11. Pulse Brewing Follow Up (Part 1)

Wow, there’s been awesome response to my previous post on pulse brewing and a couple great questions/points have been brought up. There are three that I would like to address over the next few posts. First, pulse brewing for pour over methods like the Kalita Wave and Gino. Second, should a barista pull the filter off a Chemex or Hario at the end of the pour over before letting the water drain into the cup. Finally, the migration of coffee particles and how this impacts water flow. I will give fair warning, this post will be a little more intense on the coffee geek scale than the last.


1. Kalita Wave & Pulse Pouring

I will confess, I have never used a Kalita Wave. Fortunately, I have been able to observe this brew method over the past few weeks since the “Taste is King” pour over competition (we won’t discuss my performance, but let’s just say I’m a lot better at math). Let’s compare a Chemex to a Kalita Wave:

20140926_120446428_iOS

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It’s obvious that the Wave is much shorter than the Chemex. Also, the Wave has a flat bottom with three holes to allow coffee drain into the cup and the Chemex is conic and has one point of drainage at the bottom. With the Chemex, water flows through the coffee bed and is prohibited from moving sideways by the glass wall that the filter is sealed to. Therefore, it can only flow downward and eventually leave the coffee bed through the filter at the bottom. Although not in the picture, the Wave has a wavy filter (Kalita’s marketing department is the pinnacle of originality) and this filter permits the sideways flow of water. Well, who cares whether water can move sideways or not? The big difference is that the Wave controls flow rate (remember Q?) by the limiting the size of the holes at the bottom of the dripper—whereas the Chemex filter controls flow in the Chemex. Let’s detour really quick to one of the equations that describes the flow of liquids:

Q (Flow Rate) = A (Area) x V (Velocity)

It’s getting intense, but the best way to think about this is to remember spraying someone with a water hose. Hoses are pretty pathetic by themselves if you want to win a water fight, but slightly plug the end with your thumb and BAM! Instant, unending watery doom for your competitors. Why is this? Well, flow rate must always remain the same in the above equation, but you’ve just decreased the area the water can flow through, so to balance the equation the velocity must increase. Therefore, giving you much more range and intensity with your hose.

Back to sideways flow, since the Wave allows this, the flow is not governed by the filter. Therefore, I theorize that coffee extraction is not significantly dependent on height of the water column (hydraulic head) in the filter, but is instead dependent on the velocity of the water passing over the coffee grounds, which is why the Kalita Wave must be pulse brewed. As an illustration, another one of my awe-inspiring iPad drawings.
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As water is introduced to the Kalita Wave, it hits the grounds and disperses in all directions. I believe it is a combination of the water temperature and the speed at which the water passes over the coffee grounds that ultimately determines the overall coffee extraction. This is why pulsing is required, because if a continuous pour is done, the water’s velocity essentially becomes zero as soon as it hits the standing water in the Wave and yields an undesirable cup of coffee.

So, unlike the Chemex and Hario V60—in which extraction (I believe) is governed significantly by the hydraulic head—the Kalita Wave’s (and similar methods) extraction is governed by the pulsing and temperature of the water. I’m hoping to provide experimental evidence for these theories in the future by using some of the equipment available to me at the University of Cincinnati, so be looking for that in the coming months. In the mean time, keep making fantastic coffee and let me know your own experiences with your pour overs and if you think I’m completely nuts (truthfully, I am always open to correction)!


3 thoughts on “11. Pulse Brewing Follow Up (Part 1)

  1. It seems to me that the Hario achieves a similar result for “sideways” movement with the ridges that run down the inside of their cone holding the filter away from the sides of the cone and thereby allowing flow through the sides of the filter as well as the bottom. I am unclear as to whether any of this really matters as long as proper contact time is achieved between the water and the bed of coffee. I have not seen the Kalita Wave in person, however, I have watched videos online. I am not an engineer by any means, but in your hose example if the supply flow rate is increased the ouput will increase as well. It would be the same if you put a cap over the end of the hose and poked 3 holes in it or rubber banded a coffee filter over the end of the hose. The ouput flow would increase with the input flow rate. It seems to me with a pour over regardless of how the output flow rate is limited, by the filter (, by holes in the bottom of the cone, or by a thumb over the spout, our supply flow rate and therefor head pressure is only as even as our pour, and the head pressure created by our pour will equally increase or decrease the output flow/extraction regardless of the method of limiting that output flow. Am I off in this thinking?

  2. Chris, great comment. You bring up a good point and looking back, the hose metaphor my not have been the best choice on my part.

    First, I may argue that the Hario V60 ridges do not actually allow for sideways movement, but instead force water trickling down the sides back towards the center of the coffee bed. This is for the exact reason you noted, which was to ensure correct water/coffee contact time. The main reason I don’t believe it allows for sideways movement is because when the filter is pre-wetted, before adding your coffee grounds, the filter conforms to the ridges. Therefore, leaving almost no void space for the coffee to move sideways into (especially when compared to the Wave).

    You are definitely correct in your assessment of the water hose. What I should have been more clear on is that for the Kalita Wave, hydraulic head is not the controlling factor for extraction compared to the Chemex. Imagine two buckets, one of the buckets is three times the height of the other (this will be our Chemex) and has one small hole in the bottom. The second bucket is short (like our Kalita Wave) and have three small holes in the bottom. Now if we fill the first up with water, we will see a significant change in water retention time (the amount of time a single water particle stays in the bucket) as the bucket goes from full to fully drained. However, the second bucket is much shorter, so the hydraulic head difference can only be a third of the first bucket, but the outlet area is tripled. So let me use an equation for orifice flow, which is essentially what this is:

    Q = Cf*A*√(2∆h/density)

    Where:
    Cf = flow coefficient, ranges between 0.6-0.9. We’ll choose 0.75 for this example.
    A = Orifice area (i.e. size of our holes) We’ll assume each hole has an area = 1.
    ∆h = Hydraulic head, for bucket 1 it will be equal to 3 (three times as tall) and bucket 2 will be equal to 1.
    density will be assumed to be 1.

    Q1 (bucket 1) = 0.75*1*√(2*3/1) = 1.84 flow units
    Q2 (bucket 2) = 0.75*3*√(2*1/1) = 3.182 flow units

    Realize this is the absolute theoretical maximum flow rate. What do we see? The Wave has a much higher flow than the Chemex. Now let us look at one more equation:

    Hydraulic Retention Time (Or for our case, water/coffee contact time) = Volume/Flow Rate

    HRT1 = 3 Volume units (assuming it has three time the volume of bucket 2) / 1.84 flow units
    HRT1= 3/1.84 = 1.63 time units

    HRT2 = 1 volume / 3.182 flow units
    HRT2 = 0.314 time units.

    HRT1/HRT2 = 1.63/0.314 = 5.2!

    This mean that at the maximum flow rates for both methods, the water/coffee contact time for the Chemex is roughly 5 times that of the Wave!

    So you are absolutely correct in saying that both are dependent on hydraulic head, but since flow rate does not share a linear relationship with hydraulic head, the major factor controlling the Wave’s retention time is the area of the outlet because the hydraulic head cannot range significantly due to how short it is. Whereas, for the Chemex, the hydraulic head is allowed to range significantly due to its height, which is the major factor for determining water/coffee contact time.

    This is why I believe a continuous pour for the Chemex will minimize over-extraction because maintaining a steady flow rate is key to even extraction of the coffee bed. However, with the Kalita because the water is in and out so quickly because of the three larger holes, what really matters is how well the pulse pour is executed because the contact time is much less than Chemex and V60 methods.

    I hope this answers your question, please let me know if it does not or if you have other thoughts on my exercise here. I am by no means an expert, but I am trying to provide a little more science to this field of opinions. Thanks again for your comment!

  3. Thanks for the reply. I understand what you are saying about the taller Chemex. I have no experience with the Chemex at all, but what you are saying makes sense. Regarding the Hario. I think I have to somewhat disagree regarding the ridges/sideways flow. After you pre wet the Hario filter if you look around the edges of it, you can see that it is still suspended away from the walls of the ceramic cone. in fact, you can reach down into the filter and push against the sides in between the ridges and find that the cone is being held away from the walls all the way down. With the tension that you can feel I believe even with the coffee placed in the filter it is still held away from the sides.

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