Abstract ED

I recently was notified by a friend that an interesting article had dropped. Coffee and volcanoes! How exciting is that? I cracked open the article and was initially excited, then increasingly dismayed. Here is the article: Study: Why a spritz of water before grinding coffee yields less waste, tastier espresso. I found very little to be excited about on the first read through, and felt some internal panic welling up. This was for sure going to lead to some poorly thought out, widely spread content. Here is the paper for reference: Moisture-controlled triboelectrification during coffee grinding. All “quotes” are from the paper.

Background

For the past few years that I have been involved with grinding coffee at home and brewing specialty coffee while on a low budget. I have heard that adding in a few drops of water will reduce some of the static electricity in the process whereby the finer coffee grounds stick to the burrs and internals of the grinder. Personally I have never actually tried this, as I find the bits sticking to the surface to be quite useful in removing fines from my overall grind. I’ve just tapped off the fines afterwards and run a finger to dislodge the remainder in the grind receptacle. Seeing the article title, I thought, perhaps someone has finally quantified what the effective static reduction is like!

Seeing also the note about a volcanologist involved in the work, I was hopeful that the analogies would prove meaningful. I read through the article as it meandered along in some of the past work from the lab and found that they had identified two general factors contributing to the charged particles. The article does a reasonable job summarizing the paper, in that the team tested a number of variables and collected a slew of data. This part was surprising. The paper was written such that a regular audience could parse the work and the writers at Ars Technica could summarize each segment. I then took a look at the paper.

Matter

The highlights on the site are fairly straightforward, but the print version is confusing. The paper opens up with what is presumably a visual abstract with some key words overlain an image of some ground coffee on some unidentified mechanism. There is a brief background on the sources of charge (fracture and friction), with reference to some industrial interest in further studies on surprise charge. The authors claim to address three things: coffee parameters affect charge, demonstrating most of the charge is from the fracturing process, and charge is also dependent on moisture content. The introduction ends off with

“In the context of both understanding the fundamentals of triboelectrification and bolstering our efforts toward brewing more reproducible and sustainable coffee, this article offers strategies to control the charging of coffee particles and posits opportunities therefrom.”

The Results and Discussion section broke down those three aspects as follows

1. Charge parameters. They demonstrate that beans are charged via contact with typical coffee setup materials. It is unclear why they use the Starbucks Blonde Roast (blend) here. They then claim that coffee must be ground, typically to grain size of 100 nm to 2 mm. I was unable to find their reference here, as I usually grind coarser for a French press. Perhaps it is a reference to espresso grinds only. They skip ahead to state that finer grind sizes resulted in aggregates held together by electrostatic forces. They ground 1 g at a time and found both the net charge, and the charge distribution.
   • On origin. They used several Mexican coffees with different roasters and roast levels and found three different net polarities, “But those coffees were roasted by different roasters to different colors, and the data suggest, perhaps unsurprisingly, that origin alone does not dictate the polarity of charge.” I can confirm, this is unsurprising and the point on origin is not meaningful as one farm produces differently than the next, even if they are immediate neighbours.
   • They repeated this for 30 coffees and found a weak relationship between Agtron value (roast level) and charge. Instead, they pinpointed the 2% moisture content as the possible culprit (R-squared = 0.41). This runs counter to volcanic ash and an interesting point! It also indicates that they were able to measure moisture content of these coffees.
   • Two suggestions were provided. First, dark roast tends to be more brittle, and more associated with negative charge. The second, is that water content plays a role.
   • The distribution of negative particles is skewed towards fines and “boulders”. I did not get the same interpretation from the selected figures in the paper. In one of the images, the negative charge has a greater peak in the larger grain size range, but in the other, the two peaks are quite similar. An allusion to ion scavenging is made here. To that note, I wonder what their target grind size was as they later explain different roasts resulted in different size distributions. However, I didn’t see a distribution for neutral grains.
2. Roast parameters on charge.
   • Moisture. They roasted a 12% moisture green bean to a variety of different final products. They repeated for a 9% moisture green bean and found that the roast parameters were more important than the initial product moisture content for overall charge to mass ratio. The final moisture of the roasts fell between 1 – 3%.
   • Grind size. They used a different set of coffees from the moisture test, showing two coffees with net positive/negative charge and found that finer grinds correlated to higher charges. They also did a test using the same grind size across 13 coffees and showed darker roasts typically resulted in smaller mean diameter and more negative charge. “Figure 4C offers one explanation as to why darker coffees may yield slower espresso shots for the same brew parameters. It may not only be the increased volatile content but also the reduced bed permeability.” That is, lower grain size from the same grind is pulling slower shots. Note that your barista should be aware of the properties of their coffee beans that day, and should have each roast dialed in for the day. Because of the grind size difference with the roast levels, they repeated the experiment such that the same size distributions were had for each roast. They still found that negative charges correlated more strongly with darker roasts
   • Granular mechanisms. This targets the question of, how does the grind mechanism contribute to electrification? The authors pre-ground the coffee and then put it back through to evaluate the effects of frictional charging from grind interactions and coffee/grinder contact. There was about a 90% reduction in charge after testing on seven coffees by passing through a larger grind setting. Presumably they neutralized the coffee grounds prior to regrinding.
3. Addition of water. “Anecdotally, baristas have observed that the incorporation of small amounts of liquid water onto the whole-bean coffee prior to grinding results in seemingly reduced charging. In our hands, it also resulted in near-zero grounds being retained by the grinder, an observation that has implications for reducing waste and increasing quality of beverages. Perhaps we will revisit this in a future study, but for now, we are more interested in whether the addition of water neutralizes the effects of fracto- and triboelectrification or modulates particle aggregation via capillary forces.”
   • They found that humidity affected the charge once relative humidity was greater than 60%.
   • For seven coffees, they progressively added more water. Increase in water addition took place between 25 – 40% RH. They found that water content of 20 ul/g resulted in close to 0 net charge on the grounds and suggested either a cooling effect, or rapid ion transfer.
   • They also tested this with saline solutions to evaluate the effect of ion transfer. They found that the results were similar, so electron transfer in the dry case is more likely the cause of charging.
   • Here I find my major issue with the article. “To deduce whether the coffee particles were forming neutral aggregates”. This comes rather out of left field, perhaps they meant to create a new section? It seems that they were trying to answer what the net 0 charge consisted of, but never quite managed to say so. If this plays a significant role with volcanic ash, I feel that it would have helped the reader had they included more content in the Introduction. They found that the size distribution was improved with the addition of water, and found mostly neutral components in comparison to the “dry, field” grind result. The caption does not clarify what “field” and “no field” refer to, likely the method in which they sorted the bins. Water also reduces clumping.
4. Then they brewed some espresso using parameters that were unlike any of their prior parameters. 18.0 g dry mass coffee was used to produce 45.0 g liquid coffee extract, ground at setting 1.0, tamped at 196 N, and brewed using 94°C water, kept at 7 bar static water pressure with a 2-s pre-infusion. They found that without added water (unclear how much), the shot took less time and had a slightly lower TDS (total dissolved solids). The first drops enter the cup at the same time (~10s). They then fit a sigmoidal function to a hypothetical situation where the flow will achieve a constant and then looked at the flow rate for 3 samples of wet vs dry ground. “In other words, this is conclusive [bolding by me] evidence to suggest that dry-ground coffee is producing a bed with markedly more porous pathways.” They note that lower pH corresponds with higher S (more acidic coffee is less charged). They speculate on the role of internal moisture prior to roast.

That’s a lot! They went on to note that they made some assumptions about the burrs (in this case, their contribution is fixed, perhaps this is why they didn’t use other machines) and relate the Seebeck coefficient to the amount of charge from fracturing. They state that “Agtron number is proportional to the electronic band gap, both of which are reduced with darker roasts.” Without reference, so I will have to take their word for it. They related this to the Seebeck coefficient and oxidized organic molecules having low-laying orbitals. This seems to be a pretty big addition to the general knowledge of organic molecules, so I am surprised they did not highlight this more in the Introduction.

“Furthermore, light, medium, and dark coffees and their roast profiles can feature markedly different charging regimes, even for coffees with the same Agtron values. This highlights that modern roasting paradigms are highly artisanal and pose fundamental challenges for using commercially sourced coffee in academic settings.”

Alright. This feels like pandering.

“One could imagine the development of in-ground mineralization to make designer brewing water in situ without the risk of damaging boilers in espresso settings.”

I really don’t know where they were going here. Are they suggesting adding/removing heaps of salt and hardness to accommodate for water in advance and pulling shots with distilled? This is an interesting point I wish they elaborated upon.

Methods

I’ll highlight some interesting aspects I saw in the Methods section.

A pipette was used to introduce water onto whole-bean coffee, and the coffee was shaken in a sealed container to ensure homogeneous distribution.

Whole-bean coffee was stored in H2O-impermeable vacuum bags and kept at −20°C. The coffee was allowed to equilibrate to room temperature in the vacuum bag before grinding

I am curious to see how long the coffee was shaken to “ensure homogeneous distribution”. I also see that they didn’t comment on the effects of freezing the coffee prior to use, as freezing beans is quite controversial amongst coffee enthusiasts.

Notably, they did not provide a method to clean the grinder in between experiments, nor is there data indicating the mass loss during the process.

Commentary

“Many commercial coffee roasters treat their roast profiles as proprietary, and it is impossible to back calculate the precise profile from examination of only the roasted whole beans. There is academic value in standardizing roast profiles across the industry, thereby allowing for direct comparisons between coffees. But we are not advocating for this on the industrial scale, as that would sterilize an artisanal aspect of the industry. Instead, we developed our own profiles with the aim of isolating roast through the development of systematically “darker” coffees.”

This segment indicates that the authors care about the artistry of specialty coffee. Unfortunately, this theme runs through much of the paper and I found it to strongly influence how the paper was written. I then wonder, who was this paper written for?

As a consumer and coffee enthusiast, I am left with the following thoughts and questions

• Lack of testing. Only ONE grinder was used during this test. I think they selected a sensible one if they were only going to use one, it’s reasonably accessible to different shops and the settings they used were straightforward. I do imagine perhaps there were complications justifying it on a budget (industry grinders are astoundingly expensive if there was no specific target goal for the research that served direct benefits). This is evident in James Hoffman’s video where he discusses the paper and interview with one of the authors, where he ends with a suggestion that at home brewers should fill in the gap.
• Not understanding the difference between specialty crops and variability between farms that could be adjacent to each other. Given how many external individuals were involved in the work, this should have been caught early on.
• Use of the standard dark/light roast ratings via Agtron was useful, but I wonder if there was the opportunity to develop a new standard for roast levels here. Perhaps based on water content before and after roasting?
• Freezing? This changes how the coffee works. Why did they freeze things? Why did they not evaluate the effect of “resting” coffee? How fresh were the roasts? Why didn’t they roast more of their own sample? Why choose to use a number of different roasts? Was it just a PR stunt? Clearly they had collaborators, going back to the first thought, why not borrow a few grinders for controls?
• I think there is the possibility that this work can be really extended and become useful for your average hobbyist. For example, nailing down the variable effect on extraction or charge scaled to average grind diameter and how variable the results can be if you have an even vs uneven grind
• Who is this paper actually written for? The authors refer to the “artisanal process” a fair amount, and note some complications in terms of getting roast profiles. They also do not use grinders across the hobbyist market, nor test out burrs made of different materials (which should have a notable effect).

My thoughts as someone who attempts to write and share science via papers:

• Absolute statements. The authors make a number of absolute statements that are not understood to be true, even by a regular audience. One such example is, “All coffee is ground”. Coffee beans are typically harvested, dried, then roasted to produce what the consumer sees as “coffee beans”. These can then be further processed by grinding to make a standard “coffee” drink, but can also be coated in chocolate, or have some of the volatiles extracted via CO2.
• Lack of structured testing. Opportunistic testing makes sense when one is in the field, or has limited time/funding/equipment. The authors picked and chose one strand from each of the experiments to continue on with the next step, resulting in very little data. Perhaps there were limited controls on the experiment that they do not address.
• Magical proposals. I am still baffled as to where the idea of “aggregates” comes from. This might be a matter of coming from a very different field, but if it is as intuitive as it seems, then it is surprising it wasn’t proposed earlier in discussion of volcanic ash. The role of neutral aggregates is not elaborated upon in terms of some of the other tests they did. Perhaps this lies somewhere in the realm of volcano science? The concept of aggregates was not surprising in and of itself, but it is unclear what fraction of aggregates were formed, and how they affected the final production when the aggregate ratio decreased. It’s worth noting here that I don’t possess an espresso grinder and perhaps all the “fines” that I’m thinking of are actually the espresso grind size, and the goal was to limit the loss of coffee. Which is a great target! Coffee prices are going up and grams can add up. That said, they could have been clearer in the introduction when mentioning potential industry impacts that this was one of their intentions.
• Awareness. I think this article was actually great in terms of awareness for how one changes one parameter at a time when trying to perfect a coffee brew. The authors are potentially well on their way to changing how the standard espresso at a coffee shop is pulled, but given the sterilization of specialized coffee in recent years, perhaps their target was truly the at home tinkerer.

The only profound thing this article has demonstrated is how easy it is to blow up on the internet if you have the right connections or work on a topic that touches the lives of many. Does anyone remember how science indicated red wine was both good and bad for your health?

Sketchy Science Segment

As an additional Sketchy Science segment, here is my daily brew using a size 2 V60. This uses ~25 g of coffee (pure speculation on my part, it’s been a while since I checked) and ~380 mL of water. I use straight tap water with ~35 mg/L sodium in it with a CaCO3 hardness of 149 mg/L. I couldn’t find the pH on a public site, but a sample test with the Master Freshwater API Test kit indicates I have a water pH of ~7.4 straight from the tap. I have two kettles, one electronic to boil quickly, and one gooseneck for a controlled pour.

1. Boil 600 mL of water in an electric kettle to 100 deg C.
2. Grind coffee to around ~2 mm while heating water in kettle.
3. Transfer 400 mL water into a stovetop gooseneck kettle (this drops the temperature and bypasses the slow and inefficient heating on the stove). This typically drops my temperature to around 85 C.
4. Pre-wet filter so it sticks to the plastic V60 (I used to use a ceramic, but had too much heat loss from it). Dump excess water.
5. Pour ground coffee into the center of the wet filter, tap sides to even out the coffee bed. (At this stage, I find it most useful for fines to get stuck on the side of my grinder so very little ends up in my pour). Poke a shallow indent in the middle of the coffee bed
6. Gently pour from the gooseneck into the indent, and let the water slowly flow over, covering the coffee bed. This takes around 30 – 50 mL of water. Relatively fresh coffee should now “bloom” and release CO2 during this process. The goal is to get all of the grounds somewhat damp for the actual pour. Let sit for around 30 s.
7. Pour in a circular motion until you reach just over the half way mark of the size 2 filter. Give a gentle swirl in a clockwise motion to settle the agitated grounds so they funnel along the V60 grooves. Swirling counter to the groove direction will result in a lumpy bed.
8. While the water is still in motion, top off the water in a circular motion to ¾ up the filter. Let drain. This uses up around 200 mL
9. Walk away and futz around for a minute
10. Come back and do a second pour of around 125 – 150 mL. No swirling required. Let drain
11. Walk away and pack your bag for work
12. Pour ~100 mL. The water should be fairly slow to pass through now as the filter is getting clogged. Wait until you can see the sludge at the top and the filter is very slow to drip.
13. Optionally top off with the now ~40 – 50 deg C water and let drain. I heard in passing somewhere that the last little bit is actually the sweetest aspect. I can’t say I disagree when I decant out coffee earlier during the process.
14. Swirl the carafe gently to mix, then transfer into your preferred drinking vessel. Put a small amount aside in hopes that someone else will drink and appreciate your work.

Notes: I wanted to compare my water parameters to Third Wave Water, but I was unable find the information easily on their site. My guess is that it’s buried in a video somewhere. Since I first heard of their inception, they seem to have produced a number of water adjusters for multiple types of brews… I did find SCA water standards which their main product is inspired by and found Calcium Hardness 50 – 140 ppm, Alkalinity 40 – 70 ppm, pH 6 – 8 (huge range of acceptable pH, target of 7.0). My water hardness seems quite a bit higher than the standard, which I can certainly taste and experience when drinking water from the tap. https://sca.coffee/heritage-coffee-standards

Disclaimers: I have followed James Hoffman on YouTube for years, and I am really not a fan of his “science” work. I mostly appreciate what a big fan of coffee he is and usually only watch content when I am interested in the idea (clickbait titles). I typically end up disappointed when I see a basic Excel spreadsheet with no error bars or only one test, then avoid watching his content for several weeks in a row. I found his content on how to do coffee cuppings very interesting though, do check that out! I am told that the content is usually quite high quality in terms of robustness, so I must be lucking out. The commentary on AeroPress was quite interesting RE: industry insights however, and it’s still on my list of reading content to check out his Coffee Atlas. As always, check your sources folks. Clearly there is a lack of research in the realm of coffee, perhaps it’s time other scientists start branching out to bring in related ideas.

Conclusions

Here are some questions I would love answered, or perhaps answers are out there and I haven’t stumbled upon them yet.

• Why do we break the crust during cupping? What is the actual goal here?
• What is the perfect number of swirls and swirl motion to get a flat bed during a pourover if we consider grain size distribution, water temperature, and the standard V60 size 2 with Hario filters after the initial bloom?
• Is there a perfect way to age a Coffee Sock? Surely the fabric and weave alter over time.
• What is the most efficient way of cleaning a French Press without dumping grounds into the sink and flooding the garbage with water?
• What’s up with Third Wave Water? What kind of testing did they participate in to develop their water conditioners?
• We never did find out if the espresso was tastier at the end of the paper. Is it time to quantify taste preference in coffee for the average person vs the elitists?