Coffee Data Science
Separating Heat and Water in Espresso
Towards a better understanding of flow
When brewing espresso or coffee in general, we associate heat and water as so intertwined as not to be discussed separately, particularly in how the two flow through coffee grounds. This idea is particularly true in espresso where water enters from the top and leaves through the bottom meanwhile the basket itself radiates heat during the shot.
Starting with water input for espresso, hot water enters the coffee puck, and typically, the heat differential is at least 50C. Assuming 100C water goes into 20C coffee, we could have an 80C difference in water temperature.
Usually, it takes the same amount of water to enter the puck before water starts exiting, so the temperature of the water should drop by roughly by half by the time the water gets to the bottom of the puck. In raw measurement, this has been shown to be true, and the bottom won’t necessarily get to the same temperature as the top by the end of the shot.
This temperature differential means there will be a differential in extraction rate because higher temperature causes a higher extraction rate. So the top of the puck will extract faster than the bottom leading to the top having a higher chance of being over-extracted while the bottom has a higher chance of being under-extracted.
In terms of heat flow, the moment the metal of the filter basket touches the grouphead, heat is conducting down the side of the basket faster than through the coffee bed. The heat conductivity of metal is much greater than coffee, so the coffee grounds at the side of the basket will be at a higher temperature than in the middle.
During the shot, the coffee on the sides of the basket will be warmer causing them to extract slightly faster. This side heating contributes to side channeling leading to under-extracted shots.
As a community, we got closer to a better understanding of this potential when there was a trend of microwaving coffee before grinding. Often, this would raise the temperature of the coffee by a few degrees, and it changed extraction. In my own investigations, shots later in the afternoon compared to the morning had a higher extraction yield. I suspect this was due to a slight increase in the ambient temperature in the afternoon. Now, I think that microwaving coffee beans helped bring coffee closer to the temperature of the brew water, which helps extraction.
This line of thinking led me down the road of thermal pre-infusion, which is allowing the coffee to warm up closer to the brew water temperature. I started by locking in the portafilter, with the prepared basket, into the machine for 5 minutes. This increased extraction yield and improved taste. However, some profile changes were needed to be optimal.
While this is impractical for a coffee shop during peak traffic, it is possible to use such a technique for the people who order straight espresso. The concept makes a good case for a spouted portafilter and for leaving the portafilter in the machine after a shot because both retain heat and allow the coffee to heat faster.
There is one more unexplored variable which is CO2 solubility. This changes greatly dependent on temperature where a lower temperature will lead to more solubility. It is not known how CO2 solubility affects extraction, but one of the confusing variables in my current shot profiles with thermal pre-infusion is that the pressure does not go above 2 bars for a flow rate of 0.5 ml/s to 1 ml/s flow in most cases. I currently use a ramp bloom profile where I manually move on from the ramping pre-infusion to the bloom once the filter is covered.
I haven’t been able to explain the lack of pressure aside from some visual evidence of bubbles of gas coming out of the coffee during the bloom phase. Previous data indicate that 9 bars is not optimal for espresso extraction, and now I’m doubting if pressure much higher than Moka pot pressure is necessary. However, my shot time is 2 or 3 minutes.
I have previously theorized that the moka pot could be the most efficient method for brewing espresso if done right. By using a low flame, the moka will slowly warm up as will the coffee, and before the water starts to flow through the coffee, steam will pre-heat the coffee. So as long as the flow is slow, the coffee will approach the optimal range of extraction yield (18% to 22%) in the 1:1. Once the coffee starts sputtering, I pour out the coffee because that contains all the tastiness.
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Further readings of mine:
My Second Book: Advanced Espresso
My First Book: Engineering Better Espresso
