Filter and Immersion

Coffee Fundamentals

Course 1 · Chapter 7 · Filter and Immersion: Press, Drip and Cold Methods

Two Ideas, Three Drinks

Chapter 6 split brewing into four families. Here are three of them in practice: immersion (the grounds sit in the water), filter or gravity (water runs through the grounds and a filter) and cold brewing (immersion with cool water over many hours). The facts come from Wikipedia's articles on the French press, drip coffee and cold brew coffee.

Method	Family	Contact	Grind
French press	Infusion	Steeped, about four minutes by some accounts	Coarse
Drip / pour-over	Gravity	Water passes through	Medium-fine to medium (chapter 5)
Cold brew	Infusion, cold	12 to 24 hours	Coarse

The French Press

Coarse grounds steep in hot water, then a mesh plunger pushes them to the bottom. The article says some writers give the optimal time as around four minutes, which sits inside the 4 to 7 minutes chapter 6 took from another article. It describes the right grind as "about the consistency of cooking salt": finer grounds reduce permeability, can seep past the filter, and tend to over-extract and turn bitter. A starting recipe given is about 30 g of grounds to 500 ml of water at 93 to 96 °C, adjusted to taste.

The article lists strengths and weaknesses in scattered sentences. It is simple, compact and portable, and can deliver an excellent taste. Fine grounds can pass the filter and end up in the cup, and coffee left sitting on the used grounds may become astringent and bitter, though some drinkers like that. It cites a 2012 study of cafestol extraction in different brews but does not explain the result, so I make no claim about oils or cholesterol effects (check).

A short history

Year	Event (from the article)
1812	Benjamin Thompson invents a drip-press brewing device, an early ancestor
1852	Mayer and Delforge patent a forerunner in France that did not seal around the filter
1924	A patent by Marcel-Pierre Paquet dit Jolbert is published (5 August)
1928-1929	Moneta and Calimani add a spring to seal the filter and patent it in the US in 1929
1958	Faliero Bondanini patents his version, made under the Melior brand

Drip and Pour-Over

Hot water is poured onto the grounds, seeps through and carries flavour with it, while the grounds stay in the filter and the brew collects below. Pour-over is the usual term for manually brewed drip coffee. In automatic machines the article says water is heated in a metal tube, rises up a hose by thermosiphon action and is sprayed onto the grounds.

Year	Event (from the article)
1908	Melitta Bentz invents commercial paper coffee filters in Germany
1941	The Chemex is listed among filter systems
1944	Willy Brand develops an automatic drip brewer with circular paper filters in Switzerland
1954	The Wigomat, one of the first electric drip brewers, is patented in Germany
1970s	Drip makers largely replace percolators, which tend to over-extract
2004	The Hario system is listed

Paper or metal?

Filter	Effect and trade-off (from the article)
Paper	Clear, light-bodied coffee; traps oils and essences; thrown away with the grounds
Permanent (metal, mesh, ceramic)	Does not normally remove the oils; cuts cost and waste but needs cleaning

Technique: bloom and pour

The article describes blooming: pour a small amount of hot water over the grounds and wait for about 30 to 45 seconds before continuing, which releases carbon dioxide and can improve the taste. Pour slowly in a circle; it says rapid pouring gives a "more watery consistency" and slow pouring can give a "richer taste". It gives no water temperature, total pour-over time or coffee-to-water ratio for drip coffee, so use the chapter 6 figures as a start (check).

Cold Brew

Coarse-ground beans are soaked in water for 12 to 24 hours; the water is usually room temperature, though chilled water can be used. The grounds are removed with a paper filter, a fine metal sieve or felt. The result is a concentrate, diluted with water or milk and served hot, chilled or over ice. Chapter 6 gave 8 to 24 hours from another article; this one says 12 to 24, and I did not resolve the difference (check).

On chemistry, the article says brewing at a lower temperature for 24 hours gives higher caffeine content than a hot brew of equal volume, compared against 6 minutes at 98 °C (the quote was cut short in my extract, so I have not verified the full claim). It says cold and hot brews have similar acidity but cold brew has lower titratable acid concentration, and attributes the different flavour to never meeting heated water. It does not give ratios, shelf life or a comparison with iced coffee (check). A slow-drip variant, sometimes called Kyoto-style, drips water through grounds at room temperature over many hours.

Code: Ratios, Temperatures and Time

The script below checks the press recipe against the 15 to 18 to 1 ratio from chapter 6, converts temperatures, and compares cold and hot contact times. I ran it to get the output shown.

def ratio(water_ml, coffee_g):
    return water_ml / coffee_g

print(f"French press: 30 g in 500 ml = {ratio(500, 30):.1f}:1")
print(f"A 1 litre press at that ratio needs {1000 / ratio(500, 30):.0f} g of coffee")

def c_to_f(c):
    return c * 9 / 5 + 32

for c in (93, 96, 98):
    print(f"{c} C = {c_to_f(c):.0f} F")

for lo, hi in ((12, 24),):
    print(f"Cold brew steeping: {lo} to {hi} hours = {lo * 60} to {hi * 60} minutes")
    print(f"Compared with a 6 minute hot brew: {lo * 60 / 6:.0f} to {hi * 60 / 6:.0f} times longer")

# Output
French press: 30 g in 500 ml = 16.7:1
A 1 litre press at that ratio needs 60 g of coffee
93 C = 199 F
96 C = 205 F
98 C = 208 F
Cold brew steeping: 12 to 24 hours = 720 to 1440 minutes
Compared with a 6 minute hot brew: 120 to 240 times longer

What the output shows

The press recipe works out to 16.7 to 1, inside the 15 to 18 to 1 range from chapter 6, and 93 to 96 °C matches the 199 to 205 °F the article states. Cold brew takes roughly 120 to 240 times as long as the 6-minute hot brew it was compared with, which is why it uses a different temperature and a coarse grind. The ratio here treats 1 ml of water as 1 g.

Not Yet Verified, and Sources

Sources. Wikipedia articles French press, Drip coffee and Cold brew coffee. The extracts I read appeared complete, though I saw them through a summarising tool, not the full page text.
Conflict. Cold brew time is 12 to 24 hours here and 8 to 24 hours in the Coffee preparation article from chapter 6. Not resolved.
Not in the pages read. Drip water temperature, pour-over time and ratio, cold brew ratio and shelf life, the cafestol result, and a full account of the caffeine comparison (all marked check).
Weak support. The drip coffee article's automatic-machine section is marked on the page as needing expansion.
Not tested. I made no coffee; the script is arithmetic only.

Exercises

Exercise 1

A friend's press coffee tastes bitter and gritty. Using the chapter, give two likely causes and a fix for each.

→ Solution

Exercise 2

Use the script to work out the coffee for a 750 ml press at the article's 30 g per 500 ml, and the ratio you get. Then add a line converting 94 °C to Fahrenheit and decide whether it falls inside the article's range.

→ Solution

Exercise 3

Compare paper and permanent filters on body, oils and upkeep. Then write a one-paragraph plan for a pour-over using the bloom step, saying which numbers come from this chapter and which you are guessing and must mark "(check)".

→ Solution

Quick Reference

Press: coarse like cooking salt; about 30 g per 500 ml; 93 to 96 °C; about four minutes
Drip: paper gives clear, light body; bloom 30 to 45 seconds
Cold brew: coarse; 12 to 24 hours; a concentrate you dilute
Dates: paper filter 1908, Chemex 1941, press patents 1852, 1924, 1929, 1958

Coming next

Coffee Fundamentals 8 covers espresso and the drinks made from it.