There are many unknowns in coffee. Regardless of how scientific some of us can make it sound, most of the brewing practices are a product of tradition, not science. At best, it’s practical observation, a very good first-step in scientific inquiry, but hardly definitive.
Let’s take the agitation of grounds during brewing, sometimes called turbidity. One analogy expressed is that of a clothes washing machine, which features an agitator, to swirl the clothes around during the wash cycle. This is to get the detergent thoroughly mixed with water, and to help powders to dissolve. It also gets clothes to rub against each other, the modern equivalent to rubbing against a washboard.
What does agitation do during coffee brewing? In order to brew at all, it is a given that the grounds must all come in contact with hot water. If the grounds don’t get wet, you’ve wasted them. A poorly designed automatic coffee brewer will leave dry grounds. Sometimes, it’s not the coffee brewer’s fault. End users may have overloaded the grounds basket. The grounds may have been too coarsely ground – good drip brewing involves a controlled backup (think rush hour traffic) so that the grounds are under water throughout the brew cycle. The grounds may be too fresh. I know, roasters are always stressing the importance of freshness, but just-roasted coffee foams up like beer, and the grounds floating atop the foam are chemical loafers – let’s call them “supervisors”.
So, agitation follows the washing machine analogy to this end. Everyone who’s brewed using a manual drip machine has probably swirled their brewer around in order to help get all the grounds good and wet and settled.
But, the analogy gets lost after that. Unlike powdered detergents, there are not coffee grounds that need to be dissolved. The grounds do not, as far as I know, brew by rubbing against each other. Supposedly, coffee extract is removed from the grounds using heat, hence the importance of ultra-hot, just under boiling brewing temperatures.
So what happens next? How does designing an automatic brewing sprayhead to power wash the grounds result in more extraction or affect it at all?
Lacking any credible scientific research, we can only turn to observation and precedent. The famed Vacuum coffee brewer, which dominated brewing during the first half of the 20th Century, has plenty of turbidity during its relatively short contact time. Once the hot water shot up the tube and into its upper bowl, the brewing water/grounds mixture bubbled nicely, like a hot tub with its jets on full. Perhaps this was observed as part of its ability to turn out rich coffee within three-to-four minutes. I always thought this was due to its using fine grind coffee.
Curiously, an equally revered brewing method features the least turbidity. The French press uses a coarse grind. In fact, if you grind fine, you just might break it trying to force the press down. Once you initially stir it, you’re supposed to place the press inside, virtually ensuring no-agitation throughout most of the contact time. Let’s see, no agitation, the coffee is ready in three-to-four minutes, using a coarse grind.
How can that be?
I’m not saying that turbidity is not a factor. What I’m saying is we still don’t really know why it is a factor, and, even if it is, it’s still not the only way to make coffee. I’m suggesting there’s a lot more to discover about just what brews a great cup of coffee.