本期Roast Magazine刊登讀者投書 by subman - 咖啡討論

小弟不才我看了5/6月號的烘焙雜誌 "Roasting Science- Looking Closely at
Your Curves"一文, 文章提及一些可能造成烘焙問題的狀況, 以及相關的曲線呈現,
覺得頗有收穫, 但有些想要補充的地方, 寫了讀者投書, 在這一期被刊登出來,
跟各位版友分享一下:

先貼一下Roast Science一文的pdf檔連結: http://ppt.cc/cM6y

再貼上我的讀者投書內容, 有點長, 大家慢慢看啦 XD

Dear Editor,

It was such a beneficial journey reading the article, Roasting Science –
Looking Closely at Your Curves by Kathi Zollman in Roast Magazine 2012
May/June. The influence of the pattern of roasting curves on the development
of coffee flavors and how roasting problems could be amended through
adjusting roasting curves are thoroughly discussed. However, aside from
examining on roasting curves to modify roasting outcomes, I’d like to
elaborate more on other crucial influential factors of coffee roasting in
relation to my experience and knowledge.

Referring to roasting curves while speaking about coffee roasting is
convenient and easy to understand. Yet, regarding roasting curves, as the
single guidance of the development of coffee roasting flavor may not be
appropriate. In the following article, four other influential factors of
roasting: temperature measurement, the ratio of heat transfer (or
supply) by conduction and convection, "off-flavors" mainly due to scorching
and tipping of bean surface, and roast condition (batch size or the
content of water of green beans), are discussed respectively.

First of all, the temperature measured over thermoprobe is not 100% reliable.
Temperature measured varies when different roasting machines are used or when
roasting applies different roasting conditions, such as different batch
sizes. Also, where the thermoprobe is placed affects the degree of
temperature being measured. For instance, in a drum roasting system,
temperature is measured higher when the thermoprobe is situated closer to the
bottom of the drum. Once thermoprobes are placed in different positions,
issues like unaligned temperature measurement would happen. Hence, even if
roasting curves turned out alike, coffee flavor may still differs.
Furthermore, the temperature measured at first pop commonly varies under
different roasting machines. It is to our knowledge that the temperature of
bean surface is a proper guidance of the degree and development of coffee
roasting. Nevertheless, the temperature measured by thermoprobe cannot simply
be considered as the “true” bean surface temperature. Instead, an
interactive relationship between the three factors - bean surface
temperature, the temperature of the heat flow, and the temperature of
the drum - must be considered simultaneously in terms of temperature
measurement. As a matter of fact, the influential degree of these three
factors differs under different batch sizes, not to mention the temperature
being measured. Under such circumstances, roasting curves depicted based on
temperature measured over the roasting process can only be regarded as
references toward temperature change instead of the “absolute” guidance of
the variation of bean surface temperature.

Secondly, coffee flavor is also affected by the ratio of heat transfer (or
supply) by air conduction and convection. Even with a perfectly drawn
roasting curve, once an excessive ratio of conductive heat is applied,
scorching and tipping of bean surface as well as uneven roast that may result
in burnt, unbalanced flavors and metallic taste are likely to be presented.
On the other hand, an excessive amount of hot airflow will then lead to vital
damage of coffee aroma and flat flavors. However, the ratio between heat
conduction and heat convection is not likely to be observed simply by looking
at roasting curves.

Thirdly, off-flavors contribute to a certain level of negative effect on a
cup of coffee. Scorching and tipping of bean surface often is then the main
contributors towards off-flavors. A great difference of temperature between
the bean itself and the environment it is contacted to mostly causes such
concern. Within the drum roasting system, excessive drum temperature, which
is not detectable via the temperature measured by thermoprobe, is the major
reason of scorching and tipping. Unfortunately, such problem remains
invisible although a nice roasting curve is performed. To my own experiences,
infrared thermometer is a useful tool to apply in order to inspect on the
drum's temperature during the roasting process to avoid such issue.

Last but not least, the size and water content of coffee bean determines the
way of roasting. Commonly speaking, green beans with lower water content or
of smaller size have a smaller temperature gradient between its core and
surface. Comparing to green beans with opposite conditions, a relatively
shorter roasting time is required to reach the same roasting level. Thus,
roasting curves that well suit a certain type of green beans may not be
appropriate for the other one.

In conclusion, roasting curves surely are important guidance toward coffee
roasting. Yet not the only one, many other essential factors should also be
taken into consideration. For example, temperature, ratio of conductive and
convectional heat, causes of off-flavors, as well as size and water content
of coffee beans. Without any doubt, no matter which matter is adapted as one’
s roasting guidance, “cupping” is certainly the tool to be employed as the
final judgment of the quality of the coffee.


Jeff Chang
Cafe de Impression
Kaohsiung City, Taiwan


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