Professional barista Francesca Lapierre breaks down the science behind pressure in espresso brewing and how modern baristas are using pressure profiling technology to achieve the perfect pull.

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The modern coffee consumer is more discerning than ever. Espresso consumers, in particular, look for very specific telltale signs of a good brew, like a thick, substantial layer of crema, a rich body, and the perfect balance of acidity and sweetness.

It takes a skilled barista to pull the perfect shot from a traditional espresso machine, but what exactly do they do that’s so important? In reality, there are several core aspects of top-class espresso brewing.

High-quality coffee, the right grind size, and the correct tamping technique are fundamental skills, but they’re pointless without the right water temperature and barometric pressure. In fact, barometric pressure – or the weight of air at sea level – is considered one of the most significant contributors to a perfect espresso shot.

Traditional espresso machines use barometric pressure to pull and push hot water through a tamped puck of coffee grounds. This pressure needs to be just right to ensure that enough water comes into contact with the grounds to trickle through, while facilitating a degree of resistance that prevents channelling and uneven extraction.

Pressure is generated by the machine’s boiler, which produces the steam that drives water through the coffee grounds. Some machines, like the Elektra Indie, have dual boilers, ensuring that there’s always enough steam available.

To achieve an espresso shot that is neither weak nor bitter, nine bars is typically seen as the rule-of-thumb. More recently, however, innovative baristas and prosumers have taken to “pressure profiling”, manipulating pressure at various points in the extraction process to improve the final result.

Pressure profiling has changed what baristas, roasters, and consumers understand about espresso, but it hasn’t replaced the nine bar method. Ultimately, this dichotomy is one of science and artistry, representing the nature of a barista’s work.

Early espresso machines were lever-pump operated and relied on steam pressure to extract espresso. However, this method could only achieve around one or two bars of pressure and didn’t produce much crema.

The science behind the perfect espresso pressure

It wasn’t until 1945 that Giovanni Achille Gaggia solved the pressure problem, developing a spring-loaded lever that could build up to 10 bars of pressure. When the lever was pulled by the barista, hot water was poured onto the coffee puck, and when released, it brought the spring down to push the water through the grounds.

This created enough barometric pressure to produce decent crema, and despite its shortcomings, the handheld lever and spring mechanism was a pivotal invention in the history of espresso.

As time went on, other manufacturers developed electric pumps, eliminating the need for levers. The first “automatic espresso machines” could maintain a constant nine bars of pressure, resulting in a flavourful brew with a good balance of sweetness and strength.

Science tells us that on a scale of pressure, nine bars represents “peak flow”. In other words, as pressure increases, more and more liquid gets through the puck until it crosses the nine bar threshold.

After nine bars, pressure begins to compact the cake and water struggles to trickle through. As such, maintaining pressure at nine bars produces a sweet, pleasant espresso that holds its richness and deep flavour, containing enough water to prevent bitterness and enough caffeine to satisfy the consumer.

As mentioned, there are alternatives to the nine bar tradition. For all its benefits, it doesn’t allow the barista or prosumer to explore all the possible nuances of high quality coffee. However, if one brewing variable is adjusted, the others must follow accordingly.

For example, if pressure is reduced, grind size must be correspondingly adjusted. If it’s pushed even higher, extraction time is actually increased, implying that there’s more resistance between the coffee puck and the water. 

This may seem counterintuitive – because more force is being applied to the grounds — but it actually has nothing to do with the water. In reality, gases released under higher pressure make it more difficult for water to pass through, resulting in a bitter, over-extracted shot.

Conversely, stale coffee grounds work just fine with a higher pressure setting, precisely because it has less carbon dioxide to release. In fact, it’s been found that stale coffee brewed at a higher pressure produces a shot that’s comparable to fresh coffee brewed under nine bars of pressure.

It’s clear, then, that there’s a direct correlation between pressure and coffee solubility. As such, light roast coffee beans – which are typically denser – are harder to extract, requiring a higher rate of pressure.

The pressure profiling revolution

In the last decade or so, innovation in espresso machine technology has made it far easier for baristas and prosumers to explore pressure profiling. Today, machines can be set to specific flow rates, with the standard typically being between 6.5-7g water per second.

The ability to adjust this makes it relatively simple for any barista or prosumer to play with grind size, taste, and caffeine extraction from their beans. Naturally, then, pressure seems like a good place to start for anyone interested in exploring the nuances of specialty coffee.

However, not all modern espresso machines – professional or domestic – are capable of pressure profiling. The vast majority of consumers still rely on the nine bar rule, but companies like Carimali and Bellezza are bringing more and more innovation to the market.

Using proportional–integral–derivative (PID) controllers, these machines can measure changes in pressure during brewing, allowing the user to accurately adjust it over the course of extraction. Machines like the Bellezza Francesca pair the PID controller with a shot timer, making it far easier for the user to keep track of the changing brewing variables.

Interestingly, innovation in pressure profiling isn’t limited to traditional espresso machines. The very best superautomatic espresso machines now incorporate pressure profiling into their automated functions, albeit using volumetric (fluid) pressure rather than barometric pressure.

In the case of the Carimali BlueDot Power, the ability to program the water pump enables businesses to adjust the speed at which the machine can produce a variety of drinks. Once that’s done, all the user needs to do is press a button.

The Heylo Coffee Module is another example of variation in pressure profiling technology. Unlike traditional espresso machines, the Heylo doesn’t heat water in a boiler. Instead, it uses induction technology, but is still equipped with controls for temperature and pressure profiling. It can even alert the user when it senses that something is wrong with the recipe.

It seems, then, that pressure profiling will remain a desirable feature in espresso machines in future, regardless of how that pressure is generated.

And, though experimenting with it may not always result in the most delicious flavour and desired strength, it allows the barista or prosumer to gain intimate knowledge with their grounds, contributing to the never-ending quest for coffee quality.