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[기고] 생두 수분함량과 로스팅
(뉴웨이브커피로스터스 유승권)

원두 품질을 QC(Qualty Control)하기 위해 로스터가 알아야할 변수들에는 여러가지가 있습니다. 산지, 품종, 프로세스, 크기(스크린 사이즈), 수분, 밀도, 고도, 디펙트(결점두), 등급, 수확시기(Fly Crap, Main Crap), Resting-Time 등의 다양한 변수들이 산지마다 다르게 적용되기 때문에 일정한 패턴이나 통계를 찾아보기가 어렵습니다.

생두 구매를 결정하기위해 로스터들은 커핑이나 생두의 외관, 핸드픽-디펙트 등을 거치지만, 생두의 수분이나 밀도를 확인하는 경우는 적습니다. 일반적으로 국내에 새로 입고된 생두의 수분은 NEW-Crop인 경우 10% 중반~11% 후반을 유지하는데, 도정 후 계약 지연이나 운송 등의 문제로 창고보관기간이나 Resting-Time이 너무 긴(90일 이상) 경우, 수분 함유량이 8~10%로 떨어질 수 있습니다. (예외적으로 브라질의 NEW-Crop은 9% 후반~10% 중반을 유지합니다.)

로스팅 프로세스에서 수분은 전도체로서 생두 내부로 열을 전달하고 Boiling 되면 기화되면서 Bean 내부 압력을 만들어, 팽창과 1st crack 같은 물리적인 반응뿐만 아니라 메일라이드(Mailard) 반응과 카라멜라이제이션(Caramelization) 같은 화학반응에도 영향을 주기 때문에 매우 중요한 변수로 작용합니다. 이외에도 최근 새로운 이슈로 떠오르는 수분활성(Water-Activity)은 생두 수분과 밀접한 관계를 가지고 있는데, 수분 함량이 높을수록 미생물이 활동하기쉬워 생두 보관 기간이 짧아지는 원인이 되기도 합니다.

생두수분함량_유승권 (2)생두 밀도는 부피:무게 비율로 생두 품질과 밀접한 상관관계를 가지고 있습니다. 생두무게는 셀룰로오스(cellulose, 70%)와 유기물질(30%)로 구성되는데, 밀도가 높다면 더 많은 유기물질을 함유하고 있기 때문에 뚜렷한 Coffee Flavor의 다양성과 복합성을 가짐으로써 추출시 Cup의 캐릭터도 명확해집니다.

예를 들어 부피가 같지만 무게는 100g/ 200g으로 다른 생두의 경우, 100g에서 추출할 수 있는 유기물질은 30g이지만 200g에서 추출할 수 있는 유기물질은 60g으로 차이가 납니다.

이러한 밀도차이의 원인으로는 커피 생육과정에서 영양분(비료)의 공급이 잘 이뤄지지않아 미성두가 발생하거나, 커피체리 선별/밀도분류가 잘 이뤄지지 않은 경우에 생두 품질이 균일하지 않습니다. 이밖의 원인으로는 커피나무의 재배 고도나 온도, 그늘재배 여부, 수확기간, 가뭄과 질병 등이 있습니다. 한편 밀도가 낮은 생두는 펄핑이나 탈곡을 하는 동안 잘 깨지게 됩니다.

생두수분함량_유승권 (1)

로스팅 프로세스에서 밀도가 높은 생두는 열 저항력이 높기 때문에 로스팅 시간이 상대적으로 늘어나게 됩니다. 이 경우 시간을 줄이기 위해 대부분의 로스터들이 보다 강한 화력으로 로스팅을 하는 경우가 있는데, 이는 생두의 외부와 내부가 서로 다르게 로스팅되는 원인이 되기도 합니다.

이와는 반대로 밀도가 낮은 생두는 열 저항력이 낮기 때문에 로스팅 시간이 상대적으로 짧아집니다. 아까와는 반대로 로스팅 시간을 맞추기위해 약한 화력으로 로스팅을 하는 경우, 언더(Under)나 베이크(Bake)와 같은 로스팅 디펙트를 만들 수 있습니다. 따라서 로스터는 생두의 수분과 밀도를 미리 확인해 로스팅 프로파일에 적용시킬 수 있어야 합니다.

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