The optimal espresso curve

But before I plunge into that, I want to make the argument that there is an arguably optimal pressure curve for espresso recipes.

You can see it as the pressure profile that occurs naturally when constant flow water is used to make an Allongé recipe coffee on the Decent. The resulting pressure is a reflection of the declining puck resistance over time, as the puck loses material to the espresso drink.

This curve has the following characteristics:

• a preinfusion that takes about 10 seconds before pressure builds
• a peak pressure around 8.6 bar
• an end pressure around around 4 bar
• a total espresso making time around 30 seconds

The "four mother" recipes

1. the more soluble it becomes ("more easily gives up material to water")
2. the more integrity the coffee puck will have (it is less likely to fall apart and channel badly during espresso making).  

1. increase time: the amount of time that the beans are in contact with water  
2. increase water flow rate through the grounds bean puck
3. increase the water temperature, which increases the efficacy of the water to extract material (but not always the good flavors)

Finer grind size also increases extraction, but is not an independent variable. There is typically an appropriate grind size for each type of recipe.

This gives us:

Which gives us these insights:

• Classic Italian espresso machines are hard-wired to give their best results with Italian-style roasted beans (medium dark to dark).  FYI: I consider the common "flat 9 bar" profile to be a simplified (and less good) version of the Lever profile.  Lever Profile results in typical extraction yields of 19% to 21%, that are the lowest of the 4 recipes, since flow rate is lowest and water contact time is the shortest, with this recipe.
• Light roasts are known to fall apart quickly and channel during espresso making when using the Lever (or flat 9 bar) profile. This is likely why those whose machines can only do that recipe of espresso will tend to compensate by either making Ristretto (short time) shots, or putting a lot more coffee into the basket.  If you don't do this, you get very acidic, generally under-extracted espresso, because you haven't appropriately compensated for the lower solubility of the beans you're working with.  
• The Blooming recipe works well on medium to light roast beans, but with ultralight roasts, the puck tends to fall apart at the end of Blooming shots, as ultralight beans don't have enough puck integrity to survive the last stage.  Rao tries to work around this by having the last stage flow profiled, so that if the puck integrity fails, at least the flow rate doesn't go super-fast. The long preinfusion demands a super-fine ground.  Blooming results in the highest extractions, both because of the long water contact time, but also because this recipe requires the finest ground beans, which also increases extraction.
• The Allongé recipe works best with ultralight roasts because it demands so little puck integrity, and the fast flow rate require coarse grounds in order to not go over-pressure.  The Allongé recipe follows the findings from the widely quoted Hendon research paper https://www.sciencedirect.com/science/article/pii/S2590238519304102 that coarse grinds, and fast flow, result in relatively high extractions and good consistency. The typical EY of 21%-23% is likely due to the coarser grind than Blooming. But because Hendon didn't use a flow-profiling or pressure profiling espresso machine, his espresso suffered from rapid loss of puck integrity, and so he ended up advocating for shorter duration espresso recipes.  By using constant flow (or decreasing pressure) it's possible to use the coarse grind/fast-flow approach but to pull the shot to the normal 30 second duration.
• Each recipe has an ideal bean roast level.  If you use dark roasted beans for any non-lever recipe, you'll likely over-extract it.  Light beans on Lever will often under-extract and/or channel.
• People with manual lever machines are able to make all these recipes, and have long been able to make delicious espresso with light roasted beans.  Typically, this has been either a Blooming shot, or they've cleverly made 50 seconds Lever profile shots, thus extending the water contact time over the entire duration of the recipe.

Roast Levels

• dark roast: dark chocolate flavor, some "burnt forest" flavors. Often served very thick.  Taste profile similar to dark baking chocolate.
• medium-dark: a mix of chocolate flavors, both dark and medium, with little (if any) burnt flavors, and some layering of different styles of chocolate.
• medium: less dark chocolate flavors, but no burnt flavors.  Can include layers of different chocolate roast level flavors.
• medium-light: same as medium (layers of chocolate), but with some fruity (tropical and red fruits are typical) or floral notes added as well. Can have a "milk chocolate" flavor.  Taste profile is similar to expensive "single origin chocolates".
• light: no chocolate flavors. A "well developed" light has other Maillard/Caramelization flavors such caramel, toffee, dulce-de-leche, cooked pear. A "less developed" light roast will not have any flavors arising from caramelization. Fruit, floral, meat, and more. An amazing panorama of flavors is possible with light roasted beans, which is why so many coffee experts favor this roast style.  
• ultralight: intense fruit flavors, intense flowers.  Often tastes more like expensive tea than traditional coffee.

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This article is a work in progress, and represents my current thinking. I very much would like to discuss it and you're welcome to revise my chart above with your ideas.  Here's the photoshop file: