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January 18, 2020 8 min read

One of the coffee production’s most interesting steps is roasting- taking a mostly uninteresting green coffee seed and converting it into an amazingly aromatic and surprisingly complex bean.  The coffee seed itself is almost flavorless, sans the vegetal taste, but freshly roasted coffee is intoxicating, evocative, and delicious.  How exactly does this happen?

Specialty roasters worldwide tend to be self-trained. Many of these individuals mastered their trade by way of meticulous trial and error. Every roaster has their own aesthetic, philosophy, and style. They know how to reproduce drinks they enjoy, but even they don’t always fully understand the complexities of the entire process. This doesn’t imply that roasted and delicious coffee is a rarity; rather, this is indicative of a bright future for high-quality coffee roasts, because there is much to develop and improve, which will inevitably result in enhanced roasting techniques.

Dark or Light, Slow or Fast?

In the interest of simplicity, many refer to roast level in terms of a bean’s final color (dark or light), or the period required to achieve that color (slow or fast). But describing a coffee as lightly roasted isn’t full sufficient to capture the roasting process it underwent; two of the same beans, both roasted lightly- but with other roasting conditions varying- can taste different despite outward resemblance of the beans.

An entire host of various chemical reactions transpire during the roasting process.  One of which is moisture evaporation which reduces weight. A slow roast (between 14 and 20 minutes) can lead to greater weight loss (approximately 16 to 18%) than quicker roasting, which only takes 90 seconds to achieve. Though they tend to be costly, slow roasts often produce higher-quality cups of coffee.

When roasting, to goal is to establish a balance between a few important flavor dimensions of coffee: sweetness, bitterness, and acidity. The general case is that coffee’s acidity will be minimized if roasted for a longer than usual duration. Alternatively, the more time a roast takes, the more bitter the coffee will taste.

Roasting Flavor graph

Sweetness is symbolized with a bell curve, peaking between bitterness and acidity high points. Smart roasters are able to manipulate the spot where coffee is sweetest in proportion to its roasting degree, resulting in either an acidic but sweet beverage or a muted but sweet cup, through the use of different roasting profiles.  Particularly regarding finer coffees, a great deal of care is taken to perfect the processes and techniques necessary for a cup to reach its fullest potential.

While extensive research has also been put into commercial roasting of poor-quality coffee, a majority of it is only in reference to the efficiency of the process and the approaches used to produce instantly soluble coffee. Since these beverages aren’t that flavorsome or interesting, little-to-no effort has been taken to develop sweetness or to retain flavors unique to certain varieties of coffee. 

Roasting Stages

There are numerous important stages during the roasting process, and the conditions under which beans reach these stages is known as a “roast profile.” Most roasters have their profiles meticulously tracked, allowing them to replicate roasting within tight boundaries through control of time and temperature.

Roasting levels

Drying/Green

There is between 7 and 11% water in raw coffee by weight, evenly spread through the bean’s thick structure. Like anything else cooked, coffee won’t properly brown when water is present.

Once the roaster is loaded with coffee, it needs some time to absorb enough heat so it can begin the water evaporation and drying process. There are little-to-no changes in smell or appearance during the first three minutes or so of roasting.

Yellowing

After water is released from the leaves, browning reactions begin. During this stage, coffee beans have a basmati rice scent. When expansion of the beans starts, their papery-thin skins – a.k.a. the chaff – begin flaking off. Separation of the chaff from roasting beans occurs, courtesy of air that flows through a roaster, before being collected and removed safely to prevent risk of fire. 

The first couple of roasting stages are important: if coffee isn’t dried properly, then it won’t evenly roast during subsequence stages. Although each bean’s exterior will be sufficiently roasted, its interior will basically be undercooked. The resulting coffee will have an unpleasant taste, with a mix of outside bitterness and a grassy, sour flavor stemming from the inside. Once this occurs, roasting slower won’t resolve the issue since different coffee parts will continue roasting at different rates.

First Crack

After browning reactions start to speed up, water vapor and gas (the majority of which is carbon dioxide) build up within the bean. When the pressure reaches a certain level, the bean breaks open, creating a popping sound and about doubles in volume. Coffee flavors then begin to develop.

Roasters will notice decreases in coffee temperature rates, even though they might be adding heat. If too much heat is added, stalling of the roast may ensue, essentially “baking” the coffee. The result: a low-quality cup of coffee.

Roasting Development

The surface of the coffee beans will get smoother- albeit not completely- after the initial crack stage. This part of the roasting stage establishes the bean’s final color, as well as the roasting degree. 

At this point, roasters decide at what point to end the roast- assessing when the balance of bitterness and acidity is at the right level. As the roast is transpiring, acids inside the beans degrade rapidly, while the bitterness level increases.

Second Crack

From here, beans start to crack once more, but this time, with a snappier noise. Once the second crack is reached, oils are pushed to the bean’s surface. Much acidity is lost- and a different type of flavor develops, sometimes referred to as a generic “roast” flavor. This flavor isn’t a function of the type of coffee utilized, as it is instead a by-product of burning or charring, as opposed to working with the coffee’s intrinsic flavors.

Some terms are used for darker coffee roasts- such as “Italian roast” or “French roast.” Each term is used to suggest significantly darker roasts that are both generally high in bitterness and body. However, the original characteristics of the raw coffee are almost all lost.

Quenching

Once the roast is complete, coffee needs to be quickly cooled to stop the roasting process and prevent baked (negative) flavors from developing. When roasting in small batches, this is usually accomplished with cooling trays, which hastily draw air to rapidly cool the coffee down.

coffee quenching

When roasting large batches, air by itself often isn’t sufficient – water mist gets sprayed on the coffee to accelerate cooling further.  As the steam evaporates, heat is released from the beans. When this process is performed properly, it won’t have any impact on quality per se but can reduce shelf life slightly. Sadly, though, many companies add a surplus of water to enhance bean weight, thereby artificially increasing their “supply”.  This isn’t just bad for the coffee’s quality; it is also unethical.

Coffee Compounds to Be Conscious of During Roasting

Coffee Sugars

Green coffee often contains a substantial amount of simple sugars.  At certain roasting temperatures, these sugars can be very reactive. Once water evaporation from the bean transpires, the sugars begin to respond to heat in various ways. 

A number of these sugars undergo caramelization responses, producing caramel notes that become apparent in the resulting coffee.  Be mindful, though, that sugars that respond in this manner aren’t as sweet, which surprisingly can make the coffee more bitter than it would have been before this caramelization. 

Some sugars react with proteins in coffee- known as aMaillardreaction. This umbrella term addresses the browning responses between sugars and proteins generally, be it when meat is browned on the grill, or when coffee and cocoa are roasted.  TheMaillard reaction is responsible for hundreds of different compounds in coffee that give it both its distinctive taste and aroma.

Coffee Acids

The contents of green coffee include a number of acid types, many of which taste pleasant but others that don’t. One primary roasting objective is to get unpleasant acids to react without producing a negative taste while simultaneously preserving the pleasant tasting acids.  Several acids remain stable during the process of roasting – quinic acid, for one – which gives the beverage a clean and satisfactory finish.

Roasting Coffee at Home

Ever since the middle of the 20th century, many people have purchased unroasted coffee and roasted it at home. Home roasting is fairly inexpensive and fun, but it can be a bit of a challenge to accomplish the same roasted coffee quality delivered by commercial roasters.

Roasting at home allows for producing small coffee batches, allowing you to explore a variety of green coffee varieties.  You will likely endure surprise successes and horrible attempts during this endeavor- try to treat coffee roasting as a hobby rather than as an approach to saving a few bucks over purchasing pre-roasted beans. It likely won’t be worth the aggravation if it feels like a chore.

More companies than ever these days sell green coffee on the internet. Although raw coffee’s shelf life is longer than its roasted counterpart, you are discouraged from purchasing it in bulk. Over time, raw coffee degrades and won’t yield the same flavors. 

Here are a couple tips for selecting a green coffee:

Tip #1: When choosing a green coffee, ensure whatever you’re purchasing can be genuinely traced. There are tons of low quality green coffees for sale that aren’t worth your time or money.

Tip #2: Consider purchasing green beans from a company thatalsosells roasted coffee. This will let you benchmark your roasting, allowing you to contrast roasted beans against commercial ones to monitor your progress.

Home Roasting Machines

You can roast coffee with just about anything capable of generating sufficient heat. Case in point: raw coffee beans can be placed on baking sheets and left in an oven until they are brown. Mind you, the outcome will be quite awful – an uneven roast with some of it burning from conduction from the sheet. This approach of roasting fails because it neglects the requirement for coffee bean agitation and movement during the process to achieve even roasting. It is easier to effectively roast coffee with a wok; however, a lot of stirring time is needed, making this process frustrating and tiresome.

Coffee wok roasting

Most people begin with something slightly more sophisticated. Some use heat guns to agitate coffee beans consistently during the roast. Others use modified popcorn machines, which can be bought second-hand inexpensively. These machines can roast coffee quite reasonably: they are capable of roasting small coffee batches very fast – in less than five minutes.  However, they won’t produce an even roast. Those who are partial to darker roasts tend to achieve some success with popcorn machines, though.

However, if you truly wish to roast coffee at home properly, it will require a roaster specifically designed for it.  Begin with a fairly cheap and basic machine to determine if coffee roasting is something you enjoy. Beginning with a low-expectations approach is fun and simple- and you won’t have any regrets if you opt to let professional roasters handle this aspect of the coffee preparation process.

Hot Air Roasting

Though on a much smaller scale, hot air roasting machines mimic fluid bed commercial roasters. They function very similarly to popcorn poppers: hot air moves and agitates beans around a roasting chamber, yielding a balanced roast while offering the heat required for browning. You can also regulate fan speed to slow down or speed up the process. Though they are cheaper than most drum roasters, hot air roasting machines are an ideal starting point, especially those unfamiliar with coffee roasting.

Drum Roasting

One of the most popular and economical devices you can use is a drum roaster. The design of these machines is fairly simple, comprised of a cylindrical drum that rotates. Heat is applied either through the middle or right beneath the drum via a conduit. Heat may originate from gas flames or electric heating components regulated with (and even without) profile controllers.

coffee drum roaster

Centrifugal forces push beans against the sides, transferring a surplus of heat on portions of a bean.  When using a drum roaster, conduction (approximately 25%) transfers heat, but the majority of heat transfer occurs from convection (which is approximately 75%). Temperature is measured with a pair of system gauges: one measures flame temperature and the other measures the temperature of the ambient bean inside the drum. This is usually done with a thermocouple.

Drum roasters are consistently effective at producing an even roast, although scorching/tipping can occur if excessive heat is used or if a rotating drum’s speed is too high. 

Drum roasting can differ substantially in capacity. “Sample roasters,” which usually have a small capacity of between 200 and 500 grams and are used mostly for quality control and laboratory purposes. Several commercial units are capable of roasting between 120 pounds (one bag) and more than 5000 pounds in one batch.


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