분자구조를 보면 향의 성격이 보인다.
사람마다 몸무게가 다르듯이, 향기 성분에도 체중
화학에서는 그 성분의 체중을 분자량(Molecular Weight)라고 부르며, 이는 향기의 중요한 성질인 휘발성(Volatile)을 결정하는 요인 중 하나
여러 가지 다른 변수가 있을 수 있지만, 일반적으로 분자량이 작을수록 휘발성이 커서 공기 중으로 쉽게 날아간다.
아세트알데하이드와 말톨
과일향을 내는 아세트알데하이드는 분자량(44)이 비교적 작고 가벼워 휘발성이 크다. 즉 향이 쉽게 날아간다.
캐러멜향을 내는 말톨은 분자량(126)이 크고 무거워 휘발성이 적다.
커핑 시 이런 무거운 분자들을 인지하기 위해 슬러핑(Slurping) 기법을 이용해 입자를 작게 만들어 입안 전체에 골고루 분사시키기도.
커피 향기 물질
분자 구조를 보고 향을 짐작하는 것은 전문가도 불가능하다.
아라비카 커피 생두의 화학적 조성은 총 다당류 함량이 50.0~ 55.0%, 지방질 12.0~18.0%, 단백질 11.0~13.0%, 총 클로로겐산 5.5~ 8.0%, 올리고당 6.0~8.0%, 무기질 3.0~4.2%, 카페인 0.9~1.2%, 트리고넬린 1.0~1.2%로 구성되어 있다.
로부스타 커피 생두의 경우는 총 다당류 함량이 37.0~47.0%, 지방질 9.0~13.0%, 단백질 13.0~15.0%, 총 클로로겐산 7.0~10.0%, 올리고당 5.0~7.0%, 무기질 4.0~4.5%, 카페인 1.6~2.4%로 구성되어 있다.
커피 생두는 로스팅(Roastingㆍ볶음) 공정을 거치면서 화학적인 조성이 급격하게 변하며 커피 특유의 향을 생성한다.
로스팅 공정은 4단계로 나눠지는데, 제1단계에서는 가열된 대류열에 의해서 생두의 표면 온도가 상승되면서 100℃가 될 때까지 수분이 서서히 증발하고, 제2단계에서는 가열이 더욱 진행되어 130℃가 되면 커피가 황색으로 서서히 변하면서 팽창하기 시작하며 140℃에서 탄산가스 및 향기 성분이 발생한다. 제3단계에서 온도가 150℃로 상승하면 커피 생두의 중심 부분이 열리면서 팽창음이 나기 시작하고 흡열반응은 최고에 도달한다. 제4단계에서는 발열반응으로 전환되면서 향기 성분이 본격적으로 생성되기 시작한다. 온도가 190℃가 되면 조직에 미세한 균열이 생기기 시작하며, 200℃부터는 향기 성분 생성이 더욱 활발하게 진행되다가 220~230℃에서는 로스팅을 중단하고 신속히 냉각시켜 가열반응을 끝낸다. 이 단계에서 향기 성분의 생성이 최고에 도달하며 더 진행되면 탄화작용으로 향기 성분이 감소한다. 일반적으로 로스팅 공정이 완료되면 중량의 14~18%가 감소하고, 부피는 2~3배로 팽창하며, 다량의 탄산가스를 방출하게 된다.
볶은 커피에는 약 800여 종의 휘발성 향기 성분이 있다. 메독시피라진이 커피 향의 본질
커피 생두는 바람직하지 않은 향기 성분을 함유하고 있지만, 로스팅 공정을 거치면서 수많은 휘발성 성분들이 나타나 향기 성분을 지니게 된다. 현재까지 연구된 바에 의하면 커피의 향기 성분들은 피리딘, 퀴놀린, 피라진, 알코올, 페놀 등이 존재하는 것으로 밝혀졌으며, 특히 메독시피라진은 커피의 특징적인 성분으로 알려져 있다.
커피 원두에는 다량의 탄수화물이 포함되어 있으며, 이 중 5~10%의 당분이 존재한다. 그 대부분은 자당(설탕)이고, 그 밖에 포도당 및 과당 등이 있다. 로스팅 공정을 통해 이 당분들의 일부가 캐러멜化하여 단맛 성분으로 남고, 나머지는 쓴맛과 향 성분으로 분해된다.
커피의 신맛은 로스팅 과정과 많은 상관관계가 있으며, 水溶性(수용성) 산이 최대치를 나타낸 뒤 계속 가열하면 함유량이 점점 떨어진다. 그래서 약하게 볶은 커피에 많은 산이 함유되어 있다. 또 추출과정에서도 미묘한 차이가 날 수 있으며, 커피 신맛에 영향을 주는 성분들은 지방족 산들이다.
커피의 떫은맛을 내는 타닌은 뜨거운 물에서 분해되거나 변질되어 저온에서 잘 녹는 성질이 있다. 그래서 두 번, 세 번 가열해 추출하면 카페인의 양은 현저히 줄어드는 대신 타닌이 많이 나와 떫은맛이 강해진다.
카페인 등의 알칼로이드 물질, 클로로겐산 등 폴리페놀류, 미네랄(칼슘, 마그네슘), 그리고 탄수화물(당분, 전분과 섬유질) 등은 로스팅 온도가 높아짐에 따라 캐러멜화하여, 이들이 물에 녹아 커피 특유의 쓴맛을 구성한다.
일반적으로 신맛이 강한 커피는 쓴맛이 감추어지고 쓴맛이 강한 커피에서는 신맛이 부족하기 쉽기 때문에, 두 가지 맛의 절묘한 조화가 필요하다. 쓴맛의 경우 볶는 시간, 강약, 그리고 추출 시간 및 온도 등에 영향받는 매우 복잡한 맛이라고 표현할 수 있다
커피의 쓴맛은 카페인 때문이 아니라 커피 원두를 볶을 때 발생하는 두 종류의 산화방지물질이 좌우한다는 연구 결과가 나왔다. 독일 뮌헨기술대 토마스 호프만 교수 연구진은 커피가 쓴맛을 내는 주요 원인이 산화방지물질인 클로로겐산 락톤과 페닐린데인 성분에 있다고 발표했다. 클로로겐산 락톤은 커피 원두를 중간 정도로 볶을 때 생기며 페닐린데인은 에스프레소처럼 바싹 볶을 때 발생한다. 호프만 교수는 제조 과정에서 커피 원두를 많이 볶을수록 커피가 더 쓴맛을 낸다고 설명했다. 그는 “대부분 카페인 함유량이 커피의 맛을 좌우하는 것으로 알려져 있지만 실제 영향을 미치는 비중은 겨우 15%에 불과했다”라고 덧붙였다. 연구진은 아울러 커피를 우려낼 때 가해지는 압력과 온도가 높을수록 쓴맛이 더 나온다는 사실을 밝혀냈다. 호프만 교수는 “커피의 쓴맛을 내는 성분이 어떻게 형성되는지 알아냈으므로 그것을 줄이는 방법에 대해서도 연구할 것”이라고 말했다.
coffee flavor
Like many folks all over the world, I start my mornings with a cup of coffee…or three. I’ve had a few cups that were phenomenal, and many cups that I choked down just so I could stay awake. I attribute my experiences to baristas being “good at making coffee” or good beans. But what makes coffee flavor good or bad? Where does flavor really come from?
Heads up! I mention Ritual Roasters of San Francisco a lot in this article. They were kind enough to metaphorically hold my hand through the whole research process. They even let me do a few cuppings (coffee tastings) with them.
Growing:
Like the coffee bean itself, flavor starts on the tree. In his book The World Atlas of Coffee, James Hoffman asserts that the quality of coffee peaks on the tree and that all further processing is with the goal of preserving as much of that quality as possible.6 However, that is not to say that eating a coffee bean off the tree will taste of coffee as we know it, in fact its really not even terribly pleasant. A green bean tastes very vegetal and a lot like a dirt flavored un-popped popcorn kernel. Rather, a high quality, ripe coffee bean has all the constituents that allow it to become a high quality cup of coffee.
Some factors of the raw beans that affect the final cup include varietal, density, and ripeness. There are over 120 different species of coffee, but only two are commercially relevant: Coffea arabica and Coffea camephora (also known as Coffea robusta). Arabica is prized for its flavor and comprises 60% of the global coffee harvest.5 Robusta is relegated to the world of commodity coffees, though you can still get a decent cup of coffee from robusta beans.6
As you can surely guess, ripe beans make for better coffee than unripe beans. As such, how beans are harvested will indirectly affect your morning cup o’ joe. Beans can be harvested by hand or by machine. Hand harvesting is labor intensive and tedious, but allows the workers to pick only the ripe beans off the tree. However, workers too can harvest whole trees at once by stripping all the cherries regardless of ripeness. Machine harvesting collects all the beans at once, which relies on knowing when you get the most ripeness at a given time. Harvesting unripe and overripe beans together with ripe beans, although cheaper and faster, lowers the overall quality.6
Processing:
From picking, the beans move on to their first stage of processing. How the beans are dried makes a big difference in the coffee flavor profile, though varietals are still distinguishable.4 All initial processing methods, which are referred to using the generic term “wet processing”, comprise the basic steps of drying to 11-12% moisture, removing the fruit flesh, and removing the parchment.6
Figure 2: Anatomy of a coffee cherry. Image adapted from Wikimedia Commons
The main wet processing methods are listed below:6
The Natural/Dry Process: Whole coffee cherries are laid out in the sun and turned regularly to dry. Once dry, the cherries are depulped.
The Washed Process: Ripe cherries are mechanically depulped to remove the majority of the fruit flesh and then submerged in water to undergo a fermentation. The fermentation step is to facilitate the removal of any remaining flesh by allowing microbes to break it down. The beans are then rinsed to wash off the loosened flesh and laid out to dry in the sun.
The Pulped Natural Process: Ripe cherries are depulped and laid out to dry in the sun.
The Honey (Miel) Process: Ripe cherries are partially depulped and laid out to dry in the sun. This method is popular in Central America.
The Semi-Washed Wet Hulled Process: Cherries are depulped and dried to just 30-35% moisture and then have the parchment removed before undergoing a second drying to 10-11%. This process is popular in Indonesia.
In all processes, once the beans are dried, the beans will be rested for a period of 1-2 months before shipping. This resting is thought to produce beans that ship better and age better. The parchment will be removed (the exception being the semi-washed wet hulled process) after resting, but before shipping.4,6
The most traditional method of drying is the Natural/Dry Process and involves just spreading the cherries out along the ground. Poor air circulation leads to longer drying times and spoilage. The use of raised beds has greatly improved the ability to produce high-quality coffee with the dry process.2
In terms of coffee flavor, dry processed coffees tend to be fuller in body, sweeter, and more complex than wet processed coffees. However, beans processed in this method tend to produce lower quality coffee because the cherries are susceptible to molding and over-fermenting, both of which can produce undesirable flavors.2
Wet processed coffee produces cleaner, brighter coffee flavors and is generally the preferred method of coffee professionals. Without the cherry covering the bean, drying can occur much quicker, allowing the varietal characteristics to come through.2 Jeremy Kantor, another employee of Ritual Roasters in San Francisco, described washed coffees as being a fresh orange, and naturally processed coffees as a prune. Both are fruity he said, but it’s a much different kind of fruit.4
The semi-washed process gives the beans a deep, swamp green color, reduces the acidity and increases the body of the resultant brew. Furthermore, semi-washed coffees are associated with flavor descriptors such as tobacco, spice, wood, must, and earth, though whether or not it is a good thing is a matter of opinion.6
No single method of processing is best for every region, different processes work better in different parts of the world. Furthermore, some regions are restricted to only using the dry process due to limited water resources or limited equipment.2
Roasting:
Roasting the beans is where the coffee flavors we’re all familiar with start to come through. You’re likely familiar with the notion of light, medium, and dark roasted coffees. These terms arise from the degree of color change caused by the roasting process, but they are not fully indicative of the coffee flavors each roast possesses.
Figure 3: Appearance of beans at different stages of roasting.
Temperature of bean (F): 1 – 75; 2 – 270; 3 – 327; 4 – 345; 5 – 370; 6 – 393; 7 – 401; 8 – 415; 9 – 426; 10 – 435; 11 – 444; 12 – 454; 13 – 465; 14 – 474; 15 – 486; 16 – 497. 7-9 are first crack, 12-14 are second crack.12
Roasting is much much more than just waiting for the color to change. Longer roasting times allow the bean’s interior to undergo changes and become more flavorful. Roasts that are too short and hot can produce weak, acidic coffee.5 Roasts that are too long will lose the unique characteristics of the beans.
The first stage of roasting involves the drying of the green beans. High moisture content inhibits the maillard reaction taking place. Changes to the bean are relatively slow at the beginning of the roast as water is driven off by evaporation. As the beans become sufficiently dry, they begin to turn yellow and the chaff begins to separate from the bean. Chaff will be removed by air to a separate chamber of the roaster as chaff can easily burn and impart a smokey/burnt flavor to the coffee.6
Browning reactions begin to take place more quickly at this point.
Sucrose, which can be up to 9% of the bean’s weight5, undergoes carmelization between 338-392F. During the breakdown of sucrose, CO2 is produced, causing the bean to expand and crack. This process is referred to as “first crack” and is considered to be the point at which the roasting process has met its minimum length to produce quality coffee.11
Trigonelline is one of the compounds that contributes significant bitterness to coffee. It begins to degrade at temperatures of 378F. Balancing trigonelline with sugar is key to a good roast. One compound that trigonelline degrades into is nicotinic acid; also known as niacin or vitamin B3.12 Nicotinic acid contributes a different kind of acidity to coffee, providing a “clean” finish to the cup. It melts around 457F, and is thought to influence other reactions throughout the roasting process.11
Chlorogenic acids account for 4-9% of the total dry weight. The roasting process breaks down chlorogenic acids into other compounds, though the effect of acids themselves on coffee flavors is unclear. During a light roast, 45-54% of chlorogenic acids break down; medium roasts experience a 60% breakdown; dark roasts can lose up to 100% of their chlorogenic acid.10
Within the classificaton of chlorogenic acids is quinic acid. Quinic acid contributes a sharp brightness to coffee, has a relatively low melting temperature of 325F, but is actually stable throughout the roasting process.11
As the roast is allowed to progress, more browning reactions will take place and the varietal notes and grassy characteristic of the green coffee will be replaced by the coffee flavors of Maillard products.
High heats, temperatures of 446-536F, cause the breakdown of the cellulose and lignocellulose matrix of the beans. These temperatures are found with dark roasts. The breakdown causes the cell walls to degrade and lignocellulose to volatalize, resulting in the “second crack”.11 Second crack is considered the maximum extent to which one should roast coffee. Roasting beyond this point the beans can cause the beans to combust. During second crack beans will develop a glossy black sheen as the oils escape through the fractured matrix to the bean exterior. A roast this dark has lost all varietal characteristics.6
Brewing:
Brewing coffee is a process anyone who has worked in a lab should understand. Both because scientists drink the most coffee out of any profession, and because its simply a water extraction of ground sample (aka beans). When using different brewing methods, pour over to French press, Aeropress to Mokapot, the variables being altered are: grind size (surface area), extraction time, water temperature, and ratio of water to coffee.
Grind size can influence your cup by determining how fast the extraction takes place. Finer grinds will extract more quickly, while coarser grinds will take longer to extract the same amount.4,6 Your brew method also influences how fine the grind should be. A French press requires a coarse grind to reduce the amount of sediment in the final cup. With a pour over, a medium-fine grind is used because the filter should catch all but the smallest of particulates. Paper filters can also impact coffee flavors as they can absorb much of the lipophilic compounds present and thus prevent them from reaching your cup.5
Short extractions produce highly acidic coffee (ever have a gross acidic espresso? Yeah) because the acids are some of the first compounds to solubilize.4 Over-extracting your grounds with a long brew time produces bitter astringent coffee.
To illustrate this point, Aaron van der Groen (Ritual Coffee Roasters of SF) poured me what he called a “rainbow shot”. By separating the espresso shot into 6 segments, it allowed me to see (or taste) what coffee flavors come through during different stages of brewing. Using 9.5 bar of pressure, the total espresso extraction took 40 seconds. The first cup held the first 15 seconds, only 6 of which was coffee actually pouring into the cup. This is over-extracted, but emphasizes the coffee flavors that come through. Cup 1 tasted very bright and acidic, with a saltiness. Cup 2 was acidic and fruity. Cup 3 had a rounded, mellow flavor with chocolate notes. Cup 4 was sweet and full of roasty flavors. Cup 5 was weak and is the typical cut off for an espresso. Cup 6 was very bitter and astringent and had a layer of proteins that had precipitated out of solution floating on its surface.
Figure 5: The Rainbow Shot
Freshly ground coffee should be used whenever possible as pre-ground coffee can stale very quickly. Staling causes coffee to taste bland and cardboard due to the loss of aromatic compounds and oxidation of lipids.6
Water quality also has a big impact on the quality of the coffee produced, after all it does make up ~98.5% of each cup.7 Alkaline water can cause the coffee to taste bitter and flat. Brewing with hard water can affect the rate at which compounds are extracted, but water that is too pure also produces inferior cups.4 Another factor of water that influences the coffee quality is its temperature, with higher temperatures extracting the grounds more rapidly than cold water.
Serious brewing requires precision as the ratio of grounds to water is very important. A ratio of 1g:15g coffee to water is considered to be a good starting point and can be adjusted to suit your tastes (I like my coffee on the stronger side, so more total dissolved solids). The ratio, as well as the extraction time and other factors listed previously, affect the total dissolved solids (TDS) content of the cup. Only about 26% of the grounds are actually water soluble.4
Figure 7: Total Dissolved Solids v Percent Extraction.
Drinking & Perception:
All these factors and more combine to make coffee the delicious incredible beverage that it is! The aroma of coffee is due to the delicate interplay of over 800 different chemical compounds.8 Some compounds considered to be especially important to overall aroma are listed in the table below.
| Compound | Odor Descriptors |
| 2,3-Butanedione | Buttery |
| 2,3-Pentanedione | Buttery/Caramel/Nutty |
| 1-Octen-3-one | Mushroom |
| 2-Hydroxy-3-methyl-2-ciclopentene-1-one | Sweet/Caramel |
| Propanal | Ethereal/Pungent/Earthy |
| 2-Methylpropanal | Spicy/Floral |
| 3-Methylpropanal | |
| 2-Methylbutanal | Buttery |
| 4-Methylbutanal | Buttery |
| Hexanal | Green/Vegetal/Woody |
| (E)-2-Nonenal | Buttery |
| Methional | Baked Potato |
| Methanethiol | Sulfur/Creamy |
| 4-Methyl-2-buteno-1-thiol | Tobacco/Roasted |
| 2-Methyl-4-furanthiol | Meat |
| 5-Dimethyl-trisulfide | Sulfur |
| 2-Furfurylthiol | Roasted |
| 2-Furanmethanthiol | Smoke/Roasted |
| 2-(Methylthiol)propanal | Soy Sauce |
| 2-(Methylthio-methyl)furan | Tobacco/Roasted |
| 3,5-Dihydro-4(2H)-thiophenone | Tobacco/Roasted |
| 2-Acetyl-2-tyazoline | Roasted |
| 4-Methylbutanoic | Sweet/Acidic |
| β-Damascene | Cooked Apple/Fruity/Sweet |
| 4-Hydroxy-2,5-dimethyl-4(2H)-furanone (furaneol) | Caramel/Sweet |
| 2-Ethyl-furaneol | Caramel |
| 4-Hydroxy-4,5-dimethyl-2(5H)-furanone (sotolon) | Spicy |
| 5-Ethyl-4-hydroxy-4-methyl-2(5H)-furanone (abexona) | Spicy |
| 2-Ethyl-4-hydroxy-5-methyl-4(5H)-furanone | Sweet/Caramel |
| 2-Methoxyphenol | Phenolic/Burnt |
| 4-Methoxyphenol | Phenolic |
| 4-Ethyl-2-methoxyphenol | Phenolic |
| 4-Vinyl-2-methoxyphenol | Clove |
| 4-Ethenyl-2-methoxyphenol | Phenolic |
| 3-Methylindole | Coconut |
| Vanilline | Vanilla |
| 2,3-Dimethylpyrazine | Nutty/Roasted |
| 2,5-Dimethylpyrazine | Nutty/Roasted |
| 2-Ethylpyrazine | Nutty/Fermented/Cocoa |
| 2-Ethyl-6-methylpyrazine | Potato |
| 2,3-Diethyl-5-methylpyrazine | Nutty/Roasted |
| 2-Ethyl-3,5-dimethylpyrazine | Nutty/Roasted |
| 3-Ethyl-2,5-dimethylpyrazine | Earthy |
| 3-Isopropyl-2-methoxypyrazine | Earthy |
| 3-Isobutyl-2-methoxypyrazine | Earthy |
| 2-Ethenyl-3,5-dimethylpyrazine | Earthy |
| 2-Ethenyl-3-ethyl-5-methylpyrazine | Earthy |
| 6,7-Dihydro-5H-ciclopentapyrazine | Nutty/Roasted |
| 6,7-Dihydro-5-methyl-5H-ciclopentapyrazine | Nutty/Roasted |
| 3-Mercapto-3-methylbutyl formate | Cat/Green/Blueberry |
| 3-Mercapto-3-methylbutanol | Nutty/Roasted |
Source: Counter Culture Coffee (3)
When aroma is experienced alongside the taste of the brew, you get the full coffee flavor. The variety of flavors that a coffee can possess is staggering. During my second cupping with Ritual Roasters in SF, the flavors ranged from intensely fruity to savory. The aromas reminded me of teriyaki beef jerky, toasted nuts, and pungent fruit.
Because aroma plays such a huge role in the perception of coffee flavor, you won’t taste your coffee as well if your nose is stuffy or if you have a cold. Additionally, not everyone perceives the same thing when tasting. All you can really do is prepare and drink coffee the way YOU like it.