Coffee extraction – a review of various parameters and their influence on the physicochemical character and taste of coffee. Under this title in English, a current research article has been published, which we summarize here for you.
For over a decade, more and more research has been conducted on coffee. As coffee makers, we believe that coffee is less an art than a craft. Whether it's roasting, brewing, pulling espresso – you can learn it all.
However, to understand what exactly happens in the process, scientific studies help us. They are pragmatic, detail-obsessed, demystifying, but also always somewhat cumbersome for non-scientists.
We are not natural scientists, but we enjoy diving into most coffee topics. So we thought we would read selected papers for you and try to summarize the highlights in simpler German words.
The discussed paper for download:
The authors are:
Nancy Cordoba, Mario Fernandez-Alduenda, Fabian L. Moreno, Yolanda Ruizb
Published 2020 in:
Trends in Food Science & Technology, Volume 96, February 2020, Pages 45-60
What is it about?
How coffee flavor gets into the cup. So about coffee extraction.
What is it really about?
The authors discuss various parameters and their influence on coffee brewing. About roasting, grinding, water quality, etc. All of this has a significant impact on coffee extraction. They say that the flavor notes in brewed coffee are due to volatile and non-volatile components that are created during roasting.
The authors then focus on what role various parameters can play in the extraction process. Brewing methods and their extraction parameters were analyzed, as well as their effect on the physicochemical properties and flavor profiles of brewed coffee.
Key insights into coffee extraction
- the more we understand what exactly happens when brewing coffee, the more we can predict which flavors will end up in the cup
- most brewing methods were developed to produce different flavors
- there are increasingly more brewing methods gaining acceptance, but scientific data is lacking to explain what exactly happens in each case
- the methods differ primarily in the pressure applied, the water-to-coffee ratio, water quality, contact time, the distribution of particle sizes, and temperature
- what is still missing would be a thorough explanation of mass and energy transport to explain the relationship between extraction variables and flavors
One by one: the essence of coffee extraction
Summarized below are statements from the paper with our comments.
Although coffee extraction takes only a few minutes, it is responsible for a large part of coffee quality
- and we must never forget this: from harvest, through post-harvest processes, to import and roasting, it usually takes around +/- 5 months. And then we brew the coffee in 3 minutes and can fail there
Coffee brewing methods differ depending on geographical, cultural, and social context, as well as individual preferences, of course.
- on my coffee travels, I am repeatedly amazed at what you can make filter coffee with. You only need a medium that filters the coffee. A fun machine is the chorreador, a traditional brewing method from Costa Rica.
However, what remains constant: the direct contact between water and roasted and ground coffee, so that the water can bring the flavor into the beverage. This is followed by subjective assessment, but it depends on objective standards – if I like more body (subjective), I dose the powder amount higher (objective)
- I like this distinction between subjective and objective, as it can help us speak about taste more neutrally while also seeing that the recipe needs to be adjusted.
Specific extraction variables such as extraction time, water composition, temperature, pressure, particle size, and the ratio of dosage to water influence flavor. However, the influence of physical conditions has received even less attention in research. This is why there seem to be many myths in coffee extraction
- wonderful! We humans have a habit of making things explainable. And anyone who brews coffee repeatedly might wonder why the coffee tasted better or worse the next day. It's quite possible that the reason was different from what we thought. The researchers talk, for example, about "permeability" and "diffusion" (p.2) as further research questions to better analyze the brewing process. Understanding all the details helps us brew coffee even better.
On page 2, they say that due to so many factors, it is difficult to brew high-quality coffee in a reproducible way.
- definitely. And that's why we brew the same coffee many times in our courses and pay attention to the details. We also participate in brewing championships – so we have the perfect training to always brew consistently.
How do you get good coffee?
The coffee quality of a finished beverage is inherent in the green coffee. Chemical substances, also known as aroma precursors, which only manifest themselves during roasting, bring the taste into the cup.
- the aroma precursors that we can create through controlled post-harvest processes and fermentations, we have discussed in detail here.
These substances are transformed into volatile and non-volatile components during the roasting process. Many of them are subsequently dissolved during extraction, depending on particle size, among other things.
Adjusting one of the parameters naturally results in the beverage tasting different.
Methods of coffee extraction
The authors distinguish different brewing methods.
Decoction method: (larger amount of coffee with longer contact between coffee particles and water)
- boiled coffee, Turkish coffee, percolator, vacuum
Infusion: hot water flows through a coffee bed, short contact time of the coffee grounds with individual water portions
Pressure (pressure is responsible for pressing water through a coffee bed)
- Moka, Espresso, French Press
- Although, French Press is a combination of steeping – pressure plays no role in our opinion.
TDS and extraction rate
Even though the coffee brewing chart remains widely used, its applicability is rapidly decreasing because of new coffee brewing methods.
N. Cordoba, et. al., 2020, p.49.
The Brewing Control Chart dates from the 1950s and has changed little. The authors consider it no longer contemporary, as it does not address flavors and only marginally includes today's existing brewing methods. It essentially reduces brewing to extraction and strength.
Further instruments are needed that analyze flavor, not just dissolved particles
- and of course, it still requires the human, the sensorially trained palate, with which flavor can be understood.
- at the end of the day, it's about enjoyment – and so far, humans do the best job of distinguishing what tastes good and what doesn't.
pH and acidity
pH: number of hydrogen ion concentration
total titratable acidity measures all protons
Total acidity and pH have long been used to determine acidity in a beverage. The perceived acidity is primarily attributed to the following acids:
Acetic acid, malic acid, formic acid, lactic acid, chlorogenic acid, quinic acid
The connection between pH and titratable acidity is not yet completely understood.
- but: we measure lower pH values in a Kenyan coffee than in a Brazilian coffee
- less pH AND less perceived acidity
9 facts about coffee extraction to (almost) impress everywhere
The concentration of fats in unfiltered coffee (e.g., espresso) is higher than in filter coffees. A filter retains many fats.
Fats play an important role in flavor formation because they create an emulsion that can store and transport aromas. They also improve the texture of the beverage.
CGA (chlorogenic acids) are relatively high in the beverage and are responsible for antioxidants, are anti-inflammatory, anti-carcinogenic, and anti-mutagenic.
A maximum of 30% of bitterness comes from caffeine and trigonelline.
Sugar, organic acids, caffeine are extracted super efficiently in the first few seconds, more than 90% of them.
With filter coffee – in the first 2 minutes, approximately 65-75% of the available flavor potential dissolves.
In cold brew: 400 minutes (approximately 7 hours) are enough for us to be in a balance.
At higher temperature, the kinetic energy of water molecules is higher. More mobility increases the ability to dissolve components from the coffee bed.
During brewing, coffee grounds expand by 20-23% after being wet for 10-15 minutes.
- We have noted more on the topic of extraction and strength here.
- A basic prerequisite for extraction is the grinding of coffee. This article covers it in more detail than all others on the net.
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