The Science of Brewing Coffee
When coffee arrives at your doorstep, it has traveled thousands of miles and has transformed from an agricultural product into freshly roasted beans ready to be ground and brewed. It’s essential to recognize the effort, time, and energy that goes into the labor of things like growing variables, processing, and roasting that factor into creating an exceptionally high-quality coffee. However, the truth is that none of that means it will be the best coffee you’ve ever had if it isn’t correctly brewed. You are the last factor in this long, energy-intensive coffee supply chain when preparing your coffee at home. You become active in bringing the specialty coffee to life and ultimately influencing the taste, for better or worse. A better understanding of the brewing process can help turn that specialty-grade bean into something delicious every time you brew.
Brewing with a Hario v60 pour over, Photo by Michael O’Neal
What is Brewed Coffee?
Brewing is the process by which coffee grounds are introduced to water and where the soluble compounds that create the delectable flavors and aromas are dissolved into the water. Brewed coffee is about 98% water and 1-1.5% dissolved solids from the grounds. In the coffee industry, we like to talk about TDS or Total dissolved solids, which about brewed coffee can be thought of as coffee flavor or the amount of coffee in your coffee. TDS determines the concentration or strength of brewed coffee. The higher the TDS, the stronger your brewed coffee.
What is Brewed Coffee?
Brewing is the process by which coffee grounds are introduced to water and where the soluble compounds that create the delectable flavors and aromas are dissolved into the water. Brewed coffee is about 98% water and 1-1.5% dissolved solids from the grounds. In the coffee industry, we like to talk about TDS or Total dissolved solids, which about brewed coffee can be thought of as coffee flavor or the amount of coffee in your coffee. TDS determines the concentration or strength of brewed coffee. The higher the TDS, the stronger your brewed coffee.
The Science of Brewing Coffee
For our purposes, extraction and brewing will be used interchangeably to describe the brewing process. While each method is unique, generally, for brewing to occur, you must add water to your coffee grounds. Once you’ve introduced water to the grounds, the wetting stage begins by displacing gases like carbon dioxide (CO2) from inside the coffee cells into the surrounding liquid and atmosphere. We often refer to the wetting stage as “the bloom.”
The blooming phase requires adding a small amount of water to your coffee grounds at the beginning of the brewing process. If you are brewing with 20g of coffee, you would add roughly 40g (double) of water for the bloom. Once your grounds have been evenly saturated, allow the brew to sit for 30-45 seconds before adding more water. While the coffee sits, you’ll notice bubbling as the CO2 is released from the coffee grounds. We call this degassing.
Why do we bloom? CO2 is a byproduct of the roasting process, and before brewing, it is trapped inside the coffee. CO2 essentially inhibits proper extraction by pushing water away from the grounds. We want our water to permeate the coffee grounds instead. You’ll want to bloom your coffee to encourage the release of trapped CO2 before you begin the extraction phase. By blooming, you make it easier for the water to pull out flavor. Once these gases have been released, the extraction stage begins as the coffee particles absorb the water, dissolving soluble solids and aromatic compounds into the slurry. The slurry is the mixture of coffee grounds, water, and gases that have yet to pass through the filter during the brewing process.
During extraction, numerous chemical reactions take place that transfer the coffee solubles from the coffee grounds into the water. The following three reactions can and will happen simultaneously, and each depends on brewing variables such as temperature, time, and agitation, which we will discuss later.
Hydrolysis is a general chemical reaction that occurs when a compound is broken down by water. Through hydrolysis, large molecules (carbs, acids, proteins) that cannot dissolve into the water are broken down into smaller molecules that can be.
Dissolution occurs when compounds dissolve into the water (the solvent) to form a solution–this happens naturally when water moves over and through the coffee grounds during brewing. Chlorogenic, acetic, malic, and other acids are a few compounds dissolved through brewing.
Diffusion occurs through osmosis when solubles move from areas of high concentration (the coffee grounds) to areas of lower concentration (the water). Once the coffee particles are saturated with water, the osmosis process draws the coffee chemicals out of the grounds and into your brewed coffee. After wetting, your coffee grounds are more concentrated inside than outside, so by osmosis, the water that was absorbed into the grounds leaves, pulling desirable flavors out of the coffee bean and into the brewed liquid.
What is Under, Over, and Ideal Extraction?
During the brewing process, different flavors are extracted at different times. Generally speaking, acidic and fruity flavors are extracted first, followed by sweet and bitter flavors. The goal with brewing is to pull as many desirable flavors (acids, fats, sugars, and fruity flavors) from the coffee grounds as possible without taking it too far and pulling out undesirable coffee flavors that dominate the pleasant ones (bitterness and astringency). Think of extraction as a scale we try to balance to get the right flavors and taste of coffee.
Under-extracted coffee is a scenario where not all desirable flavors have been extracted from the grounds. The water hasn’t had enough time to break down and pull out the sugars, which balance the sour acids extracted quickly. An under-extracted brew will likely taste sour or sharp, have an oily body, and have a quick finish.
Over-extraction occurs when you’ve taken your brew too far. A tipping point is crossed as extraction continues and undesirable flavors are extracted. In this scenario, the water has broken down sugars and plant fibers, contributing to bitterness. These undesirable flavors, such as bitterness, alter and mask the more desirable traits like sweetness. An over-extracted brew will taste dry, astringent, and bitter.
Ideal Extraction is our balanced scenario when all desirable flavors are extracted from the coffee grounds (acids, fats, oils, sweetness, balanced with bitterness), and the process stops before the coffee becomes too astringent or bitter. This is our goal every time, and makes a great cup of brewed coffee. The cup will have more of a rounded, creamy, or syrupy body with optimal sweetness and a pleasant, lingering finish.
Brewing Variables that Influence Extraction
Factors that will affect the efficiency and quality of the brewing process and will ultimately impact the flavor perception of the final product are:- Contact time & grind size
- Type of water
- Water temperature
- Turbulence (how much agitation or stirring occurs)
- Filtration type
- Brew method
By understanding these variables you, as a home brewer, can adjust to get the ideal extraction and your perfect cup of coffee.
Brewing Variables that Influence Extraction
Factors that will affect the efficiency and quality of the brewing process and will ultimately impact the flavor perception of the final product are:- Contact time & grind size
- Type of water
- Water temperature
- Turbulence (how much agitation or stirring occurs)
- Filtration type
- Brew method
Contact Time & Grind Size
Contact time is the length of time the grounds and water are in contact with each other. As we already outlined above when we discussed extraction, generally speaking, the longer the coffee particles and water are in contact with each other, the higher the overall extraction will be. In addition to the contact time, the extraction rate is also influenced by the particle or grind size of the ground coffee.
Smaller, more finely ground coffee particles have more surface area exposed to the brewing water so that the dissolvable solids are more readily available, and the distance that the coffee flavor particles must travel to the surrounding slurry is shorter. Think of the difference in the time it takes for a large piece of ice to melt versus the time it takes for ice chips to melt. Smaller pieces will more quickly dissolve into water. This means that coffee ground into smaller particles, usually called “fine,” will extract much more quickly than larger or “coarse” grounds. Since the extraction rate is higher for finely ground coffee particles, the overall contact time required for smaller particles to reach an ideal extraction is shorter than for coarsely ground coffee. This information is essential to consider for each brew method, as different extraction methods, such as gravity-based filter brewing, full immersion, or espresso preparation, require considerably different particle sizes for optimal extraction.
Choosing the proper grind size for each brew method is a critical part of successful brewing, and it will affect every other variable in the brewing process, such as which brew method to choose in the first place, the type of filter you’ll select, contact time, and the desired quantity of coffee to be brewed. Keep in mind that all of the brewing variables are interdependent. Here are a few tips when grinding:
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Shorter contact times and smaller batch sizes require a finer grind size.
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Longer contact times and larger batch sizes require a larger grind size.
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Espresso and pressurized extraction methods often require an excellent grind size due to the short brew times and rapid extraction of soluble solids, aided by pressure and more stable heat.
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Gravity-based filter methods (pour-over brewers, automated drip machines, steep and release drippers) typically require medium-fine, medium, or medium-coarse grind sizes depending on batch size, contact time, and filter type. The larger the batch, the coarser the grind is needed.
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Full-immersion methods (French press and cold brew) usually require a medium-coarse to coarse grind size depending on filter type, batch size, and contact time. Since these methods take longer, usually 4 to 5 minutes for hot brewing and 12 to 24 hours for cold brewing, they need a larger grind size for the longer total brew time.
Water
Water is one of the most critical factors in brewing coffee, but it often gets overlooked when evaluating variables and troubleshooting potential problems. Brewed coffee is over 98.6% water, and espresso is usually around 90% water. This means that no matter how great your coffee beans, brewing equipment, and technique are, you’ll likely end up with a lackluster cup of coffee if you start with poor-quality water.
Can I use tap water to brew coffee?
It depends on your tap water. Remember that most water is not just water but has minerals and other particles dissolved in it to give it a TDS (total dissolved solids) level. The Specialty Coffee Association recommends using clean and fresh water that is 150 ppm (mg/L), free of odor and has a neutral pH level for best brewing results. While 150 ppm TDS is the target, the recommended range can be as low as 75 ppm or as high as 250 ppm. In this range, water usually tastes like mountain spring water and is optimal for brewing coffee. If there are too few dissolved minerals (< 75 ppm), the water can taste flat and lack depth. Conversely, too many dissolved solids (> 250 ppm) can produce water that tastes muddy, muted or dull. The SCA also recommends using water at a hardness level of around 70-80 ppm (mg/L) of combined calcium and magnesium. This concentration has enough mineral content to provide sufficient body and mouthfeel to a coffee while limiting the potentially chalky textures or buildup problems within brewing equipment at higher levels. By using good, fresh, and clean filtered water, you’ll be able to prevent any issues that might otherwise arise from using overly hard water.
It can be challenging to track down the specific TDS and hardness measurements of your water since it varies widely from city to city, but it can generally be assumed that the following water sources will have levels that fall within these ranges:
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Filtered Tap Water: 50-800 ppm (varies widely by geographical location)
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Distilled Water: 0-2 ppm (TDS too low)
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Bottled Drinking Water: 2-250+ ppm (varies by brand)
If you are going to use tap water we recommend using a gravity-based carbon filter (like a Brita, PUR, or Soma) to remove chlorine. Chlorine is added to most municipal water to make it safe to consume, but it can also bind with phenols from the coffee to produce chlorophenol. This awful-tasting chemical will undoubtedly detract from delicious coffee flavors. Using something like a Brita filter will ensure the water you use for coffee is safe, and chlorophenols won’t affect the taste. Unfortunately, carbon filters, like the Brita, do little else to remove dissolved chemicals and minerals from water, so the TDS and hardness of tap water can still fall above the ideal range, potentially producing other off-flavors or causing limescale to build up on brewing equipment.
How do you measure the TDS of your water?
The best way to measure the TDS of your water is to purchase a TDS meter. They are relatively affordable, starting at around $20, and are widely available. Note that measuring the TDS simply provides a numerical value for the total quantity of dissolved materials but does not indicate which minerals and chemicals are present. As such, knowing this number can tell you if the TDS of your water might be causing taste issues with your coffee if you need to replace your water filter, or if you should consider upgrading to a Reverse Osmosis system.
Temperature
The higher the brew water temperature used, the higher the extraction rate. This is because the compounds within coffee typically dissolve more easily when exposed to more energy in the form of heat. Most coffee resources recommend an average slurry temperature between 195ºF and 205ºF for ideal brewing. Remember that this range refers to the average slurry temperature during extraction, not the starting water temperature before brewing begins. Water can lose heat quickly, especially with smaller volumes used when brewing a single cup, as the atmosphere or the material of the brewer itself can pull heat away from the water.
To get the slurry temperature ideal for the entirety of the brewing process and to compensate for heat loss, we recommend beginning brewing with a higher water temperature. We think it’s best to use a starting water temperature of 208ºF – 212ºF (98º-100ºC) for light to medium roasted coffees, effectively just below boiling. This is effective because these coffees tend to be more dense, and extracting the desirable flavor compounds is more challenging. Coffees that have reached a higher degree of development during the roast process (medium-dark to dark roasted coffees) more readily release soluble material during extraction and, therefore, don’t require as much heat to extract. In our experience, we have found that lower starting temperatures, like 195ºF – 200ºF (90º-93ºC), work just fine for dark roasts and prevent over-extracting some of the more bitter and harsh flavors.
Turbulence
A little agitation, also known as stirring, can significantly increase the extraction rate. As coffee begins to extract from the grounds, the concentration of solids dissolved into the liquid closest to the coffee particles is higher than in the areas farther away from the grounds. By giving the grounds a little stir, you are more evenly dispersing the concentration of soluble solids throughout the slurry. Therefore, the concentration difference between the coffee particles and the surrounding brew liquid is higher, encouraging extraction from the coffee particles to the surrounding slurry, making your ideal extraction a little quicker.
Filtration and Flavor Perception
The type of equipment or device used to brew coffee will significantly impact how the coffee tastes—due to the shape and size of the brewer, the batch size, and the technique by which the coffee is brewed. However, the type of filter used for each brewer will have the greatest impact on the mouthfeel, flavor clarity, and the brewer’s overall perception of flavor when enjoying the brewed coffee. This is because different filters can change what physically ends up in the cup and what stays behind. Listed below are the most common filtration types used in coffee brewing and how they can impact the flavor perception of the final product.
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Paper filters are the most commonly used filters in coffee brewing. With a few exceptions, paper filters are mostly used in gravity-based drip brewers like automatic coffee makers and manual pour-over brewers like the Chemex or Hario V60. From a flavor standpoint, paper filters produce a coffee-drinking experience with the cleanest perception of flavor. Paper filters comprise tightly bound paper fibers that retain large and tiny insoluble materials and a good amount of fatty oils produced during brewing. By reducing the amount of the fine insoluble particles and oils, called the brew colloids, you’ll have a much greater opportunity for clarity of flavor. As a result, coffees filtered through paper tend to be lighter in mouthfeel and body but contain more easily distinguished flavors.
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Metal and Mesh filters typically yield brewed coffee with a heavy mouthfeel, full body, and a low to medium flavor clarity. The perforated metal and metal wire mesh can separate the brewed coffee from the grounds, but it doesn’t hold everything back. These filters usually cannot fully restrict brew colloids, fine insoluble particles, and oils, which results in a heavier and sometimes ‘dirtier’ mouthfeel that emphasizes body over clarity of flavor. The common French press is a popular brewing method that uses this type of filter.
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Cloth Filters are typically woven tightly enough to hold back larger insoluble particles, but small insoluble particles can make it through the seams, and some oils will always end up in the cup. Cloth filters generally result in a cup with a mouthfeel and clarity between paper and metal filters. With this type of filtration, we generally taste a medium to heavy body with a medium flavor clarity. Proponents of cloth filters love them for being able to taste the oils without most of the gritty, insoluble particles found with mesh filters.
It’s worth noting that all of the above filter options have size options within their category that vary in density and can affect your desired mouthfeel or body of coffee. You can try different sizes once you’ve decided on the ideal filter type. Generally- the larger the size, the heavier the mouthfeel and the lower the clarity, as you’ll get more oils.
To summarize: Love body and a lingering finish? Try a metal or mesh filter. Love the clarity of flavor with less body? Go for a paper filter
Brewing Method
Finally, the device you choose to use for brewing coffee will also have a significant impact on the way the coffee will taste. As mentioned, this is due to the shape and size of the brewer, the batch size, filter type, and the technique by which the coffee is brewed. Brewing method selection is all about knowing what you want to get out of a particular coffee and understanding what each method has to offer. The same coffee brewed with different methods will have different perceived tastes and characteristics. Below are some of our favorite brewing methods and how they influence flavor perception.
Brewing Takeaways
1. Use good water for good coffee
2. Use paper filters for flavor clarity, mesh/metal for increased body and mouthfeel
3. If your coffee isn’t tasting right (sour or bitter) try changing your brew variables: temperature, time, grind size, and turbulence.
Cheat Guide to Perfect Brewing
If your coffee tastes too sour, bright, acidic, or weak try the following:- using a higher brew ratio (increase the amount of coffee used in relation to water)
- using a slightly finer grind
- agitating (stirring) the grounds while brewing
- increasing water temperature
- increasing contact/brew time
- using a lower coffee to water brew ratio (decrease the amount of coffee used in relation to water)
- using a coarser grind
- decreasing contact/brew time
- decreasing water temperature