Brewing pressure refers to the pressure exerted on the espresso powder during the extraction process in an espresso machine.
Applied by steam pressure (obsolete), a hand lever (still in use), hydraulics, a vibratory pump, or a rotary pump.
In many espresso machines is indicated with the indication "15 bar pump pressure (or even 18 bar) advertised. However, this is only the maximum pressure of the pump, not the actual brewing pressure. This data is about as useful as the P.M.P.O.-Indication on speakers.
The actual brewing pressure is determined on the one hand by the performance data of the pump (maximum pressure, maximum flow rate), which can be read from the characteristic curve, and on the other hand by the resistance offered by the coffee puck. This is determined by the area, height, compaction and grind, and only really builds up as the powder swells. In addition, various valves influence the brewing pressure.
Characteristic
The characteristic curve of the pump tells you at which pressure it can deliver which flow rate. The pressure is antiproportional to the flow rate. Thus one can e.g.B. The following table shows that an EX5, which does not have to overcome any resistance, can pump about 650ml/min. This situation occurs, for example, at the start of a draw, when no significant pressure has yet built up and the coffee powder is still dry.
However, as soon as it becomes wet, it begins to swell and slows down the water flowing through it. This causes a pressure to build up in the system between the pump and the puck. This pressure also weighs on the piston of the pump and reduces its stroke. As a result, the pump delivers less and less water and the pressure increases more and more slowly. At the same time the water flows faster through the coffee powder due to the pressure. If the resistance of the coffee powder remains constant, the pressure will soon reach a value at which the pump’s delivery rate matches the amount of water draining off. This is our effective brewing pressure.
If no water flows out at all, the pressure rises to about 15 bar. At this value, the pump cannot deliver any more water, so it is the maximum pressure of the pump.
Pressure reduction
The ideal flow rate for a double espresso is about 1.5g/s (z.B. 17g coffee grounds, 34g espresso in 25s from the first drop). However, if you look at the characteristic curves of the widespread vibration pumps, you will see that the resulting pressure is about 12 bar. At this pressure, however, filling must be very careful and more bitter substances are dissolved. The ideal pressure, depending on the type, is 9-10bar and must therefore be reduced.
In simple machines, a brew group valve is installed for this purpose, which is basically just a small seal on a spring opposite the water path and requires a certain pressure to be opened. The valve is therefore an additional resistance in the water path that must be overcome and thus lowers the flow rate of the pump by a certain value. As a result, less pressure can build up on the puck. There is now a higher pressure in front of the valve than behind it. If one has z.B. If the pressure at the pump is 13Bar at 1ml/s, the valve deducts 3Bar from this and there is only 10Bar in the sieve.
However, with a double espresso, 2ml/s will flow, which according to the characteristic curve usually corresponds to about 1-2Bar less. This means that with a double espresso, the pressure is always lower than with a single espresso. For this reason, valves are installed in better espresso machines that do not constantly reduce the brewing pressure, but instead open at an adjustable pressure and drain off enough water so that the pressure does not rise any further. So you limit the brewing pressure above a certain, often adjustable value. It does not matter whether a double or a single is brewed, as long as the necessary opening pressure is reached. In principle, these valves are expansion valves, but if they do not direct the excess water into a drainage tank, but back into the pipe or tank in front of the pump, they are called bypass valves.
Rotary pumps need such a valve in any case, because they can pump much larger quantities, about 3 times as much as a vibratory pump (common rotary pumps in single-group machines deliver about 150l/h). Therefore, this type of pump has such a valve built in. To avoid producing too much wastewater or pumping it senselessly through the machine, the drained water is returned to the pump inlet by the shortest possible route, still inside the pump head. This valve is only called bypass.
While many home espresso machines have expansion valves built in that can be used to reduce pressure, they are usually set to only compensate for the thermal expansion of the water after draw and are inactive during draw itself. If the opening pressure is set lower, they effectively limit the brewing pressure, but may produce waste water and, depending on the installation location, may also have a negative effect on the brewing temperature.
Hand lever
With hand lever machines, the pressure is directly dependent on the force with which the user pulls on the lever, or on the force with which the user pulls on the lever. How strong the spring is. Machines without a spring are therefore always able to generate the right brewing pressure if the force is correctly measured. However, if the powder offers too little resistance, the water rushes through very quickly at this pressure. If this is compensated for by less force, the pressure drops too much. On the machine side, nothing stands in the way of the correct brewing pressure, the operator can always achieve this by the appropriate choice of grind, powder quantity and contact pressure.
This is somewhat different with lever machines with spring. In this case, the powder is always subjected to the pressure specified by the current spring tension. If the spring is too soft, the appropriate brewing pressure may not be achieved at all. In addition, the spring force decreases the closer it relaxes. So at the end of the reference the pressure is always lower than at the beginning. Here, it may be necessary to change the spring to adjust the brewing pressure. An incorrectly operated lever machine usually produces an even worse espresso than a pump machine, because the flow time becomes extremely short if the grind is too coarse.
Nozzles
Many espresso blends react negatively to a rapidly increasing brewing pressure, as seen for example in rotary pumps with their high delivery rates. Most espresso machines therefore have a nozzle in the brew head. This nozzle offers resistance to the water, especially at high flow rates, as is the case when drawing empty or at the beginning of the draw. In this way, pressure builds up within the machine and the flow rate decreases or. is slowed down. This means that the brewing chamber fills with water more slowly, and the pressure can build up more slowly in it. The smaller the diameter of the nozzle, the flatter the increase in brewing pressure and the better the powder can preswell. However, a nozzle that is too small can also lead to the necessary brewing pressure not being achieved for products such as cafe creme or Schumli, which require a much higher flow rate than espresso, or to the brewing time becoming unnecessarily long even with very coarse grinds. Larger nozzles are therefore more likely to be used here. If you want to be able to produce both types of beverage on one and the same brewing group, you need a different type of preinfusion.