Introducing sufficient pressure within the espresso machine and knowing how to control it and the flow of water is crucial for pulling full-flavoured shots of coffee. If you’ve ever wondered why pressure builds up in the espresso machine, this guide will discuss the underlying mechanics and techniques for capitalizing on it.
A simple way to describe the process is input flow + resistance = pressure + output flow. In an espresso machine, pressure builds up when water passes through the finely ground coffee as the water encounters resistance from the coffee puck. The more resistance, the higher the pressure, which you control to determine the quality of your espresso. Now, let’s dive into how this balance plays out across different machines and technologies and how it can help you perfect your espresso.
Pump Espresso Machine: Flow and Pressure Control
The pump itself is the key component of a traditional pump espresso machine that restricts the amount of water reaching the coffee puck. While this allows significant water flow, once the water reaches the coffee puck, the finely ground coffee filters the water, and pressure rises. Most espresso machines apply around 9 bars of pressure when extracting espresso.
Vibration Pumps
Vibration pumps are commonly used in various household appliances, including kitchen equipment and cars. These pumps typically have an output of about 8 mL per second, which is more than sufficient for standard espresso extraction.
Rotary Pumps
Rotary pumps, found in most industrial equipment, can provide much higher flow rates, reaching 30 mL per second or more. However, greater flow rates don’t always mean better espresso quality, as other factors like pressure control come into play.
Water continues to flow until pressure builds up, which is usually regulated by an Overpressure Valve (OPV). After reaching the pre-set pressure level, such as 9 bars, the OPV diverts excess water to maintain stable pressure throughout the shot.
Another characteristic of pump espresso machines is the variation in pressure measurement sites. In commercial machines, the pressure gauge is usually placed after the OPV. In contrast, in home machines it is often located before the OPV, which can cause discrepancies when comparing brew pressure between machines.
Achieving a Constant Pattern by Balancing Flow
The art of espresso preparation goes beyond achieving high pressures. Too much flow in the coffee puck can lead to crushing or channelling, which results in uneven extraction. To avoid this, one can manipulate the flow rate to ensure pressure builds slowly and steadily.
Gicleurs
A giclee controls the maximum pressure that reaches the coffee puck by limiting water flow. Commercial machines often use it to slow pressure build-up, leading to higher-quality shots. It ensures that water does not hit the coffee puck simultaneously, reducing the risk of channelling.
Needle Valves
Needle valves make it easier to control the flow by allowing baristas to throttle water flow throughout the extraction process. A popular feature in machines like the Slayer Espresso is that needle valves support precise control during different stages of brewing, including pre-infusion and pressure extraction. Adjusting the flow profile can help extract more flavour from the coffee beans.
Conical Valves
Conical valves, used in machines like the La Marzocco GS3 MP, regulate pressure by controlling the water flow from the group head. This allows users to adjust pressure without changing the flow, making it easier to maintain brewing consistency.
Lever Espresso Machines: A New Concept of Pressure Application
Lever espresso machines introduce a different pressure application method compared to pump machines. With a lever machine, pressure is applied by the user or a spring mechanism, allowing nearly unlimited flow to the coffee puck with varying effects.
In these machines, pressure builds as the spring lever uncoils, following a unique pressure profile. It starts with low pressure during pre-infusion, rises to a peak, and gradually decreases. Lever machines typically don’t have OPVs because the lever mechanism or the user’s strength determines the pressure.
These machines can create well-balanced shots due to their natural pressure modulation, which enhances extraction.
Importance of the Coffee Puck in Pressure and Flow
The coffee puck’s composition plays a significant role in determining pressure and flow dynamics. Grind size, coffee dose, and puck distribution contribute to pressure resistance.
- Finer Grinds: The coffee puck becomes more resistant and requires higher pressure. However, this can also slow the flow, leading to over-extraction and bitter flavours.
- Coarser Grinds: Coarser coffee allows more flow and results in lower pressure. This can cause under-extraction, making the coffee taste sour or weak.
As extraction time passes, the resistance of the coffee puck changes, which can affect the consistency of the shot. This is why fine-tuning your grind and flow settings is essential for avoiding uneven extractions.
Pressure Profiling vs Flow Rate Control: What’s the Difference?
One of the most common questions in espresso extraction is the difference between pressure profiling and flow rate control.
- Pressure Profiling refers to the amount of pressure applied to the coffee puck during extraction. Some machines allow you to manipulate pressure during the shot, often raising and lowering it at different stages for optimal flavour. This technique is particularly common for lighter roasts.
- Flow Rate Control focuses on the volume of water hitting the coffee puck per unit of time. By adjusting the flow, you also indirectly adjust pressure. For instance, reducing flow during pre-infusion can help the coffee bloom more gently and prevent channelling.
While pressure profiling and flow rate control are related—changing one affects the other—flow rate control focuses more precisely on the water’s interaction with the coffee puck, while pressure profiling is about maintaining pressure over time.
Conclusion: Achieving Harmony in Espresso Brewing
Understanding the interaction between pressure and flow during espresso extraction is key to creating the perfect shot. Whether adjusting the grind size, modifying the flow rate, or increasing brew pressure, the goal remains: finding the right balance to extract the full flavour from your coffee.
You can achieve various espresso profiles with tools like needle valves for fine flow adjustment or conical valves for pressure regulation. And remember, it’s not just about reaching 9 bars of pressure—true mastery lies in balancing flow and resistance to make the perfect cup. Happy brewing!
