第一次爆裂的溫度

咖啡豆烘焙的過程中，第一次爆裂與第二次爆裂都是很重要的烘焙度判斷點，近來流行的淺焙尤以第一次爆裂的過程最為重要。

然而，第一次爆裂的溫度，應該只是相對溫度而非絕對溫度，所以這個相對溫度會因為量測的位置與溫度針的材質、敏感度有關。

在大多數人眼中，溫度針插入烘焙旋轉的豆堆中，量出來的溫度就是豆溫，這種直觀的看法在烘焙過程中並不是說完全正確，但也談不上完全錯；量測體溫度的顯示是一個結果，而要把這個結果回推出烘焙的過程，得先考慮量測溫度點的誤差。

首先討論溫度針的敏感度，烘焙上採用的溫度針主要是K-type，在-20℃~400℃之間量測的精度可以達到小數點第一位，就烘焙的過程來說，也算是夠用了，一般K-type的溫度針主要由兩種不同金屬導線組成，量測點則是將兩種不同的金屬導線銲接，相關的資料可以自行上網搜尋。

通常該銲接點會由金屬管包裹住，來避免銲接點在量測過程中被量測物碰撞，而這就影響了量測的敏感度，因為包裹的金屬管越粗，對溫度的敏感度就越低，之前的溫度針實驗中，兩種不同的溫度針 (細速型Φ1.6mm、傳統型Φ3.8mm)，高溫量測值 (190℃)時兩者誤差1℃，而在投入生豆後，回溫點誤差20℃。

因為溫度針鎖固在烘豆機正面板上，所以在溫度顯示上，160℃前主要是正面板的熱通過溫度針傳遞給生豆，該階段時生豆的溫度低於正面板；而在160℃後，生豆溫度逐漸接近並高過正面板的溫度，在溫度的傳遞上會造成感溫的差異。

接下來討論咖啡豆一爆的溫度，第一次爆裂簡略的說，是咖啡豆的組織經高溫脆化，且內部的壓力增大，造成爆裂，這個爆裂的過程就是累積熱量，內部壓力增大，超過脆化組織的承受，導致組織破裂，大量的壓力 (氣體)洩出。

影響一爆溫度的因素？

在機械式風門的烘豆機上，可以藉由風門來影響烘焙桶內部壓力，相較於咖啡豆的內部壓力，利用一爆前降低風門，增加烘焙桶內的壓力，來讓咖啡豆的外部壓力增大，提高一爆的溫度，另一種說法就是：讓豆體吸較多的熱才進一爆。

這種做法的優點就是可以有效的讓低溫焦糖化比例更高，也可以視為豆芯的溫度提高，豆芯與豆表的溫度提高，就結果來說，當然可以視為焦糖化比例更高，這種做法早期用在水洗豆較多，針對豆體較硬的高海拔豆都算不錯。

這同樣是有缺點的，一爆前降低風門雖然可以有效增加烘焙桶內的壓力，相對的也可能造成豆芯的溫度過高，一爆溫過度提高，這也是近來較多的高海拔衣索比亞日曬生豆很常見的問題，一般一爆溫度在195℃時，該方式可以有效提高2~3℃，但是生豆的不同，卻造成了一爆溫達到202℃，這狀況在日曬處理的生豆上，導致酸味降低，香氣變得不明顯。

而在採用變頻抽風，持續抽風，烘焙桶內部壓力為負壓的烘豆機上，相較於機械式風門的烘豆機可以輕易的影響烘焙桶內的壓力為正壓時，變頻抽風的烘豆機，烘焙桶內部的壓力仍然有，但變化幅度就不如機械式風門那麼大，雖然仍可以在一爆前降低抽風來試著提高一爆的溫度，但因為降低抽風的範圍不如機械式風門的大，改變的並不明顯，很多人會因為一爆溫度無法有效提高而認為該手法無效，但得從實際的杯測去判斷這手法的作用。

而針對高海拔 (豆體組織堅硬) 、豆體小、水分含量少的生豆，這手法基本上就不適用，因為豆體組織堅硬，水分又少的狀態，豆體要爆裂的溫度就需要更高，反倒要思考如何有效降低一爆的溫度。

而思考應該就從豆體堅硬的基礎上來推論，豆體堅硬與豆體小，水分較少，進一爆前的水份比例低........

我會提出較極端的做法，降低烘焙的總時間，豆體堅硬可以承受較高 RoR，豆體小則豆芯可以較快得到熱量；水分的維持則是利用風門的微調；進一爆前的風門則因為主要避免豆體外部壓力過大造成內部爆裂壓力不足，避免降低風門。

這種手法在機械式風門的烘豆機與變頻抽風的烘豆機上都可以使用，但就看每個烘豆師的微調部分怎麼做了。

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1st CRACK TEMPERATURE OF ROASTING COFFEE BEANS

August 02, 2020

During the roasting process of coffee beans, the 1st crack and the 2nd crack are both very important points to determine the grades of roasting. The light roast has becomes popular in recent days and 1st crack is most important during the roasting process.

Although the temperature of the 1st crack should only be a relative temperature rather than an absolute temperature, therefore this relative temperature will be effect to the thermometer material used, sensitivity of the thermometer and the position of the thermometer been placed.

The most people thought thermometer has inserted into rotating bean pile, and the temperature showed is the bean temperature. This visually intuitive is not completely correct or wrong in the process of roasting, i.e measuring body temperature the displacement digits only shown as an indication and not exact body temperature, one has to take maximum error allowance into consideration and this applies back to the roasting thermometer figures. 

Firstly, lets discuss the sensitivity of the thermometer. The thermometer use in roasting mainly is K-type, the accuracy of the measurement between -20℃~400℃ can reach up to the first decimal place which is sufficient for coffee roasting concerned. Generally K-type thermometer are mainly composed of two different metal wires, and weld the two different metal wires to the measuring point, for more relevant information can search over the Internet.

Usually the welding point is covered by a metal tube to prevent collided with the measured object during the measurement process, the metal tube size which could affects the sensitivity of the measurement, because the thicker the metal tube less sensitive towards temperature experiment, the two different types of thermometer (thin tube Φ1.6mm, traditional Φ3.8mm), when on the high temperature measured value (190℃) and the different between the two is 1℃, after the raw beans release into the roaster and the variation of turning point is 20℃ different between both thermometers.

The thermometer is fixed on the front panel of the roaster and the front panel absorb heat faster than raw beans, therefore the figure shows on thermometer which influence by heat transferred from front panel and cause inaccurate  readings before 160°C. At this stage, the temperature of the raw beans is lower than the front panel; however after 160℃ the temperature of the raw beans gradually approaches and surpass the temperature of the front panel, which more accurate temperature will be presented.

Following will discuss the temperature of the 1st crack on coffee beans, the easiest way to express 1st crack is that the compound of coffee beans get brittle after high temperature and the internal pressure also  increased excessively from the heat which causing bean to crack. The process of crack is due to accumulation of heat energy which build up internal pressure, once the internal pressure is exceeding the endurance of fragile tissues then rupture occurred and high pressure(gas) released .

Which factors affect the 1st crack temperature?

The roaster that designed with mechanical damper, which could use damper effectively effect the internal pressure of the roasting drum, similarly in compared with the internal pressure of coffee beans, reduce the damper before 1st crack to increase the pressure in the roasting drum to exert the external pressure of the coffee beans, at the same time increase the temperature of the 1st crack process. Another way of saying is: let the bean absorb more heat before it starting on 1st crack.

The advantage of this approach is that it can effectively make the caramelization ratio higher at lower temperature, it can also be expressed as the temperature inside the bean core increased. The temperature of the bean core and surface of the bean increased in result regarded as a higher caramelization ratio, this method was used for wet processed beans in the early days, and it is good for high-altitude with harder compound beans.

The disadvantage of this technique by lowering the damper before 1st crack can effectively build up  pressure in the roasting drum, it may also cause the temperature inside the bean core rise up too high, and the temperature of the 1st crack is excessively increased. This also a common problem recently happens to high-altitude Ethiopia natural processed beans, normally the 1st crack temperature is 195°C, this method can effectively increase 2~3°C, but difference in raw beans resulted 1st crack temperature rise up to 202°C. This cause the defect on natural processed beans with acidity reduced and aroma becomes inconspicuous.

While adopts variable frequency ventilation fan in roaster with continuous ventilation and the internal pressure of the roasting drum becomes negative in pressure, compared with the mechanical damper which the pressure in the roasting drum can be easily adjusted to positive. However the pressure inside the roasting drum is still exist that equips with variable frequency ventilation fan, but the range of change is not as far as the mechanical damper. Although it is still possible to reduce the ventilation before the 1st crack take place and try to push up the temperature of the 1st crack, but the reduction of the ventilation range is not as significant as the mechanical damper. The change is not noticeable and many people think that this technique is null because the temperature of the 1st crack cannot be effectively increased, result of this technique can only be define by actual cupping.

Specifically for high altitude (hard bean structure), small bean size, and low moisture content, this technique  is basically not applicable, because the bean structure is hard and the moisture content is low, the beans  required higher temperature to reach 1st crack, instead one might consider how to effectively reduce the temperature of the 1st crack.

The consideration should be based on the hardness of bean itself, the bean is hard and the bean size is small, with little moisture content, and the ratio of moisture before 1st crack is low........

I will propose a more extreme approach by reduce the total roasting time, the bean is hard and can withstand a higher RoR, also bean is small and inside of bean core can get heat up faster therefore fine tuning of damper is crucial to maintain bean moisture inside the roasting drum. To avoid insufficient internal burst pressure caused by excessive external pressure on the bean body, it should prevent lowering the damper before 1st crack starts.

This technique can be used on both mechanical damper and variable frequency fan roasters, but it depends on the fine tuning skill of each roaster.

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This article was translated by George, thank you very much.