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?

Five French Press Faux Pas

I love that rhyming title! The French press is a relatively undemanding coffee brewing process. It's hard to screw it up. That said, there are a few sure-fire ways to extract bitter coffee from your press.

Here are my top five:

1. Leaving coffee to sit in the carafe after the plunger is pressed down. Coffee will keep brewing through the screen, resulting in a bitter and over-extracted second cup.

2. Brewing with boiling water. You'll burn your coffee. It won't taste good. Don't do it. To be safe, let boiled water rest for about a minute before brewing. Advanced brewers may wish to tweak brewing temperature by adjusting the height and thickness of the poured water column. Pouring higher and thinner cools your water faster, and vice versa.

3. Over-brewing. The French press is forgiving, but it's patience is not infinite. Over-extraction will occur if you let the coffee sit. A good rule of thumb is to aim for three to four minutes contact time between grounds and water.

Some tired drinkers over-brew in a misguided attempt to jack up caffeine content. Proper brewing extracts the vast majority of the caffeine in the bean so brewing super-long only makes your coffee taste bad. If some morning (or evening) you do need an extra jolt, simply increase your grounds-to-water ratio at the start of the process. Dark roasts, by the way, do not contain more caffeine than light roasts. Since roasting does not destroy caffeine, the jolt per bean remains the same.

4. Using too little coffee. Home-made coffee bought from a local roaster costs less than a dollar per cup, so don't skimp on the coffee! Most presses ship with little black scoops that hold seven to eight grams of coffee at a time. Use about eight scoops for a 32 oz press, or four scoops to make a half press. A low coffee-to-water ratio yields an over-extracted, bitter cup -- even at a regular four minute brew time!

5. The microwave. Never re-heat coffee! It tastes terrible and savings are negligible. Instead of making a large batch of coffee in the morning, take a mini coffee break and brew by the cup. Your coffee will be tasty every time, and the mini breaks will help to keep your head clear as well.

Friends Don't Let Friends Reheat Coffee!

Why Doesn't It Taste Like It Did in the Cafe?

If the coffee you enjoy at home just doesn't taste like it did in the cafe, it might be your grinder. I've owned four home grinders over the past decade. All of those grinders have produced a substantial variety of particle sizes, and that's made it very hard to brew good coffee at home.

Particular brew methods are designed to extract desirable soluble and insoluble flavor compounds without extracting undesirable bitter flavors that leach out of the bean later in the brew cycle. If coffee particle size varies significantly, then your brew will be extracting at substantially different (and unpredictable) rates, resulting in a coffee that is partly under-extracted, partly brewed right, and partly over-extracted. Flavors that popped in the cafe are fogged, and your brew may taste a somewhat flat or bitter due to over-extraction of particles that are too small for your brew method.

An ideal (non-existent) grinder would grind 100% of the particles to exactly the size. Professional quality grinders are developed to grind as many particles as possible to the same size. Ordinary home grinders are designed to break up the beans into small pieces, but not necessarily accurately. Here's what the difference might look like:

Wealthy home aficionados can tighten up that bell curve with a shop-quality grinder at home ($1000+). Depending on the state of your palate and your wallet, a slightly less accurate home grinder might also do the trick (about $200). The rest of us must make do with the grinders we have. Here are a few tricks:

• If you have a blade grinder, and the grinding chamber floor is flat, tilt the grinder as you grind to avoid re-chopping already ground particles. Here's a blade grinder pictured in a great grinder review over at Gimme! Coffee. 

• If you have a hand burr grinder, keep an eye on the wobble in your handle over time. As you crank on that handle morning after morning, you'll start to put pressure on the metal axle itself and on the fittings that align that axle. Over time, your burrs will wobble more and more, generating an increasingly erratic grind size. Home-barista.com has a great article on maintaining the axle of the popular Hario hand grinder.

• Alter your brew methods to suit your grinder. It sounds odd, but if your grinder can produce a decently consistent turkish coffee grind, but only pebbles and dust in the French press range, learn to brew turkish!

• Finally, cheap grinders wear out. Replace your grinder when you can't stand it any longer!

Other tips? Please below and let the rest of us know!

N’Orleans Coffee Comfort in Seattle

One of the perks of launching a new product (cue: The Coffee Catcher) is anticipating the day when a stranger on the street starts telling you about “this handy new gadget” they discovered - and you wait a couple extra seconds to tell them that it’s your gadget and yes, you agree that it’s handy.

That day arrived recently for one of our Kaffeologie cohorts as she visited Seattle’s new Creole-style soul food truck,“Where Ya At Matt?”  Alongside their (delicious) Po’ Boy Sandwiches this mobile-eatery features made-to-order beignets doused with a healthy amount of confectioner’s sugar.  Of course, as a native of New Orleans owner Matt Lewis knows that beignets aren’t complete without the proper brew.  Enter Herkimer coffee.

Thanks to Scott Richardson at Herkimer, WYAM is brewing up a smooth yet fierce cup of coffee that, sitting next to a bag of beignets is enough to make you homesick for New Orleans - even if you’ve never been.  We tried sipping the coffee black and also doctored it up cafe au lait style - both ways it’s delicious mellowness finishes with a bit of bite.  (The bite is really what makes the brew fit so perfectly next to the beignets’ sweetness.)

Delicious beignets. Photo from Seattle Met.

Naturally when considering coffee + New Orleans, the French Press came up in conversation...and it was only moments until: “Have you heard about this new gadget...”

Oh yes.  Yes, we’ve heard about it.