The Science of Cold Brew Coffee: A Coffee Nerd Explains Why Cold Brew Is So Smooth and Rich

Cold brew coffee stands apart from any other type of brewed coffee. The intricate science behind cold brew is what gives it an incredibly smooth, mellow, and naturally sweet taste profile that is far less acidic and bitter than standard hot coffee.

In this comprehensive guide, I’ll break down the complex chemistry and scientific principles that make cold brew unique. Understanding the science behind extraction and flavor development will give you the knowledge to brew amazing cold brew coffee at home with confidence and consistency. So brew a fresh batch and let’s dive deep into the science of cold brew coffee.

Understanding the Science of Cold Brew Coffee

Cold brew coffee is fundamentally different from any other type of brewed coffee. There are six main pillars of cold brew coffee brewing that account for its uniquely smooth flavor profile: brewing temperature, grind size, brewing time, coffee roast, coffee origin, and water quality. Let’s dive deep into the science behind each one.

Brewing Temperature

The brewing temperature is perhaps the most critical scientific factor in cold brew coffee. As the name implies, cold brew coffee is brewed at a much lower temperature than typical hot coffee.

While drip coffee is brewed between 195 and 205 degrees (F) and espresso between 190 and 200 degrees (F), cold brew can be brewed at room temperature or chilled in a refrigerator. Room temperature is around 70 degrees (F) and refrigerator temperature around 34 to 40 degrees (F). This vastly lower temperature changes the chemistry of extraction dramatically.

When coffee grounds come in contact with hot water, it causes the cell structure of the grounds to break down rapidly. The cell walls essentially dissolve, allowing the solubles inside to escape freely into the water. This breakdown unlocks many compounds that would normally stay trapped inside the intact cell structure.

Many of these released compounds are quite bitter and acidic. Chlorogenic acids, for example, are a family of bitter phenolic compounds found naturally in coffee beans. When the grounds are subjected to hot water, the chlorogenic acids break down into quinic and caffeic acids, which are even more bitter and astringent. Trigonelline, another bitter compound, decomposes into equally bitter pyroglutamic acid and other derivatives when the grounds are heated.

As I’ll mention later, high temperatures also bring out more bitter compounds, specifically because they break down the cell walls even more over time, and the bitter result makes its way into your cup.

In cold brew, the grounds never reach these high temperatures, so the cell structure remains largely intact. The chlorogenic acids, trigonelline, and other bitter compounds mostly stay trapped inside the cells rather than getting extracted into the water. This is a key reason why cold brew coffee has a naturally sweeter and smoother taste without the acidity and bitterness.

Just as important as brewing temperature is serving temperature. Even if coffee is brewed hot, allowing it to cool before drinking will elicit a sweeter taste profile. Research shows there is a positive correlation between coffee serving temperature and perceived bitterness and astringency. Compounds like quinic acid that cause a bitter taste have been shown to elicit a stronger response on human bitter taste receptors at higher temperatures.

So the cold water used in cold brewing, coupled with the cold serving temperature, creates a mellow brew completely unlike piping hot coffee.

Grind Size

The ideal grind size for cold brew is much coarser than normal hot coffee—around 1,000 to 1,400 microns compared to 1,000 to 1,200 microns for French Press and 600 to 800 microns for drip. Espresso is extremely fine at 250 to 400 microns. The dramatically coarser grind for cold brew makes a major difference.

The main role of grind size is that it determines the surface area of the coffee grounds exposed to the water. Finer grounds have far greater surface area—more nooks and crannies are exposed for the water to penetrate and extract solubles. Coarser grinds provide much less available area for the water to interact.

This slowing of extraction is key for cold brew. Many bitter, astringent compounds extract rapidly because they are highly water soluble. So grinding too fine strips away the nuanced sweetness and brings out harsh bitterness. The coarse particles in cold brew provide a slow, gradual extraction focused on sweeter compounds.

Oils and lipids are examples of components that extract relatively slowly. These provide a perceived sweetness and round mouthfeel. Given enough time, the water can pull out these solubles from even the large cold brew grounds. So you achieve sweetness without overextraction.

Too fine a grind can ruin a batch of cold brew. The increased surface area rapidly overextracts bitter elements and creates an imbalanced, harsh brew. A uniform coarse grind is essential for flavor. And make sure you grind your beans freshly right before brewing cold brew to avoid stale flavors.

Brewing Time

While hot coffee brews in a matter of minutes, cold brew steeps for anywhere from 12 to 24 hours. This prolonged brew time allows for proper extraction even with the massive grounds and cold water. Even with coarse particles, full extraction still occurs, just at a slower pace.

The flavors you want in your cold brew come out slowly over time. Fruity acids and sweet sugars are only sparingly soluble, needing hours to dissolve fully at lower temperatures. Given the coarse grind, it simply takes this long for water to penetrate and pull out soluble compounds from the interior of the particles.

This effect is directly linked to the cold water and coarse grind. Attempting shorter brew times with a fine grind would ruin a cold brew by rapidly overextracting. You need all three elements—time, temperature, and grind size—working together to yield optimal extraction.

Coffee Roast

As with any brewing method, the degree of roast substantially affects cold brew’s final flavor. Darker roasts are generally considered ideal for cold brew, providing a rich chocolatey, caramel sweetness. Light roasts can taste grassy and tea-like if not balanced properly.

Why does roast level matter so much? It comes down to how roasting transforms the composition of the beans. When coffee is lightly roasted, it retains much of its natural acidity derived from chlorogenic and other organic acids. As roasting progresses, these compounds break down into hundreds of derivative products with their own flavors and solubilities.

In particular, roasting leads to browning reactions like caramelization and the Maillard reaction. These transform natural bean sugars into aromatic compounds called melanoidins, which give a toffee-like, caramel taste. The longer the roast, the more sweet melanoidins develop. Since cold brew highlights sweetness, darker roasts pair well with this brewing method.

Roasting also causes the chlorogenic acids to degrade into less bitter quinic acid and phenolic compounds. In darker roasts, nearly all the chlorogenic acids are eliminated. The loss of bitter precursors balances the natural sweetness of the melanoidins.

That said, light roasts contain more caffeine and can sometimes work well in cold brew. The lower temperatures highlight fruity, floral notes that would otherwise get roasted away. But take care to avoid grassy, vegetal tones from underextraction. Generally a specialty medium roast strikes the right balance.

Coffee Origin

The origin of coffee—the geographic region the beans come from—also significantly influences cold brew’s flavor. Beans from different parts of the world naturally contain varying levels of acids, sugars, phenols, and aromatics based on factors like soil, altitude, climate, and processing method.

For example, beans grown at higher altitudes like in Colombia and Kenya tend to be higher in fruity acids and floral aromatics. Their complex fruit-forward flavor mixes nicely with cold brew’s mellow sweetness. East African coffees like those from Ethiopia and Rwanda are prized for their bright, tea-like aromatics—which cold extraction can beautifully highlight IF that’s what you’re into.

On the flip side, beans from lower altitude regions of Brazil and Sumatra are deeper, richer, and heavier in the cup. They have intense earthy qualities that complement the chocolatey roundness characteristic of cold brew. Careful aging and processing of these beans balances sweetness with layered depth.

The variety of the coffee plant matters too. For cold brew, I recommend against 100% Robusta beans—they contain nearly twice as much bitter chlorogenic acid as the milder Arabica coffee varieties. You can try blending in some Robusta for body, but too much amps up astringency.

Experiment with single origin beans from leading specialty growers to experience cold brew’s range.

Water Quality

The quality and composition of the brewing water has a substantial impact on the final product. Factors like mineral content, pH, and purity all influence extraction and flavor.

Hard water tends to accentuate bitterness due to higher concentrations of minerals like calcium and magnesium. These ions bind to organic compounds in the beans, releasing more insoluble particulates into the brew.

Conversely, soft water allows fruit acids to extract more cleanly, brightening the final character. Pure spring water brings out subtleties that heavily mineralized water might obscure. Though if too soft, the lack of minerals can taste flat and flavorless.

Off tastes can result from impurities like chlorine commonly found in tap water. While harmless, the subtle medicinal taste of chlorine can clash with coffee’s flavor profile. Filtration helps reduce chlorine prior to brewing.

A moderate mineral content balances sweetness with structure. I recommend starting with high quality bottled or filtered water. If you prefer your local tap water’s flavor, use charcoal filtration—like you’d get in a Brita pitcher—to remove chlorine before brewing.

Why Is Cold Brew So Different Than Hot Coffee?

Now that we’ve covered the scientific factors behind cold brewing, let’s examine how the final product differs in taste from regular hot brewed coffee. There are four main points of distinction: acidity, temperature, bitterness, and caffeine content.

Acidity

The most prominent difference between cold brew and hot coffee is the acidity level. Cold brew coffee has very low acidity, providing a smooth, creamy mouthfeel. In testing, cold brew averages a pH of around 5.3, compared to hot coffee which averages around pH 4.9.

That may seem a small difference, but on the scientific pH scale it is substantial—about half of the difference between apple juice (pH 4) and lemon juice (pH 3). So in terms of acidity perception, cold brew is significantly mellower.

The science behind cold brew’s lower acidity includes a few key points:

Brewing Temperature: The grounds are never subjected to hot water, so their cell structure stays intact. Acidic compounds like chlorogenic and quinic acids remain locked inside rather than extracting.
Grind Size: The coarse particles have less surface area for rapid extraction of acids.
Brew Time: Though long overall, the cold temperature slows extraction so acids plateau without overextracting.
Roast Level: Darker roasts degrade more chlorogenic acids, the main source of coffee acidity.

Specifically, cold brew contains around 35% less quinic acid than hot brewed coffee. Quinic acid, an organic compound found naturally in coffee beans, is primarily responsible for coffee’s tart, acidic taste. Chlorogenic acids are bitter precursors to quinic acid destroyed by roasting. And since dark roasts are more common for cold brew, it tends to have less bitterness, as well.

Less quinic, chlorogenic, and other acidic compounds provides a cleaner, sweeter flavor not dominated by sourness or tartness. The natural sugars are left to shine rather than being masked by harsh acids.

Serving temperature also mattes. Research shows tartaric acid tastes twice as sour hot as it does cold. Just as important as brewing cold is drinking cold brew chilled.

Temperature

Cold brew coffee is, of course, brewed with cold water instead of hot. And it is almost always served cold over ice, while hot coffee is served piping hot immediately after brewing.

Temperature has an enormous impact on taste and aroma. Heating food or drinks releases more aromatic compounds, increasing overall flavor intensity. But it also volatilizes certain delicate top notes while amplifying bitter, astringent ones. Sweetness perception is suppressed.

When hot, coffee’s bitterness and acidity dominate the profile. But cooled, these aspects recede into the background while subtler notes emerge. Serving temperature matters just as much as brewing temperature.

Heating extracts more soluble compounds from beans, but not necessarily good ones. Gentle cold extraction preserves the delicate, complex flavors obscured by hot brewing.

So cold brewing highlights coffee’s inherent sweetness and nuanced aromas, rather than the roasty/bitter characteristics. And serving it chilled makes it taste mellow, delicate, and refreshing.

Bitterness

As touched on earlier, cold brew contains significantly less bitterness than hot coffee. This is due to a few factors:

Lower Brewing Temperature: Prevents bitter compounds from extracting out of the intact grounds.
Coarser Grind: Smaller surface area for rapid extraction of bitter solubles.
Extended Brew Time: Allows sweetness to extract but bitterness plateaus.
Darker Roasts: Breaks down harsh chlorogenic acids.

In particular, cold brew contains around 30% less caffeic acid, which has an intensely bitter, astringent flavor. Other bitter phenols and acids are similarly reduced.

There is no harshness or rough aftertaste. The floral, fruity notes can take center stage without being dominated by the roasty/burnt bitterness inherent to hot coffee.

And perhaps most importantly, the limited bitterness and acidity in cold brew help to make the naturally sweet sugars from the coffee grounds shine in cold brew.

Caffeine Content

Many assume cold brew contains more caffeine than hot coffee. This is false. In fact it generally contains slightly less caffeine, although there is a BIG caveat here.

Typical cold brew (2 oz coffee to 1 cup water) has around 100mg of caffeine per 8oz cup.
Typical hot brewed coffee has around 100 to 150mg of caffeine per 8oz cup.

However, with cold brew concentrate, caffeine levels can be higher. Concentrate is essentially overextracted coffee, with a higher coffee-to-water ratio. Diluting reduces caffeine, but drinking concentrate straight spikes it upwards of 300 to 500mg per serving.

Higher caffeine levels increase perceived acidity and bitterness because caffeine itself is a bitter compound. Diluting concentrate improves flavor balance as well as controlling caffeine consumption.

Wrapping Up: How to Use Science to Brew Better Cold Brew

Based on all the scientific factors we’ve covered, here are some tips for using science to optimize your cold brew brewing:

Grind Size: Use a coarse grind of between 1,000 and 1,400 microns for reduced acidity and bitterness. A consistent particle size is key.
Brew Time: Steep for 12 to 24 hours to extract sweetness fully but avoid overextracting harshness. Dialed in steeping time enables proper extraction.
Water Temperature: Brew with pure cold water, around 40 degrees (F) or below. This preserves delicate flavors. You can also brew at room temperature, but the taste will vary.
Roast Level: Try a chocolatey dark roast for balanced sweetness and body. But medium roasts can work too. Light roasts in cold brew tend to be too floral for most coffee drinkers.
Coffee Origin: Experience fruitiness from African beans or richness from Central/South American beans, like in coffee from Brazil.
Water Quality: Filtered, mineral-balanced water allows subtleties to shine.
Serving Temperature: Chill and serve cold brew over ice. Coldness helps bring out the natural sweetness.
Dilution: Dilute concentrate to taste. More water equals more mellow flavor.
Storage: Keep cold brew chilled & consume it within 14 days. Acids develop over time if left out.
Grind Fresh: Grind beans immediately before brewing to avoid them going stale.
Sanitation: Clean equipment prevents mold and contamination.
Weigh Grounds and Water: Use an accurate scale to perfect your coffee-to-water ratio.

Understanding the scientific principles behind soluble extraction allows crafting a smooth, sweet cold brew every time. Proper technique brings out coffee’s hidden qualities obscured by hot brewing.

Controlling time, temperature, dilution, and grind size lets you balance sweetness against bitterness and acidity. When optimized through science, cold brew’s lower acidity, mildness, and nuance make it a refreshing, tasty treat.

Frequently Asked Questions

Why does cold brew taste sweeter than hot coffee?

Cold brew’s sweeter taste comes mainly from lower levels of acids. Acids like quinic and chlorogenic acid dominate the flavor of hot coffee, masking natural sugars. Cold brew’s cold water and coarse grind minimize extraction of these bitter acids. The reduced acidity allows more sweetness to shine through in your cup. The reduced bitterness also helps and is a product of the low brew temperature.

Is cold brew healthier than hot coffee?

Potentially yes, as cold brew contains fewer acids that can bother sensitive stomachs. The pH is not that much higher—5.3 as opposed to 4.9 for hot coffee—so it’s not that much healthier overall. But lower acidity and absent heat stress means less irritation for those prone to heartburn or reflux issues from coffee. Milder acids may mean less stomach upset.

How does grind size affect cold brew extraction?

Grind size determines the surface area of the coffee grounds exposed to water. Coarser grinds have less area exposed, slowing the extraction of solubles. This allows sweeter compounds to extract without overextracting bitter ones. Too fine a grind risks rapid overextraction of harsh acids and phenols. A uniform coarse grind maximizes sweetness and flavor balance.

Can you cold brew different types of coffee beans?

Yes, you can cold brew any coffee bean variety. However, the roast level and origin will affect the flavor. Darker roasts provide more sweetness while lighter ones have more acidity. African beans give fruity flavors, while Central/South American offer chocolate notes. Blending varieties can create complexity. But a medium-dark, Central/South American bean is a safe start.