Here’s something you hear often here in UChicago: that’s all good in practice, but how does it hold up in theory? In this article, we’ll briefly go through the theory of coffee brewing. In our next article, we’ll give you the step by step guide to brew with a french press, but here, we’ll run through it conceptually. We believe that the coffee brewing process is made much richer (and the coffee better, perhaps because of cognitive biases) if you have an idea of what exactly you’re doing to produce a rich, flavorful cup.
Firstly, when brewing coffee, we cannot emphasize enough the importance of freshly ground coffee. Coffee, in its pre-roasted green bean form, lasts for nine months before losing its flavor. Roasted coffee retains its flavor for maybe at most four weeks, and ground coffee lasts for a mere nine minutes. For the sake of your palate, try your best to grind coffee right before you brew it, and store whole beans in airtight containers. When you buy coffee, buy it from a local roaster, and check out its freshness by looking at the roast date.
After you’ve ground the coffee, you’ll think of adding the hot water. But once you do, you’ll see foam appearing. This is known as ‘blooming’. Freshly roasted coffee still has a lot of carbon dioxide in the cellulose matrix that is the coffee, and the addition of hot water forces out the carbon dioxide, causing the formation of a ‘crust’.
The reason why we bloom for a short period of time (to drive out the carbon dioxide) is because when carbon dioxide is coming out, water can’t access the grounds as easily to dissolve the tasty and aromatic compounds to produce coffee.
Let’s go a little into what happens during extraction. Coffee contains many soluble compounds (acids, amino acids, sugars and alkaloids) and they all dissolve at different rates. During extraction, water is entering the cellulose matrix of the coffee grounds, and dissolving compounds present in the grounds. A little acronym we keep in mind while thinking about extraction is SaRTT (almost like the French Humanist, Sarte).
- Surface Area (grind size, surface available for water to extract the dissolved compounds, affects speed of extraction)
- Ratio (weight of coffee to hot water affects strength of brew)
- Time (length of brew time affects how much is extracted)
- Temperature (water temperature affects speed of extraction)
We’ve briefly touched on all of these in the previous articles, but maybe not so much in the context of brewing . All the factors is SaRTT affect the extraction rates of the different compounds in coffee, thus affecting the proportions of the different compounds and eventually the taste and flavor profile of the cup.
The last step in brewing is the separation of the coffee grounds from the liquid nectar (containing all those delicious dissolved compounds) of coffee. Most brew methods use a form of filter (paper or metal), and the choice of filter affects the flavor and feel of the coffee. For example, paper filters trap more oils and fines as compared to wire mesh (of a french press), producing a cleaner cup.
It’s all a little abstract and science-y at this point, but come back and read the step-by-step guide we have in the next article and maybe things will make sense.