原文：

　　Basic Chemical Reactions Occurring in the Roasting Process

　　by Carl Staub

　　sourced from the SCAA Roast Color Classification System developed by Agtron - SCAA in 1995

　　Many thermal and chemical reactions occur during the roasting process: decarboxylation, dehydration of quinic acid moiety, fractionization, isomerization, polymerization, and complex sugar reactions. The principal thermally reactive components are monosaccharides and sucrose, chlorogenic acids, free amino acids, and trigonelline. Both aravinose and calactose of polysaccharides are splitoff and the basic sulfur containing and hydroxyamino acids decompose. Carbohydrates both polymerize and degrade, liberating thermally unstable monosaccharides decomposing 20-30% of the polysaccharides, depending on the degree of roast.

　　Sucrose: Disaccharide of d-Glucosyl and d-Fructosyl Moieties

　　Sucrose is the principle sugar in coffee. The melting point of pure crystalline sucrose is in the 320-392 degrees F with 370 degrees F most commonly accepted. Degradation of dry sucrose can occur as low as 194 degrees F. and begins with the cleavage of the glycosidic bond followed by condensation and the formation of water. Between 338 and 392 degrees F, carmelization begins. It is at this point that water and carbon dioxide fracture and out-gassing begins causing the first mechanical crack. These are the chemical reactions, occurring at approximately 356 degrees F, that are exothermic. Once carmelization begins, it is very important that the coffee mass does not exotherm (lose heat) or the coffee will taste "baked" in the cup. A possible explanation is that exothermy of the charge mass interrupts long chain polymerization and allows cross linking to other constituents. Both the actual melting point of sucrose and the subsequent transformation, or carmelization, reaction are effected by the presence of water, ammonia, and proteinatious substances. Dark roasts represent a higher degree of sugar carmelization than light roasts. The degree of carmelization is an excellent and high resolution method for classifying roasts.

　　Cellulose: A Long Linear Polymer of Anhydroglucose Units

　　Cellulose is the principle fiber of the cell wall of coffee. It is partially ordered (crystalline) and partially disordered (amorphous). The amorphous regions are highly accessible and react readily, but the crystalline regions with close packing and hydrogen bonding may be completely inaccessible. Native cellulose, or cellulose 1, is converted to polymorphs cellulose III and cellulose IV when exposed to heat. Coffees structure is a well developed matrix enhancing the mass uniformity and aiding in the even propagation of heat during roasting. Cellulose exists in coffee imbedded in lignocellulose (an amorphous matrix of hemicellulose and lignin containing cellulose), making up the matrix cell walls. Hemicellusloses are polysaccharides of branched sugars and uronic acids. Lignin is of special note because it is a highly polymerized aromatic. Severe damage occurs to the cell walls of the matrix at distributed temperatures above 446 degrees F and bean surface temperatures over 536 degrees F The actual temperature values will change due to varying levels of other constituents. Second crack, associated with darker roasts, is the fracturing of this matrix, possibly associated with the volatilization of lignin and other aromatics. Under controlled roasting conditions, the bean environment temperature should never exceed 536 degrees F. A wider safety margin would be achieved by limiting the maximum environment temperature to 520 degrees F. These temperature limits minimize damage to the cell matrix and enhances cup complexity, roasting yield, and product shelf life.

　　Trigonelline: A Nitrogenous Base Found in Coffee

　　Trigonelline is 100% soluble in water and therefore will end up in the cup. Trigonelline is probably the most significant constituent contributing to excessive bitterness. At bean temperatures of 445 degrees F, approximately 85% of the trigonelline will be degraded. This bean temperature represents a moderately dark roast. For lighter roasts there will be more trigonelline, hence bitterness, but also less sugar carmelization. Caramelized sugar is less sweet in the cup than noncaramelized sugar, so when properly roasted these two constituents form an interesting compliment to each other. Trigonelline melts in it's pure crystalline form at 424 degrees F Degradation of trigonelline begins at approximately 378 degrees F.. The degradation of trigonelline is one of the key constituent control flags for determining the best reaction ratio.

　　Quinic Acid: Member of the Carboxylic Acids Group

　　Quinic Acid melts in pure crystalline form at 325 degrees E, well below the temperatures associated with the roasting environment. Quinic Acid is water soluble and imparts a slightly sour (not unfavorably as in fermented beans) and sharp quality, which adds to the character and complexity of the cup. Surprisingly, it adds cleanness to the finish of the cup as well. it is a stable compound at roasting temperatures.

　　Nicotinic Acid: Member of the Carboxylic Acid Group

　　Nicotinic Acid melts in pure crystalline form at 457 degrees F. Naturally occurring Nicotinic Acid is bound to the polysaccharide cellulose structure. Nicotinic Acid is also derived in soluble form during roasting. Higher levels of Nicotinic Acid for any given degree of roast are associated with better cup quality. Since it is I 00% soluble, it will end up in the cup. Nicotinic Acid contributes to favorable acidity and clean finish. It's derivation rate is one of the key constituent control flags for determining the best reaction ratio temperature and chemistry propagation rates. Additionally, the interaction of melted Nicotenic Acid with other constituents contributes significantly to the intensity associated with darker roasts.

　　Environment Temperature

　　The temperature of the roasting environment determines the specific types of chemical reactions that occur. There is a window of temperatures that produce favorable reactions for the ideal cup characteristics. Temperature values outside of this window have a negative effect on quintessential cup quality. Even within the window values, different temperatures will change the character of the cup, giving the roaster the latitude to develop a personality or style desired, or to tame the rough signature of certain coffees while still optimizing relative quality. System Energy: At any given environment temperature, the amount of energy (BTU) and the roasting system's transfer efficiency will determine the rate at which the specific chemistrywilloccur. Higher levels of both energy andt ransfer efficiency will cause the reactions to progress more quickly. There is a window of reaction rates that will optimize cup quality. This is called the Best Reaction Ratio, or BRR.

　　Best Reaction Ratio (BRR)

　　The best cup characteristic are produced when the ratio of the degradation of trigonelline to the derivation of Nicotinic Acid remains linear. The control model of this reaction ratio is a time/temperature/energy relationship. The environment temperature (ET) establishes the pyrolysis region for the desired chemical reactions while the energy value (BTU) and system transfer efficiency (STE) determines the rate of reaction propagation and linearity of Nicotinic Acid derivation to degradation of trigonelline. Because green bean density varies dramatically, under any given ET / BTU / STE format, the reaction distribution will vary. it takes longer to obtain comparable uniformity for a higher density bean. Monitoring the bean temperature offers a good method of approximating the reaction distribution during this phase of the roasting. The ideal environmental temperature, ET, for best reaction ratio, BRR, is from -401-424 degrees F, with 405 degrees F as the default value. The BTU required is determined by the systems transfer efficiency, or ability to impart the energy to the charge mass.

　　Maximum Environment Temperature (MET)

　　Establishing the thermal environment protocol for the ideal roast is a balancing act. While it is desirable to maintain the BRR temperature and energy levels until the target reactions are achieved, the BRR temperature is well above the carmelization temperature of sucrose. Because many roasting systems exhibit thermal hysterysis using simple temperature regulating schemes, care must be taken not to allow the coffee mass to exotherm. Additionally, limiting the maximum environment temperature, MET, is also important. As previously mentioned, maintaining structural integrity of the cellulose matrix is of great importance. Lower temperatures will reduce surface evaporation of constituents minimizing the capillary action that draws constituents to the surface where they would be volatilized. Hydraulic action, a function of internal pressure which is directly related to bean temperature, is already at work. By limiting the maximum temperature, losses will be minimized and the essence of coffee retained. Consequently, the MET should not exceed 520 degrees F. This roasting system bases the MET value on the actual final bean, or drop temperature, which correlates to the degree of roast.

　　譯文

　　在烘焙過程中發生了很多熱與化學反應：去碳酸基，奎寧酸的脫水，細分，異構化，聚合，以及複雜的糖反應(焦糖化)。主要的熱反應的組件有：單糖和蔗糖、綠原酸、流離氨基酸，以及葫蘆巴酰胺。多糖中的aravinose和calactose都被轉移，基本的硫化處理包含了羥氨酸分解。碳水化合物同時被聚合及分解，視烘焙的程度，20-30%的多糖會被分解，釋放出熱不穩定的單糖。

　　蔗糖: D-葡萄糖和D- fructosyl 各一半構成的雙糖

　　蔗糖是咖啡中主要的糖。純結晶蔗糖的熔點是320-392華氏度(160-200攝氏度),公認的熔點是 370華氏度(187.8攝氏度)。退化的幹蔗糖的熔點可以低至194華氏度(90攝氏度)，並隨着脫水和濃縮，糖開始分裂成糖苷結合物。 在338和392華氏度(170和200攝氏度)之間，焦糖化反應開始。正是在這一點上,水和二氧化碳破裂，由此造成的出氣造成一爆。這些都是化學反應，發生在大約356華氏度(180攝氏度)，這是放熱反應。有件非常重要的事是，一旦焦糖化反應開始，咖啡不能放熱(散熱)，否則咖啡的杯中表現會嚐起來有"烤"(baked)的味道。一個可能的解釋是, 正在加熱的咖啡豆子放熱，會中斷長聚合鏈，並使得斷掉的長鏈連接到其它成份。蔗糖和後來轉化成的成份，或是焦糖化反應，都由水，氨，以及proteinatious物質的存在而決定。重度的烘焙會比輕度的烘焙表現出更高程度的焦糖化反應。焦糖化反應的程度是一個評量烘焙的很好的標準，而且分辨率很高。

　　纖維素: 無水的長線性聚合物(A Long Linear Polymer of Anhydroglucose Units)

　　纖維素是咖啡細胞壁的主要纖維。 這是局部有序(結晶)，局部無序(無定形/非晶)。非晶區域很容易受影響並易於起反應，但結晶區域緊密堆積並且氫鍵結合，幾乎是完全行不受影響的。天然纖維素，或者叫纖維素I，當被加熱時被轉換成同質異像體纖維素III和纖維素IV。咖啡的結構是一個很發達的矩陣，這提高了質量的一致性，並可以在烘焙的過程中有助於熱量的均勻傳播。纖維素在咖啡中的存在形式爲嵌入在木質纖維素中(一種無定形矩陣，包含半纖維素和含纖維素的木質素)，這些物質組成了矩陣單元壁(細胞壁)。半纖維素酶(hemicellusloses)是由分岔糖及糖醛酸組成的多糖。木質素是特別值得注意的,因爲它是一個高度聚合的芳香物質。當分佈溫度超過446華氏度(230攝氏度)，且豆子表面溫度超過536華氏度時(280攝氏度)，細胞壁發生了嚴重的損壞。(發生這一變化)實際的溫度會因其它要素的不同而改變。與深度烘焙有關的第二爆，就是這個矩陣的破裂，可能同時伴隨着木質素和其他芳香烴的揮發。在受控制的焙燒條件下，豆子的環境溫度絕不應超過536華氏度(280攝氏度)。一個稍寬些的安全界線應該將最高環境溫度限制爲 520華氏度(271.1攝氏度)。這些溫度限制可以儘量減少對細胞矩陣的破壞，並能提高杯中表現的複雜性，提高焙燒產量以及產品貨架生命週期。

　　葫蘆巴酰胺(Trigonelline): 咖啡中發現的一個含氮基(A Nitrogenous Base)

　　葫蘆巴酰胺是百分之百溶於水的，所以最後會出現在(咖啡)杯中。胡蘆巴酰胺產生過度苦味的最主要的成份。當豆子溫度在445華氏度時(229.4攝氏度)，大約85%的葫蘆巴酰胺會退化。豆子溫度在這個溫度點時，豆子表現爲中等深度的烘焙。對於更淺些的的烘焙會有更多的葫蘆巴酰胺，因此會(嚐到)苦味，但是在這個溫度時被焦糖化的糖也比較少。焦糖化的糖在杯中表現比未焦糖化的糖甜度要差些，所以當適當地烘焙時，這兩種成份可以互相襯托，使味道更好。純結晶的葫蘆巴酰胺的熔點在424華氏度(217.8攝氏度)，葫蘆巴酰胺在大約378華氏度(192.2攝氏度)時開始退化。葫蘆巴酰胺的退化是確定最佳反應比率的關鍵控制標誌之一。

　　奎尼酸：羧基酸組羣的會員

　　奎尼酸的純結晶體的熔解從325華氏度(162.8攝氏度)開始，遠低於與烘烤環境的溫度。奎尼酸是水溶性的，表現爲些微的酸溜溜(不是發酵豆的那種壞味道)和銳利的性質，它賦予了杯中表現以更多的特性和複雜度。令人驚訝的是，它也給杯測的回味帶來了乾淨的口感(it adds cleanness to the finish of the cup as well)。在烘焙溫度下它是一個穩定的化合物。

　　煙酸: 羧基酸組羣的會員

　　煙酸的純結晶體的熔解從457華氏度(236.1攝氏度)開始。天然煙酸與多糖纖維素結構捆綁在一起。在烘焙過程中煙酸衍生出可溶性的形式。不論任何程度的烘焙，高等級的煙酸總是與好的杯中表現相關。因爲這是100%可溶性的，它終將呈現在杯中。煙酸有助於良好風味的酸度以及乾淨的回味(clean finish)。 它的衍生率是一個關鍵控制標誌，可以用來確定最佳反應率的溫度，以及化學傳播率(chemistry propagation rates)。此外，熔解的煙酸與其它組份的交互作用可以顯著增加深烘焙咖啡(杯中表現)的亮度。

　　環境溫度

　　烘焙環境的溫度決定了特定類型的化學反應的發生。有一段溫度窗，(在這個窗內的烘焙)會產生良好風味的反應，併產生理想的杯中表現。溫度值超出這個窗口會對經典的杯中表現呈負面影響。即使溫度的值在窗口內，不同的溫度仍會改變杯中表現的特性，這就給烘焙師以空間來開發個人的風格或是某種想要的風格，或是馴服某種咖啡中粗礦的個性，同時仍能把相關的質量控制到最佳。

　　系統能量

　　在任何給定的環境溫度下，能源量(BTU)和烘焙系統的傳輸效率將決定特定的化學變化發生的速率。能量和轉輸率都較高的話，反應會進行得較快。反應速率也有一個窗口，在此窗內可以最優化杯中表現的質量。這被稱爲最佳反應速率，簡稱爲 BRR。

　　最佳反應速率 (BRR)

　　最佳的杯中特徵產出於這樣的時間：葫蘆巴酰胺降解與煙酸衍生的比率保持線性。這個反應速率的控制模型是時間/溫度/能量的關係。環境溫度(ET)確立了所期望的化學反應發生的高溫分解區，而能量值(BTU)和系統傳遞效率(STE)確定了反應傳播的速率，以及煙酸衍生與胡蘆巴酰胺退化的比例的線性。因爲綠豆子的密度變化很大,在任何給定的ET / BTU / STE格式下，反應的分佈會有所不同。對於高密度的豆子，需要較長的時間來得到有可比性的一致性。在烘焙的這一階段，監測豆子溫度是一個很好的讓反應分佈類似的方法。理想的環境溫度，ET，爲了得到最好的反應比，BRR，是從401～424華氏度(205～218攝氏度)，405華氏度(207.2攝氏度)是默認值。所需的BTU由該系統的傳輸效率，或者說，把能量傳輸給豆子的能力，來確定。

　　最高環境溫度 (MET)

　　建立理想烘焙的熱環境協議是一個平衡點。儘管期望是能夠在反應的目標達到前一直維持BRR溫度及能量水平，BRR溫度會遠高於蔗糖的焦糖化溫度。因爲許多烘焙系統使用簡單的溫度調節機理，這些系統會表現出熱滯後效應，所以一定要很小心，不要讓咖啡放熱。另外，限制最高環境溫度，MET，也是很重要的。如前所述，保持纖維素矩陣的結構完整性的是十分重要的。較低的溫度可減少成分的表面蒸發，使那種將成分吸到表面並揮發掉的毛細管現象降到最小。液壓作用，一個與豆子溫度直接相關的內部壓力的作用，已經在起作用了。通過限制的最高溫度，損失會被最小化，咖啡的精華會被保留。因此, MET不應超過520華氏度(271.1攝氏度)。這個烘焙系統基於MET的值，即實際最終的豆子，或是說下豆溫度，和烘焙的程度相關。

　　作者：Carl Staub

　　源自SCAA烘焙顏色分類系統，由Agtron開發——SCAA 1995年

　　原文出自：http://www.sweetmarias.com/roast.carlstaub.html

　　翻譯：Grant, http://51coffee.blogcn.com

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