Exploring Coffee Roasting Science: The Impact of Time and Temperature on Aroma Formation
Juerg Baggenstoss, et al. Coffee Roasting and Aroma Formation: Application of Different Time−Temperature Conditions. J. Agric. Food Chem. 2008, 56, 14, 5836–5846. https://doi.org/10.1021/jf800327j
Introduction
Roasting is the transformative stage of coffee processing, where green beans are turned into aromatic and flavourful roasted coffee. The study by Baggenstoss et al. (2008) delves into the intricate relationship between roasting time, temperature, and aroma development, shedding light on how roasting conditions shape the sensory qualities of coffee. This research highlights the complexity behind creating a perfectly roasted coffee bean and its impact on aroma compounds that define your favourite cup of coffee.
Roasting Methods and Physical Changes
The researchers compared three distinct roasting processes:
High-Temperature, Short-Time (HTST) roasting.
Low-Temperature, Long-Time (LTLT) roasting.
A temperature-profile roasting method designed to mimic the gradual temperature increase of drum roasting.
These methods were tested using both a laboratory-scale fluidized bed roaster and a commercial drum roaster. Despite achieving the same roast colour, defined by lightness (L* = 21), the beans roasted under these conditions displayed significant differences in physical properties such as density, moisture content, and roast loss. HTST roasting resulted in beans with lower density, higher volume, and faster roasting progression compared to LTLT roasting.
Aroma Formation During Roasting
The study tracked the development of 16 key aroma compounds, categorised into sulfur compounds, aldehydes, diketones, and heterocyclic/phenolic compounds. Each compound class exhibited unique behaviours under different roasting conditions:
Sulfur Compounds: Sulfur-containing aroma compounds like methanethiol and 2-furfurylthiol are critical to coffee’s sensory profile. Methanethiol formed rapidly in HTST roasting but degraded at higher degrees of roast, while 2-furfurylthiol increased steadily throughout roasting, suggesting a large precursor pool.
Aldehydes and Diketones: Strecker aldehydes (e.g., methylpropanal, 2-methylbutanal) formed rapidly during initial roasting stages, peaking early in HTST conditions. Diketones like 2,3-butanedione showed higher concentrations in HTST roasting, contributing to buttery and sweet aromas.
Heterocyclic and Phenolic Compounds: These compounds, including pyridine and 4-vinylguaiacol, are Maillard reaction products contributing to roasted and spicy notes. HTST roasting accelerated their formation, resulting in higher concentrations compared to LTLT roasting.
Effects of Over-Roasting
To investigate the impact of excessive roasting, the researchers extended the profile roasting process beyond typical durations. Over-roasting generally led to the degradation of desirable aroma compounds, such as 3-mercapto-3-methylbutyl formate, while others like dimethyl trisulfide and pyridine continued to increase. This finding highlights the importance of precise control in roasting to avoid undesirable flavour outcomes.
Key Takeaways for Roasters and Enthusiasts
Roasting Dynamics Matter: Even with the same roast colour, different time-temperature profiles produce coffees with distinct physical and sensory properties. HTST roasting may yield vibrant aromas but risks volatile compound loss if over-roasted.
Aroma Precursors and Profiles: Understanding precursor behaviour and the kinetics of aroma formation can guide roasters in achieving specific flavour profiles.
Precision is Key: To replicate the flavour profiles of a traditional drum roaster in faster systems like fluidized bed roasters, careful adjustments to temperature profiles are necessary.
Conclusion
This study underscores the scientific precision involved in coffee roasting. It is not merely about reaching a desired roast colour but about understanding the complex interactions of time, temperature, and chemical transformations. For roasters and coffee enthusiasts alike, this research offers valuable insights into crafting coffee that delights the senses and honours the bean's potential.
Explore more about the science of coffee roasting and the meticulous craft that goes into every cup at Fission and Bloom.
