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roasting process

glossary: roasting process

learn about the coffee roasting process, including key phases like carmelization, and how each coffee roast profile is developed. understand the intricacies of the coffee beans roasting process and discover what makes each roast unique. get detailed insights into the coffee roasting phases that define the flavor and aroma of your favorite brew.


A measurement of forced air inside the roaster, can be variable and is typically used to varying degrees in all roasting types (i.e., it is not exclusive to air roasters). May be manipulated manually by employing dampeners.


Prevailing, and often uncontrolled atmospheric and weather conditions in the roastery such as temperature and relative humidity.


A now-infrequently used style of dark roasting to impart additional roast color and smoke/roast notes to a coffee without risking a fire. Typically executed by closing the airflow dampener and simultaneously reducing or completely cutting the gas supply to a drum roaster, usually well into and beyond second crack. Not to be confused with the aroma of roasted coffee.


the amount of green or roasted coffee that forms a single roast


Used to refer to a coffee roasted with some combination of too low of heat and/or too slowly, expressed sensorially as flattened acidity and muted flavor profile.


data captured by the bean probe.


These are a set of actions roasters perform, between roasting batches of coffee to create and maintain the consistent operational temperature of the roasting system so that each batch roasts as closely as possible to the last. There are many different ways to achieve the same end – a consistent charge temperature, a consistent roast time and adherence to roast profile, and a consistent result in the cup. 


Two or more different coffees (lot, origin, processing, etc.) mixed together in specific ratios to create a synergy of flavors that match a desired flavor profile. Blends may be created before or after roasting, each with differing advantages and drawbacks.


A non-technical term for a disc-like cavity visible on the surface of the bean which occurs during roasting. Blowouts may be caused by trapped moisture and/or carbon dioxide gas heating under the bean’s outer layers and escaping the cellulose rapidly. This build up and release may happen if the bean is exposed to excessive heat after first crack, or with improperly dried green coffee.


The oxidation of sugars which, in coffee roasting, often results in a sweet, nutty flavor. This is a non-enzymatic browning reaction. The reaction needs heat to occur, as it requires not only the breakdown of sugar, but also the removal of water from the material, the latter being achieved by evaporation. However, unlike the Maillard reaction, caramelization is pyrolytic

The reaction time and temperature depend on the type of sugar(s) involved. Caramelization results in the recomposition of carbohydrates (sugars) into long polymer chains giving coffee its brown color. The brown colors are produced by three groups of polymers: caramelans, caramelens, and caramelins, the process also results in the release of volatile aromatics, such as diacetyl, that gives caramel its distinctive smell.


Silver skin of the green coffee that comes off during roasting due to the expansion of the bean volume. The silver skin adheres to the bean and is held in place by the seam along the coffee bean face. As this opens up during roasting, the silver skin, or now chaff, is released into the roasting chamber. Coffee chaff is light and the airflow of a roaster usually carries it to a separate chaff can for collection. It is also flammable and may smolder for hours if ignited and left unattended.


to load, or to begin the roast. Charging the hopper means loading it with green coffee while charging the drum or roaster means beginning the roast.


A visual indicator of the loss of moisture from the bean towards the end of the drying stage of the roast. Green coffee becomes progressively more yellow in appearance, associated with the beginning of the Maillard stage. Color change in the coffee will continue throughout Maillard Reactions from yellow to orange to light brown, and after first crack will progressively darken towards blackish hues if left to continue roasting.


Heat transfer in the roaster via direct contact


Heat transfer in the roaster via hot air


The evaluation of a coffee sample using a standardized method of assessing attributes associated with aroma and taste and a standardized form to collect the resulting data, used as a quality control measure to assess sensory characteristics, inherent green coffee defects, roast defects, production roast quality, etc.


A quality control tool used to capture evaluations and scores regarding specific coffee lots. Different forms may be used in conjunction with specific protocols in order to evaluate a particular set of attributes and/or for a particular purpose (e.g., one might use a different set of protocols and forms for evaluating production roasts and sample roasts).


the release of gases, such as carbon dioxide, from the cellulose structure of roasted coffee beans. This phenomenon is most evident during the first few days after roasting.


Usually used to refer to the coffee’s progression or duration after the beginning of first crack (sometimes “Time After Crack” or “Post-Crack Development”). Considered one of the three main roast stages. The coffee is at the most exothermic and fragile state during this stage of the roast; the beans release moisture, gases and thermal energy into the drum. The development stage is considered a crucial time during which the chemical reactions that have taken place previously, including sugar and acid development and the roaster’s influence on the body of the resulting roasted coffee, are further expressed and the coffee’s final flavor profile is resolved. The development stage ends when the coffee is dropped into the cooling tray, effectively completing the roast.


To discharge the roasted beans from the machine at the end of the roast, e.g., I dropped the roast at 12 minutes or the drop temperature was 420 F


Heating of a material (green coffee) at higher temperatures, which often produces gases. In coffee roasting, as one example, as the temperature continues increasing, this process involves a next step, pyrolysis.


The first roast stage, lasting from the initial charging of the drum/chamber until the beginning of Maillard stage. This phase, which is evidently endothermic, is marked by a lightening of the coffee color and a noticeable grassy/vegetal smell from the beans. This phase is often referred to as the “enzymatic stage,” “dehydration stage,  or simply “stage one” of the roast.


A chemical reaction that relies on a constant transfer of energy (in roasting, heat) in order to occur. This constant uptake of heat energy depletes the environmental temperature, therefore, in order to allow this reaction to be sustainable, a continuous input of heat needs to be supplied to the roasting drum/chamber


A process or reaction that releases energy, usually in the form of heat, from a system (the coffee bean) into the environment (roast drum/chamber). There are many exothermic phases during coffee roasting, the first, most obvious one being the phenomenon known as first crack.


A confusing term which can refer to either the ambient temperature in the space housing your coffee roaster or, more frequently, to refer to the measurement of convective heat measured by the exhaust, environment, and/or return probe in the roasting system.


A roast defect where one side (face) of the coffee bean is overly darkened usually due to increased contact time with the drum, often caused by a combination of incorrect drum speed (too fast or too slow) and/or an overfilled roasting chamber.


The phenomenon of both audible and visual cracking of the roasting coffee bean. Above approximately 380 F (depending on variables including green coffee metrics and probe placement), most of the moisture in the bean will have evaporated, leaving a pocket of highly pressurized steam trapped at the center of the bean. When the pressure is too high for the now-fragile cellulose structure to withstand, it pushes out of the bean, usually at either end (the tips). In all but the rarest of circumstances, all successfully roasted coffee undergoes first crack.


The experience of multiple attributes that comprise the taste and aroma of coffee, creating a unique flavor of that particular coffee lot/blend, which can often be attributed, in part, to place of origin, botanical variety, processing method(s), and/or roast degree.


A series of many sugar browning reactions between sugars and either proteins or amino acids, catalyzed by heat (usually beginning at or around 302 F). The Maillard reaction contributes color to roasting coffee and complexity of flavor through the types of sugars, the amount of perceived acidity, and the structure of the viscosity. The Maillard reaction is responsible for nothing short of the development of coffee’s essential sensory character.


The second stage of roasting, after the drying stage, during which the Maillard reaction(s), Strecker Degradation, and number of other browning and complex pyrolytic reactions take place, radically modifying the chemical make-up of the bean. The advent of this stage is noted by the roaster when they see the beans change from green to yellow. The smell changes from hay to baking bread and finally roasted coffee. While the Maillard reaction and caramelization continue, the end of the Maillard stage is typically signalled by the onset of first crack. (See also: Color Change).


Roasted coffee packaging allowing air and gasses to flow out of but not into the package. (See also: Degassing)


A process by which organic compounds of condensed structure, mainly carbohydrates and proteins, are decomposed at high temperatures. Extreme pyrolysis leads to carbonization. Pyrolysis in coffee roasting is usually seen to occur at/after first crack and second crack. (See also: Caramelization)


A system of protocols and practices used to establish and assess the predetermined quality of a product. In coffee roasting this can include specific roast goals (adherence to the roast profile, measuring roast loss per batch, checking the specific roast color per lot per batch, cupping for specific flavor profile consistency, etc.)


A coffee bean whose intrinsic chemical make-up lacks the appropriate chemical compounds to complete the caramelization and browning reactions in the roaster. May be caused by underipe beans, or those chemically underdeveloped for other reasons. Many quakers may be sorted prior to export by flotation channels at the wet mill or density sorting tables at the dry mill, as browning is caused by sugars, proteins and other heavy chemical composites may be absent and thus contribute to lack of density. Although this defect is not noticeable in green coffee, these beans are evident after roasting in the cooling tray. They are pale blond to cinnamon in color and distinctly different from properly roasted coffee beans; sensorially they may express as raw or rancid peanuts, oily, or fatty, and can have significant negative consequences in a coffee’s flavor profile.


Using water to assist in cooling a completed roast batch. Many large, industrial roasters may choose to use water quenching after roasting in addition to cooling the coffee in a cooling tray. Coffee beans continue to roast internally even after they have left the drum, until the temperature has reduced throughout the whole bean. Thus, there may be a need to cool larger batches as quickly as possible in order to achieve roast consistency. If performed discretely, the water will evaporate on the beans’ surface, adding no additional weight or moisture composition to the coffee.


a measurement of the change in temperature over a given time period. E.g., the RoR at the beginning of Maillard Stage was 10F degrees every 30 seconds. Sometimes called “heat delta.”


Many roasters will suggest that you rest coffee a number of hours or days prior to first brewing. This allows the coffee to degas – releasing gases, especially carbon dioxide, that have built up in the bean as a result of roasting and can contribute to bitter and other undesirable flavors.


The color of the finished roast. Although there have been many names given to different degrees of roast; light, medium, dark, city, french roast, etc., none actually describe a metric or objective measurement. Spectrophotometers or similar color reading devices can objectively measure reflectivity and may provide repeatability and consistency to achieve the same degree of color/roast for every batch.


The charted visualization of data from temperature probe(s) collected during the roast, plotted as time (y axis) over temperature (x axis), and a part of a complete roast profile. (See also: S Curve).


The difference between the weight of a batch of green coffee and the weight of the same batch once roasted (sometimes called “post-weight” and may be expressed as a percentage). This figure is important for multiple reasons, including roast data, but also inventory, forecasting and costing your product.


A roast profile can be thought of as a recipe. It is the purposefully created blueprint of the manipulation of the coffee roasting process, using the application of heat and airflow, to influence the temperature of a batch of coffee over the time of the roast. The roast profile may include (but is not limited to) its roast curve, rate of rise, roast stage times, temperatures, and roast stage percentages, end time, roast color, roast loss, etc.


Three convenient divisions of a roast into segments, referred to in this glossary as Drying Stage, Maillard Stage, and Development Stage, often divided into time ratio percentages.


The name given to the ‘normal’ roast curve produced by plotting the roast data of time (y axis) over temperature (x axis) on a graph. Left to its own devices coffee introduced into a hot drum/chamber will reduce the environmental temperature until the beans and drum/chamber air come to equilibrium. With enough energy input, the coffee, as an endotherm, will ascend in temperature until it exotherms at first crack, releasing energy and moisture. A roaster operator can influence this process and collate the data to create a roast profile.


a small batch, often around 100g, usually roasted to a specific roast profile which encourages minimal manipulation by the roaster (operator and machine). These roasts are usually lighter in color than a standard production roast and are roasted on a sample roaster. These roasts are typically created for the purpose of evaluating the quality of the green coffee. Sample roasts of different coffees are often cupped side-by-side. As such, among the more important qualities of good sample roasts is consistency, particularly of roast loss, roast color, and roast length.


caused either by excessive drum or environment temperatures (e.g., too hot of a charge temperature) and/or a slow drum speed, scorching is caused by conductive heat contact from the drum or bean-to-bean. This roast defect presents as dark, burnt patches on the front or curved surfaces of the bean. The result is an ashy, burnt, ‘roasty’ taste to the coffee even if it is not dark in overall roast color


The second audible exothermic reaction the coffee beans undergo when roasting. A faster, shallower sound that first crack occurring at around 440 F, second crack is a further marker of pyrolysis and indicates an increased fracturing of the cellular matrix. Coffees taken to, through, and/or beyond second crack are typically considered “dark” in roast color.


This term can refer to coffee from one lot, farm, location, mill, or simply country depending on the person using it. It is the opposite of a blend (although, based on the loosely defined nature of S.O.s, one might create a “single origin blend” of coffees from the same farm or country, e.g.).


Roasted coffee which has lost its freshness and flavor profile. Roasted coffee begins to stale the moment it exits the roaster. Coffee staling, as with all staling, is due to contact with oxygen and the process of oxidation. The more intense the exposure to oxygen, the faster your coffee will oxidize. There are many ways, via chemical intervention (e.g. nitrogen flushing), packaging and storage protocols (e.g., One-way valve bags), to help to slow this process down. However, you cannot fully arrest coffee staling. 


a term used to describe a phenomenon when the batch of coffee being roasted resists heat absorption. The rate of temperature rise of the coffee will slow and may dip into negative temperature deltas over time. It may also, in the most extreme cases, slow to such a degree as to not achieve first crack. In other cases, it may achieve first crack, but the slower portion of the roast may contribute to lower acidity, hollow flavors and a less complex cup. These characteristics are summed up by the nomenclature baked coffee.


This chemical process is dependent on the Maillard reaction, and the availability of amino acids present in the green beans. The carbonyl-grouped molecules created as a result of the Maillard reaction interact with amino acids, contributing to melanoidin production (heterogeneous, nitrogen-containing brown polymers that absorb light). The melanoidin reaction complex is also responsible for the production of aromatic compounds such as aldehydes and ketones.


a tongue-in-cheek reference to coffee’s flash point, around 475F.


A roast defect where the narrow, fragile tips of the coffee beans are burnt by applying too much heat too quickly. These burns are caused by heat affecting the most fragile part of the bean, and may not be evident on other parts of the bean, or conversely, may be seen in conjunction with scorching or facing.


This roast defect occurs when the chemical reactions responsible for protein and sugar browning either did not have enough time to be fully expressed, or when the coffee has had little time to develop the results of these reactions after first crack. Examples of causes may include roasting too hot and/or too fast, or dropping the coffee from the roaster into the cooling tray too quickly after first crack. The result in the cup is green, grassy, peanut kernel type flavors, with sharp sour acidity and little body. The flavors will be uncomplex and lack a true representation of the coffee’s potential.

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