Coffee Data Science

The Untold Story of Steam Pre-infusion for Espresso

The unintended benefit to some lucky machines

Robert McKeon Aloe
Nerd For Tech


Recently, I compared my Decent Espresso (DE) machine to my Kim Express spring-driven, manual espresso machine. I found a large discrepancy in performance with the Kim being the clear winner. I wanted to dig deeper. So I further examined the coffee sneeze.

My theory is that the Kim Express can not only have a higher water temperature than the DE; it must have some other advantage. I found that the Kim does steam pre-infusion, which I believe is the secret weapon. I don’t think this was intentional for the Kim or any other machine using steam pre-infusion, but I believe it is an overlooked and under-studied variable.

Kim Express on the left, DE1Pro on the Right. All images by author unless stated.

For the Kim, the optimal water temperature is between 116C and 123C, so when you pull down the lever to let water into the piston, the first parts of that water immediately turn into steam. This steam starts shooting through the coffee puck very rapidly, and in less than 20 milliseconds, water comes through.

As the water rushes through the coffee, the water exposed to air is vaporizing because the water temperature is far above boiling. Once the bottom of the puck has coffee coming out, the pressure inside the puck is the same as the boiler, so the water temperature can stay high.

This is different from an E61 machine because the water temperature is lower, usually capped at 105C. Also, there is not this chamber to suddenly fill with steam to cause the sneeze. The coffee sneeze is the start of the steam pre-infusion.

Let’s take a trip down memory lane to better understand the history of steam pre-infusion.

La Macchina

This work is in the public domain in its country of origin and other countries and areas where the copyright term is the author’s life plus 70 years or fewer.

Moriondo invented the original espresso machine which was a water boiler that brought the water to 1.5 bars of pressure. This means the water temperature was around 113C. It got the advantage of higher temperature, but not the steam pre-infusion.

La Pavoni

The original espresso machines (around 1910) by Pavoni heated water to be at pressure between 1.5 and 2 bars of pressure. That means the water temperature should have been between 113C and 122C. This is in a similar good range as the Kim Express, but the mechanism to allow water flow was still a flow control knob.

Walt Disney’s in Disneyland.

The Moka Pot

In 1933, Bialetti invented the Moka pot. Oddly enough, the design of the Moka has steam pre-infusion baked in. The Moka has water come to a boil in the chamber which forces water up through a small tube, through the coffee, and out to the top. However, before the chamber becomes pressurized, some steam is generated, and that travels up through the coffee. The amount depends on how long you heat the pot and at what heat intensity.


The first spring driven espresso machines (1948) made by Gaggia allowed for much higher pressures, but they also had the potential for steam pre-infusion. The only blocker was to maintain the water temperature in the grouphead.

The Kim Express

The unique design of the Kim is that the grouphead sits inside the boiler. This design is not used for any other closed boiler lever machines. As a result, the water temperature at the group head is much closer to the boiler, usually within 2C.

Lelit Elizabeth

In 2020, Lelit came out with the Elizabeth, a dual boiler programmable espresso machine using a vibration pump. The Elizabeth can do pre-infusion by turning on the water flow for a few seconds, or it can use the boiler for the steam wand. I thought it was doing actual steam pre-infusion, but I found out that the water is routed in the brew group so that the only benefit is the pressure, not the steam part. I was hopeful it would have such an advantage built in.

Decent Espresso (DE) Machine

My current goal is to find a way to hack the DE to imitate steam pre-infusion. I believe a better understanding on how to implement steam pre-infusion and a study of it will be helpful for not just the DE but all espresso machine design.

Some times, innovations are made without fully understanding them. I think steam pre-infusion is one of those innovations.

If you like, follow me on Twitter, YouTube, and Instagram where I post videos of espresso shots on different machines and espresso related stuff. You can also find me on LinkedIn. You can also follow me on Medium and Subscribe.

Further readings of mine:

My Future Book

My Links

Collection of Espresso Articles

A Collection of Work and School Stories



Robert McKeon Aloe
Nerd For Tech

I’m in love with my Wife, my Kids, Espresso, Data Science, tomatoes, cooking, engineering, talking, family, Paris, and Italy, not necessarily in that order.