CFRR – Extracting coffee under high pressure is a special factor when brewing espresso, so will the pre-infusion stage have any effect on the quality of brewed coffee?
How is espresso made?
Espresso is made by squeezing an amount of hot water about 90-96°C with a pressure of 9 bar through a finely ground coffee cake in a short time from 25 to 30 seconds. The amount of coffee extracted is made of:
Soluble solids include sugars, acids, caffeine, etc., which create flavor.
Insoluble solids and oils in coffee create a smooth feeling on the palate.
Espresso is divided into 2 layers: the upper layer is a crema made from CO2 gas and steam pressure in dark yellow, and the lower one is a liquid containing the characteristic flavor of coffee beans.
Creating a whole with the smooth fatty aroma of the crema layer, the balance of sour, sweet, bitter flavors, smooth and also full feeling in palate; the last sip is the lingering aftertaste is the standard for an espresso needs to achieve.
The quality of the espresso is an overall combination of factors: the amount of used coffee (dose), beans size, amount of brewed coffee, take time to extract, steam pressure, and water temperature.
In this article, we will focus on the pressure factor because most common espresso machines will use an initial pressure of 9 bars; this requires precise technical manipulations by baristas in all stages, as with high pressure acting on the surface of the coffee layer, water tends to flow through places with little resistance leading to channeling (irregular drainage channels). At the same time, this requires a good blender with a low fine particle ratio because the pressure of 9 bar pushes the finer particles to the bottom of the filter basket clogging the basket’s exit hole and leading to the combat layer. Channeling and compact layer are both factors that cause uneven extraction in espresso.
To limit these weaknesses, some modern machines have installed pre-infusion, which allows water to contact the coffee block at a pressure of 3-4 bar for a few seconds, next increasing to 9 bar as required, reducing channeling significantly. Because infiltrating with water at lower pressure will have enough time for the expanding and binding tightly coffee beans
In order to form a more homogeneous mass, limiting fine particles move downwards the filter basket causing a compact layer.
What is pre-infusion?
Pre-infusion is the first step of extraction at which little water is used to evenly wet the coffee beans layer at low pressure (maybe around 3-4 bar) for a while. It takes about 2 to 8 seconds before starting the main extraction step with a pressure of 9 bar which is essentially similar to the coffee “bloom” step in the brewing method named Pour over.
The main problem in channeling is that water will flow through places where there is little resistance to forming channels and over-extracting in that area and the rest will be under-extracted. The effect of pre-infusion in espresso extraction is resulting in more even flow, more stable extraction, and better espresso flavor.
In the image above is a display depicting the extraction of an espresso shot, the dark green line shows the pre-infusion step in 8 seconds of the brewed shot in 20 seconds with pressure gradually increasing, but not suddenly increasing, to a straight level. 6.2 bars. Some modern espresso machines have seriously taken the pre-infusion step with a meter clearly showing the pressurization during this process. It is also a competitive factor between expensive models to give the most optimal espresso shot.
When referring to pre-infusion, the term pressure-profiling refers to the time the coffee beans are absorbed evenly, then increasing the pressure to the extraction level and gradually reducing the pressure when a cup of espresso is completed. Some theories suggest that exposing coffee to hot water with a sudden high pressure can create an imbalance in the flow rate through the coffee mass, the process of gradually increasing the pressure contributing to ensuring that the extraction will be more balanced.
Experiment to extract espresso with the pre-infusion step
Compound Coffee conducted an experiment using Colombian coffee beans, caturra beans, and wet processing using respectively 20 grams of coffee to extract 40 grams of espresso without pre-infusion step and with pre-infusion in 5 seconds, 10 seconds, and 15 seconds. Waiting for the espresso to come to room temperature and then measuring total dissolved solids (%TDS) and extraction yield (%EY). The results obtained are as follows:
Based on the obtained data, it can be seen that extraction at 9 bar and without pre-infusion step resulting the extraction yield (%EY) higher than extracts with pre-infusion. So It can be concluded that having a pre-infusion step is not effective in terms of espresso extraction yield but provides more uniformity for different coffee shots.
In addition, the sensory aspect of each espresso is significantly different from without pre-infusion, being with the pre-infusion step, the results will have a much more interesting taste:
- The 5-second pre-infusion has brighter acidity and a more appetizing feel.
- The 10-second pre-infusion has bright acidity, a thicker body with richer flavors, and a short aftertaste.
- The 15-second pre-infusion has significantly reduced acidity, fuller body, and increased sweetness but the flavor will be worse.
The pre-infusion step is really necessary for espresso extraction, it is significant in limiting channeling or compact layer, contributing to more stability and uniformity in the extraction. As well as the espresso flavor is also better and smoother than shots with the pre-infusion process.
Scott Deans, 2021. What is Pre-infusion in Espresso (Does it really matter?)
What is Pre-Infusion? Clive Coffee.
Pre-infusion and it’s impact on espresso.