Cold brew is immersion brewing at room temperature or in the refrigerator over an extended period. The slow, cold extraction produces a cup with lower acidity, high sweetness, and a smooth, heavy body — characteristics that result from the physical chemistry of cold water extraction, not from any special bean or roast. Understanding the variables lets you produce consistent cold brew without guesswork.
How Cold Extraction Works
Hot water extracts coffee compounds rapidly because heat increases the solubility of most organic compounds. Cold water extracts the same compounds, but slowly and selectively. Acidic compounds (chlorogenic acids, acetic acid) are less soluble at low temperatures, which is why cold brew tastes smoother and less acidic than hot-brewed coffee cooled down. However, cold extraction also misses some volatile aromatics that are released with heat — cold brew is heavier but less fragrant than hot brew.
This is not a quality hierarchy. It’s a different beverage.
Ratios: Concentrate vs. Ready-to-Drink
Concentrate (most common for home and commercial use): 1:4 to 1:6 coffee to water by weight. A 1:4 concentrate is diluted 1:1 with water or milk before drinking. A 1:6 concentrate is diluted 1:1.5. Concentrate keeps longer in the refrigerator (up to 2 weeks) and takes less refrigerator space.
Ready-to-drink (RTD): 1:8 to 1:12. Brew at this ratio and drink directly without dilution. RTD cold brew requires more space to store and has a shorter shelf life (7–10 days refrigerated) but is simpler to serve and easier to share.
For a 1-liter batch:
- Concentrate 1:5: 167 g coffee to 833 ml water
- RTD 1:8: 111 g coffee to 889 ml water
Grind Size
Coarse — comparable to or slightly coarser than French press. Cold brew’s long contact time compensates for the coarse grind. Finer grinds extract aggressively over 12–24 hours and produce bitter, astringent results. More critically, fine grinds are nearly impossible to filter fully, leaving a murky, over-silted brew.
A coarse grind produces a sediment-free concentrate with minimal filtering effort.
Water Temperature
Room temperature (20–22°C / 68–72°F): Faster extraction, typically 12–16 hours. Slightly more acidic than refrigerator cold brew due to warmer conditions allowing more acid extraction.
Refrigerator (4°C / 39°F): Slower extraction, 18–24 hours. Smoother, lower-acid, less volatile. Better for delicate light roasts where room-temperature extraction might develop undesirable fermented notes.
Do not steep longer than 24 hours at room temperature. Extended room-temperature immersion promotes microbial growth and off-flavors.
Brewing Steps
- Add coarse-ground coffee to your vessel (mason jar, French press, Toddy cold brew maker, or dedicated cold brew pitcher).
- Add cold or room-temperature water. Stir briefly to saturate all grounds.
- Cover and steep 12–24 hours at your chosen temperature.
- Filter. The filtering step determines clarity.
Filtering
First pass: Strain through a fine-mesh sieve to remove large grounds. A nut milk bag or cheesecloth works here.
Second pass (for clarity): Strain through a paper filter. This removes fines and produces a clear concentrate. Paper filtration takes 20–40 minutes for a liter — patience required. Do not squeeze the bag or press the filter; this forces fines through.
A French press serves as a convenient single-vessel cold brew system: steep with the plunger up, then plunge and pour. The plunger’s metal mesh handles the first-pass filtering; follow with paper for clarity if desired.
What Coffee to Use
Contrary to common advice, light roasts make excellent cold brew. Their fruit-forward acids and floral notes are still present in cold extraction, producing a sweeter, more nuanced result than the chocolate-and-low-bitterness profile of dark roasts. Medium roasts offer balance. Dark roasts produce the classic cold brew flavor most people know commercially.
Single-origin coffees and blends both work. Use coffees you’d be happy to drink hot.
Storage
Concentrate: Sealed in the refrigerator for up to 2 weeks. Acidity slowly increases as the concentrate ages.
RTD cold brew: 7–10 days refrigerated. Quality peaks at days 1–3.
Do not freeze cold brew — freezing damages the delicate soluble compounds and produces a flat, thin result when thawed.
Serving
- Over ice with water: Concentrate diluted 1:1 with water, poured over ice.
- With milk or oat milk: Dilute concentrate 1:1 with the milk of choice.
- Nitro cold brew: RTD cold brew pushed through a nitrogen-charged keg or nitro charger. The nitrogen creates a creamy, cascading head. Use RTD ratio, not concentrate — the nitrogen already adds perceived body.
- Cold brew tonic: 60 ml concentrate over 120 ml tonic water and ice. The carbonation and quinine bitterness complement coffee’s sweetness.
Troubleshooting
- Bitter, harsh cold brew: Grind is too fine, or steep time too long. Use coarser grind and reduce to 14–16 hours.
- Weak, thin result: Under-extracted. Steep longer, increase dose, or grind slightly finer.
- Fermented or wine-like off-flavors: Room-temperature steep went too long (past 16 hours) or ambient temperature was too warm. Move to refrigerator or reduce time.
- Murky, silty texture: Paper filter step was skipped, or filter was squeezed. Re-filter through a fresh paper filter without pressing.