Ben Mitchell chats with mechanical engineer Greg Scace about how induction heating in espresso machines has opened up a whole world of opportunities for temperature profiling.

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The technology of espresso machines is forever changing, each time contributing to a slightly better cup of coffee. But of all the new developments to burst onto the scene in the last few years, rarely has there been as much excitement as there has been around induction heating.

Pioneered by modular espresso machine manufacturer Heylo Coffee, induction heating is a relatively simple concept: cold water is channelled through conductive metal tubes surrounded by electromagnetic induction coils that heat it up to the desired temperature. The whole process takes place in under five seconds and consumes somewhere between 600 and 700 watts of electricity – or less.

This is in contrast to traditional espresso machines that rely on a large boiler to keep the water at a constant temperature ready for brewing (typically around 86-95°C). While this works for coffee shops with high footfall, it tends to eat up more energy.

“Nearly all espresso machines have large boilers,” explains Greg Scace, a mechanical engineer who helped develop the Heylo coffee machine. “They’re essentially just reservoirs of heat. It takes a lot of energy to heat up four or five litres of water – or even one litre of water.”

Many espresso machines also use a proportional integral derivative controller (PID) algorithm to control boiler temperature. This works by monitoring the variance in the process temperature versus the “setpoint” temperature within a period of time.

However, Greg says it can make it difficult to manipulate how hot the water is during the extraction. “The old PID control works pretty well,” he says, “but it’s very hard to move that around.” That’s where induction heating comes in.

What is temperature profiling & how does it work?

Flow and pressure profiling are now well-established concepts in espresso brewing. The first involves manipulating the speed at which water flows through the espresso puck; the second is the number of bars of pressure that are exerted.

Temperature profiling, however, is relatively new. The basic idea is that baristas control the temperature of the water throughout a shot to bring out different nuances in the coffee. For example, baristas could start with a relatively high temperature, before slowly decreasing it during the extraction to change sweetness or acidity.

On a more scientific level, Greg says it means affecting the way water reacts with the soluble compounds of the coffee. “Temperature profiling makes a lot of sense if you understand how dissolvable material in ground coffee goes into the cup,” he says.

He adds that there are over a thousand compounds in coffee that are soluble in water – and each of them dissolves at a certain rate. The rate of dissolution has three main drivers:

• The temperature of the solvent (brew water)
• Surface area (the fineness of the grind)
• The concentration gradient (how much of the dissolved material is already in the water)

As the water emerges from an espresso machine group head and travels through the bed of coffee, it collects soluble material.

The rate of dissolution is limited by the amount of dissolved material already in the water. As such, the very small boundary layer – immediately adjacent to the dissolving material – is at “equilibrium solubility”.

The condition of that boundary layer is decided by the current solubility equilibrium of the surface of the dissolving coffee. It’s also affected by the amount of dissolved coffee in the flowing water.

It’s this layer that can be directly manipulated using temperature profiling. Put simply: the hotter the water, the less soluble material it can hold. In other words, the solubility of coffee increases with temperature.

Each particle within a coffee puck ultimately receives a different amount of energy. Temperature profiling gives the barista more control over this energy distribution.

“When you transfer energy from hot water to cold coffee, the ground coffee is cooler,” Greg elaborates. “When the water hits the ground particles at the top of a bed of coffee, it heats them up near the temperature of the hot water – but by doing so, the water loses energy. So the water leaving those particles and going to the next particles is colder.”

Similarly, the difference in flow rate through different parts of the puck will affect this energy balance. Even with excellent puck preparation, there will be far more water flowing through the centre of the puck than around the perimeter, keeping the centre hotter.

“The idea of temperature profiling is to manipulate that and possibly correct for it, or accentuate the differences if desired,” Greg adds. “It becomes another artistic space that you can work in.”

Tips for mastering temperature profiling

Greg notes that it wasn’t easy to develop the Heylo induction system. For every shot, approximately 65ml water must be heated from room temperature to brew temperature.

The challenge is to precisely control and maintain temperatures for 25-30 seconds at a time.

Inside the Heylo Espresso Module, water circulates around a 12cm coil in order to heat up. Just before extraction, water runs through a recirculation system to quickly reach the target temperature. This mitigates any temperature drops that could occur when water leaves the group head.

In fact, the Heylo group head itself has a 200W heating element that maintains stable temperatures.

In theory, a barista can use the Heylo to tailor the temperature to specific coffees to highlight different characteristics.

The main decider is roast level. Darker roasts are more porous and therefore more soluble. In contrast, the cellular structure of a lighter roasted bean is more intact and less porous. Therefore, darker roasts require lower-temperature water, and vice versa.

For example, an “increasing gradient” temperature profile (92ºC start, 96ºC finish) might suit a light-roasted coffee. In this case, light acidity will be extracted first, and the shot will not be dominated by acidic, sour notes sometimes associated with lighter roasts.

The finishing temperature will grant access to sugars, which typically extract at a slower rate than acids. As Greg says, this corrects the energy imbalance exerted on the coffee puck with a “flat” (constant) temperature profile.

Conversely, a “decreasing gradient” temperature profile can curtail the bitterness and astringency more easily extracted from darker roasts. This accentuates the energy imbalance and leaves those undesirable materials behind.

With older espresso machines, the boundary conditions must be reset after each shot. This results in excessive energy use and can waste valuable time. This, Greg explains, is what makes induction heating so remarkable.

“There’s so little energy wasted, and it all happens in this little period of time,” he elaborates. “It’s very easy to reproduce [results] and [the machine] is more reactive. You can [pull shots] repeatedly and with a great deal of precision.”