Coffee extraction – a review of various parameters and their influence on the physiochemical character and taste of coffee. Under this title in English, a current research article has been published, which we summarize here for you.
For the past decade, more and more research on coffee has been conducted. As coffee makers, we believe that coffee is less an art than a craft. Whether it's roasting, brewing, pulling espressos – 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 unwieldy for non-scientists.
We are not natural scientists, but we like to dive into most coffee topics. So we thought we would read selected papers for you and try to summarize the highlights in simpler words in German.
The discussed paper for download:
The authors are:
NancyCordoba, MarioFernandez-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. About coffee extraction, in other words.
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 the role that 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 happens during coffee brewing, the more we can predict what flavors will end up in the cup
- most brewing methods were developed to create different flavors
- there are more and more brewing methods gaining acceptance, but there is a lack of scientific data to explain what exactly happens in each case
- the methods differ primarily in the applied pressure, water-to-coffee ratio, water quality, contact time, distribution of particle sizes, temperature
- what is still missing would be a thorough explanation of mass and energy transport to explain the relationship between extraction variables and flavors
Step by step: the essence of coffee extraction
In summary, the following statements from the paper are presented with our comments.
Although coffee extraction only takes 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 typically takes around +/- 5 months. And then we brew the coffee in 3 minutes and can fail there
Coffee brewing methods differ depending on geographic, cultural and social context, as well as individual preferences of course.
- I am always amazed on coffee trips at what you can use to make filter coffee. You only need a medium that filters the coffee. A funny machine is the chorreador, a traditional brewing method from Costa Rica.
What remains the same, however: the direct contact between water and roasted and ground coffee, so that the water can bring the flavor into the drink. This is followed by subjective judgment, which is based on objective standards – if I prefer more body (subjective), I dose the powder amount higher (objective)
- I like this distinction between subjective and objective, as it can help us talk about flavor more neutrally while also recognizing that the recipe needs to be adjusted.
Specific extraction variables such as extraction time, water composition, temperature, pressure, particle size, and the ratio of dosing and water influence the flavor. However, the influence of physical conditions has received less attention in research. So it seems that there are many myths in coffee extraction
- wonderful! We humans have the habit of making things explicable. And whoever brews coffee again and again may wonder why the coffee was better or worse the next day. It's quite possible that the reason was different than the one we assumed. The researchers, for example, talk about "permeability" and "diffusion" (p.2) as further research questions to analyze the brewing process even better. Understanding all the details helps us brew coffee even better.
On page 2, they then say that due to so many factors, it is difficult to brew high-quality coffee in a reproducible way.
- certainly. 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 competitions – so we have the perfect training to always brew the same way.
How do you get good coffee?
The coffee quality of a finished drink is inherent in the raw coffee. Chemical substances, also known as aroma precursors, which only manifest themselves during roasting, bring flavor into the cup.
- the aroma precursors that we can promote 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 then dissolved in extraction, depending on particle size and other factors.
Adjusting one of the parameters of course means that the drink can taste different.
Methods of coffee extraction
The authors distinguish between 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 doesn't play a 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 outdated because it does not address flavors and includes only a few of 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 takes humans, 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 here, distinguishing what is delicious and what is less delicious.
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 drink. Perceived acidity is primarily attributed to the following acids:
Acetic, malic, formic, lactic, chlorogenic, quinic acid
The relationship between pH and titratable acidity has not yet been completely researched.
- but: we measure lower pH values in a Kenyan coffee than in a Brazilian coffee
- lower pH AND less perceived acidity
9 facts about coffee extraction to make an impression (almost) anywhere
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 drink.
CGA (chlorogenic acids) are relatively high in the drink and are responsible for antioxidants, are anti-inflammatory, anticarcinogenic, and antimutagenic.
A maximum of 30% of bitterness comes from caffeine and trigonelline.
Sugar, organic acid, caffeine are extracted super efficiently in the first seconds, more than 90% at that.
In filter coffee – in the first 2 minutes about 65-75% of the available flavor potential dissolves.
In cold brew: 400min (approximately 7 hours) is enough for us to be in balance.
At higher temperatures, the kinetic energy of water molecules is higher. More mobility increases the possibility of dissolving components from the coffee bed.
During brewing, coffee granules grow by 20-23% after being wet for 10-15 minutes.
- More on the topic of extraction and strength we have noted here.
- A basic requirement for extraction is the grinding of the coffee. This article covers it in more detail than any other on the web.
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