Moka Pot Coffee

One of the most frequently asked questions during brewing courses is “how do you make moka pot coffee?” The answer is that there is no single universally accepted method, but rather the best method is the one that achieves the result you want.

To do this, you need to understand the variables at play and their impact on the final taste in the cup. Get ready for a journey where the latest scientific research meets one of Italy’s greatest traditions.

The Moka Pot and its Components

We cannot discuss moka pot extraction without first understanding how it is built.
The figure shows the main components.

Moka Pot: Stainless Steel or Aluminum?

Since the 1980s, stainless steel has begun to rival aluminum as a construction material for moka pots. (2)

The first choice when you decide to make coffee with a moka pot is the material of the pot itself. The main materials on the market are aluminum and stainless steel. There are currently no scientific studies comparing moka pot extraction with the two materials, but we can still make some considerations.

Aluminum has an indicative thermal conductivity at room temperature of 2.4 W/(cm °C), whereas stainless steel is 0.17 W/(cm °C). This means aluminum can transfer a greater amount of heat per unit time given the same surface area.

One could plausibly hypothesize, pending appropriate scientific data, that an aluminum moka pot may be more responsive to temperature changes and may lead to faster increases (or decreases) in temperature and pressure in the boiler.
There is no universally better choice; choose according to the results you want, which can be inferred from the variables discussed below.

Moka Pot Coffee: Choosing the Coffee

Of primary importance for moka pot preparation is choosing the coffee itself. Getting this choice wrong would render the entire subsequent process futile.

You therefore need to know the differences between species, origins, harvesting and processing methods, and the various roasting methods and levels. This knowledge is not so much for judging a coffee before tasting it, but to make faster, more targeted choices when you aim for a specific aromatic or flavor goal.

If you have not attended a tasting course or an introduction to coffee, do not worry—finding the coffee you want still requires tasting a wide variety of types. The search for the right coffee is often enjoyable and involves exploring new aromatic and flavor landscapes, and above all discovering your own taste in coffee.

The second decision is whether to use whole beans or pre-ground coffee. In the first case you will necessarily need a manual or electric grinder, but once opened the coffee will keep much longer, and you will have more scope to change the taste of the final beverage by adjusting the grind size.

In the second case, certainly more convenient, you will have fewer possibilities to adjust the taste in the cup and the raw material will degrade more quickly during storage. (5) The recommendation, therefore, is to use whole beans if you are seeking experimentation and high quality in the cup, and to use pre-ground coffee if you are looking for convenience and simplicity.

Moka Pot Coffee: Choosing the Water

Here too, the topic is long, complex, and constantly evolving. What is certain is that water composition influences coffee extraction and the resulting sensory experience in the cup.

Soon, for those who wish to explore the topic in depth, we will publish an article focused on the important subject of how water composition affects coffee extraction!

The recommendation here as well is to experiment with a wide range of waters and select the best one for the coffee you have chosen and the resulting extraction.

How Moka Pot Extraction Works

The moka pot uses pressure generated by steam, produced from the water in the boiler and heated by an external source (gas or electric stove), to force that water upward through a bed of roasted, ground coffee held in the funnel filter. The beverage thus created is channeled upward through a tube (spout) and collected in the upper chamber. (2)

First, the boiler is partially filled with water; at this stage it is important not to exceed the safety valve, which, in the event of excessive pressure inside the boiler, vents steam to restore a safe pressure.

The ground coffee is then placed on top of the funnel filter, which is in turn set onto the boiler. Then screw the upper chamber (collector) onto the boiler to create an airtight seal that prevents lateral leakage of liquids.

The boiler thus initially contains water at atmospheric pressure and at the initial temperature (Ti). At this point the moka pot is placed on the heat source, warming the water, water vapor, and air inside the boiler.

Heating the water increases its vapor pressure and gradually forms new steam above the surface; at the same time, the air and steam are heated by the system, thus increasing in pressure (because heated gases tend to expand).

The rise in pressure of the air/steam mixture above the water pushes it downward; under pressure, the water then finds its way up through the funnel to come into contact with the coffee inside the filter. (5)

This is where the imbibition phase begins: the water gradually wets the grounds and is absorbed by them, but does not yet have enough force to exit through the spout.

During the imbibition and extraction phases, the coffee undergoes chemical transformations due to interaction with water, which fundamentally alters its properties. The invasion of water into the coffee bed during imbibition leads to the dissolution of the more soluble, lower–molecular weight compounds and the more volatile aromatic substances.

At the same time, the ground coffee particles swell due to the expansion of insoluble polysaccharides present in roasted coffee; during this phase the particles move and change their arrangement due to the flow of water. (2)

At this point, swelling of the coffee bed and particle movement progressively reduce the bed’s porosity; extraction proceeds at increasingly higher temperatures/pressures. (2) The water carrying the soluble (and partly insoluble) coffee compounds passes the spout and collects in the upper chamber.

Note that the temperature of the air–steam mixture remains, during extraction, considerably lower than the water temperature. (2) It has been shown that extraction begins at fairly low temperatures; various texts report initial extraction temperatures of about 70°C when the water placed in the boiler is at 20°C—well below the SCA’s recommended 92–96°C for brewing.

The flow continues (at progressively higher temperature and pressure) until the water level in the boiler drops below the lower end of the filter’s funnel; at that point steam is forced through the funnel toward the filter to contact the coffee and pass through the spout. This ends the regular extraction phase and begins the volcanic phase. (5)

At this point the remaining water in the boiler undergoes intense evaporation. The operator recognizes the start of the volcanic phase by the characteristic sound, and by the coffee no longer flowing neatly from the spout but spurting irregularly away from it; it is also possible to observe coarse bubbles caused by steam rising from the boiler.

The passage of high-temperature vapor (water and steam) through the ground coffee is detrimental to extract quality because, under these conditions, such fluids are very effective at solubilizing normally less soluble compounds and less volatile aromatic compounds, typically imparting bitterness, astringency, and organoleptically unpleasant sensations described as clove-like, smoky, burnt, medicinal/chemical. (2)

For this reason, the volcanic phase should be avoided when obtaining a quality cup of coffee.

Water Temperature

As we have seen previously, in traditional moka pot coffee preparation, water is placed inside the boiler at room temperature. Consequently, half of the coffee could be extracted when the water temperature is below 70°C, a temperature significantly lower than that indicated as correct for coffee extraction. (5)

It might be useful in this regard to heat the water to about 70°C before inserting it into the boiler. In this case, depending on the moka pot configuration and other variables at play, you could reach, for example, an average extraction temperature of 88°C, with the last milliliters of extraction (before the volcanic phase) estimated at 94°C. (5)

K. Fibrianto et al. performed moka pot coffee extractions on two different types of coffee at different water temperatures: 50, 70, and 100°C. According to their results, the least bitter and sweetest cups were found with water at 100°C temperature. Although they did not report the extraction methods and this data might make moka pot coffee extraction experts shudder, it might not be entirely in contrast with data from other scientific research and daily practice.

It should be remembered that Navarini et al. measured water temperature in the boiler and in the filter (just above the coffee) during the extraction phase with heat source power at 400 and 600 W, with initial water insertion at room temperature.

For the first test (400 W), an initial extraction temperature of 68.7°C was recorded in the boiler, a final temperature of 117.2°C with a total average temperature of 94.3°C. In the filter, the initial temperature was 63°C, final 95.8°C with an extraction average of 78.8°C.

For the second test (600 W), an initial extraction temperature of 70.2°C was recorded in the boiler, a final temperature of 120.6°C with a total average temperature of 97.6°C. In the filter, the initial temperature was 61.8°C, final 97.7°C with an extraction average of 80.5°C.

This data clearly shows that the water temperature in the boiler is always higher than the actual coffee extraction temperature.

Since the chosen water temperature depends, for example, on the moka pot composition, heat source modulation, and the organoleptic characteristics you want to extract from the raw material, it is essential to proceed by trial and error in creating the recipe. 70°C is often a good starting point; the range recommended by various texts, pending further studies, is between 70 and 100°C.

However, you must always remember that the higher the water temperature, the more careful you must be about the temperature of the final part of extraction, which risks becoming very high compared to the desired temperature, and that the other variables at play during extraction remain very important in determining the actual coffee extraction temperature.

Water Quantity

W. D. King performed diligent work in calculating the mechanics of extraction water temperature variation by modulating the amount of water present in the moka pot boiler (specifically, the free space left above the boiler water).

Without going into the merit of the complicated equations reported by W. D. King (5), we can state that:

The more free space is left above the water in the boiler (i.e., the less water we put in the boiler), the faster overpressure is obtained at lower water temperatures (i.e., extraction begins earlier and with a lower average water temperature during extraction) with higher overpressure during extraction.

Conversely, the less free space is left (the more water we put inside the boiler), the more slowly overpressure is obtained and at higher water temperatures (coffee extraction begins later with higher average water temperature during extraction) with lower overpressure during extraction.

As we have seen, standard filling conditions result in extraction temperatures that are too low. If we wanted to extract coffee at higher temperatures, it might be useful to minimize the free space above the water in the boiler (add more water). Unfortunately, most moka pot models on the market do not allow for significantly increasing the water level, as it would end up covering the safety valve of the pressure switch designed to release steam in case of overheating. (5)

The recommendation is to modulate the amount of water to make adjustments regarding extraction temperature, without ever exceeding the safety valve or going close to the height of the filter funnel opening.

Coffee with Moka Pot: Ground Coffee

First of all, the filter should be filled to the brim and simply leveled, without creating grooves or pressing the coffee. These procedures would risk creating greater resistance to the water during extraction with consequent strong increase in temperature and pressures involved, up to risking bringing the boiler into overpressure and causing the safety valve to activate.

Coffee grinding is of fundamental importance for coffee extraction. If using coffee beans, it is good to know that grinding it finer will result in a more bitter and full-bodied coffee, while a coarser grind will result in a more acidic and less full-bodied coffee. It is important not to grind the coffee too fine, as this would risk creating a “plug” effect with the risks described above, as well as bringing overly bitter flavors to the cup.

High or Low Flame?

Research has shown that lower energy provided by the heat source (for example a burner kept low) leads to a slightly lower average water temperature in the boiler during extraction compared to a heat source that provides more energy, where the average water temperature in the boiler during extraction is slightly higher.
The water temperature during extraction in any case decreases slightly compared to that present in the boiler due to contact with the coffee bed. (2)

From this data, someone might conclude that low flame is the best choice to maintain a lower extraction temperature. However, tests report minimal differences in measuring the average temperature for the regular extraction phase (before the volcanic phase).

For example, a heat source with a power of 400 W of energy led to an average extraction temperature of 78.8°C compared to 80.5°C with a power of 600 W.

It must be remembered that the greater the rate of temperature increase, the greater the pressure, since pressure increases more quickly and therefore the flow rate of outgoing water increases accordingly. (5)

This brings us to one of the fundamental factors regarding coffee extraction: the contact time between water and coffee.
The greater the contact time between coffee and water, the greater the extraction of coffee substances. Greater extraction tends to lead to more bitter tastes in the cup, while lower extraction leads to acidic tastes; typically, a balance between these tastes is sought in the cup.

Consequently, while it is true that a lower flame used in preparing coffee with a moka pot leads to slightly lower extraction temperatures, it is also true that the same practice leads to a longer extraction time, resulting in the extraction of a greater quantity of substances.

The recommendation is to use a slightly coarser grind if you decide to use a lower flame and, instead, a slightly finer grind if you use a higher flame. To create the recipe, you must therefore be able to calibrate the flame/grind combination, always being careful not to use a grind that is too fine, which would result not only in over-extraction of substances but also in the much more dangerous blockage of the water outlet inside the pressurized boiler, with the “cork” effect described earlier.

End of Extraction

The final phase of extraction is crucial for the final taste in the cup. Some people lower the flame as soon as coffee begins to emerge from the spout, thus modifying the extraction times and temperature, while others maintain the same power until the end.

You must be very careful not to reach the volcanic phase at the end of extraction. In this regard, it is useful to calculate the time needed to remove the moka pot before the volcanic phase using a given amount of water and coffee, a specific grind, and a certain flame power. There are two main schools of thought on this matter.

The first proposes removing the moka pot from the heat source several seconds before the volcanic phase to avoid providing new energy to the extraction. In this way, as the water flow proceeds through the spout, the pressure in the boiler naturally dissipates, thus ending the extraction.

A second school of thought proposes continuing the extraction until a desired moment, at which point the boiler is immersed in water and ice, greatly reducing the thermal energy of the moka pot and thus ending the extraction.

Bibliography:
1- N. Caporaso et al. (2014). Neapolitan coffee brew chemical analysis in comparison to espresso, moka and American brews.
2- Navarini L.et al. (2009). Experimental investigation of steam pressure coffee extraction in a stove-top coffee maker. Applied Thermal Engineering, 29(5), 998-1004.
3- G. Concetto. (2007). Experimental analysis of the Italian coffee pot “moka”. American Journal of Physics – AMER J PHYS. 75. 43-47. 10.1119/1.2358157.
4- Carolyn F Ross et al. (2006) Effect of storage conditions on the sensory quality of ground arabica coffee
5- Warren D King. (2008) The physics of a stove-top espresso machine
6- K. Fibrianto et al. (2019) Effect of Mokapot brewing temperature on sensory profiling of Dampit and Tulungagung ljo coffee