Is it better to have as many holes as possible in your spray head; hardly any to make more agitation; or somewhere in between?
To achieve even extraction in batch brewing, it’s important to get all the grounds evenly wetted — meaning exposed to the same amount of solvent, with the same flow rate, and with no channeling to cause localised over-extraction.
The major difference between hand brewing and a typical batch brewer is how the water reaches the coffee to achieve this. In hand brewing, we usually use the flow from a kettle to deliberately agitate the grounds, pouring in circles to effectively ‘stir’ the bed of coffee during pouring. A typical batch brewer doesn’t have moving parts, so instead sprays the coffee through multiple holes, trying to disperse the water evenly over the whole bed.
The classic design for this is a flat plate, perforated with several holes, that drips water in thin streams onto the coffee. Some recent batch brew designs use different kinds of spray heads however, that spray fewer, thicker streams of water onto the coffee bed. These claim some advantages over traditional designs including increased agitation, but have proved contentious.
Turbulence or Agitation?
Firstly, it’s important to differentiate between two separate concepts, both sometimes called turbulence. Turbulent flow refers to random currents within a flow of liquid, which we discuss in detail in this blog post. In coffee brewing, this turbulence takes place on a small scale, for example by affecting the way water flows through the space in between coffee particles.
However, in the context of filter coffee brewing, the word turbulence often also refers to fast flows of water physically moving particles in the coffee bed on a much larger scale, in a similar way to stirring. This type of movement can also be referred to as agitation, which is the term I’ll use in this post.
The Importance of Agitation
Agitation is effective at ensuring all grounds are evenly wetted. It can break up dry clumps of coffee, disrupt channels from forming, and make sure no grounds are left floating on the surface or stuck to the sides of the filter. This is particularly important during the bloom, or pre-wet, in preventing any coffee from remaining dry. This is most effectively done by stirring, or physically spinning the filter cone — but neither of these is an option in a typical batch brew, meaning that any agitation that takes place can only be achieved by the flow of water. Ted Lingle recognises this in the Coffee Brewer’s Handbook, saying that “For uniform wetting and extraction…the brewing water must lift and separate each particle” (TR Lingle, 1996).
The Tradeoff Between Even Coverage and Agitation
According to Scott Rao, even coverage is key in a batch brewer, and the best spray head designs “distribute water over the coffee bed in numerous small streams, causing less localized channeling” (S. Rao, 2016). In fact, Scott manufactures his own replacement spray heads for various brewers, specifically to offer users spray heads with a larger number of holes for more even coverage. However, this approach results in thinner streams of water that will agitate the slurry less. We asked Scott what affect this approach has on agitation in the coffee bed.
“There is a tradeoff between dispersion and turbulence,” explains Scott. “You can get a high extraction with a kettle because the concentrated flow from the kettle creates a lot of turbulence.” In a machine with a stationary spray head, however, you can only choose the number, size, and positioning of holes. “The best option will vary with the flow rate and the bed size,” Scott says. “With our FETCO [replacement sprayhead], 19 holes was better than 12. We lost a little turbulence but gained better coverage.”
One approach to increasing turbulence without sacrificing even coverage is to use mechanical approaches to mimic hand brewing, using robotic arms or rotating filter cones to allow the water streams to travel over the whole coffee bed, such as in UCC’s ‘Dripmaster’. While this approach is promising, the mechanical challenges and associated cost mean this approach isn’t available in mainstream commercial brewers.
There is however a simpler approach: adjusting the flow rate to control turbulence, by using a pump to control the flow of water through the sprayhead. Increasing the flow of water through the head increases the force with which the water hits the slurry, promoting agitation. Using ‘pulses’ during brewing means that the overall brew time can be controlled independently of the flow rate during each pulse.
This approach is being investigated by UK CIGS and UKBrC champion Gordon Howell, as part of his work on the forthcoming Tone Touch 03 single serve brewer. In this brewer, there was an upper limit to the possible flow rate, due to technical factors. They found that in this case, fewer holes in the sprayhead allowed higher extractions because of the increased agitation, especially with the deeper beds in conical filters like V60s.
This discovery prompted them to look at flow rate in more detail, and build control of this into the brewer. They plan to offer the barista control over the flow rate during different parts of the brew, to allow agitation without the flow digging channels into the coffee bed. “Once you’ve done the work of creating agitation, then you can slow down the flow to elongate the brew and get out more sweetness,” Gordon explains. “We are focused now on a three phase approach, with a bloom phase, a ‘turbulence’ phase, and a ‘development’ phase.” The idea is that different flow rates and pulse patterns could be programmed at each stage, allowing the barista to control the level of agitation.
Scott Rao has also been working on a similar approach by adapting the Decent espresso machine to work as a filter brewer. “We can pump water at any pressure we want, so that divorces the hole size from the flow rate, which has opened up some interesting options,” he says, “but if there is too much turbulence, fines will clog the filter.”
The key here is finding the balance between too much force, causing choking, or channeling and indentations in the spent coffee bed, and too little force, meaning inadequate agitation. The right setting is a matter of trial and error, dependent on the brew basket geometry, but also on other factors such as the coffee density, meaning different coffees might need different flow rates, Gordon says. “The barista needs to be in control.”
T.R. Lingle, 1996. The Coffee Brewing Handbook, second edition, pp 29-33
S. Rao, 2016. Batch Brew Basics Part 1: The Setup (Blog post). https://www.scottrao.com/blog/batch-brew-basics-part-1-the-setup