Brewing Coffee (a Framework) updated 2/8/10
The Chemistry of Coffee Brewing...
When you brew coffee, hot water acts as a solvent, washing the soluble solids out of the coffee grinds and into the brew. If you dissolve table salt in water, you have a soluble solid. Brew methods that use paper filters have only the soluble solids in the cup. Some brew methods also allow insoluble solids to enter the brew. French Press, Espresso or Turkish Coffee are three methods that will have insoluble particulates suspended in the liquid. Insoluble solids will precipitate out of the brew over time, if you don't disturb the liquid. Hence your mug of French Press coffee might taste gritty nearing the end, and there is muddy residue in the bottom of the cup. Suspended solids add a sense of body to the cup, but can also add bittering tastes. Insoluble solids are bonded with the water molecules, and will not separate over time.
There are five main factors that control brewing results. First is the brew recipe, the ratio of water-to-coffee. More on that below. Second is the particle size of the coffee: finer grind means more surface area of the bean is exposed to the water. Third is the temperature of the water, ideally between 198-204 f, since water is a better solvent at near-boiling temperature. The fourth factor is contact time, how long the water and coffee are in contact with each other. And lastly is agitation, since stirring the coffee-water infusion increases extraction rate of soluble solids. Other factors influence the brew, but these are the main ones.
Most people find that when 20% of soluble solids are extracted from the coffee grounds, the brew has the best flavors. Too much extraction (too fine grind, too long brew time, too hot water, too much coffee in the recipe) and the brew is bitter. On the other hand, under-extraction results in a thin, weak cup. Simply using more coffee grinds cannot fix other brew problems: If you use 20 grams coffee and 350 ML of water and 4 minutes steep time to achieve 20% extraction (it should), using 40 grams coffee with a contact time of 1 minute to compensate will not result in a better cup.
Knowing these simple theories might help you troubleshoot that next bitter, weak, or a flat tasting cup. Here are a few general tips too:
1. Rinse all paper filters with hot water to wash away loose paper fibers that will give an off taste in the cup, especially when brewing small amounts.
2. Preheat your brew device, your French Press, Filtercone, etc. You can do both by simply heating extra hot water, and washing/preheating in one step.
3. Keep brewing equipment clean! Old sediments easily make for rancid flavors in the cup. A good rule of thumb is if you smell an odor from your coffee making equipment, clean it. If you cannot remember the last time you cleaned your brewer, clean it.
Here are some ideal coffee/water/time ratios for different brewing methods:
The ideal brewing practice is:
- Grind immediately before you brew.
- Adjust grind to brewing method and use a good grinder.
- Use good clean water. If your water does not taste good, neither will your coffee.
- Pre-Rinse your paper filter. This reduces paper taste, which can be more of an issue when brewing small amounts of coffee.
- Make more. For a somewhat mysterious reason, filter-brewed coffee tastes better the more you make.
- Coffee is fresh for 10 minutes or less. Try to make the right amount of coffee so you are drinking fresh brewed coffee more often.