As a result, it has been confirmed that homogenizing the condition of green beans by preheating at an appropriate temperature helps to add a positive effect on expression of flavor and balance of taste.
For each sample, water activity, water content, and density of green been were measured. Water activity is a measure of the activity of free water, which is pure water remaining in green beans. It also indicates the activity of microorganisms in green beans. Generally, normal range is 0.2 ~ 0.6Aw. When the water activity is too low or high, it may affect the shelf life of roasted coffee beans. In addition, the total amount of water, which is generally recognized as a measure of freshness, measures the total amount of free radicals and binding water (binding water: water that has been bound with protein, sugar, etc.). Lastly, we will also measured the density of green coffee bean to determine if physical changes had occurred as a result of preheating. Using these three indicators, we can determine if moisture profile of green coffee bean changes during preheating, and how this change affects the roasting profile and sensory difference.
- Green Bean: Costa Rica Las Lajas Black Honey
- Sous-vide Machine: Anova Precision cooker
- Roaster: Stronghold S7 (*Used to reproduce the same roasting profile)
- Measuring tools
- Water activity: Aqualab Pawkit (error range: ±02 Aw)
- Water content/density: SINAR Bean pro (error range: water ±3 STD% mc; density ± 1.0g / ℓ)
- Sensory evaluation
- 4 skilled panelists who have completed calibration attended the cupping
- Mahlkonig W401 Grinder was used to medium grinding degree (Level 5 out of level 1 ~ 9)
- Preheating green coffee bean
- Packing: subdivided into 1kg each in heat-resistant zipper (can be heated up to 100℃)
- Temperature classification: RT / 50℃ / 70℃ / 90℃
- Preheating time for each temperature set: 10 minutes / 20 minutes / 30 minutes
- After preheating, samples were rested for 30 minutes before roasting
- Immediately after sous-vide: water activity, water content
- Roasted at the same profile of RT sample using Stronghold S7
- Sensory Evaluation
- Calibrated with RT samples
- Cupping was performed for 1 cup of each sample
How are the physical properties of green coffee beans changed?
2. What are the effects of preheating on roasting and sensory performance?
During roasting, there was no significant change in the profile itself, except the turning point (TP) was slightly advanced than that of the RT sample. It was surprising to find out that the reproducibility was much better than what we had expected. The reproducibility of total time was about ± 1 second and a discharge temperature was ± 2 ℃. The Stronghold S7 has a mechanism to reproduce the roasting profile based on RoR (Rate of Rise) and discharge beans based on the end temperature. This study proved that Stronghold’s technology regarding these processes is quite advanced.
The sensory evaluation was carried out for the 10 kinds of samples in a cupping manner. Rather than focusing on detailed flavors, the evaluation was focused on how overall balance and preference changed in comparison to the RT sample. Panels have freely discussed about overall tastes and were asked for the preference of each sample. The results are as follows.
Table 2. Sensory evaluation result
Conclusion and Future Progress
The conclusion of this test is summarized as follows.
- Homogenization of green beans by preheating at an appropriate temperature helps to develop positive flavor and balance of taste.
- Excessive preheating rather breaks the homogenization of green beans and affects flavor.
In the next study, we will further develop this research by focusing on the preheating temperature of 70 ℃. We would like to find out the optimum roasting point at this most preferred and stable temperature.
In this study, beans were roasted to about 62 (high, Agtron standard). In the next study, I would like to tune the roasting profile more precisely and find out which point results in the best development and expression flavor.