Brewing with a quality espresso machine or an auto-drip coffee maker will provide you with a reasonably stable temperature. The same is not true for most manual brew methods. Immersion methods lack the continual flow of hot water through the coffee bed and consequently these methods can become heat sinks. The severity of which is determined by the type of material they’re made from, how thick the walls of the brewer are, how much your preheat, etc. Below we’ve looked at some of the more common systems used: ceramic cupping bowls, glass cupping bowls, ceramic immersion dripper, Aeropress, and the classic glass french press.
In order to brew within the common volume of each system our brew ratio for the Bonavita dripper and french press have been scaled up. This allows us to compare how scaling up your ratio when using the same material will impact your brewing temperature. The fixed variables were our all Brazilian blend Heritage, ground on a medium-fine grind setting using coffee burrs. We used a brew ratio of 16:1 across all brew systems (regardless of batch size). With a starting temperature of 202f you can see the temperature profiles each brew system created.
We can now compare the traditional glass french press (300ml) and the glass cupping bowls (180ml), and the ceramic immersion dripper (300ml) to the ceramic cupping bowls (180ml) to observe how scaling your brew ratio impacts your temperature profile. it became clear that scaling up brew ratios within systems made from the same material would result in higher brewing temperatures. The higher the batch sizes had more thermal energy available and were less susceptible to declining temperatures.
We know in theory that longer contact times, higher brew temperatures, and increased turbulence or flow rate (or drip elements in our case) will raise extraction. The Aeropress and immersion droppers had average brew temperatures in this test but finish their brew cycle with a drip element; letting all of the liquid pass through the coffee bed whether by external force or through gravity. This is likely why we observe these two as having the highest levels of extraction. Among the remaining three full immersion brews it is the system with the highest brew temperature, our glass french press, that comes third highest. The two styles of cupping bowls extracted the least despite having more than twice the brewing time.
The cupping bowls are quite interesting. We followed a standard procedure by brewing for 4 minutes, breaking and skimming the crust, and allowing them to cool beyond 10 minutes before testing. This supports the idea that breaking the crust effectively stops extraction; and explains why having twice the contact time was not enough to counteract the lowered brewing temperature when compared to the other full immersion method; the french press.
Ultimately the most compelling aspect of the test is one you’ll have to assess for yourself - taste! The quality of the acidity across all brews was very similar as if each had successfully extracted the same acids in similar concentrations. What really set them apart was whether or not that acidity was supported by a sweetness and a fullness of body. In an attempt to substantiate what we tasted we turned to some coffee constituent tables found in the SCAA brewing handbook on sucrose levels under various brewing conditions.
The work done by Ted Lingle for the SCAA’s Brewing Handbook indicates that Sucrose is extracted most efficiently at higher temperatures (around 201f). Retaining that higher temperature will see the sucrose levels continue to increase up to at 14 minutes at least. This aligns with what we were tasting. The brew systems that could maintain a higher brew temperature for their brew cycle were sweeter (The french press was notably one of the sweetest coffees despite its lower extraction yield). Although this work isn't conclusive, within this test we saw that higher brew temperatures were more influential than extended contact times if our goal was to extract as much sweetness as possible. We used reasonably high temperatures and contact times from 1-10 minutes in this test. The point in which a lower brew temperature can be held long enough for the sweetness to reclaim dominance in the cup is work left to the next instalment of this work.
Using a refractometer to target and repeat an ideal result is an invaluable resource, but nothing can substitute for your sense of taste when assessing the balance of constituents you’ve extracted and if they’re pleasing to the pallet.
Have any experience with offsetting temperature and time for new results - get in touch and let us know what you've found!
Yours in Coffee - Pilot.