The Chemistry of French Press Foam

I used to think that foam at the top of a French press was a matter of personal preference: some drinkers prefer foam's texture and astringency, while others insist it clouds the palate and inhibits flavor clarity. What does it matter? It turns out that the presence or absence of foam during the brew cycle may affect the final flavor of the brewed coffee itself.

Here's why:

Coffee beans hold deposits of fatty acids (lipids) that are released during extraction into the brew. Lipids are all hydrophobic, so in the press you'll notice them separating themselves from the water and gathering in small pools of oil at the top of your cup. 

Certain lipids also have special chemistry that makes coffee foam possible. Sterols make up only about 2.2% of the total lipids in the bean, but they are amphiphilic, which is a fancy way of saying the sterol molecule has one end that likes water (water soluble) and one end that hates water (water insoluble).

A sterol is a physically flat molecule with an hydroxyl (HO above) group bonded to four ring structures of carbon known collectively as cholestane. Chemically, sterols are part of the steroid family and are related to hormones like testosterone and estrogen. One of the sterols in coffee is cholesterol.

Amphiphilic chemicals are a must for forming bubbles, as their opposite ends allow them to line up and form bubble walls that are insoluble on the outside and soluble on the inside. As carbon dioxide bubbles form during brewing, amphiphilic sterols effectively give water-hating oils a place to hide - within the walls of forming bubbles. 

Here's the kicker. Extracted coffee oils collect against the super fine coffee grounds that are suspended in the water. Some of those fines then get trapped inside bubbles as a part of the foam structure on the top of brewed coffee. In other words, the foam on top of a press likely acts as a chemical scrubber, stripping away fines suspended in passing convection currents and holding them in the foam structure itself.

Does any of this make a tastable difference? Yes. Expert baristas Tim Wendelboe and James Hoffman have popularized a French press variant in which foam is scooped from the surface prior to pressing the coffee. Video here. The resulting brew does taste significantly cleaner and less muddy.

Photo by Alex Negranza of Why Not? Coffee

To my knowledge, no detailed chemical explanation of the Wendelboe/Hoffman variant has been proposed before. One physical explanation has been suggested: saturated coffee grounds at the top of the press over-extract when forceably moved through the brewed coffee by the plunger. The theory goes that the Wendelboe/Hoffman technique removes these grounds and thus avoids over-extraction.

On close examination, I don't think that explanation holds up. While physical agitation does increase extraction rates, both traditional methods and the Wendelboe/Hoffman brewing variant involve gentle but significant agitation when the crust or bloom is stirred in (known as the break). After the break, relatively few grounds are left floating at the top of the brew and pressing is a gentle process in both cases. In other words, most particle agitation -- and thus over-extraction -- would occur in both methods. Furthermore, over-extraction alone does not explain the significantly cleaner mouthfeel yielded by the Wendelboe/Hoffman variant. It seems like the only significant difference between the two methods is the treatment of the foam after the break.

To me, French press foam chemistry provides a much more satisfying and robust explanation for the significant cleaning effect of the Wendleboe/Hoffman brewing method. By removing foam before pressing, fines that would otherwise be agitated out of the foam and into the brew during pressing (press with foam, and look at the difference in foam color and structure before and after pressing. Where did the brown color go?) are safely removed from the brew and from your cup.

So how does the Coffee Catcher - a physical fines filtration system at the bottom of the press - compare or contrast with the chemical scrubbing process that I've suggested lies behind the Wendleboe/Hoffman technique? I've been tasting both and thinking hard about both - and the results are for another blog post!

This post depended heavily on a number of sources, particularly James Hoffman's excellent 2006 post on coffee chemistry.

Here are a few further reading links:

Introduction to Lipids in Coffee, by James Hoffman

Sterols article at Wikipedia

Sterols and their Conjugates, at the Lipids Library (useful for understanding esters of sterols)

Cholestane stub at Wikipedia

Discussion of the Wendelboe/Hoffman technique at Home-Barista.com

Chemistry of Soap at about.com (soap is also amphiphilic)

Finally, one interesting theoretical question for extra credit: esters of sterols make up another 3.2% of coffee lipids. Esters, in my very limited understanding, tend to be inert. If so, does the brewing process break down these esters, releasing free sterols that form foams? If not, do esters of sterols contribute to bubble formation regardless?