The Science of Cold Brew Extraction: Variables and Chemistry

Cold brew coffee has evolved from niche category with a storied history to a global phenomenon. In this five-part series on cold brew coffee, Dr Steffen Schwarz, the founder and director of Coffee Consulate, examines the history of cold brew coffee, the science of cold brew extraction, the chemical and sensory profile, hygiene and food safety challenges, and practical recommendations for coffee professionals. Part two focuses on key parameters and scientific insights for cold brew extraction. By Dr Steffen Schwarz

Brewing coffee is fundamentally a chemical extraction process: water acts as a solvent to pull soluble compounds out of ground coffee. In hot brewing, heat greatly accelerates this extraction. In cold brewing, we rely on time and other factors to compensate for the lack of heat. As a result, cold brewing is a delicate dance of multiple variables. Small changes in any factor – say, grinding a bit finer or steeping a few hours longer – can tilt the flavour and strength of the final coffee. Recent pilot studies have shed light on the optimal balance of these variables. Below is a break down the key parameters and scientific insights for cold brew extraction.

Coffee Bean Variety & Roast Level

The type of coffee and how it is roasted set the stage for flavour. Lightly roasted beans retain more acids and bright aromas, whereas dark roasts have more roasted, bitter compounds and fewer delicate notes. For cold brew, experts generally recommend avoiding excessively dark roasts. Dark roasts can yield a flat or harsh brew, as many of their soluble compounds (like certain aromatics) were burned off or converted during roasting. A medium roast often works best, providing sweetness and complexity without too much bitterness. In fact, consumer research indicates roast level is one of the predominant factors influencing cold brew flavour.

Some roasters even develop specialty ‘cold brew roast’ profiles – these roasts tend to be shorter or slightly lighter than espresso roasts, aiming to preserve some acidity and fruitiness that cold water can extract. Interestingly, roasting for cold brew might involve ending the roast a touch early once development is sufficient to avoid overproduction of heavy Maillard-reaction flavours (smoky, ashy notes) that could dominate a cold brew. Allowing beans to rest (de-gas) after roasting is also important; one study suggests that the time after roasting can affect cold brew flavour consistency. In summary: start with high-quality beans, and if in doubt, a medium roast Arabica is a reliable choice for a balanced cold brew.

Grind Size

Grind is crucial for any brewing method, and cold brew is no exception. A coarser grind is typically used for cold brew than for hot brew. Why? Coarse particles have less surface area, extracting more slowly – this helps prevent over extraction during the many hours of contact time. Fine grinds, by contrast, could quickly release all their soluble content, including bitter compounds, and also make filtration difficult (muddy coffee and clogged filters).

Most recipes call for a coarse grind (similar to French press). Research confirms that grind size affects extraction kinetics: finer particles lead to faster and higher extraction yield. In one experiment, grinding a bit finer increased the concentration of certain compounds significantly in the same steep time. However, simply going finer to speed up cold brew can be a double-edged sword – it might reduce the clarity of flavour and increase sediment. A practical approach is to grind coarse but ensure consistency (minimal ‘dust’ that over-extracts). Some advanced techniques involve two grind sizes mixed (a few fine particles to kick-start extraction and lots of coarse for stability). No matter what, ‘grind uniformity’ will help the extraction proceed evenly. As one study’s fishbone diagram illustrated, grinding (particle size, surface texture, and fines/dust content) is one of the major cause-and-effect factors governing cold brew extraction.

Brew Ratio (Coffee Dosage)

How much coffee to use per unit of water – the brew strength – is a defining choice in cold brew. A higher dose (say 1:5, very strong) yields a concentrate that can be diluted, whereas a lower dose (say 1:15) yields a ready-to-drink strength directly. Surveys of baristas and home brewers found a wide range in use, but most prefer between 50–100 grams of coffee per litre of water. Common café practice is around 80g/l, which provides a robust brew that can be served straight or on ice without tasting weak. Using more coffee not only strengthens flavour but also can slightly increase extraction efficiency (up to a point) because the water is saturated with solubles faster. However, beyond a certain ratio, returns diminish – water can only dissolve so much.

The Coffee Brewing Control Chart used for hot coffee often targets around 18–22 percent extraction yield; interestingly, cold brew might reach similar yields but perceptions differ due to the chemical makeup. In one expert recommendation, an optimal extraction for cold brew is about 70 percent of the total possible solubles – meaning we intentionally don’t extract everything, to avoid harsh components. Choosing the right dose helps achieve that balance.

For professionals, consistency in dosing is key to a repeatable recipe. Measuring by weight (using scales) is far more accurate than by volume (scoops) , especially because coarsely ground coffee is fluffy. In summary, decide if you want a concentrate or a direct drink, and adjust the coffee-to-water ratio accordingly – most settle in the 1:8 to 1:12 range (which corresponds to~85g/l give or take) for a pleasantly strong cold brew.

Water Temperature

Temperature is perhaps the single most defining factor of cold brew. By definition, cold brew uses ‘cold’ water — but that spans a range from just above freezing to ambient room temp.

Extraction rate roughly doubles with each 10°C increase in temperature (Arrhenius’s law), so even a few degrees matter. Colder water (0-4°C) will extract very slowly, requiring the upper end of brew time (18-24+ hours) , but it may preserve volatile aromas better and minimise microbial growth during brewing. Room temperature water (20-25°C) extracts much faster – some recipes call for as short as four to eight hours at room temp – but could draw out more bitterness if left too long. One study noted that two hours at 20°C can produce a decent cold brew if other factors are optimised. The authors of that study suggest carefully considering. In practice, many shops brew at ‘cool room’ temperature (~15-20°C) for a shorter time, then chill the brew. In a large survey, the most preferred extraction temperature was 8°C (refrigerator temperature), followed by around 20°C. Using refrigerator-cold water (around 4-8°C) and letting it steep overnight (12-16 hours) is a popular method that balances extraction and flavour. If you brew at ambient conditions in a hot climate, be mindful that 30°C room temperature will act almost like a slow hot brew – extraction could finish in two to three hours, which might actually help with reducing contamination risk as the brew does not sit as long.

Overall, temperature and time trade off: lower temperature = longer time needed, and vice versa. Finding the right ‘time/temperature equilibrium’ is crucial so that you extract enough flavour but not so much that you get harshness or risk spoilage. For example, one optimised approach is 80g/l at 15°C for ~2 hours, achieving about a 70 percent extraction yield– enough for a flavourful brew without overdoing it. If that sounds short, note that laboratory tests have shown many compounds in coffee reach their maximal extraction in just a few hours at room temperature. We will explore those specific extraction kinetics shortly. The bottom line: colder brewing is gentler but slower; a slightly warmer ‘cold’ brew can be faster but must be cut off sooner.

Brew Time

Hand-in-hand with temperature is the duration of brewing. Traditional cold brew recipes often say “steep for 18-24 hours.” However, emerging science suggests this may be longer than necessary. In controlled experiments analysing extraction over time, researchers found that most coffee solubles extract within the first three to six hours of cold brewing. For instance, one study monitored key chemical markers: formic acid levels stopped increasing after about 40 minutes; chlorogenic acids peaked by ~2 hours; caffeine content plateaued after ~2.3 hours; lactic acid was done in 20 minutes; trigonelline by 40 minutes. Essentially, beyond a certain point, the concentration of these compounds in the water does not get much higher – equilibrium is reached. Another pilot study noted that caffeine and 3-CQA (3-caffeoylquinic acid, a major chlorogenic acid) achieved ~100 percent extraction by six to seven hours under their test conditions.

So why do people brew for 16-plus hours? At colder temperatures, these time scales stretch out — those studies were often done at room temp or with stirring to see the maximum potential. In a refrigerator at 5°C with no stirring, it might indeed tak e the better part of a day to approach full extraction. Also, the last few percentages of extraction (which contain minor compounds) might contribute subtle flavours; some brewers feel a longer steep gives a more rounded profile. But there is consensus forming that ‘longer is not always better’ for cold brew.

Prolonged extraction can lead to over-extraction, manifesting as overly bitter or astringent notes. As one expert panel put it, nothing is gained by extending a cold brew from four hours to 20 hours, except perhaps more bitterness.

In fact, leaving coffee grounds soaking too long can start extracting undesirable components like certain lipids or higher molecular-weight compounds that add roughness to the taste – or even begin leaching woody, papery flavours from the cellulose in coffee bean structure. From a practical view, many shops find 12 hours a convenient default (overnight). But if aiming for optimal quality and safety, some are experimenting with shorter brew times (four to eight hours), especially at mild room temperatures, to produce a cleaner flavour and then promptly removing the grounds to halt extraction.

The key is to know when to stop the brew. One sign might be tasting a small sample at intervals until you sense the flavour has peaked (before bitterness creeps in). The research-backed guideline of ~70 percent extraction degree is useful: once you have extracted about two-thirds of the coffee’s solubles, you likely have captured the good flavours and can leave the rest behind.

Turbulence (Agitation)

Unlike hot brewing, where rapid water flow or vigorous pouring is common (think of pour-over or espresso pressure), cold brew is often done with minimal movement – the coffee and water just sit together. However, introducing some agitation can significantly affect extraction speed. Turbulence means anything that increases the contact between water and coffee, such as stirring, shaking, or ultrasonication (using ultrasonic waves to agitate particles). Research has shown that agitation can shorten brew time and increase yield. For example, gently stirring the brew a few times during an immersion can help prevent dry pockets of grounds and ensure uniform extraction.

In one study, scientists compared no agitation, intermittent stirring, and ultrasonic agitation for cold brew. After one hour of extraction, the batch with ultrasonication had extracted far more compounds: ~71 percent more chlorogenic acid, ~26 percent more caffeine, ~21 percent more acetic acid, etc. compared to the static control. Even simple constant stirring gave notable increases (eg, +26 percent chlorogenic acid over static). These are striking differences – essentially, you can get the equivalent of many extra hours of extraction by adding movement.

Ultrasonic baths (commonly used for cleaning jewelry or lab samples) have been adopted by some coffee researchers to produce a complete cold brew in under an hour. Of course, most café set ups won’t include an ultrasonic machine, but the principle holds for manual techniques: stirring your cold brew at least once or twice during the steep can help extract more evenly and quickly. Be cautious, though – too much agitation, especially toward the end of extraction, might also introduce fine sediment into the filtered coffee or extract harsher components.

A good practice for immersion is to stir initially when combining coffee and water (to fully wet the grounds), then perhaps one gentle stir midway through the brew. Drip cold brew, by contrast, usually should not be disturbed – you rely on gravity drip, and stirring the bed could cause it to drain too fast. For those experimenting with vacuum or pressure, turbulence is inherently part of those processes (as fluids are being actively moved through the coffee). In summary, agitation is a powerful lever: use it to accelerate extraction if needed but apply judiciously to avoid muddiness.

Water Composition

Often overlooked, the chemistry of the water itself plays a role in brewing. Minerals in water (like calcium, magnesium, bicarbonate, etc.) can affect flavour extraction and stability. In hot brewing, hard water can suppress extraction of certain flavours or make coffee taste flatter. For cold brew, a survey indicated most people prefer to use soft or medium-hard water. Soft water (low mineral content) tends to extract higher perceived acidity and clarity, whereas very hard water might under-extract some acids and could result in duller flavour. Water pH is also a factor – typically tap or bottled water is near neutral pH ~7. Coffee itself is acidic, so the brew’s pH will drop into the five range regardless. Using water with extreme pH (very alkaline or very acidic) would be problematic, but in general normal drinking water works. If consistency is key, using filtered water or a certain mineral recipe (some cafés use reverse osmosis water with minerals added for optimal coffee brewing) can help replicate results every time.

Additionally, water freshness matters – if using cold tap water, let it run a bit so it’s well aerated and not stale from pipes. Some brewers even boil water then cool it, not to add heat to the brew but to de-aerate or sterilise it; however, boiled then cooled water can lack oxygen and taste flat, so it’s not usually necessary if your water supply is good quality. A final note: oxygen content in water might influence oxidation of coffee compounds. Cold brew is less prone to rapid oxidation than hot coffee (since it is not hot and is often covered during brewing) but starting with cold water that has not been sitting around open can reduce the dissolved oxygen that could oxidize some flavour compounds over the long steep. While these water details are subtle, they can subtly influence the outcome in a professional setting. The general consensus: use clean, filtered water of moderate hardness for the best cold brew results.

As we tune all these variables, what we end up with is a very different chemical profile in the cup compared to hot coffee. So, what is the composition of cold brew, and how does it translate to taste? In Part III, we will examine the chemistry and sensory qualities that distinguish a cold brew from your classic hot drip coffee.

Dr Steffen Schwarz is the director of the Mannheim, Germany based Coffee Consulate, which he founded in 2005 to meet the increasing demand for training. In addition to extensive workshop programmes, the company is engaged in research and product development, and manufactures barista tools, teaching, and analytical materials.