The Science behind French Press Brewing

What is with the Bubbles?

When you add hot water to your ground coffee, it can bubble furiously or more sedately.  This is referred to as 'blooming'. It helps to understand what is causing the bubbling and how it might influence your brewing decisions.

When coffee beans are roasted, carbon dioxide (CO₂) is trapped within the beans as part of the roasting process. Some of this CO₂ reacts with water inside the beans to form carbonic acid (H₂CO₃). It is very transient and unstable and almost immediately breaks back down into CO₂ and H₂O.  The fate of carbonic acid is influenced by the duration and intensity of the roast.

Implications for Flavour

• Lighter Roasts: Retain more CO₂ and thus more potential for carbonic acid to be formed and subsequently CO₂ to be released. This can contribute to a brighter, tangy acidity in the flavour profile.
• Darker Roasts: Have less CO₂ and carbonic acid, leading to reduced acidity and a flavour profile characterised by deeper, roasted, and sometimes smoky notes.

What is the Blooming Point?

Now that you understand what the bubbling is, what do you do about it? 

It is important to set your coffee up for bubbling success by adding sufficient water to make even contact with your coffee grounds.  Stirring briefly should help. This ensures that the CO₂ is released in order for an even extraction when you add the remainder of your hot water.  This is less critical for French Presses but will help get an even extraction.

There is more to getting the perfect extraction but the top tip here is to keep notes of your steps and timing so that you can refine it to suit your own taste.  

Blooming vs Degassing

Degassing is the natural release of carbon dioxide (CO2) from roasted coffee beans over time. As beans release this trapped CO2, they gradually become stale, affecting flavour and aroma. This process happens faster when beans are ground, which is why whole beans stay fresh longer than pre-ground coffee.  

 

Extraction

Several factors influence a successful extraction.

1) Grind Size: Ensure that you're using a medium to coarse grind size.  Finer grind sizes can lead to over extraction as the smaller grind will allow the bitter notes to be extracted faster 

2) Steep time: Test different times but always ensure that the French press was warmed before adding your coffee as this will give your boiled water to cool slightly to an optimal temperature and when you add it to your french press it will also remain at the same temperature giving you a more even extraction

3) Water: Hard water can impact your extraction due to the other chemical compounds involved.  If possible, aim to use tap water that has been through a charcoal filter.  This is affordable and easier to achieve in your home.  Water is a huge topic on it's own and beyond my skills and knowledge.  If anybody is interested I can point you at some research on the topic.

Comparison of French Press Coffee Extraction Levels

Aspect	Under Extraction	Normal Extraction	Over Extraction
Taste	Sour, overly acidic, lacks sweetness	Balanced, complex, sweet, and smooth	Bitter, harsh, overly astringent
Body	Thin, watery	Full-bodied, rich	Heavy, overly dry, or gritty
Aroma	Muted, grassy, or unripe	Aromatic, deep, and nuanced	Dull, burnt, or woody
Appearance	Light, pale brew	Clear, deep amber color (with slight sediment)	Dark, muddy, or overly cloudy
Cause	Coarse grind, short steep time, cool water	Proper grind size, brew time, and water temp	Fine grind, long steep time, too hot water

 

Key Factors to Control:

• Grind Size: Medium-coarse for French press.
• Steep Time: Around 4 minutes (some Baristas have found more sweetness is achieved by steeping for 6 minutes but I would recommend starting out with 3-4 minutes and testing it for your own preference).
• Water Temperature: ~92–96°C (195–205°F).

 

 

 

Photo by Alex Alvarez on Unsplash