Science of Coffee Brewing

Science of Coffee Brewing

This is our main article on brewing coffee. You will be able to apply this information to all brewing methods. We have tried to tie all the key aspects together to help build a more comprehensive understanding of the how and why of coffee brewing.

We attribute a great deal of our understanding on this topic and much of the technical content to the seminal piece Some Aspects Of Espresso Extraction by Jim Schulman. 

The topics of this page are:

  1. Categorising the flavours present in brewed coffee.
  2. The factors affecting extraction.
  3. Practical tips for brewing.

1. Categorising the flavours present in brewed coffee

Ted Lingle was credited with first categorising the flavours present in brewed coffee by molecular weight which importantly, directly relates to their solubility in water. The categories are usefully described by Jim Schulman as follows (in order of decreasing solubility):

  • Enzymatics (fruit acids, florals)
  • Maillard compounds (nuts, toasted grain, malt, wood, tannin)
  • Sugar browning (sweet, vanilla, caramel, chocolate)
  • Dry distillates (burnt sugars, tobacco, smoke, ash)

The proportion of these different groups in the cup dictates the overall cup flavour profile. Their varying solubility explains why the cup profile is changed by various brewing methods.

So for example, this explains why a ristretto made with a light roasted coffee would likely taste sour and why too long a steep time in your plunger brings out more of the darker, ashy flavour in dark roasted coffees.

We will now talk about extraction in more detail.

2. The factors affecting extraction

Before we discuss factors affecting extraction we must first define two important measures that relate to extraction.

  • TDS - Total Dissolved Solids
  • Extraction yield

The TDS is the % soluble coffee solids in the cup - simply the strength of the concentration of the coffee/water mixture i.e. the strength of the flavour. Typical figures are about 1.5% for filter coffee and 15% for espresso. TDS can be measured directly with a portable coffee refractometer. From TDS measurement, you can easily back-calculate the extraction yield.

The extraction yield is the % of soluble coffee solids extracted from the dry coffee - this dictates the flavour profile in the cup. At about 20% extraction yield both filter and espresso methods produce the best balanced cup profile (but there is some leeway here in the range 18-21%). Note that only about 30% of dry, ground coffee is water soluble; the remainder generally being cellulose. In summary, coffee seems to taste best when about two-thirds of the available solubles are extracted.

The factors affecting extraction yield - which will affect your flavour profile:

  1. grind fineness (surface area)
  2. coffee/water contact time
  3. agitation (stirring)

What do these all have in common? They all affect how much of the soluble coffee present in the grounds is dissolved in the brew water.

The factors affecting TDS - which affects the strength of flavour:

  1. dose of dry, ground coffee
  2. volume of water in cup

The extraction yield has a secondary influence on the TDS but should be kept somewhat separate in your considerations (but not entirely) because with all other variables held constant, an increase in extraction yield will also result in an increase in TDS. More on this in the detailed guidelines linked below.

Notice here we have omitted temperature from the discussion. While it could be included, it has been assumed constant in this discussion at the optimal temperature of 95°C, just off boiling. The reason for leaving temperature out of the discussion is that the concept of extraction yield really relies on factors quite separate from temperature and furthermore, there is widespread historical agreement on the optimum coffee brewing temperature and achieving a consistent brew temperature is relatively easy for most everyday coffee brewing circumstances.

3. Practical tips for brewing

So how do we put the knowledge above to good use in our favourite brewing method? Read on below.

Tips for all brew methods (from espresso to pour over):

  • Have a stopwatch ready with 1 second increments.
  • Have some digital scales available with 0.1g increments.
  • Weigh your dry ground coffee going into your brewing device, including your espresso portafilter!
  • Weigh your hot water for filter methods (plunger, Aeropress, Chemex, V60, Clever Dripper and siphon). 
  • Weigh your shots for espresso.
  • Remember that darker roast coffees will more easily reach higher extraction yields than lighter roast coffees.
  • Maintain consistent temperature each time.
  • Taste the difference. Use the flavour categories above to discern where you arrived in the extraction spectrum for a particular coffee. For filter and immersion methods adjust the grind, steep time or agitation (one at a time) to increase or decrease the extraction yield and adjust the ground coffee dose or water volume to affect the TDS (strength of flavour).

Detailed guidelines for each brew method:

Full immersion methods - Plunger, Aeropress and Clever Dripper

Drip filter methods - Chemex, V60, Bonavita and Vietnamese filter

Stove-top espresso methods - Bialetti and Atomic

Espresso - commercial and domestic machines

Tagged : Workshop, How-to, Brewing, Extraction

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