Why We Roast Coffee Beans ?
Coffee beans are the seeds of the cherries of the coffee tree. Each cherry typically contains two beans whose flat sides face each other. When steeped in hot water, raw, or “green,” coffee beans offer little in the way of what one might relate to as coffee taste and aroma.
Roasting green coffee creates myriad chemical changes, the production and breakdown of thousands of compounds, and, the roaster hopes, the development of beautiful flavors when the beans are ground and steeped in hot water. Among its many effects, roasting causes beans to
- Change in color from green to yellow to tan to brown to black.
- Nearly double in size.
- Become half as dense.
- Gain, and then lose, sweetness.
- Become much more acidic.
- Develop upwards of 800 aroma compounds.
- Pop loudly as they release pressurized gases and water vapor.
The goal of roasting is to optimize the flavors of coffee’s soluble chemistry. Dissolved solids make up brewed coffee’s taste, while dissolved volatile aromatic compounds and oils are responsible for aroma. Dissolved solids, oils, and suspended particles, primarily fragments of bean cellulose, create coffee’s body. (coffee roasting)
Green Coffee Chemistry for coffee roasting
Raw coffee beans are dense, green seeds consisting of about one-half carbohydrate in various forms and one-half a mixture of water, proteins, lipids, acids, and alkaloids. Roasters do not need to know much about green coffee’s chemistry to roast delicious coffee, but I offer the following summary to familiarize readers with the primary components of green coffee. (coffee roasting)
A raw coffee bean’s structure is a three-dimensional cellulose, or polysaccharide, matrix containing approximately a million cells. Coating the cellulose strands within that matrix are hundreds of chemicals that the roasting process will transform into the oils and soluble material that determine brewed coffee’s flavor. Green coffee’s cellulose structure contributes half of its dry weight. The cellulose contributes little to coffee flavor but does trap some volatile compounds, which are responsible for aroma, and adds to brewed coffee’s viscosity, increasing its perceived body. (coffee roasting)
Sugars, dominated by sucrose, make up 6%-9% of a green bean’s dry weight and provide sweetness in the cup. Sucrose also contributes to development of acidity, as the caramelization of sucrose during roasting yields acetic acid. (coffee roasting)
Lipids, primarily triglycerides, make up approximately 16% of green coffee’s dry weight. Although lipids are not water soluble, brewed coffee contains some, especially when the brewing method uses either no filtration (e.g., cupping) or a very porous filter (e.g., espresso, French press, or metal-or cloth-filter drip). Lipids in brewed coffee help retain aroma and contribute to coffee’s mouthfeel. Higher lipid content is generally associated with better green-coffee quality. (coffee roasting)
Unfortunately, lipids also present challenges to quality, as they are vulnerable to oxidation and rancidity during storage of roasted beans.
Proteins and free amino acids make up 10%-13% of green coffee by dry weight. Amino acids and reducing sugars in coffee beans interact during roasting in nonenzymatic browning reactions known as Maillard reactions. These reactions produce glycosylamines and melanoidins that contribute to coffee’s bittersweet flavor, brown color, and roasted, meaty, and baked aromas. (coffee roasting)
Alkaloids: Caffeine and Trigonelline (coffee roasting)
Two alkaloids, caffeine and trigonelline, each account for approximately 1% of green coffee’s dry weight and are responsible for much of coffee’s bitterness and stimulating properties. Caffeine contributes approximately 10% of coffee’s bitterness and most of its stimulant effect. The coffee plant produces caffeine as a defense against consumption by insects. A coffee tree planted at a high altitude would probably produce beans with less caffeine because of the lower risk of insect attack. (coffee roasting)
Trigonelline is perhaps the greatest contributor to coffee’s bitterness, yields many aromatic compounds, and degrades to pyridines and nicotinic acid during roasting. Nicotinic acid is also known as niacin, or vitamin B3; a mere 7 oz (200 g) of brewed coffee, depending on roast degree, contains 20-80 ml of niacin, 26 which is likely responsible for coffee’s documented anti-cavity effect (coffee roasting)
Ideally, water should account for 10.5%-11.5% of green-coffee weight. If moisture content is too low, bean color is typically faded and the cup has notes of hay and straw. A roaster must apply heat cautiously to low-moisture beans, as they are likely to roast too fast. If moisture content is much higher than 12%, green coffee is prone to developing mold and may taste grassy in the cup. Water slows heat transfer within beans, and it requires extra heat input to evaporate. Roasting very moist beans therefore requires extra energy in some combination of added time and roasting power. (coffee roasting)
Organic acids, primarily chlorogenic acids (CGAs), constitute approximately 7%-10% of green coffee’s dry mass. CGAs contribute to coffee’s acidity, sourness, astringency, and bitterness. Robusta coffee’s higher CGA content is likely responsible for its significantly greater bitterness. For both the coffee bean and the coffee drinker, CGAs offer antioxidant benefits. Other organic acids in coffee include citric, quinic, caffeic, malic, acetic, and formic. (coffee roasting)
Gases and Aromatics
Volatile aromatic compounds provide coffee’s aroma. Green coffee contains more than 200 volatiles but offers little aroma. Roasting creates the vast majority of coffee’s aromatic compounds, and so far, researchers have identified over 800 volatiles in roasted coffee. (coffee roasting)
- Data on green-coffee composition refers to the genus Coffea species arabica only. The chemical compositions of Coffea robusta and other species of coffee differ, sometimes significantly, from that of arabica (coffee roasting)
Green Coffee Processing and Storage
Green-coffee processing affects cup quality as well as how one should roast beans. Once a bean has been processed, a roaster must carefully control its packaging and storage conditions to prevent degradation of quality before it’s roasted. (coffee roasting)
Primary Processing Methods
Washed, natural, and pulped natural are the three primary processing methods of specialty coffee.(coffee roasting)
The Washed, or wet, process consists of the following steps:
- Pulping of the cherry to remove the skin.
- Removal of the sticky mucilage layer by fermentation or mechanical means.
- Washing of the beans to remove loosened mucilage.
- Drying of the beans in parchment, either mechanically for 1-2 days or in the sun for 3-16 days.
The natural, or dry, process consists of partially or completely drying the coffee cherries on the tree and then husking the cherries to remove their skins. Alternatively, the cherries are picked when ripe and then dried before husking. (coffee roasting)
In the pulped/natural process, the cherries are pulped to remove their skin and set to dry with the mucilage layer intact. This method delivers a sweeter, cleaner cup than does the traditional natural process. Washed processing produces cleaner, more acidic, more consistent, and generally more-prized coffee than natural processing does. Washed coffees also tend to be denser and require more aggressive roasting. The dry process can take several weeks and yields coffee with less acidity, more body, and earthier flavors than washed coffee. Arid growing areas often use the natural process because it requires much (coffee roasting)
less water than the washed process. Natural-processed coffees burn more easily during roasting, so one should use lower charge temperatures and gas settings when roasting those beans.( coffee roasting)
Green Coffee Storage (coffee roasting)
Until recent years, all coffee was packaged in burlap (jute) sacks and shipped in containers, arriving at roasters months after the coffee was processed. Roasters and importers frequently had the experience of cupping a coffee at origin, and perhaps cupping and approving a “pre-shipment sample,” only to receive coffee ruined by exposure to poor atmospheric conditions in storage or in transit. (coffee roasting)
In the past ten years, several small, quality-driven roasters have spearheaded a revolution in green-coffee packaging and transport. Many roasting companies, even some of the smallest ones, now buy coffee directly from farmers, share cupping and green-grading information with the farmers, and demand speedy delivery of coffee in packaging designed to preserve its freshness and quality. Such packaging is costly but justified, given the ever-increasing premiums paid for specialty coffees. (coffee roasting)
The following is a survey of the more prominent packaging options:
Burlap (jute) bags are the most common and economical option for packaging and transporting green coffee. Jute is a renewable resource, and the bags are cheap; their use requires no special skills or equipment beyond those that are standard at any dry mill or exporting operation. Burlap sacks do not protect coffee from moisture or odors, however, so the coffee is vulnerable to damage during transport and storage. (coffee roasting)
Vacuum sealing is the best available packaging for green coffee. Vacuum-sealed bags protect beans from moisture, odors, and oxygen, dramatically slowing the respiration, and therefore the aging, of green coffee. Before vacuum sealing, care must be taken to measure beans’ water activity to prevent development of mold during storage. Vacuum packaging costs approximately USD 0.15-0.25 per pound (EUR 0.45-0.75 per kilogram), requires special equipment and skill to implement, and often delays shipment of green coffee, so it is not without its costs and risks.( coffee roasting)
Water Activity and Moisture Content
Water activity (aw) is a measure of the strength of the bond between water and the dry material of a coffee bean or other food product. The aw level indicates how likely moisture is to migrate into or out of a bean, which in turn affects how beans interact with their storage environment and how fast they degrade during storage. (coffee roasting)
Water activity differs from moisture content, which is the percentage, by weight, of water in green coffee. The two measures correlate, though their correlation may decrease when moisture content rises above 12%. Both characteristics influence cup quality, the degradation rate of green coffee during storage, and the risk of microbial growth during storage. (coffee roasting)
Physical Changes During Roasting
Roasting causes beans to change color, lose moisture, expand, and become brittle. While all professionals label roast levels based on bean color, there is no consensus on exactly what roast level each name indicates. (coffee roasting)
The first stage of roasting is commonly known as the “drying phase,” although beans lose moisture at similar rates throughout most of the roasting process. During the first few minutes of roasting, degradation of chlorophyll causes beans to change color from green to yellow. As roasting progresses, the beans change from yellow to tan to light brown, primarily due to Maillard reactions. Late in a roast, as the beans approach first crack, the brown color deepens due to caramelization. In a dark roast, carbonization may turn beans black. (coffee roasting)
The microstructure of green coffee is relatively organized and dense, with oils.
coating the cellulose matrix. As coffee roasts, the generation of steam and carbon dioxide (CO2 ) increases pressure within the beans, forcing their structure to expand and pores to enlarge. A couple of minutes before first crack, beans expand enough to begin freeing the silver-colored skin, or chaff, trapped within the folds of their center cracks. When the cellulose can stretch no farther, fissures form within beans and on their surfaces, violently expelling water vapor and gases, creating the popping noises of first crack.
Specialty roasters seeking a light or medium roast typically drop beans between the end of first crack and the beginning of second crack. After first crack, gas production continues, rebuilding pressure within the bean cells. Simultaneously, the bean structure becomes more brittle, setting the stage for second crack. While the primary cause of first crack is the buildup of steam pressure, accumulation of CO2 is the main driver of second crack. Just before or after the onset of second crack, oils bleed to the bean surfaces; almost all roasters would regard this as an objective indicator of a dark roast (coffee roasting)
Inner Bean Development (coffee roasting)
Bean expansion and the release of water vapor and gases during the cracking phases weaken beans’ cellulose structures and make them more porous and brittle. The darker, more porous, and more brittle the inner beans are, the more developed they are. Sufficient inner-bean development is a prerequisite for great grind quality, high extraction, and elimination of undesirable savory flavors. (coffee roasting)
Bean Size, Density, and Weight Loss
Coffee loses 12%-24% of its weight during roasting, depending on initial moisture content, roast degree, and inner-bean development during roasting. The lightest palatable roasts are probably those dropped during the latter stages of first crack and typically have weight loss, or shrinkage, of 11%-13%. About 30 seconds after first crack ends, shrinkage is roughly 14%-16%, while at the onset of second crack, shrinkage is around 17%-18%. Dark, oily roasts may have shrinkage of 22% or more. The light roasts currently popular in the specialty industry lose an average of 14%-16% of their initial weight. (coffee roasting)
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