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Why is tamping necessary for espresso machines but not mokas?

Kingston's How to Make Coffee: The Science Behind the Bean, on how to use a moka, says:

  1. Fill the funnel with enough finely ground coffee so that it forms a rounded dome. Tap the funnel on the counter to even out the grounds.
  2. Without tamping or compressing, make sure that the rubber gasket is in place and then screw the top half of the macchinetta onto the pot.
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Espresso machines rely on pressure to extract coffee solutes over a very short (~25 seconds) extraction time. Tamping is a critical aspect to achieving this. By tamping ideally, you eliminate channels by which water might pass through the grounds. Elimination of those channels is important for two reasons. First, channels mean that water only touches some of the grounds but not most, meaning you get very little coffee extracted. Second, tamping evenly forces water to travel through the entire bed of grounds; this is a critically important aspect to how the machine builds pressure. If channeling is present, water will simply drip through the grounds. You will not get 9-12 bars of pressure (or anywhere near that).

Moka pots are different because they build pressure below. Water is heated in the bottom chamber where it rises to dampen the bottom layer of grounds in the basket above. Because the moka pot doesn't use a pump, it requires steam to build up to create pressure. The pot is designed with a certain volume of space between the water reservoir and the grounds basket to facilitate this. If you tamp, you create resistance that the steam pressure might not be able to penetrate--at least not without causing an explosion of grounds and forcing a lot grounds up into the top chamber and making your brew sludgy.

If a moka pot filtered the coffee, something else bad could happen if you tamp. If there were a filter above the grounds, the aforementioned upward explosion of grounds caused by excessive pressure buildup would push grounds up into the filter, clogging the filter. This could cause the filter to get clogged. Best case, a steam outlet valve lets out the excess pressure and your brew is just ruined. Worst case, the steam outlet valve could get clogged and your pot could explode. Basically, even with completely effective safety features and special filtering features, tamping a moka pot would make it impossible to achieve a consistent brew because the amount of clogging of the filter would cause pressure during brewing to vary unpredictably. This isn't a problem with espresso because with espresso, a pump pushes the water through and because the machine itself controls the temperature of the water.

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An espresso machine sends the water through the coffe grounds under high pressure, typically 9 bars. If you had an uncompacted bed of grounds, the water would just bore through on a path of least resistance (aka. channeling), leading to a very uneven extraction.

A moka pot, on the other hand, only uses steam pressure to force water up through the coffee grounds, a much weaker force (only about 1 bar). If you tamped the coffee, it would provide too much resistance, the water would not be able to get through, or only very slowly. Due to the weak force, channeling is not a problem, the puck will get soaked evenly.

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A Moka (usually) has a safety valve that will prevent the pot from exceeding a target pressure. It’s balanced in a way that the pressure that builds when using a Moka pot is just right when the chamber is filled flush with right grind size. If you use either too fine coffee (erm... happened to me once), fill the water chamber higher than the valve, or restrict the flow by tamping, you will either trigger the valve or - in case of a moka without valve - have the pot explode.

It’s of course up to the individual user’s risk tolerance to follow or ignore the general rules, but don’t come back here to complain.

An espresso machine on the other hand is balanced for a compacted load of coffee in the chamber. The two machines are just designed differently.

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