Coffee Science
The Science of Sip
The chemistry behind your cup — and how it makes you a better brewer.
Extraction
Extraction Fundamentals
Water dissolves soluble compounds from ground coffee. The trick is controlling how much.
Extraction & TDS
Water dissolves coffee compounds in order: acids first, then sugars, then bitter compounds. Extraction yield is the percentage of the coffee bed that ends up in your cup. The industry has targeted 18 – 22% since Lockhart mapped dissolved solids to cup quality in 1957.[1][2]
TDS (Total Dissolved Solids) measures concentration. Filter coffee reads 1.2 – 1.5% TDS; espresso sits around 8 – 12%.[2] A strong (high TDS) shot can still be under-extracted if the ratio is off.
Under< 18%
Ideal18 – 22%
Over> 22%
Sour, thin, saltySweet, balanced, complexBitter, harsh, astringent
Variables that affect extraction
Four levers control extraction. Adjust one and you'll compensate with another.
Grind Size
Finer = more surface area = faster extraction
Water Temp
90 - 96 C is the sweet spot for most methods
Contact Time
Longer brew time = more compounds dissolved
Brew Ratio
Coffee-to-water ratio shapes strength and flavor
Extraction yield = how much you took from the coffee. TDS = how concentrated the cup is. You need both to diagnose a brew.
Chemistry
The Chemistry of Taste
Bitter, sweet, acidic, heavy-bodied — each maps to specific compounds in the bean.
Bitter & Sweet
Caffeine accounts for only 10 – 15% of perceived bitterness.[3] The real drivers are chlorogenic acid lactones and Maillard reaction products, both of which increase with darker roasts. Caffeine stays constant across roast levels.[4]
Green arabica beans contain up to 9% sucrose.[5] Roasting destroys the sugar but creates caramelization by-products that trigger sweet receptors. Light-to-medium roasts preserve more sweetness; dark roasts let bitterness dominate.
Acidity & Body
Acidity is what separates a lively cup from a flat one. Citric acid reads as citrus, malic as apple crispness, phosphoric as clean effervescence.[4] Higher altitude and lighter roasts produce more acidity.
Body comes from lipids, proteins, and suspended fines. Paper filters trap them for a clean cup. Metal filters let them through for a thick, coating mouthfeel.
Want body? French press. Want clarity? Paper filter. It's the single easiest variable to change.
Temperature
Temperature & Perception
The same coffee tastes different hot versus cold. Real neuroscience explains why.
Temperature changes taste
Hot coffee releases volatile aromatics — that rich smell of a fresh pour. As it cools, aroma fades but sweetness and fruit notes emerge. Cuppers taste at multiple temperatures to track these shifts.
Cold brew tastes smoother because the TRPM5 ion channel, involved in bitter and sweet perception, is less active at low temperatures.[6] Your sensitivity to bitter compounds drops.[7]
Cold brew's pH (4.85 – 5.13) is actually comparable to hot brew.[8] “Lower acid” is a myth — fewer dissolved compounds and a different acid balance explain the perceived smoothness, not lower acidity.
Cold brew isn't lower in acid — the pH is about the same.[8] Your taste buds just respond differently at cold temperatures.
Milk
Milk Science
Milk doesn't just dilute coffee — it changes it through protein binding, fat coating, and selective flavor masking.
How milk changes coffee
Casein proteins bind to chlorogenic acids, the main bitter compounds, blocking them from your taste receptors. Milk cuts bitterness in a way water can't.
Fat coats your palate, adding body and smooth mouthfeel. Whole milk does this best; skim gives you protein binding but less texture. Lactose (~4.8% of milk) adds sweetness without sugar.
Delicate notes — florals, bright fruit — get buried. Chocolate, caramel, and nuts survive and often improve. This is why medium-to-dark roasts suit milk drinks.
The 70 C rule
Milk proteins denature above 70 C. Sweetness drops and you get a scorched, sulfurous edge. Steam to 60 – 65 C and stop.
Lattes flat or burnt? The milk is too hot. 60 – 65 C — warm on your wrist, nowhere near scalding.
Roasting
The Roast Curve
Where a dense green seed becomes something you'd want to drink.
From green to brown
Roasting drives off moisture, triggers the Maillard reaction, caramelizes sugars, and builds volatile aromatics. Too fast scorches the outside; too slow bakes out flavor.
At ~196 C, steam and CO2 fracture the bean with an audible crack. How long the roaster continues after first crack determines the balance between origin character and roast character.
Green
Light
Medium
Dark
First crack at ~196 C — the moment roast flavors begin developing
Light
Origin character up front. Floral, fruity, tea-like notes with bright acidity. The most transparent window into where the coffee grew and how it was processed.
Medium
Origin and roast in balance. Caramel, chocolate, brown sugar sweetness with moderate acidity. Works well across most brew methods.
Dark
Roast character takes over. Smoky, bittersweet, toasted flavors, low acidity, full body. Origin nuances fade — you're mostly tasting the roast.
No “best” roast level. Espresso favors medium-dark; pour-over flatters lighter roasts. It's personal.
Water
Water Quality
Coffee is 98% water. What's dissolved in it shapes what ends up in your cup.
Water & minerals
The SCA recommends 150 mg/L TDS, pH ~7.0, and calcium hardness near 68 mg/L.[10] Magnesium extracts fruity, sharp flavor compounds; calcium pulls rounder, heavier notes.[9] The balance between them shapes flavor as much as grind size or brew time.
Too soft
Under-extraction. Flat, muted flavors.
Too hard
Over-extraction. Chalky, harsh, and hard on equipment.
A carbon filter strips chlorine and leaves enough minerals for good extraction. Filtered tap water is all most people need.
Freshness
Freshness & Degassing
Fresh-roasted coffee needs rest before it brews well.
The degassing window
Roasting traps CO2 inside the bean. In the first 24 – 48 hours, beans off-gas rapidly — the dramatic bloom when you pour water on fresh grounds. Excess CO2 repels water: in espresso it creates channeling; in filter methods it causes uneven extraction. Most coffee needs a few days of rest.
PeakDays 7 – 14
GoodDays 15 – 21
StaleDay 22+
Vibrant, aromaticBalanced, reliableFlat, papery
Resting & staleness
Filter methods are forgiving — brew from 5 – 7 days post-roast. Espresso amplifies CO2 under pressure, so most baristas wait 10 – 14 days.
Past its window, coffee goes flat. Bloom disappears, oils turn rancid, papery flavors replace complexity. Whole beans hold up longer than pre-ground — grinding exposes surface area to oxygen.
Brewd tracks freshness for you:Peak7 days or lessGood8 – 21 daysStale22+ days
Buy what you'll use in 2 – 3 weeks. Airtight container, room temp, out of sunlight. Freezing an unopened bag long-term works; daily freeze-thaw doesn't.
Sources
References
[1] Lockhart, E.E. (1957). The Soluble Solids in Beverage Coffee as an Index to Cup Quality. Coffee Brewing Institute, New York.
[2] Frost, S.C., Batali, M.E., Guinard, J.-X. & Ristenpart, W.D. (2020). A Sensory and Chemical Comparison of Brewed Coffee Made at Different Extraction Yields. Journal of Food Science, 85(10), 2786–2800.
[3] Hofmann, T., Frank, O. & Blumberg, S. (2006). Molecular Definition of Black Coffee’s Taste by Means of the Taste Dilution Analysis. Chemistry & Biodiversity, 3(3), 274–287.
[4] Farah, A. (2012). Coffee Constituents. In Coffee: Emerging Health Effects and Disease Prevention (ed. Chu, Y.-F.), pp. 21–58. Wiley-Blackwell.
[5] Ky, C.-L. et al. (2001). Caffeine, trigonelline, chlorogenic acids and sucrose diversity in wild Coffea arabica L. and C. canephora P. accessions. Food Chemistry, 75(2), 223–230.
[6] Talavera, K. et al. (2005). Heat activation of TRPM5 underlies thermal sensitivity of sweet taste. Nature, 438, 1022–1025.
[7] Green, B.G. (2016). Temperature Effects on the Perception of Bitterness. Chemical Senses, 41(7), 537–545.
[8] Rao, N.Z. & Fuller, M. (2018). Acidity and Antioxidant Activity of Cold Brew Coffee. Scientific Reports, 8, 16030.
[9] Hendon, C.H., Colonna-Dashwood, L. & Colonna-Dashwood, M.W. (2014). The Role of Dissolved Cations in Coffee Extraction. Journal of Agricultural and Food Chemistry, 62(21), 4947–4950.
[10] SCA (2015). Water Quality Handbook, 2nd ed. Specialty Coffee Association of America, Long Beach, CA.