"4M" is something that I have been heard of as a requirement set for tasty espresso.
What are these four important quality factors that affect the flavor of final espresso cup?
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4M is a term mainly used among Italians for espresso. It is a placeholder for four words that start with "M" in Italian that affects espresso preparation. Some information can be found here.
These M's are:
Some people add the fifth M, referring to "Maintenance" (keeping equipment clean, espresso machine descaled, having burrs in the grinder regularly replaced etc.)