Slow Feeding when Grinding Coffee in Espresso Grinders

Slow feeding – the slowed-down supply of coffee beans to the grinder – can significantly influence the particle size distribution of the ground coffee and thus the extraction of espresso. In this study, we investigate this effect with laboratory measurements of particle distribution, extraction parameters (brew time, TDS, extraction yield), and sensory evaluations.

Based on previous research, including by Lance Hedrick, Jonathan Gagné, Christopher Hendon, and Samo Smrke, we expect that slower bean feeding reduces the amount of fines and makes the ground coffee more uniform. Our experimental series with various specialty coffees (Apas, Limontitla, Chirinos, Hamasho) confirm: Slow feeding with the same grinder setting leads to coarser ground coffee with a significantly lower proportion of fines, which increases the flow rate and speeds up extraction.

To achieve comparable extraction times, the grind size must be adjusted finer accordingly. The results show differences in extraction behavior: Slower feeding can increase puck permeability (resistance), resulting in shorter brew times at the same grind size, but also affects puck structure and thus taste.

Sensorially, slow feeding tended to lead to clearer, more flavor-transparent and complex cups, and slightly less body, depending on the bean variety and roast freshness. The discussion links these findings to the role of triboelectric charging (static electricity) and the well-known "popcorning" effect with individually fed beans. Finally, practical implications for grinder design and barista workflows are derived.

We performed slow feeding by hand, as well as with an ingenious slow feeding tool from a Swiss startup called

Introduction

The particle size distribution of coffee grounds is a key factor for espresso extraction. In particular, the proportion of very fine particles ("fines" <100 µm) significantly influences the flow rate of water through the coffee puck (The role of fines in espresso extraction dynamics | Scientific Reports). Samo Smrke et al. (2024) showed in a systematic study that a higher proportion of fines reduces the permeability of the coffee bed and leads to slower flow rates and longer extraction times. At the same time, a finer grind size (with more fines) increases the extraction yield and intensity of espresso to a certain extent, until an excess of fines leads to clogging and uneven extraction. The art therefore lies in creating a balanced particle spectrum that provides sufficient extraction surface and enables a stable flow rate.

In addition to obvious parameters such as grinder geometry and setting, the type of bean feeding is increasingly coming into focus. In the coffee scene, slow feeding is understood as the deliberate slowing down of bean feeding into the grinder. Instead of putting all the beans of a dose into the grinder hopper at once, with slow feeding they are added gradually – sometimes even individually. This procedure is intended to improve the quality of the ground coffee, but how exactly?

A well-known phenomenon when single dosing beans is called “popcorning”. The term describes individual beans bouncing around in an empty or almost empty hopper, causing them to pass uncontrollably between the burrs. As Jonathan Gagné explains, a single bean in an empty hopper can bounce around and potentially slip through a larger gap between the burrs, as there is no bean stack pushing it down (Grind Quality and the Popcorning Effect – Coffee ad Astra).

This popcorning results in an overall coarser grind and a slightly broader particle distribution with more coarse particles ("boulders"). Such larger particles contribute less efficiently to extraction due to their smaller specific surface area. Gagné's measurements showed that while the effect is limited for typical dosing amounts (for a 10 g dose, bean-by-bean ground samples were about 0.08 mm coarser in median than with a full hopper) and the amount of fines can even be slightly lower. Nevertheless, these results illustrate that the feeding rate influences the grinding curve: a full bean supply in the hopper leads to a more homogeneous grinding process, while individual beans tend towards a more bimodal distribution.

In addition to mechanical effects, static charge also plays a role during grinding. Due to friction and fragmentation of the beans, particles become triboelectrically charged (Grinding coffee with a splash of water reduces static electricity and makes more consistent and intense espresso | ScienceDaily). These charges lead to agglomeration (clumping) and adherence of coffee powder to the grinder and chute. Christopher Hendon and colleagues investigated the causes and effects of these triboelectric effects in 2023/24. They found that beans with a higher residual moisture content are less charged during grinding, which leads to less scattering and more consistent particle release. Wetting the beans before grinding (Ross Droplet Technique) can thus reduce static charge and, in their experiments, resulted in more uniform ground coffee and more intense espresso extraction. This finding underlines that triboelectric effect management (e.g., through bean moisture or ionization) significantly influences the distribution and dispersion of the ground particles.

Against this background, the question arises: Can the deliberate slowing down of bean feeding – slow feeding – offer similar advantages? Anecdotal reports from the specialty coffee community and initial experiments suggest that slow feeding can reduce fines production and improve extraction (Impact of slow feeding is nuts : r/LanceHedrick - Reddit). However, systematic scientific investigations on this are still lacking. In our report, we build on the aforementioned works and use scientific methodology to investigate the influence of slow feeding on ground coffee distribution, extraction behavior, and sensory properties. In doing so, we particularly address the hypotheses from Gagné's popcorning analysis and Smrke's fines study and examine to what extent slow feeding shifts the particle size distribution (e.g., lower proportion of fines) and what extraction dynamics (flow, puck resistance) result from this. In addition, we consider possible differences depending on bean addition (manual vs. continuous feeding) and roast profile to classify the observed effects.

Methodology

Experiment Overview

Two main experimental series were carried out. In the first test (January 2025), we qualitatively investigated the influence of slow feeding with two different coffees (Apas (medium roast) and Limontitla (lighter roast)). In the second test (April 2025), quantifying measurement series followed with Apas coffee, including TDS measurements to calculate the extraction yield. Additionally, we conducted accompanying particle size analyses at the ZHAW and evaluated sensory differences between shots with and without slow feeding for four coffees (Apas, Limontitla, Chirinos, Hamasho) in tastings.

Equipment and Conditions

Both tests used a DF64 flat burr grinder (64 mm). In the first test, a DF64 (1st generation) was used, in the second, a DF64 Gen 2. A La Marzocco Linea (commercial espresso machine) served as the espresso machine. The temperature was set to 93 °C. IMS Competition 24.5 g baskets were used in a La Marzocco 58 mm portafilter. The input dose for all shots was 18 g of coffee, the target output amount approximately 45 g of espresso (ratio ~1:2.5), unless otherwise specified. Each shot was prepared with identical puck preparation (distribution with WDT/Moonraker, leveling and tamping with 12 kg pressure).

Slow-Feeding Mechanisms

To ensure slow bean feeding, two methods were used:

1. Manual Slow Feeding by gradually dosing by hand. For this, the 18 g were added in small portions to the hopper while the grinder was running, supported by light tapping on the dosing cup. The feeding duration was about 50–70 seconds per dose (instead of ~5–10 s for normal "all at once" grinding).
2. Mechanical Slow Feeding using an Electric Slow Feeder attachment (Crema Loop Slow Feeder ). This 3D-printed device is placed on top of the DF64 hopper and uses a rotating disc to feed the beans individually and controllably into the grinding chamber. With the Slow Feeder, we could set a constant feeding rate (approx. 1 bean every ~1–2 seconds) to ensure consistent conditions.

Experimental design first test

For the coffees Apas (Natural, Brazil) and Limontitla (Washed, Mexico), several shots were pulled under different grinding and bean feeding conditions. First, a reference shot with normal feeding was created, i.e., all beans at once into the running grinder ('without slow feeding'). Afterwards, shots were made with manual slow feeding and with the slow feeder tool, with the grind size initially remaining unchanged compared to the reference shot.

Since we observed that slow feeding significantly increased the flow rate (shots ran too fast), we then varied the grind size to bring the flow time back into the target range (~25–30 s). Specifically, for Apas, for example, the grind size was gradually adjusted from an initial value of about "20" (on the DF64 scale) to finer (~11.5). Each condition (e.g., Apas without SF, Apas with SF unchanged, Apas with SF finer) was tested several times to check reproducibility. The achieved shot times, output quantities, and brew rates were noted. Sample doses of the ground coffees were packed airtight and prepared for particle analysis.

Experimental design second test

Building on the first results, a more precise investigation was carried out with Apas coffee (new roast batch from 25.03.2025). Three scenarios were directly compared:

• (A) Normal feeding (no slow feed) set to a target output of 1:2.5 in ~25 s,
• (B) Slow feeding at the same grind size as (A), and
• (C) Slow feeding with a finer grind size adjusted to also achieve approx. 25 s extraction time. For each scenario, five shots were pulled consecutively.

In scenario A, a grind size setting of 21 (DF64 scale) resulted, while in scenario C, the grind was significantly finer (value ~10.5). Immediately after extraction, the TDS value (Total Dissolved Solids, in %) was measured for each shot with a VST refractometer and the extraction yield (%) was calculated (based on input, output weight and TDS). This gave us mean values of extraction yield and brew time for the conditions with and without slow feeding.

Particle size analysis

The collected ground coffee samples from Test 1 (approx. 13 g each) were sent to the Zurich University of Applied Sciences (ZHAW) for particle distribution measurement. The (ZHAW) used the Camsizer X2 from Retsch Technology to measure the particle size distribution (PSD) of coffee grounds. This device is based on dynamic image analysis and is particularly suitable for analyzing particles in the size range of approximately 0.8 to 8,000 µm.

For each sample, we obtained characteristic values of the particle distribution, in particular the median x50 (median particle diameter in µm), the fines content Q<100µm (mass fraction of particles <100 µm, in %) and the peak width of the main fraction (width of the particle spectrum, here defined as the range encompassing 60 % of the coarse particle mass). This data allows an objective comparison of grind sizes and distribution widths between the different bean feeding variants.

Sensory Tasting

To evaluate practical effects on taste, we conducted two descriptive tastings with the KM-Espresso Score Sheet. Additionally, another discriminatory tasting was performed with filter coffee.

In this report, we summarize the results of the descriptive tastings with the KM-Espresso Score Sheet. These are also explained in our test video. Further tastings are necessary to verify the results.

For each coffee, espresso shots with and without slow feeding were compared. The espresso samples (temperature ~60 °C, 1:2.5 ratio) were evaluated by the tasters according to a simplified scheme: bitterness, sweetness, acidity, balance, aroma, body, texture, aftertaste – each on a scale from 1 (weak) to 6 (strong) with half points.

The individual attributes served for discussion; however, the focus was on an overall judgment of sensory quality and possible differences in clarity, complexity, and body between slow-feed and normal-feed shots. The tastings took place immediately after extraction.

Results

Particle Size Distribution (ZHAW Analysis)

The laboratory analysis of the ground coffee samples clearly confirmed that slow feeding influences the particle distribution. Table 1 shows the characteristic values for Brazil Apas and Mexico Limontitla under four conditions: without slow feeding (Baseline), manual slow feeding (constant grind size), slow feeding with device (constant grind size), and slow feeding with device and adjusted finer grind size.

Table 1: Particle size distribution (median x50, fines content <100µm, and 60%-main peak width) for Apas and Limontitla under different bean feeding conditions. GS = set relative grind size on the DF64 scale.

Even comparing the Baseline (all-at-once) to the slow-feeding conditions with the same grind size shows clear trends: Slow feeding shifts the median x50 upwards – meaning the particles become coarser on average.

• For example, x50 for Apas increased from ~250 µm to ~299 µm (+20%) with manual slow feeding.
• At the same time, the fines content (<100 µm) drastically decreased: from ~33% to only ~24% for Apas (or from 33% to ~28% for Limontitla).
• Even with the electric feeder (device), there was a reduction in fines compared to the reference (Apas ~26%, Lim. ~29%).

These results support the hypothesis that slow, portioned feeding of the grinder reduces fines production. Interestingly, the effect was even slightly stronger with manual slow feeding than with the continuous feeder – possibly because truly individual beans were manually fed, while the automatic feeder maintains a constant small "bean flow". Regardless, the trend was clear: Slow feeding makes the ground coffee coarser and significantly reduces the fines fraction at an identical grind size.

The width of the main particle fraction (60% peak width) changed only moderately. It tended to be slightly larger under slow feeding (Apas: 193 → ~210 µm; Lim: 178 → ~182–186 µm), which indicates a somewhat broader distribution of coarse particles. This is consistent with popcorning resulting in some beans producing slightly coarser "boulders" (Grind Quality and the Popcorning Effect – Coffee ad Astra). However, at the same time, the scatter in the fines range significantly decreased, which can overall be perceived as a more homogeneous distribution (i.e., less pronounced bimodal character). Gagné's finding that grinding bean-by-bean can result in a slightly tighter distribution similar to a high-quality grinder is reflected here in that the undesirable fine-particle "shoulder" of the distribution is trimmed.

Finally, Table 1 also illustrates the data for the "finely adjusted" slow-feed conditions (ASF_444, LSF_444). Here, based on the excessively fast flow rates, the grind size was significantly readjusted (Apas from MG20 to ~11.5; Limontitla from 16 to 8). As expected, this shifted x50 strongly towards finer particles (178 µm for Apas, 164 µm for Limontitla) and increased the fine particle content (approx. 38% each). These values are even higher than the original fine particle content of the baseline. This shows: To achieve the same extraction time despite slow feeding, the coffee had to be ground significantly finer – which again

Fig. 1: Influence of slow feeding on particle size distribution for Apas (blue) and Limontitla (green). Left: Median particle size x50; Right: Proportion of fine particles <100 µm. Shown are baseline (no slow feed), slow feed (manual), slow feed (device) at the same grind size, and slow feed with finer grind size to compensate for flow time. Data source: Kaffeemacher PSD Data 2025

Extraction Times and Yields (with/without Slow Feeding)

The extraction metrics measured in the test series clearly reflect the changes in particle distribution mentioned above. In the first test, it was immediately apparent that, with an unchanged grind size, shots ground with slow feeding ran much faster than the reference. For example, Apas without slow feeding extracted in approx. 25 s, whereas with slow feeding (same grind size 20), the puck was so permeable that the liquid reached the target volume after only ~10 s – an extreme difference. Accordingly, these shots also had a significantly lower strength (concentration of dissolved solids).

By gradually adjusting to a finer grind, the flow time could be extended again; ultimately, only a grind size of ~11–12 slowed the slow-feed shot to ~25–30 s. Limontitla showed analogous behavior: at MG 16, the slow-feed shot ran ~5 s faster than the normal one (27 s → 22 s), and only with a drastically finer grind size (MG 8) did a slow-feed shot reach ~25 s. These observations are consistent with the expectation that fewer fine particles increase puck permeability – water flows through more easily, which shortens the extraction time.

The second test quantified these effects for Apas. Table 2 shows the average results of the three scenarios (A: no slow feed, B: slow feed same grind size, C: slow feed finer grind size):

Table 2: Mean values (n=5 shots) of extraction parameters for Apas in the second test. Without slow feeding vs. slow feeding at identical grind size (21) vs. slow feeding with adjusted grind size (10.5) for approx. 25 s brew time. TDS and yield measured with refractometer.

In the baseline scenario (without slow feeding), the set grind size of 21 resulted in an average brew time of ~26 s and an extraction yield of ~20.9%. This served as a reference. When coffee was fed slowly at the same grind size (scenario B), the brew time dramatically dropped to ~10 s. The extraction yield correspondingly decreased to only ~17.4%. Such a short espresso clearly indicates underextraction (TDS ~6.8% compared to ~8.2% normally).

These figures quantify the effect previously observed qualitatively: slow feeding, with unchanged settings, makes the puck so permeable that standard extraction fails. Only significantly finer grinding in scenario C brought the parameters back to the initial level (∅ 24 s, 20.9% yield). Interestingly, despite a much finer grind, the achieved yield was not higher than in the standard – apparently, the factors compensate each other, or such a fine grind prevents effective extraction due to clogging. Theoretically, finer grounds have a larger surface area and should therefore extract more efficiently. However, overly fine grounds make it difficult for water to flow through the puck.

It is noteworthy that even with practically identical yields, the sensory properties (see below) differed, indicating altered extraction dynamics (e.g., different flow profile, layer extraction in the puck, etc.).

In summary, the extraction data confirm: Slow feeding significantly increases the flow rate, provided the grind size is not adjusted. To maintain the desired extraction time, therefore, the coffee must be ground correspondingly finer, which, however, increases the proportion of fines again. Our previous tests and measurements of particle distribution curves have shown that a narrower main peak has positive effects on taste. However, if it is too narrow, the grind size must be set so finely that an overshooting effect occurs, leading to a very large fine peak.

Sensory Results (Tasting)

The tastings of the different coffees substantiated some of the analytical findings but also revealed nuanced differences depending on the coffee and setting.

• Apas, medium roast (Brazil, Natural): In direct comparison, the espresso with slow feeding had a clearer aroma profile. Panel descriptions included "cleaner, more organized taste" and more pronounced sweetness and acidity, while the shot without slow feeding had a bit more body and "impact." For Apas, slow feeding did not lead to a better result in scoring. What the coffee gained in taste balance, it lost in quality at the level of texture, aftertaste, and body.
• Chirinos, medium-light roast (Peru, washed): Here, the clearest advantage of slow feeding was observed. The grind-corrected slow-feed shot extracted in 23 seconds and delighted with pronounced floral notes and high overall complexity. Tasters noted "much more floral" and awarded the slow-feed espresso the highest average score (32.5 out of 36 points; without slow feeding 27.5 out of 36 points). The balance of acidity, sweetness, and texture was excellent here, and the coffee was rated better in all categories.
• Hamasho medium-light roast (Ethiopia, natural): This coffee was very challenging to dial in with slow feeding. The grind size had to be set very fine, which led to clogging or channeling multiple times. In terms of taste, Hamasho did not gain in quality with slow feeding. As with Apas, Hamasho also showed slightly more clarity and complexity in its aroma profile, and likewise, the coffee lost some texture and body due to slow feeding. However, setting the grinder and finding the right grind size was so challenging with slow feeding for this coffee that it cannot be ruled out that the tasted shots also suffered from visually imperceptible micro-channeling. For a coffee with a density structure like Hamasho, an adaptive flow profile or a lever machine profile that counteracts the rapidly decreasing puck integrity may be useful.

In summary, the sensory results show: Slow feeding measurably changes the taste profile. Frequently mentioned keywords were clearer, more defined, more floral, sometimes associated with slightly less body/bitterness. However, the extent depends on the specific coffee and the fine-tuning. Especially with complex, high-quality beans like Chirinos, slow feeding seems to favor a more complete extraction of desired aromatic compounds (higher intensity and clarity), while with demanding setups (Hamasho, very light roast), the benefits only come to fruition with optimal adjustment.

The results of the sensory evaluation correspond with the analytical data in that a changed proportion of fines and flow behavior indeed have sensory effects – for example, more fines could tend to lead to more astringent and bitter notes, while fewer fines and a more even flow emphasize finer aromas. These correlations are discussed in the next section.

Discussion

The present results confirm that slow feeding has a significant impact on grind and extraction. The mechanisms and implications are interpreted in detail below.

Effects on Grind and Extraction Behavior

Slow feeding essentially relieves the burden on bean grinding: Instead of many beans being pressed between the burrs simultaneously, they pass through the grinding zone individually or in small groups. Our particle analysis showed an associated reduction in the proportion of fines by ~5–10% absolutely (relatively ~20–30% fewer fines). This means that during slow grinding, fewer particles are severely shattered, and more particles remain in the comparatively coarse range. One explanation is that individual beans in the grinding gap experience less shear forces from neighboring beans – they are crushed, but possibly not further fragmented by repeated collisions. In addition, each bean can be processed by the motor at a constant speed and torque with a lower feed rate. Gagné noted that bean-by-bean grinding on his experimental setup allowed for a more consistent grinder RPM, which contributed to a slightly tighter distribution. In our setting (DF64 direct grinder), the motor's performance during normal feed was sometimes audible as "struggling" and thus possibly slightly slowing grinding behavior under full load. With slow feeding, the grinder hums at a continuous tone and seems to grind the beans effortlessly. The beans may spend less time in the grinding chamber, resulting in less friction and "grinding cycles." The result is a coarser median and less fines.

The consequence for the coffee puck is higher porosity or permeability. Fewer fines mean more voids between the particles and a lower proportion of "quasi-colloidal" substance that could clog the pores. Smrke et al. supported this: additional fines clog the pore space and significantly reduce permeability. Our flow data are fully consistent with this – slow-feed shots literally shot through the puck at identical grind size because the water resistance was lower. Interestingly, despite strongly differing flow rates in scenarios A vs. B (26 s vs. 10 s), the total extracted solubles were proportionally lower (~17% yield), suggesting that underextraction indeed occurred, and not merely the same amount of extraction was achieved in a shorter time. The water simply had too little contact time and surface area to dissolve sufficient ingredients.

Puck Resistance and Flow Dynamics

A more homogeneous, but coarser, grind behaves differently under pressure. On the one hand, pressure builds up more slowly in the brew chamber (because water seeps through faster, more liquid initially escapes before full pressure is applied). On the other hand, the puck may tend to remain more stable because fewer fine particles migrate. In espresso pucks, fines often play the role of a "cement" that, deposited in upper layers, influences flow (fines can migrate and cause local density differences). Fewer fines could therefore also counteract channeling, provided the particle distribution is otherwise evenly distributed in the puck. Our observations with Chirinos support this: the slow-feed puck apparently showed no channeling, although it was ground extremely fine – possibly because the lower absolute number of fines did not lead to local blockages and pressure peaks that favor channels to the same extent. Apas also benefited in some ways: while there was no obvious channeling problem in the standard, slow feeding made it possible to grind finer (MG ~12 instead of 21) without overpressure or extraction breakdown – the puck withstood the fineness, presumably because the proportion of fines remained relatively moderate.

However, there is also a limit: if, as required for Hamasho, you have to go extremely fine to even reach ~25 s, then the absolute proportion of fines and thus potentially the negative effects (clogging, channeling) increase again.

In Hamasho's case, an extraction with slow feeding into the grinder was only successful with a very fine grind, which effectively meant that the puck contained significantly more fines than without slow feed. This complicated the entire extraction behavior, as no intermediate grind setting could be found that provided sufficient resistance in the puck, but then did not lead to clogging, or was not too coarse and led to premature destruction of puck integrity (extraction times under 18 seconds, despite fine grind).

For Apas and Chirinos (medium and medium-light roasts), the grind size could be significantly refined, and stable flow behavior was still achieved – here, the proportion of fines was virtually "replaced" by overall finer grinding, which apparently led to a more efficient extraction of desired substances (higher clarity, sweetness, etc.) without drifting into over-extraction. For Hamasho (lighter, difficult to extract), however, slow feed required a grinding spectrum that was almost outside the sensible range (very many fines needed to have enough resistance, which negated the hoped-for advantages).

Additionally, it must be considered that slow feeding can change the dosing volume. Coarser particles mean a lower bulk density – the puck could be fluffier and more voluminous for the same mass, which increases the puck height. This, in turn, can influence extraction (higher puck = longer path, different pressure distribution).

It is striking that all three mentioned beans, as well as Limontitla, which was also tested, tended to lose their puck integrity significantly faster. This was observed in extractions with the Decent espresso machine and led to higher flow and lower back pressure. Even the Apas espresso, which as a medium roast should show higher puck stability, behaved like a lighter roast from the middle of the extraction. As conclusions and subjects for further investigations, adaptive flow profiles or profiles that significantly reduce pressure from the middle of the extraction should be used here. It is possible that coffees ground with slow feeding and having an overall lower median grind size release a large part of their extraction potential significantly faster. Then, an adapted flow might wash out fewer unwanted notes from the fine particles that contribute to bitterness and astringency.

Static Charge and Particle Dispersion

One aspect closely linked to slow feeding is the avoidance of scattering. With a full bean load, many particles are generated at once, colliding with each other and with surfaces – ideal conditions for triboelectricity. That slow feeding may reduce static charge is an interesting side effect: Since only a few particles are continuously generated, charges could dissipate more easily into the grinder's metal before large accumulations form. Our experiments did not directly measure this, but indirectly, the grind dispersion with slow feed was visibly different: the coffee grounds landed more evenly in the cup/portafilter and formed fewer clumps. This aligns with Hendon's research, which suggests that a reduction in charging reduces clumping.

Although we did not use water RDT, slow feeding may work similarly by reducing the collision frequency of particles, thus generating less charge. Hendon et al. showed that even a small change in conditions (e.g., more moisture) noticeably improves espresso quality – in our context, slow feeding could therefore also contribute to sensory clarity because the grounds fall looser and unclumped into the portafilter. An unclumped coffee bed can be distributed and tamped better, making the puck density more homogeneous. The result is more even extractions and fewer zones of different flow rates.

To verify this hypothesis, we have ordered a meter for electrostatic charge.

Influence of Bean Format: Popcorning vs. Bean Slide-down

Slow feeding should contribute to a more even particle distribution, especially with single-dose grinders. While in espresso grinders with bean hoppers, beans are continuously pushed down, and thus "bean pressure" is exerted on the burr and the lower beans throughout the grinding process, this is not the case with single dosing. Towards the end of grinding, we always see some popcorning when only the last straggling beans jump on the burr.

We can infer the extent of the effect on particle distribution with only a few beans in the grinding chamber from our slow feeding measurements. Single dosing therefore always results in a more uneven grind distribution than grinders with bean hoppers or when using the slow feeding method.

Interestingly, newer grinder designs (e.g., Weber EG-1, Lagom P64 with optional slow feeder) address this issue.

It is sometimes argued that slow feeding reduces frictional heat in the grinding chamber. If high temperatures develop in grinders during grinding, this can negatively affect taste (temperatures above 40 degrees are required for this).

We were able to rule out this conclusion with a series of measurements. For this purpose, we measured the temperature of the dispensed grounds both after grinding with slow feeding and without. In a "normal" home use (we checked 6 shots in a row), a slight increase in temperature was noticeable over the shots for both feeding methods. However, the temperature of the grounds obtained with slow feeding was only on average one degree colder than the "normal" grinding. With this small difference, we do not assume an effect that has taste implications.

Theoretical Classification of Mechanisms

If you put all the puzzle pieces together, the following hypothesis emerges: Slow feeding changes the grinding conditions from a highly dynamic, stochastic multi-bean system to a quasi-stationary single-bean system. In a multi-bean system (normal dosing), there are intense bean-to-bean interactions: crushing, rubbing, abrasion of cellulose and roast fragments – many fines are created early in the grinding process and can even be re-ground. In a single-bean system, on the other hand, each bean is largely ground in isolation; once through, it exits the gap before the next one arrives. This results in fewer additional fines per bean. The particles that are created tend to be the "natural" fragments of the bean, without these fragments being further ground into dust because the next bean does not immediately push through. This picture is consistent with the finding that roller mills (multi-stage gentle grinding) produce fewer fines – slow feeding, in a way, mimics part of this effect by giving the disc grinder more control and less randomness.

Conclusion

Our scientifically sound investigations prove that slow feeding – the slow supply of beans during grinding – has a real and relevant influence on grind quality and espresso extraction. In summary, we were able to show:

• Slow feeding significantly reduces the proportion of fine particles in the grinds with the same grinder setting, which indicates gentler fragmentation of the beans.
• This results in higher permeability of the coffee puck: the flow rate increases, and espresso flows through faster. To achieve the desired extraction time, a significantly finer setting is necessary.
• With an adjusted grind size, slow feeding enables espressos with equally high extraction yields. Initial sensory results indicate more complex aroma structures, better flavor balance and more transparency, partly at the expense of texture and body, as well as some dryness in the aftertaste. Further tests are needed, and blind tastings with larger panels are necessary.
• Since there are initially fewer fine particles with slow feeding, the grind size must be set significantly finer. This can lead to problems. On the one hand, many grinders are not able to grind reproducibly that fine. On the other hand, the entire structure of the coffee cake changes during extraction, and thus also the puck integrity. For some coffees, an even extraction with a traditional 9-bar flat profile was almost impossible to achieve without channeling. It needs to be further investigated here whether brewing profiles with falling pressure or flow lead to desirable results.
• Possible negative effects (underextraction) occur especially when the grind size is not adjusted accordingly. A certain amount of fines also seems necessary for a balanced extraction ("happy medium of fines" according to Rao).
• The benefits of slow feeding depend on the roast level and bean profile: For straightforward roasts, it can increase clarity; for very light roasts, correct adjustment is more critical to achieve sufficient extraction at all.

Relevance for practice: For roasters and baristas in the specialty coffee scene, these results offer valuable insights. Grinder designers could integrate slow-feed mechanisms – some manufacturers are already doing this, such as Option-O with the recently introduced Preliminary Crushing System (PCS). Our data confirm that such innovations are not just gimmicks, but can bring measurable quality advantages. Even without special equipment, a barista can experimentally feed beans more slowly or even try "single bean grinding" to potentially improve extraction for problematic shots. However, it must be considered that slow feeding slows down the workflow – in a busy café environment, it is not always practical to spend 30–60 seconds per shot on grinding alone. Here, a balance must be struck: For a championship shot or perfecting a high-end espresso, slow feeding can be a valuable tool, but in everyday life, benefits must be weighed against time investment.

Who benefits from slow feeding? Of course, slow feeding opens up another factor that can influence taste. If you are currently dealing with other elements of espresso preparation, then don't open another door in parallel. The basis for everything is good standard recipes and perhaps dealing with flow and pressure profiles. However, if you are already advanced with these topics, or your machine perhaps does not offer such possibilities, then slow feeding could be an exciting field of experimentation for you.

Please write in the comments what you discover!

• Our coffee

Sources:

The central data in this article was collected by our Kaffeemacher test team at the House of Coffee in Basel. Andrea Perin, barista trainer at our coffee school, should be mentioned as the leading figure, who pulled all samples and espresso shots. The test series was led by Michel Indelicato as head of our coffee school and Benjamin Hohlmann, as the author of this report. Sensory analyses were carried out by sensory experts, Q-Graders and national barista champions (Nadja Schwarz, Michel Indelicato, Felix Hohlmann, Philipp Schallberger, David Wistorf, Benjamin Hohlmann).

The discussion was based on current scientific papers and expert reports, including those by

• Hendon et al. (2024) on electrostatic charging during grinding (Grinding coffee with a splash of water reduces static electricity and makes more consistent and intense espresso | ScienceDaily)
• Gagné's analyses on the popcorning effect (Grind Quality and the Popcorning Effect – Coffee ad Astra) as well as
• Smrke et al. (2024) on the role of fine particles in espresso extraction (The role of fines in espresso extraction dynamics | Scientific Reports).
• Lance Hedrick's recent YouTube video on slow feeding should also be mentioned.

These and other sources are referenced in the text.

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