ArticlePDF Available

The effect of bean origin and temperature on grinding roasted coffee

Springer Nature
Scientific Reports
Authors:

Abstract and Figures

Coffee is prepared by the extraction of a complex array of organic molecules from the roasted bean, which has been ground into fine particulates. The extraction depends on temperature, water chemistry and also the accessible surface area of the coffee. Here we investigate whether variations in the production processes of single origin coffee beans affects the particle size distribution upon grinding. We find that the particle size distribution is independent of the bean origin and processing method. Furthermore, we elucidate the influence of bean temperature on particle size distribution, concluding that grinding cold results in a narrower particle size distribution, and reduced mean particle size. We anticipate these results will influence the production of coffee industrially, as well as contribute to how we store and use coffee daily.
Content may be subject to copyright.
1
Scientific RepoRts | 6:24483 | DOI: 10.1038/srep24483
www.nature.com/scientificreports
The eect of bean origin and
temperature on grinding roasted
coee
Erol Uman
1
, Maxwell Colonna-Dashwood
2
, Lesley Colonna-Dashwood
2
, Matthew Perger
3
,
Christian Klatt
4
, Stephen Leighton
5
, Brian Miller
1
, Keith T. Butler
6
, Brent C. Melot
7
,
Rory W. Speirs
8
& Christopher H. Hendon
6,9
Coee is prepared by the extraction of a complex array of organic molecules from the roasted bean,
which has been ground into ne particulates. The extraction depends on temperature, water chemistry
and also the accessible surface area of the coee. Here we investigate whether variations in the
production processes of single origin coee beans aects the particle size distribution upon grinding.
We nd that the particle size distribution is independent of the bean origin and processing method.
Furthermore, we elucidate the inuence of bean temperature on particle size distribution, concluding
that grinding cold results in a narrower particle size distribution, and reduced mean particle size. We
anticipate these results will inuence the production of coee industrially, as well as contribute to how
we store and use coee daily.
Second only to oil, coee is the most valuable legally traded commodity. ere are two biologically dissimilar
species of coee grown for consumption; Coea canephora (robusta) and Coea arabica (arabica)
1
. Whilst robusta
is both less chemically complex and less avoursome than arabica, it benets from being feasibly grown at low
altitude and is pest resistant. However, over 60% of the global coee consumption is of arabica. In 2014, Brazil
and Colombia combined to produce over 3.5 million tonnes of green arabica
2
, with Ethiopia and other African
and Central American producers also making signicant contributions. Including countries like Vietnam which
almost exclusively produces robusta, global coee production amounts to 8.5 million tonnes annually.
With the exception of unusual green coee medicinal and dietary preparations, coee is not typically con-
sumed as a solid but rather an extract from the roasted seed
3–9
. Coee beans are imported, roasted, ground
and then brewed (including instant coee) in coee shops and homes. In such a valuable industry, the quality
and yield of the product is paramount. However, there are many variables that inuence the avour, yield and
overall enjoyment of this mass consumed beverage
10
. e challenges associated with ensuring coee quality can
be divided into two categories i) variables associated with the country of origin and ii) variables associated with
consumption.
Besides typical botanical inuences including climate and altitude, there are two general considerations that
aect the coee at the origin: the variety of coee (e.g. Typica, Pacamara, Geisha)
11
and the processing method
(i.e. washed, pulped and natural). e variety denes chemical characteristics of the bean, and also the conditions
in which it may be grown. Ideally, the fruit of the coee bean should not ripen more rapidly than the ovum devel-
ops, otherwise the seed is lacking chemical complexity. Conversely, the fruit should be able to ripen in variable
climate conditions thereby permitting the formation of the seed. Genetic variety hybrids are now ubiquitous and
oen feature the best of both of the parent varieties
12,13
.
Irrespective of the variety, all coee is processed in one of three general methods. e washing (or wet) process
is the most common, and uses water to remove the skin and fruit of the cherry, leaving only the seeds to dry in the
1
Meritics Ltd., 1 Kensworth Gate, Dunstable, LU6 3HS, United Kingdom.
2
Colonna and Smalls, 6 Chapel Row,
Bath, BA1 1HN, United Kingdom.
3
St Ali/Sensory Lab, 12-18 Yarra Pl, South Melbourne, Victoria, 3205, Australia.
4
Mahlkönig GmbH & Co.KG, Tilsiter Str. 142, 22047 Hamburg, Germany.
5
Has Bean Coee Ltd., Unit 16, Ladford
Covert, Staord, ST18 9QL, United Kingdom.
6
Department of Chemistry, University of Bath, BA2 7AY, Bath, United
Kingdom.
7
Department of Chemistry, University of Southern California, Los Angeles, CA 90089, United States of
America.
8
School of Physics, The University of Melbourne, Victoria, 3010, Australia.
9
Department of Chemistry,
Massachusetts Institute of Technology, Cambridge, MA 02139, United States of America. Correspondence and
requests for materials should be addressed to C.H.H. (email: hendon@mit.edu)
Received: 11 December 2015
Accepted: 16 March 2016
Published: 18 April 2016
OPEN
www.nature.com/scientificreports/
2
Scientific RepoRts | 6:24483 | DOI: 10.1038/srep24483
sun. e pulped (pulped natural) processing method removes the skin from the cherry, but does not fully remove
the mucilage. is then forms a sun-hardened sugar-rich shell around the parchment (the thin protective layer
for the seed). e natural process is simply the sun-drying of the coee cherries with both seed and fruit intact.
Whilst the processing method used has a profound impact on avour, the chemical mechanisms which dic-
tate these dierences are not well-understood. Regardless of the cherry processing method, aer drying the
beans are hulled, which exposes the bean by removing all the dry parchment, mucilage, or skin. e green coee
beans are then transported to roasteries, where the roaster develops a roast prole with the aim of producing the
most avoursome cup to their palate. e roast prole is a two variable problem of temperature and time, but
due to limitations of roasting equipment and the inhomogeneity of heat transfer into green coee
14
, the devel-
opment of a roast prole is more artistic than scientic, although there is certainly room for improvement in
this area.
e roast prole presented in Fig.1 shows the measured roaster temperature as the roasting progresses for the
particular Tanzanian coee listed in Table1. e chemical constituents of roasted coee depend on the tempera-
tures of green coee molecular decomposition. e generation and concentration control of these compounds is
achieved through ne tuning of the roast prole
15–17
. Whilst most compounds in roasted coee are likely Maillard
products (an example of which is not shown in Fig.1)
18
, we present various pathways that permit the formation
of acids, phenolic compounds, and also the cleavage of cellulose into sugar-related products like levoglucosan.
e le-most process in Fig.1 shows an example of decomposition of a chlorogenic acid (a group of molecules
contributing to 66% of the acidity in green coee) through low temperature hydrolysis, in which the formation of
products depend on the water content within the seed
19,20
.
Undoubtedly the extent and quality of extraction is dictated by the accessibility of the organic molecules con-
tained within roasted coee. Many factors inuence the total amount, and relative proportions of the dierent
organic molecules extracted, including temperature of brew, water chemistry and water-to-coee ratio
21–24
. Here,
however, we are specically concerned with physical method of increasing accessible surface area; i.e. the eect
of the grinder.
Figure 1. e roast prole for the Tanzanian Burka (Has Bean). In this case, 10 kg of the Burka coee
was roasted in a 12 kg Probat Roaster. e temperature was monitored with a probe in the headspace of the
oven, and hence the hot air rapidly cools due to thermal energy transfer to the green coee. e temperature
trajectory throughout the roasting process determines the decomposition of organic materials in coee. ree
illustrative decomposition reactions are shown that are representative processes throughout the heating process.
At lower temperature a chlorogenic acid (le) may decompose through either hydrolysis or pyrolysis into quinic
acid, acetic acid and the phenolic compound 3,4-dihydroxybenzyl alcohol
40
, or quinic acid, carbon dioxide and
3,4-dihydroxystyrene
41,42
. Oxalic acid (centre) may decarboxylate to either CO
2
or in the case of incomplete
combustion CO
2
and formic acid
19
. At higher temperatures cellulose can undergo hydrolysis to smaller sugar
derivatives including glucose and levoclucosan
43–45
. Both the temperature and time determine the chemical
composition of the roasted coee: In this case, the coee was removed from the oven aer 9 m 54 s as this time
was determined to result in a soluble, sweet and favourably acidic product.
Farm/Estate Origin Varie ty Roaster Roast Agtron colour
Las Ilusiones (W) Guatemala Caturra and Bourbon Round Hill Espresso 62
Santa Petrona (W) El Salvador Pacamara Has Bean Espresso 59
Burka (W) Tanzania Red Bourbon Has Bean Espresso 59
Sasaba (N) Ethiopia Mixed Heirloom James Gourmet Filter 68
Table 1. Details on the four coees (Coea arabica) that were ground in this experiment: two African and
two South American. (W) and (N) indicate washed and natural processing methods, respectively. ‘Roast’ is used
as an indication for whether the coee was roasted for lter (lighter) or espresso (darker) style coee, and can be
quantied by the ‘Agtron colour’ as determined by the Agtron spectrophotometeric measurement
46
. All coees
examined here would be considered light/medium roasted relative to typical commodity grade coee.
www.nature.com/scientificreports/
3
Scientific RepoRts | 6:24483 | DOI: 10.1038/srep24483
Whilst routine in the pharmaceutical industry, it is challenging to both design and execute a grind to a homo-
geneous particle size in a coee shop. is, however, is of critical importance in coee brewing because variable
accessible surface area causes the small particles to extract more rapidly relative to larger ones. As a result, brew-
ing coee is challenging with variable particle size, especially in espresso-style pressurised brews, where packing
eects become important
25,26
. Given the importance of particle size, we assess if bean origin, cherry processing
method, and roast prole have any signicant eect on the particle size distribution of the ground coee.
Additionally, it was suspected that the temperature of the beans could also inuence the bean fracturing
dynamics, and therefore the nal size distribution. Whilst ideally the beans and burrs would both be brought to
the desired temperature, controlled active heating or cooling of the burrs is not presently feasible. To investigate
the temperature eects we pursued the controlled cooling of the coee itself. Given that many people store coee
in the refrigerator or freezer (if devoid of water vapour this is a chemically reasonable method of storage), we
examine if varying bean temperature results in an observable modulation of grind distribution.
Methods
For this study, it was assumed that the most important property of the ground coee which can vary in the grind-
ing process is the distribution of particle sizes. Whilst it is possible that particle shape may have an eect on the
nal extracted brew, it is dicult to see how this can be reliably controlled on the micrometer scale, and it is likely
that most ground coee has a similar spread of particle shapes.
e rst set of experiments explored if the origin, type, or processing method of the bean had any eect on the
particle size distribution, when ground under identical conditions. e second set of measurements explored if
bean temperature at the time of grinding had any eect on produced particle size distribution.
To probe these eects we employed laser diraction particle size analysis of roasted coees ground on a
Mahlkönig EK 43 coee grinder.
Laser diraction particle size analysis. e laser diraction particle size analysis was performed on the
multiple wavelength Beckman Coulter LS13 320 MW. e instrument has a built in dark eld reticule which is
used to ensure correct optical alignment. An alignment check was carried out prior to every run to ensure the
optimum accuracy of the particle size distribution. Particular care was taken to ensure correct optical alignment
because ground coee contains particles sizes spanning 3 orders of magnitude, including components larger
than 100 m, which can be challenging to measure with diraction based techniques since they rely on distance
measurements in reciprocal space.
Grinding. e Mahlkönig EK 43 grinder, shown in Fig.2, was selected for this study because it is designed to
have minimum retention time between placing the coee in the hopper and subsequent grinding. Like all grind-
ers, the EK 43 burrs are replaceable and are susceptible to becoming misaligned (where the two grinding discs
are not perfectly parallel). We had access to three separate EK 43 s on the day of testing; two which were tted
with so-called coee burrs and one with Turkish burrs (Fig.2). Burr alignment can initially be assessed audibly
by closing the burr aperture with the grinder turned on, causing them to ‘chirp. e pitch of the chirp provides
insight into the alignment, with deeper chirps indicating more contact between the burrs and therefore better
alignment. Assessing the smoothness and spread of ground particle distribution can also give information on
burr alignment, though it is dicult and slow to reliably adjust alignment based on this information. We have
Figure 2. e EK 43 grinder, (a) consists of two burrs; one stationary and one mobile. e hopper-to-shoot
path is linear resulting in minimal retention of ground coee in the burrs and shoot. ere are two types of
burrs: Turkish, (b) and Coee, (c). e primary dierences between the two burrs are emphasised in blue in
(b,c), respectively. e at triangular ends are intended to polish the particulates. For this study we employed
the Turkish burr set. Photographs taken by Spencer Webb.
www.nature.com/scientificreports/
4
Scientific RepoRts | 6:24483 | DOI: 10.1038/srep24483
provided one example of particle size distributions from a burr misalignment in Figure S1. Ultimately, we elected
to use the grinder that was producing subjectively marketable espresso shots that day, as determined by a resident
qualied Q-grader and shop owner (Maxwell Colonna-Dashwood)
27
. Experiments herein were performed with
the Mahlkönig EK 43 grinder spinning at 1480 rpm and grinding with Turkish burrs.
Coee origin and processing. To determine if bean origin has an eect on particle size distribution aer
grinding, beans were tested from four countries: Guatemala, El Salvador, Tanzania, and Ethiopia. e beans had
been roasted by roasteries listed in Table1, between seven and sixteen days prior to the grinding test, and so had
sucient time for CO
2
degassing, but were still considered ‘freshly roasted’. Further details of the four coees
considered in this study are presented in Table1. All beans were allowed to equilibriate to room temperature (at
the time, 20 °C and 79% relative humidity), densities of the roasted coee beans were not measured. e grinder
burr aperture was kept constant for all coees throughout the experiment, xed at 2.7 (arbitrary units) on the
stock EK 43 dial. For each measurement, 20 grams of coee was ground, and the grinder was allowed to cool for
10 minutes aer each grind (returning to room temperature).
Coee temperature. For temperature studies, we selected the Guatemalan coee because this particular
Guatemalan crop is representative of contemporary speciality grade coee (i.e. it has a favourable balance of acid-
ity, oral complexity and overall taste). e four temperatures were achieved using the following method: 20 g of
whole roasted coee beans were placed into a paper cup, covered, and placed into either liquid nitrogen, a tub of
dry ice, the freezer and on the counter top. No visible condensation of atmospheric water was observed on any
of the samples cooled below 0 °C. e beans were equilibriated at each temperature for 2 hours prior to grinding.
e grinder was switched on 5 seconds before grinding and the beans were taken directly from their climates
and fed into the hopper. e EK 43 is rated to grind 1200–1500 g/min, suggesting each 20 g dose of coee was
exposed to ambient conditions for no longer than 1 second. To prevent condensation of atmospheric water onto
the surface of the ground coee, the ground particulates were immediately placed into sample vials for laser dif-
fraction particle size analysis. Absorption of atmospheric water proved not to be a problem, as duplicate samples
which were exposed to atmospheric moisture as they equilibrated to room temperature, showed no dierence to
those that where sealed immediately upon grinding.
Each data set was obtained in triplicate, and each temperature was obtained in duplicate thereby generating
6 data sets per temperature. ANOVA was employed for determination of similarities in particle number distri-
butions with consideration of the bean origin, processing method, roast and roaster included. e output of this
statistical analysis is included in the supporting information.
Do Dierences in the Green Bean Aect the Final Grind?
e physical structure of roasted coee beans is a complex composite of materials, containing high molecular
weight brous molecules interspersed with amorphous and partially crystalline domains of a vast array of smaller
organics. e extremely complex structure of both the roasted beans and grinding apparatus makes accurate
rst principles modeling a daunting prospect, and so fracturing is best studied experimentally (in line with pre-
vious studies of grinding other amorphous materials)
28–32
. at said, it could well be expected that the specic
mix of chemicals that give dierent coees their distinctive avour may change the way in which the bean is
fragmentised.
To investigate this, we elected to sample four speciality grade coees. e selection spans the variables of ori-
gin, variety, processing method and roast prole, and is a representative cross section of contemporary speciality
coee. e four coees described in Table1 were ground at ambient conditions using the stipulated methods.
Here we are concerned with the deviations in grind prole as a function of coee origin, although before
embarking on these experiments it was unclear what the grind prole looked like. e EK 43 produces particles
ranging from 0.1 m to 1000 m, and whilst we have elected to present most of the data on a logarithmic scale,
the linear scale is shown for the Tanzanian coee in the upper panel of Fig.3. All grind proles appear as a
skewed-Gaussian shape. In this case, we present the particle number distribution in the shaded blue region, and
the integral in grey. We can arbitrarily dene the ne particulate cuto, graphically represented as a purple dashed
line = n where:
.=099cumulativenumber
(1)
n
0
Here, n is a diameter in m. From the upper panel of Fig.3, the Tanzanian n = 70 m (mode = 13.0 m, where the
mode is the most frequent size occurrence). Given the skewed nature of the distribution, the mode is helpful in
assigning key features of the distribution. However, it is not only the number of particles that contributes to the
extraction of coee, but also the available surface area obtained from these particles.
e grind proles for the four coees examined here are shown in the middle and lower panels of Fig.3. ey
are presented on a logarithmic scale to accommodate the surface area contribution from the large particles. e
surface area is estimated using a spherical approximation for the particles
33
, and is shown by the dotted line. Here,
the data appears distinctly bimodal because the ne particulates contribute to the majority of the accessible sur-
face area (modes ii and v), but large particulates (one/two orders of magnitude larger in diameter, iii and vi) are
also present. ese have an inuence even at low concentrations.
ere are minor dierences in the grind proles: e proles shown in black and purple share similar par-
ticle number modes (i), and have a ne particulate cuto of 76.4 ± 3.5 m. e proles shown in red and blue
produced a slightly ner particle distribution with a number mode (iv) 1.3 ± 0.7 m) more ne than the black/
purple coees, and a ne particulates cuto of 69.6 ± 3.1 m. In summary, the coees appear to produce a very
similar grind distribution irrespective of the variables associated with bean production. Full ANOVA details are
www.nature.com/scientificreports/
5
Scientific RepoRts | 6:24483 | DOI: 10.1038/srep24483
presented in Table S1. It should be noted that all of the beans considered here are roasted relatively ‘light’ com-
pared to typical consumer grade coee (although on the ‘Agtron Gourmet Scale, these coees all are catagorised
as light-medium roast). We can only speculate how heavily decomposed beans (e.g.dark’ or French roast) may
deviate from these results; further experimentation is required to elucidate that eect.
For espresso, the coee grinds can be thought of as a granular material, where the increase in pressure during
tamping jams the materials
34–36
. e variability in particle size plays a signicant role in the accessible surface
area, but also in the vacuous space in which the water may ow through. From the work of Herman
37
, it is appar-
ent that large particles install signicant order of neighbouring small particles, which increases local density and
therefore can result in inhomogeneous water ow through the espresso puck. However, given the subjectivity of
coee avour and the preferences of practitioners working in the industry, it is not clear if there is an ideal particle
size distribution: We only hope to shed light on the surprising consistencies between coees.
Do Dierences in the Roasted Bean Grind Temperature Aect the Final Grind?
Temperature changes in amorphous materials can lead to well dened glass transitions, where the material
changes from rubbery and exible to being hard and brittle
38
. Some solids can also undergo shattering transi-
tions, where there is an increased fragmentation rate as particle size decreases, resulting in production of greater
numbers of ne particles
39
. is property is instigated by both temperature and crack velocity. It is understood
that crystalline materials progress towards this shatter transition point with decreased temperature, because the
strain on the lattice becomes proportionally larger with decreased lattice kinetics. However, roasted coee is a
complex material and glass or shattering transition points are unlikely to be constant across macroscopic regions
of the bean, if present at all. erefore, while it is reasonable to expect that a change in temperature will aect the
grinding result, describing how and why this occurred is problematic. Experiment provides the simplest and most
reliable route to assessing how temperature inuences ground coee particle size.
e lower the original bean temperature, the colder the produced particles will be at every stage of grinding.
However colder bean fragments will absorb heat from their surroundings more quickly due to the larger tem-
perature gradient, eectively reducing the indicated temperature dierence between the samples. erefore, the
observed change in grind prole should be considered a lower limit on the eects of grinding at reduced tempera-
tures. Given the inhomogeneous nature of the beans, it is likely that cooling the burrs (and hence further reducing
the temperature of the particles as they are ground) would smoothly continue the trend observed in Fig.4.
Some fraction of particles are produced in their nal size from the initial fracturing of the whole bean (or large
portion thereof), and so are truly produced at the stated temperature. However, experiments using a single impact
event (i.e. hitting a cold bean with a mallet), show that only a small amount of small particles are produced on
initial bean fracturing, so most particles do have some time for thermalisation before further fracturing occurs.
0
1
0
1
110 100 1000
Particle Diameter (µm)
Relative particle number frequency
Surface Area
Counts
0
1
05
01
00
Particle Diameter (µm)
70
number
iiiiii
iv vvi
1%
0
1
0
1
0
1
Relative surface area contribution
Figure 3. Upper panel: e particle size distribution as a function of number (cumulative) of physical
particles (shown in blue) and the integral of this data (shown in grey), yields 99% of the particles with a
diameter of 70 μm or less. e ne particular cuto is depicted as a purple dashed line. Middle and lower panel:
e grind proles of the four coees examined here. e cumulative number and surface area contribution are
shown in solid and dashed lines, respectively. e Tanzanian, Ethoipian, El Salvadorian and Guatemalan proles
are shown in black, purple, red and blue, respectively. Data modes i-vi are included for visual aid: i - 14.3, ii -
27.4, iii - 282.1, iv - 13.0, v - 27.4 and vi - 256.9 m.
www.nature.com/scientificreports/
6
Scientific RepoRts | 6:24483 | DOI: 10.1038/srep24483
Even with some particle thermalisation due to room temperature burrs, the initial bean temperature has a
signicant eect on the modal particle size distribution (Fig.4a) reducing the mode by 31% as the beans are
cooled from room temperature to 200 °C, as shown in Fig.4b. Additionally, the distribution generally becomes
narrower as the beans are cooled (Fig.4c) with the biggest change occurring between room temperature and
19 °C beans. e room temperature grind prole is also distinctly less Gaussian-like, with the development of
a hip at approximately 9.5 m. is detail could indicate that some components of the bean undergo a shattering
transition between 20 °C and 19 °C, and studies are ongoing into the origin of this feature.
To probe the reversibility of this transition, we performed the same room temperature experiments with coee
beans that had been cooled to liquid nitrogen temperatures and then allowed to reheat to room temperature. It
appears that if there is a transition, it is reversible as there were no notable dierences between the two samples.
is is not surprising given the very low water concentration in roasted coee: e thermal contraction and
re-expansion of coee did not play a signicant role in the grind prole obtained from either test set.
Applications and Concluding Remarks
In busy coee shops, it is common practice to reduce burr grinding aperture as the day progresses in order to pro-
duce a consistent cup of coee. From work presented here, we propose that this phenomenon is a direct product
of the grinding burrs (and potentially beans sitting in the hopper in grinders other than the EK 43) becoming
increasingly warm as the grinder is used. e particle warming at the interface between the coee bean and warm
burr - which can certainly be much higher in temperature than explored in this study - shis both the mode and
spread of the particle size distribution. us, as the grinder gets warm a ner grind setting may be required to
obtain the same eective surface area as the same coee ground on cooler burrs. However, we also observe a dif-
ference in the shape of the distribution with temperature, which indicates that simply grinding ner with warm
burrs will not produce the same result as grinding coarsely with cold burrs. e impact on taste and preference is
not the focus of this study, but is certainly an interesting avenue to explore in the future.
e distinct lack of dependence on origin and processing method is comforting for coee shops that serve cof-
fees from multiple origins, and also for roasters who develop and market blends (mixtures of origins). One grand
challenge with blended coee is to produce a product where each desired component is equally soluble, such that
the cup of coee tastes appropriately extracted. Consider the traditional blend of Brazilian and Ethiopian coees:
e two are combined to obtain the body and nuttiness from the Brazilian, and the fruit and complexity from the
Ethiopian. But such results are only obtained if both beans reach terminal extraction at similar rates. Here, we
1
0
Particle Diameter (µm)
-79 ºC -19 ºC
-196 ºC
20 ºC
110100 1000
Relative particle
number frequency
Surface Area
Number
11
13
15
-250 -150 -5
05
0
Bean Temperature (ºC)
Number
mode (µm)
9
Skewness
0.8
1.2
17
15
14
16
1
0
Relative surface
area contribution
Number
mean (µm)
a
d
b
c
Figure 4. e temperature dependence on the grind prole of the El Salvadorian coee, (a). e temperatures
were achieved by grinding liquid nitrogen, dry ice, freezer and room temperature coee, respectively. e
ne particulate cuto is schematically shown, with exact values corresponding to; 196 °C = 61 ± 3 m,
79 °C = 63 ± 3 m, 19 °C = 73 ± 3 m and 20 °C = 70 ± 3 m. e mode of the number distribution, (b)
shows a clear and non-linear trend of increasing mode with increasing temperature. e distribution skewness
is inversely proportional to temperature. From a avour perspective this is a favourable feature because the
surface area to volume ratio becomes increasingly signicant for the smaller particles. e mean particle size,
(d) is discontinuous with temperature likely indicating a transition between freezer and room temperature.
www.nature.com/scientificreports/
7
Scientific RepoRts | 6:24483 | DOI: 10.1038/srep24483
have minimised one variable by showing that at least the accessible surface area is kept constant whilst grinding,
thereby placing the chemical problems associated with blending solely on the roast prole.
From a physical chemistry perspective, the temperature dependence presents many interesting questions.
Given the minimal dierence between liquid nitrogen and dry ice temperatures and the reversibility of the cool-
ing, we question whether it is possible in the future to cryogenically store roasted coee at these temperatures.
Indeed, water content in the roasted bean is of paramount importance at these temperatures, as water expansion
may lead to be fracturing. Also, prolonged exposure to water can result in the solvation of avoursome molecules,
thereby decreasing the lifetime of the frozen product. But if these variables were managed, there are a host of
subsequent implications for the storage and relative quality assessment allowing for access to direct year-to-year
comparison of crop quality. From a consumption perspective, cooling of coee beans signicantly decreases the
rate of mass loss through volatile sublimation/evaporation. us, coee that is ground and brewed cold could
potentially demonstrate increased aroma and or avour in the eventual brewed cup.
From an industrial perspective, the yield of extraction is paramount. Grinding colder coee beans produces
a more uniform particle distribution, with a decreased particle size. While the decreased particle size will tend
to speed up extraction due to the larger surface area, the increased uniformity should minimise the amount of
wasted bean, which is discarded without being extracted to completion. Whilst active cooling of either the coee
beans or burrs is energy consuming, the benet of cold coee grinding may oset this cost with more ecient
extraction from the smaller particles.
References
1. Denoeud, F. et al. e Coee Genome Provides Insight into the Convergent Evolution of Caeine Biosynthesis. Science 345, 1181
(2014).
2. Http://www.ico.org/, International Coee Organisation, Date Accessed: 26/02/2016 (2016).
3. Farah, A., Monteiro, M., Donangelo, C. M. & Lafay, S. Chlorogenic acids from green coee extract are highly bioavailable in humans.
J. Nutri. 138, 2309 (2008).
4. Stelmach, E., Pohl, P. & Szymczycha-Madeja, A. e content of ca, cu, fe, mg and mn and antioxidant activity of green coee brews.
Food Chem. 182, 302 (2015).
5. Daglia, M., Papetti, A., Gregotti, C., Bertè, F. & Gazzani, G. In vitro antioxidant and ex vivo protective activities of green and roasted
coee. J. Agric. Food Chem. 48, 1449 (2000).
6. Cliord, M. N. in Chemical and physical aspects of green coee and coee products, 305 (Springer, 1985).
7. Charurin, P., Ames, J. M. & del Castillo, M. D. Antioxidant activity of coee model systems. J. Agric. Food. Chem. 50, 3751 (2002).
8. Shimoda, H., Sei, E. & Aitani, M. Inhibitory eect of green coee bean extract on fat accumulation and body weight gain in mice.
BMC Comp. Alt. Med. 6, 9 (2006).
9. Watan abe, T. et al. e blood pressure-lowering eect and safety of chlorogenic acid from green coee bean extract in essential
hypertension. Clin. Exp. Hyperten. 28, 439 (2006).
10. van Doorn, G., Colonna-Dashwood, M., Hudd-Baillie, . & Spence, C. Latté art inuences both the expected and rated value of
mil-based coee drins. J. Sens. Stud. 30, 305 (2015).
11. Https://www.stumptowncoee.com/varieties/, Stumptown Coee oasters, Date Accessed: 26/02/2016 (2016).
12. Carneiro, M. F. Coee biotechnology and its application in genetic transformation. Euphytica 96, 167 (1997).
13. Sera, T. Coee genetic breeding at iapar. Crop Breed. Appl. Biotechnol. 1, 179 (2001).
14. Fabbri, A., Cevoli, C., Alessandrini, L. & omani, S. Numerical modeling of heat and mass transfer during coee roasting process.
J. Food Eng. 105, 264 (2011).
15. Johnston, W. . & Frey, C. N. e volatile constituents of roasted coee. J. Am. Chem. Soc. 60, 1624 (1938).
16. Gianturco, M. A., Giammarino, A. S. & Friedel, P. Volatile Constituents of Coee-V. Nature 210, 1358 (1966).
17. Franca, A. S., Oliveira, L. S., Mendona, J. C. F. & Silva, X. A. Physical and chemical attributes of defective crude and roasted coee
beans. Food Chem. 90, 89 (2005).
18. Somoza, V. & Fogliano, V. 100 years of the maillard reaction: why our food turns brown. J. Agric. Food Chem. 61, 10197 (2013).
19. Flament, I. & Bessière-omas, Y. in Coee avor chemistry. (John Wiley & Sons, 2002).
20. Moon, J. ., Hyui Yoo, S. U. N. & Shibamoto, T. ole of roasting conditions in the level of chlorogenic acid content in coee beans:
Correlation with coee acidity. J. Agric. Food Chem. 57, 5365 (2009).
21. Hendon, C. H., Colonna-Dashwood, L. & Colonna-Dashwood, M. e role of dissolved cations in coee extraction. J. Agric. Food
Chem. 62, 4947 (2014).
22. Shimizu, S. Caeine dimerization: eects of sugar, salts, and water structure. Food Funct. 6, 3228 (2015).
23. Colonna-Dashwood, M & Hendon, C. H. in Water For Coee. (Self-Published, 2015).
24. Navarini, L. & ivetti, D. Water quality for Espresso coee. Food Chem. 122, 424 (2010).
25. Möbius, M. E., Lauderdale, B. E., Nagel, S. . & Jaeger, H. M. Brazil-nut eect: Size separation of granular particles. Nature 414, 270
(2001).
26. Hong, D. C., Quinn, P. V. & Luding, S. everse brazil nut problem: competition between percolation and condensation. Phys. ev.
Lett. 86, 3423 (2001).
27. Http://www.coeeinstitute.org/, Coee Quality Institute, Q-Coee System, Date Accessed: 26/02/2016 (2016).
28. itajima, ., Cai, G. Q., urnagai, N., Tanaa, Y. & Zheng, H. W. Study on Mechanism of Ceramics Grinding. CIP Ann. - Manuf.
Technol. 41, 367 (1992).
29. Hamdi, H., Zahouani, H. & Bergheau, J. M. esidual stresses computation in a grinding process. J. Mater. Process. Technol. 147, 277
(2004).
30. Inasai, I. Grinding of Hard and Brittle Materials. CIP Ann. - Manuf. Technol. 36, 463 (1987).
31. onig, W. A Numerical Method to Describe the inematics of Grinding. CIP Ann. - Manuf. Technol. 31, 201 (1982).
32. Li, X. & Bhushan, B. Measurement of fracture toughness of ultra-thin amorphous carbon lms. in Solid Films 315, 214 (1998).
33. Corrochano, B. ., Melrose, J. ., Bentley, A. C., Fryer, P. J. & Baalis, S. A new methodology to estimate the steady-state permeability
of roast and ground coee in paced beds. J. Food Chem. 150, 106 (2015).
34. Iwashita, . & Oda, M. in Mechanics of granular materials: an introduction. (CC Press Inc., 1999).
35. Li, X. & Li, X.-S. Micro-Macro Quantication of the Internal Structure of Granular Materials. J. Eng. Mech. 135, 641 (2009).
36. Corwin, E. I., Jaeger, H. M. & Nagel, S. . Structural signature of jamming in granular media. Nature 435, 1075 (2005).
37. Herman, A. Shear-jamming in two-dimensional granular materials with power-law grain-size distribution. Entropy 15, 4802 (2013).
38. Lubliner, J. in Plasticity eory, Ch. 2, e Physics of Plasticity Section. (Dover Publications, 2008).
39. Mcgrady, E. D. & Zi, . M. “Shattering” Transition in Fragmentation. Phys. ev. Lett. 58, 892 (1987).
40. Sugumaran, M., Semensi, V., Dali, H. & Nellaiappan, . Oxidation of 3,4-dihydroxybenzyl alcohol: A sclerotizing precursor for
cocroach ootheca. Arch. Insect. Biochem. Physiol. 16, 31 (1991).
www.nature.com/scientificreports/
8
Scientific RepoRts | 6:24483 | DOI: 10.1038/srep24483
41. Fran, O., Zehentbauer, G. & Hofmann, T. Bioresponse-guided decomposition of roast coee beverage and identication of ey
bitter taste compounds. Eur. Food es. Technol. 222, 492 (2006).
42. Lentner, C. & Deatherage, F. E. Organic acids in coee in relation to the degree of roast. J. Food Sci. 24, 483 (1959).
43. Minamisawa, M., Yoshida, S. & Taai, N. Determination of biologically active substances in roasted coees using a diode-array hplc
system. Anal. Sci. 20, 325 (2004).
44. Minowa, T., Fang, Z., Ogi, T. & Várhegyi, G. Decomposition of cellulose and glucose in hot-compressed water under catalyst-free
conditions. J. Chem. Eng. Jap. 31, 131 (1998).
45. aufman, P. B., Csee, L. J., Warber, S., Due, J. A. & Brielmann, H. L. in Natural products from plants. (CC Press Inc., 1998).
46. e Agtron value for roasted cofee is an spectrophotometeric measurement of reected near-I light. Higher values indicate lighter
roasted coee. For reference, an ‘Italian’ or dar-roasted bean typically has an Agtron value of ca. 30.
Acknowledgements
e authors would like to acknowledge Beckman Coulter and Meritics for their generous donation of the LS13
320 MW Laser Diffraction Particle Size Analyzer. Their equipment and expertise enabled this project. We
appreciate the contributions from Mahlkönig mbH, who provided insight into their grinder constructions.
All authors are also humbled by the generosity of Colonna and Smalls and their patient customers who were
deprived of their daily coee whilst these experiments were performed in store. We would like to acknowledge
the continued support and interest in our work from the global specialty coee industry. Finally the authors
are thankful for S. Webbs photography of the EK 43 burrs, and the lyrical insights from A. omas Murray and
C. Derek Molloy.
Author Contributions
E.U., B.M., M.P., C.K., L.C.-D. and M.C.-D. contributed to the experimental design and execution. S.L. roasted
the Has Bean coee and provided the roast prole. K.T.B., B.C.M. and R.W.S. aided in both experimental design
and data interpretation. C.H.H. conceived the study and wrote the rst dra of the paper. All authors contributed
equally to subsequent revisions of the paper.
Additional Information
Supplementary information accompanies this paper at http://www.nature.com/srep
Competing nancial interests: e authors declare no competing nancial interests.
How to cite this article: Uman, E. et al. e eect of bean origin and temperature on grinding roasted coee.
Sci. Rep. 6, 24483; doi: 10.1038/srep24483 (2016).
is work is licensed under a Creative Commons Attribution 4.0 International License. e images
or other third party material in this article are included in the article’s Creative Commons license,
unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license,
users will need to obtain permission from the license holder to reproduce the material. To view a copy of this
license, visit http://creativecommons.org/licenses/by/4.0/
... The paper [1] states that one of the most common problems in the preparation of coffee drinks in coffee machines is the unsatisfactory quality of coffee, which is associated with improper grinding of coffee beans, and the taste of the coffee drink depends on the size of the ground particles and the uniformity of coffee grinding. The paper [2] examines the influence of the origin of coffee beans and the temperature during the grinding of roasted coffee. It is noted that the extraction depends on the temperature, the chemical composition of the water, as well as the available surface area of the coffee. ...
... random residues due to three factors: grinding time, geometric grain size, grain moisture, respectively. A comparison of the residuals of the presented models with the residuals of models (1) and (2) shows that simplifying the model obviously reduces its accuracy, because these residuals are more than αβγδν ψ , а αβγδν ψ > i αβγδ ε . ...
Article
In order to assess the impact of each of the factors that affect the quality and uniformity of grinding coffee beans and to compare the impact of these factors, it is worth establishing a quantitative indicator of this impact. To solve this problem, dispersion analysis was used as a method of organizing sample data according to possible sources of dispersion. The chosen method made it possible to decompose the total dispersion into components caused by the influence of factor levels. Grinding time, geometric dimensions of the grain, moisture content of the grain, speed of rotation of the motor shaft were selected as factors influencing the homogeneity of grinding. The justification and assessment of the reliability of statistical conclusions about the informational significance of indicators affecting the homogeneity of coffee grinding was carried out to ensure the highest possible probability of the obtained result.
... The percentage of coffee solids extracted is determined by the rate and quality of the extraction. The extraction rate is dependent on the specific extraction surface (particle size), the temperature of the solvent/solid interaction, the agitation of the brewing matrix, and the solid/solvent ratio (Rao, 2010;Uman et al., 2016). The final solute concentration is then determined by the extraction rate and the extraction time (Rao, 2010). ...
... However, all the brewing parameters were identical between AWE and CWE in order to isolate the effect of the temperature of the brewing. The higher concentration of soluble solids in the AWE coffees, brewed at 25°C, than the CWE coffees, brewed at 4°C, agreed with previous literature that higher temperatures increase the extraction rate (Rao, 2010;Uman et al., 2016). ...
Article
Full-text available
The shelf‐life of cold and hot water extraction coffees based on sensory and chemical profiles and microbial growth was examined, which also allowed the study of the influence of extraction temperature on the chemical and sensorial profiles of coffee. The shelf life of refrigerated cold‐ and hot‐brewed coffee was limited not by microbial stability but rather by deterioration in sensory attributes. Further work is recommended to elucidate the mechanisms of coffee staling in a refrigerated environment, with particular interest in the degradation products of chlorogenic acid, as a significant decline in chlorogenic acid concentration was found over the storage period. Cold‐extracted coffees were found to be chemically and sensorially different beverages from coffees extracted at high temperatures. Additionally, the cold‐brewed coffees had greater sensory flavor stability over the storage time than the hot‐brewed treatment. Practical application This study advances the industry's understanding of the shelf life of ready‐to‐drink bottled cold coffees and demonstrates that lower brewing temperatures lead to greater flavor stability over shelf life. The findings also provide brewing parameters that can help guide product developers in modulating the flavor of commercial cold coffees.
... Recently, we demonstrated parameters that control charging by grinding commercially sourced coffees and measuring the charge-to-mass (Q/m) ratio of the grounds using the process presented in Figure 1A. 2 As whole beans are fractured into small grains with broad size distributions ( Figure 1B), 10 particles may acquire charge densities comparable to those of volcanic ash and thundercloud ice, through both fracto-and triboelectrification. 11,12 Generally, the polarity and magnitude of charge loosely depends on the roast level or color of a coffee, and we observed that dark roasts charge largely negative, and lighter roasts charge positively, Figure 1C. Residual moisture levels, a property typically inversely proportional to color, were found to be the primary determiner of charge polarity, where a positive-to-negative charging transition occurs at moisture contents less than $ 2%. ...
Article
Full-text available
Coffee grinding generates electrostatically charged particles, causing clumping, spark discharge, and beyond. When brewing, the particle aggregates affect liquid-solid surface accessibility, leading to variable extraction quality. Here, we study four charge mitigation strategies. De-electrification is readily achieved by adding small amounts of water to whole beans or by bombarding the grounds with ions produced from a high-voltage ionizer. While these techniques helped reduce visible mess, only water inclusion was found to impact coffee extracts prepared as espresso. Wetting whole beans with less than 0.05 mL/g resulted in a marked shift in particle size distribution, by preventing clump formation and preventing fine particles from sticking to the grinder. This particle size shift results in at least a 15% higher coffee concentration for espresso extracts prepared from darker roasts. These findings encourage the widespread implementation of water use to de-electrify coffee during grinding with the benefit of increased coffee extraction efficiency.
... According to the International Coffee Organization (ICO), 1.7 million 60 kg bags of coffee were consumed worldwide from 2020 to 2021, with coffee consumption being highest in Europe, followed by Asia and North America. 1 This demonstrates that coffee has a high economic status and is one of the most legally traded products worldwide. 2,3 The coffee production is dominated by the coffee species Coffea arabica L. (arabica), which occupies a percentage of 57%, followed by a production of 43% Coffea canephora Pierre ex A. Froehner (canephora). 4 Green coffee beans are produced in more than 70 countries, including cultivation, harvesting, and post-harvest treatment. ...
Article
Full-text available
Background Coffee contains a plethora of constituents with some of them being especially important either due to their physiological effects or as quality markers. As quantitative proton nuclear magnetic resonance spectroscopy (¹H‐NMR) has been established as a fast and reliable analytical tool its application was evaluated for the simultaneous quantitation of lactic acid, acetic acid, formic acid, caffeine, caffeoylquinic acid (CQA) isomers, N‐methylpyridinium, trigonelline, and 5‐hydroxymethylfurfural (HMF) in aqueous extracts of roasted Coffea arabica samples. Results Simultaneous quantitative determination was achieved by an automated analysis based on the PULCON methodology (pulse length‐based concentration determination). The method was validated regarding linearity, accuracy, precision, limit of detection (LOD), and limit of quantitation (LOQ). Recovery rates were between 76% (CQA) and 116% (HMF), and precision was between 1.7% (caffeine) and 10.3% (HMF). The LOD varied between 0.06 g/kg (HMF) and 1.35 g/kg (caffeine and CQA), with the LOQ being between 0.22 g/kg (HMF) and 4.87 g/kg (CQA). To verify the results of the ¹H‐NMR method, caffeine, trigonelline, HMF, 3‐CQA, 4‐CQA, and 5‐CQA were additionally quantitated by HPLC‐DAD and the results were compared. The described ¹H‐NMR method was additionally applied to coffee samples that contained different coffee defects. Results showed only slight changes in the concentrations of the analytes by adding defective beans to defect‐free coffee. Discussion The developed 1H‐NMR approach was proven to be fast (30 min), reliable, and precise. Thus, it is well suited to analyze several coffee constituents of interest in a large number of samples in, for example, quality control.
... The amount of caffeine in ground coffee is influenced by several factors; one of the factors that can affect it is the roasting process. As the volume of the coffee bean increases and the free space inside the coffee bean increases, the low temperature and short roasting time can lead to an increase in the caffeine content, which is beneficial for the extraction of caffeine crystals during grinding [24]. However, the caffeine content decreases with high temperature and prolonged time or when the roasting speed is increased because the larger lumen of the coffee bean allows the caffeine to be sublime easily. ...
... The distribution is controlled by grind setting and bean temperature. 28 In flat-burr grinder architectures such as the configuration housed within the Mahlkö nig EK 43 (a grinder with steel 98 mm burrs; Figure 1C), the grind setting is determined by the separation between rotating metal plates. Finer grind settings result in more fracturing events, longer coffee-burr contact time, and the production of more fines (sub-100 mm particulates) and smaller boulders (post-100 mm particulates). ...
... In contrast, the standard bean failed at a more extensive force range, suggesting they will fail at different forces when grinding a mass of these beans. This indicated that a peaberry lot could break simultaneously once the required force is attained in the grinding process, providing a more even ground coffee as seen in other materials (Derossi et al., 2018;Uman et al., 2016). ...
Article
Full-text available
In many coffee‐producing countries, the ellipsoidal‐shaped seeds called peaberries are often labeled as a defect because of their shape and reduced size, going against the market demand for large‐sized standard coffee beans. Nevertheless, the peaberry natural occurrence on the coffee plantations is significant, accounting for 5%–7% of the total harvested coffee for Coffea arabica L. and Coffea canephora, the most planted species worldwide. Nevertheless, recent growth in the peaberry market has happened due to exceptional cupping scores for this specific bean type; however, the relationship between these scores and the shape of the bean was not yet recorded in the literature. Therefore, this research aimed to evaluate and compare the impact of the shape and size of the peaberry against the standard beans in different postharvest processes: Drying, roasting (colorimetry and inner roasting profiles), grinding (compressive and shear force tests) and overall quality by cupping analysis. Coffea arabica L. var. Cenicafé 1 was used throughout all the experiments, where advanced methods were used to increase the accuracy of the results and deeply characterize the process behavior. The results of this research allow to understand the peaberry postharvest behavior better and add significant value to this often‐underrated bean condition. The peaberries demonstrated shape influence in the different evaluated parameters, allowing them to dry faster, roast evenly, avoid burnt spots, and collapse at homogeneous forces while attaining the same high cup scores as a standard coffee bean. Practical applications Understanding the peaberries' shape and size influence different postharvest processes is crucial to comprehend their value. By gaining insights, into how these unique beans interact with stages of processing, producers can control and predict their behavior leading to more consistent and optimized results. The findings of this research provide an opportunity to fully incorporate peaberries into the coffee product without considering them as defective beans or expanding market trends. Generally, peaberries were often seen as undesirable because they did not match the bean size and shape. However, their exceptional performance in drying, roasting, grinding and cupping dispels these misconceptions, highlighting their value and contribution to coffee quality. As a result, coffee producers can embrace peaberries as valuable beans without discarding or downgrading them. This newfound appreciation for peaberries does not only reduce waste, but it also diversifies coffee offerings to meet consumer preferences and enriches the industry.
Article
Full-text available
This study examined the influence of ground coffee granulometry and particle distribution on extraction parameters. They have been investigated the physicochemical properties, and the bioactive and volatile compound content in coffee obtained by a conventional filter method, the French Press, as a function of particle size and distribution. Some samples have been used for the extraction the directly the grinding machine, set at different grinding grade, and other samples have been seed before the usage in order to reproduce samples at different particle size class very homogeneous. The results showed that bioactive and volatile compounds are released differently in the beverages depending on the specific particle size. The results have been demonstrated that a homogeneous grind was more deficient in bioactive compounds and total dissolved solids than a classical, bimodal grind. Moreover, extraction from a very fine homogeneous grind was poorest with respect to these compounds, despite the greater surface in contact with the solvent. Conversely, bimodal grinds obtained conventional by the grinding machine, which were more heterogeneous from a granulometric point of view, were found to be richer in volatile organic and bioactive compounds. The study highlights that the grind plays a key role in producing well-extracted coffee and, therefore, in making the most of the potential inherent in the roasted bean.
Article
Full-text available
In an espresso-style extraction hot water (90±5 °C) is driven through a coffee packed bed by a pressure gradient to extract soluble material from the coffee matrix. Permeability is a key parameter affecting extraction as it determines the flow rate through the bed and hence brewing and residence time. This may alter bed-to-cup mass transfer and therefore impact brew quality.In this work a methodology that will allow estimation of the permeability of coffee packed beds in steady-state was developed. Fitting measured flow rate – pressure drop data to Darcy’s law resulted in permeability values in the range of 10-13-10-14 m2. Disagreement between the experimental and theoretical permeability, as estimated from dry measurements of particle size distribution and Kozeny-Carman equation, was found. Bed consolidation may have a larger effect on the packing structure than the mere decrease in bed bulk porosity. The Kozeny-Carman equation, corrected with a porosity-dependent tortuosity according to a power law, gave a good fit of the data
Article
Full-text available
Coffee is a valuable beverage crop due to its characteristic flavor, aroma, and the stimulating effects of caffeine. We generated a high-quality draft genome of the species Coffea canephora, which displays a conserved chromosomal gene order among asterid angiosperms. Although it shows no sign of the whole-genome triplication identified in Solanaceae species such as tomato, the genome includes several species-specific gene family expansions, among them N-methyltransferases (NMTs) involved in caffeine production, defense-related genes, and alkaloid and flavonoid enzymes involved in secondary compound synthesis. Comparative analyses of caffeine NMTs demonstrate that these genes expanded through sequential tandem duplications independently of genes from cacao and tea, suggesting that caffeine in eudicots is of polyphyletic origin.
Chapter
This chapter summarises the vast literature on the composition* of green coffee beans paying particular attention to those components which are peculiar to coffee. The corresponding data are given for roasted beans and where possible for soluble powders. Attention is focused on compositional factors that might be determinants of acceptability, and situations where the data are incomplete or contradictory with the intention of provoking thought, comment and further investigation.
Article
The present study investigated whether consumers’ expectations and perceptions concerning milk‐based coffee drinks would be influenced by: (1) the presence/absence of latté art on the froth of the coffee, and (2) shape‐taste symbolism (i.e., angular versus rounded shapes presented on the froth). An online survey conducted using photographs of cups of coffee revealed that the presence of latté art did indeed influence people's expectations concerning the value of the drink. Follow‐up research revealed that people were willing to pay more for a milk‐based coffee drink that had latté art as compared to a similar drink served without art. In a third experiment, an online survey revealed that an angular shape, relative to a more rounded shape, influenced people's expectations concerning the likability, bitterness and quality of the drink. A final experiment (Experiment 4) revealed that shape influenced people's perception of the quality and estimated price of the coffee. Taken together, the various results reported here demonstrate that the presence of latté art influences how much people expect, and are willing, to pay for a café latté. As such, adding art to, and the type of visual design on, a customer's drink should be considered by those serving café latté as an effective means of increasing value. Practical Applications The addition of latté art to milk‐based coffees is an interesting, and somewhat recent, phenomenon. The inclusion of latté art can help baristas differentiate their product from those of others. The results reported here suggest that the addition of latté art influences how much people expect, and are willing to pay for milk‐based coffees. As such, for the cafe owner thinking about how to increase profits, the experiments reported here suggest that people are willing to pay between 11–13% more for coffee with latté art than for those without it.
Article
Sugars and salts strongly affect the dimerization of caffeine in water. Such a change of dimerization, considered to be crucial for bitter taste suppression, has long been rationalized by the change of “water structure” induced by the additives; “kosmotropic” (water structure enhancing) salts and sugars promote dimerization, whereas “chaotropic” (water structure breaking) salts suppress dimerization. Based on statistical thermodynamics, here we challenge this consensus; we combine the rigorous Kirkwood–Buff theory of solution with the classical isodesmic model of caffeine association. Instead of the change of water structure, we show that the enhancement of caffeine dimerization is due to the exclusion of additives from caffeine, and that the weakening of dimerization is due to the binding of additives on caffeine.
Article
The flavoursome compounds in coffee beans exist in the form of aprotic charge neutral species, as well as a collection of acids and conjugate salts. The dissolution and extraction of these organic molecules is a process dependent on the dissolved mineral content of the water. It is known that different rates and compositions of coffee extraction are achieved through the control of the water 'impurities', Na+, Mg2+ and Ca2+, which coordinate to nucleophilic motifs in coffee. Using density functional theory, we quantify the thermodynamic binding energies of five familiar coffee-contained acids, caffeine, and a representative flavour component, eugenol. From this, we provide insight into the mechanism and ideal mineral composition of water for extraction of flavoursome compounds in coffee.
Article
Granular media differ from other materials in their response to stirring or jostling - unlike two-fluid systems, bi-disperse granular mixtures will separate according to particle size when shaken, with large particles rising, a phenomenon termed the 'Brazil-nut effect'. Mounting evidence indicates that differences in particle density affect size separation in mixtures of granular particles. We show here that this density dependence does not follow a steady trend but is non-monotonic and sensitive to background air pressure. Our results indicate that particle density and interstitial air must both be considered in size segregation.