while titratable acidity reflects the amount of base required to neutralize the acid to a defined endpoint. These two indicators are not the same and cannot completely replace sensory evaluation.

Two cups of coffee with similar pH levels can still provide different sour perceptions due to differences in acid composition, buffering capacity, sweetening compounds, bitter compounds, and serving temperature.

5. How roasters control acidity and fruit flavors

Starting with green bean quality

Recording the variety, growing region, ripeness, processing method, moisture content, and storage conditions helps the roaster understand the differences between raw material lots. One should not expect a single profile to produce the same result across all coffee lots.

Developing targeted roast profiles

It is possible to experiment with different profiles on the same coffee lot to evaluate the impact of heating rate, development time after first crack, and roast degree. Changes should be controlled, data recorded, and chemical mechanisms should not be inferred solely from the temperature curve.

Cupping under consistent conditions

Evaluate the samples using the same coffee-to-water ratio, grind method, cupping time, and serving temperature. Record acidity intensity, acidity quality, sweetness, body, aftertaste, and balance separately.

If determining the actual citric or malic acid content is required, appropriate chemical analysis methods must be used. Cupping helps describe sensory attributes but cannot replace quantitative measurement of individual acids.

6. Summary Table: Acids, Origins, and Sensory Impact

7. Conclusion: Acidity is the result of a chain of transformations

Citric acid and malic acid play a significant role in the sour structure and fruity perception of coffee. However, their levels are influenced by many factors, starting from fruit development, continuing through processing, roasting, and finally extraction.

Growing altitude and temperature can alter the chemical composition of green beans, but they do not establish a universal rule for acid increase or decrease applicable to all growing regions. During roasting, the transformation of each acid depends on the thermal history and its interaction with the bean’s chemical matrix, rather than a single temperature threshold.

For growers and roasters, the most reliable approach is to combine raw material data, process control, systematic cupping, and chemical analysis when needed. This forms the basis for understanding and adjusting acidity without equating flavor perception with a single chemical index.

Frequently asked questions about acids in coffee

Does citric acid create a lemon flavor in coffee?

Citric acid contributes to bright acidity, but the lemon flavor is perceived through the interaction between acidity and various other aroma compounds. The entire lemon flavor cannot be attributed solely to citric acid.

Does malic acid begin to decompose when the bean reaches 130°C?

Such a conclusion cannot be drawn solely from the melting point of pure malic acid. Transformation within the coffee bean depends on the material matrix, moisture content, thermal history, and reaction conditions.

Do coffees grown at high altitudes have higher acid content?

Not always. Altitude interacts with temperature, variety, shade cover, farming conditions, and processing. Acid content must be evaluated based on specific samples and conditions.

Does dark roasting eliminate all acids in coffee?

No. Roasting transforms the acid composition, but different acids may change in different ways. The roast level also alters bitterness, roasted aromas, and the sensory balance of the beverage.

How to measure citric and malic acid in coffee?

Chemical analysis methods such as High-Performance Liquid Chromatography (HPLC) can be used, depending on research objectives and sample preparation protocols. Measuring pH or tasting alone is insufficient to determine the specific content of each acid.