Every cup of brewed coffee is a solution — water in which soluble compounds from ground coffee have been dissolved. But not everything in roasted coffee is soluble. Approximately seventy percent of a roasted coffee bean’s dry weight consists of insoluble material — cellulose, plant fiber, and structural compounds that remain as spent grounds after brewing. The remaining thirty percent is soluble and available for extraction, and within that soluble fraction, different compound classes dissolve at different rates, at different temperatures, and to different degrees. Understanding what dissolves and why is the scientific foundation for every brewing decision — grind size, temperature, time, and ratio — that determines whether a cup tastes excellent, mediocre, or undrinkable.
The Soluble Fraction
The approximately thirty percent of roasted coffee’s dry weight that is water-soluble includes organic acids, sugars, Maillard reaction products, caffeine, melanoidins, and hundreds of volatile aromatic compounds. These are the molecules that create flavor, aroma, body, and color in brewed coffee. The goal of extraction is to dissolve an appropriate proportion of these solubles — typically eighteen to twenty-two percent of the coffee’s dry weight — to produce a balanced cup.
Not all of the soluble fraction is desirable. The first compounds to dissolve — primarily organic acids — contribute brightness and fruity character. The middle fraction — sugars, caramelized compounds, and lighter Maillard products — contributes sweetness, body, and complexity. The final fraction — heavier bitter compounds, astringent phenolics, and over-extracted tannin-like molecules — contributes harshness and unpleasant dryness. Optimal extraction captures the first two fractions fully while minimizing the third. The quantitative framework for measuring this balance is explored in our article on extraction yield and measuring brewing efficiency.
Why Compounds Dissolve at Different Rates
Molecular Weight and Polarity
Smaller, lighter molecules dissolve faster than larger, heavier ones. The organic acids that extract first — citric, malic, acetic — are relatively small molecules that dissolve readily in water. The sugars and Maillard products that follow are larger and more complex, requiring more energy and time to dissolve. The bitter compounds that extract last are the heaviest and most resistant to dissolution, requiring extended contact time or aggressive extraction conditions to enter the brew. This molecular weight gradient is the fundamental reason extraction proceeds in a predictable sequence from bright and acidic through sweet and balanced to bitter and astringent.
Temperature Dependence
Solubility increases with temperature for virtually all coffee compounds. Hotter water provides more kinetic energy, accelerating the rate at which molecules break free from the coffee matrix and enter solution. But the temperature sensitivity varies between compound classes — lighter acids dissolve efficiently even at moderate temperatures, while heavier compounds require higher thermal energy. This differential temperature sensitivity is why brewing temperature affects not just how much dissolves but what dissolves, shifting the flavor balance of the cup toward different compound classes depending on the thermal energy applied.
Surface Area and Particle Size
Dissolution occurs at the interface between the coffee particle and the surrounding water. Increasing the surface area — by grinding finer — increases the number of dissolution sites and accelerates extraction. Decreasing the surface area — by grinding coarser — slows extraction. This is why grind size is the most direct control over extraction rate: it determines how much of the coffee’s interior is exposed to water and therefore how quickly solubles can dissolve. The practical application of grind size management is explored in our article on how grind size affects coffee flavor.
The Role of Water Chemistry
Water is not a passive solvent — its mineral composition actively influences what it can dissolve and how efficiently. Calcium and magnesium ions in water facilitate the extraction of flavor compounds by interacting with the negatively charged molecules on the coffee particle surface, helping to pull them into solution. Water that is too soft — lacking these mineral ions — extracts less efficiently, producing thin, under-flavored cups. Water that is too hard may extract excessively or introduce mineral flavors that mask the coffee’s character.
Bicarbonate content affects the acid balance of the brew. High bicarbonate water buffers the organic acids that contribute brightness, suppressing perceived acidity and producing a flatter, less vibrant cup. Low bicarbonate water allows acids to express fully, sometimes to the point of perceived sharpness. The ideal water for coffee brewing provides enough calcium and magnesium to facilitate extraction while keeping bicarbonate low enough to preserve the acid structure that gives coffee its liveliness.
Solubility and Freshness
The solubility characteristics of coffee change after roasting. Freshly roasted coffee contains significant CO2 trapped within its cellular structure — a byproduct of the chemical reactions that occur during roasting. This CO2 creates a physical barrier to water penetration, reducing the effectiveness of extraction in very fresh coffee. As the coffee degasses over the first few days after roasting, its solubility improves because water can access the soluble compounds more readily without being repelled by outgassing CO2.
This is why many brewing professionals recommend a rest period of two to seven days after roasting before brewing — to allow sufficient degassing for water to extract efficiently while the coffee is still fresh enough to retain its volatile aromatics. The dynamics of degassing and its implications for brewing timing are explored in our article on understanding coffee degassing after roasting.
Practical Implications
The science of solubility translates into practical brewing guidance. If your cup tastes sour and thin, more solubles need to dissolve — grind finer, increase temperature, or extend contact time. If it tastes bitter and harsh, too many solubles have dissolved — grind coarser, decrease temperature, or shorten contact time. If it tastes flat despite adequate extraction, the water chemistry may be suppressing acidity or the coffee may have lost volatile compounds to staling. Each of these adjustments operates on the same underlying principle: controlling what dissolves, how much dissolves, and how quickly it dissolves.
Conclusion
Coffee brewing is applied solubility science. The compounds that create flavor, aroma, and body dissolve from ground coffee into water in a predictable sequence governed by molecular weight, temperature, surface area, contact time, and water chemistry. Understanding these principles transforms brewing from guesswork into a logical process where every variable has a clear purpose and every adjustment produces a predictable effect. The cup is a solution — and the quality of that solution depends on understanding what dissolves and why.
Daniel Almeida is a member of the editorial team at Saiba Money, where he contributes to the research, writing, and review of educational content focused on coffee culture, production, and brewing methods.
He works collaboratively to ensure that all published articles are accurate, clearly structured, and accessible to a broad audience. His interests include agricultural development, global coffee markets, and the science behind brewing techniques.
Daniel is committed to delivering reliable, well-researched information that helps readers better understand coffee from origin to preparation.