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Chapter 14
Protecting the
FlavorsdFreshness as a Key to
Quality
Chahan Yeretzian
1
, Imre Blank
2
, Yves Wyser
2
1
Zurich University of Applied Sciences, Wa
¨denswil, Switzerland;
2
Nestec Ltd., Nestle
´Research
Center, Lausanne, Switzerland
1. THE SECRET TO GREAT COFFEE IS THE PEOPLE WHO
MAKE IT
Defining the quality of coffee is by no means a simple endeavor and several
renowned publications and coffee experts have offered various definitions and
discussions on the subject (Illy and Viani, 2005). However, the definition has
remained elusive and with the mounting importance of the specialty coffee
community, a rational approach to quality is becoming increasingly important.
Indeed, the specialty coffee movement can best be described as the uncom-
promising quest for the highest quality in the cup.
It all began 50 years ago, when Alfred Peet opened a coffee store in
Berkeley, April 1, 1966 (www.peets.com). Noticed by only a few quality
aficionados, Alfred Peet can be considered the pioneer of the specialty coffee
movement. Although the term “specialty coffee” did not exist at the time, a
revolution was brewing. His coffee was unlike anything Americans had ever
tasted beforedsmall batches and fresh beans. His philosophy was that there
should be the shortest distance possible between the roaster and the customer.
Freshness was at the heart of his vision and quality concept. Since then
freshness has remained a focus for all those who strive to deliver the highest
quality.
Alfred Peet inspired and guided the founders of Starbucks. In 1971, Jerry
Baldwin, Gordon Bowker, and Zev Siegl founded Starbucks in Seattle, selling
fresh-roasted whole beans to local customers. Freshness was a central moti-
vation to the founders of Starbucks, introducing a larger American public to
the specialty coffee movement. More than 15 years later, in the late eighties,
Nespresso launched portioned coffee and introduced high quality coffee with a
personalized touch to European customer in their homes.
The Craft and Science of Coffee. http://dx.doi.org/10.1016/B978-0-12-803520-7.00014-1
Copyright ©2017 Elsevier Inc. All rights reserved. 329
The term specialty coffee was first used in 1978 by Erna Knutsen (www.
scaa.org/?page¼RicArtp1). She described “special geographic microcli-
mates” that “produce beans with unique flavor profiles, which she referred to
as Specialty Coffees.” Underlying this idea of coffee appellations was the
fundamental premise that specialty coffee beans would always be well pre-
pared, freshly roasted, and properly brewed.
In 1982, the Specialty Coffee Association of America (SCAA) was foun-
ded and then in 1998 the Specialty Coffee Association of Europe (SCAE)
followed. In 1982, Paul Songer published an article in the Specialty Coffee
Chronicles, the Newsletter for members of the SCAA entitled “A Question of
Freshness.” The closing lines of his articles unequivocally positioned freshness
at the center of the specialty coffee movement: “Freshness of the coffee that a
roaster or retailer sells and serves is a direct reflection of the standards and
abilities of that operation. It will determine one’s competitiveness in the
marketplace and the ability of the consumer to experience a product that is
unique and worth seeking out. The bottom line is flavor. For specialty coffee,
flavor means freshness.”
Besides addressing the development of the freshness concept from the
perspective of the coffee specialist, it is also worth to briefly review freshness
from the consumer’s perspective and how this may have evolved over the last
decade. Although very little has been published on this subject, Pe
´neau (2005)
explored the concept of freshness in fruits and vegetables; some of the
learnings are of interest also for coffee. She concluded that freshness “is best
described by a level of closeness to the original product, in terms of distance,
time and treatment.” Interestingly, negations (absences of negative attributes)
were widely used to describe freshness, whereas people less familiar with the
origin and processing of products used negations more often to describe
freshness than those more familiar with these aspects of the products. Here we
will attempt to develop a concept of freshness that is based on positive attri-
butes rather than defining freshness by the absence of negatives. This also
follows the evolution of the concept of quality introduced by the specialty
coffee movement that increasingly focuses on positive quality attributes of the
cup versus the notion of lack of defects in green bean, which is still the
dominant concept of quality in the coffee business in general. It is worth
mentioning here that oxidation or oxidized flavor is not an intrinsic defect that
appears as an attribute in cup tastings. However, it is related to freshness, as it
may be introduced during storage and aging.
We would also like to mention here that the concept of freshness, as it will
be developed and discussed in this chapter, refers to the freshness of roasted
coffee, in contrast to freshness in green beans. In fact, the timescale and the
chemical and physical processes underlying the loss of freshness in green
beans are different to those that occur in roasted beans. Once coffee has been
harvested, sorted, and graded, it is often stored as green/raw bean over a
prolonged period, from several months up to several years. During storage,
330 The Craft and Science of Coffee
there is a distinct decrease in quality, which is expressed by a flattening of the
cup quality (Selmar et al., 2008; Scheidig et al., 2007). As a consequence, the
provenience-specific characteristic features, especially those of top quality
coffees, gradually diminish. In contrast to “off-notes,” which may occur during
the course of storage and are mainly caused by oxidation processes within the
lipid fraction (Speer and Ko
¨lling-Speer, 2006), the causes of the progressive
reduction in cup quality are still unknown. The appearance of storage-related
off-notes can largely be controlled by appropriate storage of the green coffee,
with the two critical factors that need to be controlled being moisture content
and temperature. However, it must be stated that flattening of the cup quality
occurs even under optimal storage conditions. Although it is generally stated
in the coffee trade that green coffee beans can last for up to 3 years (if properly
stored), increasingly, specialty coffee roasters acknowledge that roasting
fresher green beans is beneficial to quality.
2. MEASURING FRESHNESS
Since the advent of the specialty coffee movement in the late 1960s, the
concept of freshness of roasted coffee has remained at the center of much of
the effort of specialty coffee lovers. However, despite the central role freshness
plays in the highest quality coffee and in the specialty coffee movement in
general, it appears that much of the discussion on freshness revolves around
the process of how freshness can be delivered and assured: freshly roasted,
ground within a few days, immediately extracted, and consumed. But when it
comes to defining freshness as an objective and scientifically measurable
attribute of the cup, things become much less clear. The aim of this chapter is,
therefore, to provide a quantitative and scientific answer to the question: How
can we measure freshness?
The first step to measuring freshness is to clarify what is meant by
freshness, i.e., to understand and define freshness. In the context of coffee, we
define freshness as coffee that exhibits no impairment to its original qualities.
The original point of reference referred to here is coffee that has just been
roasted. However, this initial state of an absolutely fresh coffee cannot be
defined in absolute terms. Indeed, because coffee is an agricultural product, the
initial status of a fresh coffee depends on a large number of factors, such as the
green coffee variety (genetics), the altitude, climate and soil composition of
the plantation, agronomic and harvest practices, postharvest treatment, and
green coffee storage (Yeretzian et al., 2002). This results in green coffee beans
with varied chemical compositions and physical properties. Collectively, all
these factors lead to vastly differing green beans being roasted and hence
affect the chemical and physical properties of the freshly roasted coffee.
During roasting, a range of complex chemical and physical processes occur
within the coffee bean (also see Chapter 12), leading to the formation of typical
coffee aroma compounds and inorganic gases (mainly carbon dioxide; CO
2
).
Protecting the FlavorsdFreshness as a Key to Quality Chapter j14 331
The sensory characteristics of the volatile compounds shown in Fig. 14.1
are described in Table 14.1, indicating the relevance of their aroma to the roast
and ground coffee and the corresponding brew, expressed as the flavor dilution
(FD) factor. The FD factor was introduced by Grosch et al. and is defined as
the ratio of the concentration of the odorant in the initial extract to its con-
centration in the most dilute extract in which the odor is still detectable by gas
chromatography-olfactometry (Blank et al., 1992; Grosch et al., 1993). Only
2-furfurylthiol (6) elicits a coffee-like aroma in a certain concentration range,
all other odorants smell differently from coffee.
Once roasting is completed, a multitude of physical and chemical processes
immediately start, leading to an evolution of the coffee over time. Indeed, a
freshly roasted coffee is a highly elusive product. Among the many changes
that occur over time after roasting, two are of particular importance and related
to the quality attributes of coffee. One is (1) the evolution of the aroma profile
and the other (2) the degassing of the beans. Once roasting is complete, the
clock starts ticking on both of these processes. Although the evolution of both
processes can be measured by a multitude of methods, with varying levels of
sophistication, the aim here is to establish approaches that are accurate, robust,
and simple. The goal is to find an appropriate method for monitoring changes
in aroma compositions, i.e., the aroma balance defined as the ratio of volatiles
that are relevant from a sensory perspective. These changes may include the
loss of certain volatiles, but also the formation of others, which may lead to a
OO
O
6
SH
SH
OH
O
N
N
N
N
N
NO
N
N
OH
OO
OH
OH
O
OH
OO
14
17 21 25 26
30 31 32 34 35
O
SH
5
OS
8
N
NO
15
N
N
19
FIGURE 14.1 Selected coffee aroma compounds that represent various chemical classes that
contribute to the aroma of roast and ground coffee.
332 The Craft and Science of Coffee
modified aroma composition, which may ultimately be perceived as lack of
freshness. Changes in the aroma composition may be due to various phe-
nomena, such as:
1. Volatilization: Volatile aroma molecules are lostdthe overall coffee aroma
fades in and above the cup. This can be limited by protecting the coffee
with impermeable packaging.
2. Intrinsic reactivity: Aroma molecules are often intrinsically labile reacting
with compounds naturally present in coffeedthe consequence is again that
the aroma in the coffee fades. This cannot really be avoided, even with the
TABLE 14.1 Potent Odorants Found in Arabica Coffee (Blank et al., 1992)
No Compound Aroma Quality
Aroma
Relevance (FD)
a
Powder Brew
5 2-Methyl-3-furanthiol Meaty, boiled 128 <16
6 2-Furfurylthiol (FFT) Roasty (coffee-
like)
256 64
8 Methional Boiled potato-
like
128 512
14 3-Mercpto-3-methyl-butyl formate Catty, roasty 2048 256
15 3-Isopropyl-2-methoxypyrazine Earthy, roasty 128 32
17 2-Ethyl-3,5-dimethylpyrazine Earthy, roasty 2048 1024
19 2-Ethenyl-3,5-methylpyrazine Roasty, earthy 128 128
21 2,3-Diethyl-5-methylpyrazine Earthy, roasty 512 128
25 3-Isobutyl-2-methoxypyrazine Earthy 512 128
26 2-Ethenyl-3-ethyl-5-methylpyrazine Roasty, earthy 512 32
30 3-Hydroxy-4,5-dimethyl-2(5H)-
furanone
Seasoning-like 512 2048
31 4-Ethylguaiacol Spicy 256 512
33 5-Ethyl-3-hydroxy-4-methyl-2(5H)-
furanone
Seasoning-like 512 1024
34 4-Vinylguaiacol Spicy 512 512
35 (E)-b-Damascenone Honey-like,
fruity
2048 64
a
The FD factor of 256 for FFT means that the roasty note of FFT in the 256-fold dilution of the original
coffee aroma extract was still detected by gas chromatography-olfactometry, i.e., FFT was no longer
detectable in the 512-fold diluted extract.
Protecting the FlavorsdFreshness as a Key to Quality Chapter j14 333
best packaging, but can be slowed down through storage at lower
temperature.
3. Oxidation: Aroma compounds oxidizedthe coffee aroma fades and new
volatiles with off-notes are created. This can be prevented by protecting the
coffee from oxygen and oxidative processes. The way coffee is being
packaged (e.g., the atmosphere inside the pack) and the barrier properties
of the packaging material will make a difference here.
In general, these processes are accelerated when beans are ground and
when stored at elevated storage temperature.
2.1 Loss of Inorganic Gases
During roasting, coffee beans undergo major chemical transformations, during
which a large amount of inorganic gases, mainly CO
2
, are generated. Much of
these gases remain entrapped within the porous structure of the roasted beans.
Approximately 1e2% of the weight of freshly roasted coffee can be attributed
to entrapped inorganic gases (excluding water), whereas unroasted, green
coffee beans contain no entrapped gases. These gases are mainly released
during storage, but the process already starts during the final phase of roasting.
Coffee that has been stored for some period of time will have less entrapped
gases and consequently a lower rate of gas release. Therefore, one approach
for assessing freshness is based on measuring the amount and the kinetics/rate
of gas (mainly CO
2
and excluding H
2
O) released within a given time window.
The approach taken by the research group led by Prof. Yeretzian at the Zurich
University of Applied Sciences (ZHAW) group is to measure the weight loss
of freshly roasted coffee as a direct means of monitoring the loss of freshness
over time, whereas other groups have taken alternative approaches (Wang and
Lim, 2014; Wang, 2014). A quantitative discussion of the degassing of freshly
roasted whole and ground coffee beans (Arabica and Robusta) that have been
roasted to different roast degrees and along different timeetemperature roast
profiles will be presented in a forthcoming publication.
2.2 Evolution of the Aroma Profile
Probably the most important quality attribute of coffee is its aroma and,
therefore, the most appropriate and direct approach for measuring freshness is
to examine the aroma and its evolution over time (Sunarharum et al., 2014;
Grosch, 2001, 1998; Grosch et al., 1996; Lindinger et al., 2008, 2010; Poisson
et al., 2009; Semmelroch et al., 1995; Blank et al., 1992, 1991). However, we
must acknowledge that coffee flavor is complex, elusive, and labile (Munro
et al., 2003). The most important coffee aroma compounds and nomenclature
were shown in Fig. 14.1 and Table 14.1. Once roasting is complete, the aroma
has already started to evolve (Gloss et al., 2014). This is due to physico-
chemical changes, such as evaporation, as well as chemical reactions, and
334 The Craft and Science of Coffee
interactions between aroma compounds and the coffee matrix. For example, a
comprehensive mechanistic, chemical study by Mu
¨ller and Hofmann (2007)
explored in detail the degradation of the key coffee odorant 2-furfurylthiol,
which contributes to the sulfuryeroasty odor quality of a coffee brew and was
found to reduce considerably during coffee storage.
As shown in Fig. 14.1, many aroma-active components are reactive species
bearing functional groups such as thiols, carbonyls, and enolones. Depending
on conditions such as oxygen, moisture, and temperature, they will evolve over
time and thus change the perceived aroma of a coffee. Hence, it seems obvious
to look for clues of freshness (or loss of freshness) in the evolution of the
coffee aroma profile.
As previously outlined, quantitatively assessing changes in aroma com-
pounds during storage will allow the loss of freshness to be determined. The
processes involved in such losses of freshness are complex and may occur in
two main ways: (1) a loss of highly volatile compounds; (2) through chemical
reactions, for example, as a result of oxidation by O
2
, or through intrinsic
chemical reactions between different coffee components. Many chemical
classes (thiols, diones, aldehydes, vinyl derivatives) may react upon storage.
This may lead to either a decrease or an increase in headspace concentrations
for selected compounds. Hence a loss of freshness can best be described as a
progressive imbalance in the aroma profile. Such processes have been exten-
sively discussed in the literature, with the aim of identifying marker com-
pounds for the shelf life of packaged roasted coffee. The first studies on coffee
aroma deterioration can be traced back to the 1940s (Shuman and Elder, 1943),
followed in the 1950s by work from Merritt et al. (1957) and Buchner and
Heiss (1959). Many more groups have examined the shelf life of roasted coffee
beans or of roast and ground (R&G) coffee, either from a chemical or a
sensory perspective (or both) (Spadone and Liardon, 1990; Nicoli et al., 1993,
2009; Anese et al., 2006; Marin et al., 2008).
Considering the fact that green coffee beans contain more than 10% fat,
volatile lipid oxidation products were an early focus in studies on degradation
markers in coffee. Such studies have reported a correlation between the pro-
cess of coffee going stale and the generation of n-hexanal after an initiation
phase of approximately 7 weeks of storage in air (Spadone and Liardon, 1990).
These studies also showed that other products formed by oxidative degradation
of unsaturated fatty acids in roasted coffee do not play a significant role in the
flavor of roasted coffee. However, the formation of hexanal cannot explain
the loss of freshness; at best it may be seen as an early marker of the fading of
the coffee aroma which ultimately may lead to a reduction in perceived
freshness.
Although several volatile organic compound (VOC) markers have been
suggested for monitoring deterioration in freshness of R&G coffee, the major
weakness of using absolute concentrations of such marker compounds is the
fact that the amount of any single compound depends, among other things, on
Protecting the FlavorsdFreshness as a Key to Quality Chapter j14 335
its initial concentration, which in turn is affected by variables such as blend,
roast degree, grinding, extraction, as well as other factors (Kallio et al., 1990;
Leino et al., 1992). The use of ratios of headspace concentrations of selected
VOCs is, therefore, more robust and reflects changes in the balance in the
headspace (Spadone and Liardon, 1990; Arackal and Lehmann, 1979; Marin
et al., 2008; Kallio et al., 1989). Coffee VOCs that have typically been used in
such a ratio are methanethiol, propanal, 2-methylfuran, 2-butanone, 2,3-
butanedione, 2-furfurylthiol, dimethyl disulfide, and hexanal.
3. FRESHNESS INDEX
Several VOC have been suggested in the literature as markers for aging or
staling. Already in 1992, Holscher and Steinhart reported that methanethiol
has a strong impact on aroma freshness and a strong decrease in concentration
can be seen just one day after roasting. In 2001, Mayer and Grosch published a
study in which they investigated the changes in odorant composition in the
headspace of ground coffee based on time after grinding. They reported an
approximate 50% reduction in headspace intensities for a range of aldehydes
and diketones (methylpropanal, 2-methylbutanal, 3-methylbutanal, 2,3-
butanedione, 2,3-pentandione), just 15 min after grinding. Considering the
fact that these volatiles are important coffee aroma compounds, it can be
inferred that their loss is related to a change in the aroma profile and hence a
reduction in freshness. Alternatively, some compounds are hardly present in
freshly roasted coffee and only appear when coffee ages (Parliment et al.,
1982; Baggenstoss et al., 2008). Their presence may, therefore, serve as a sign
of loss of freshness. Compounds such as dimethyl disulfide and dimethyl
trisulfide, however, only form during storage due to the oxidation of meth-
anethiol. Therefore, one approach may potentially be to measure the con-
centration of such compounds and then use these data to estimate the freshness
level of the coffee.
3.1 The Concept
A more robust and simple method to assess the freshness of coffee is to
monitor ratios of headspace concentrations of selected VOCs (Spadone and
Liardon, 1990; Arackal and Lehmann, 1979; Marin et al., 2008; Leino et al.,
1992; Kallio et al., 1990). In a previous publication, many of the reported VOC
ratios were revisited and the ones that are most robust and suited to assessing
the freshness of high quality specialty coffee were selected (Gloss et al., 2014).
These were termed “freshness-indices” as they focus on fast changes (in
contrast to aging or staling markers). Furthermore, we were aiming to find
freshness indices for which the chemistry leading to the observed changes was
well understood. Among the many potential ratios of coffee aroma com-
pounds, the freshness index dimethyl disulfide/methanethiol (DMDS/MeSH)
was identified and shown to satisfy these criteria and therefore be particularly
336 The Craft and Science of Coffee
suitable (Gloss et al., 2014). Some of the properties of these two compounds
are summarized in Table 14.2.
Methanethiol is known to be a highly volatile as well as reactive compound
(Grosch, 2001; Steinhart and Holscher, 1991; Sanz et al., 2001), examples of
which are oxidation and dimerization to dimethyl disulfide (Belitz et al., 2004;
Chin and Lindsay, 1994b). In contrast, dimethyl disulfide has both relatively
lower reactivity and volatility. Consequently, the overall evolution of this
freshness index is mainly driven by the high reactivity and volatility of
methanethiol. A further reaction of dimethyl disulfide to dimethyl trisulfide
was not observed in our analyses.
In the following sections we introduce the experimental approach to
measuring the freshness in more detail, based on the freshness index DMDS/
MeSH, and apply this to three specific applications. (1) The first is the storage
of roasted whole beans in packaging made of plastic composite film with a
thick aluminum layer, equipped with a CO
2
release valve, and stored at 22C
and 50C. (2) The second application is analogous to the first, yet with the
distinction that the packaging was not equipped with a valve. Since the
packaging was fully air-tight, we could introduce variable oxygen contents
into the packaging and examine the loss of coffee freshness (i.e., the evolution
of the DMDS/MeSH freshness index) as a function of the oxygen content. (3)
Finally, the third application deals with the loss of freshness of roasted and
ground coffee in single serve capsules.
3.2 The Experiment
The compounds dimethyl disulfide and methanethiol were analyzed by gas
chromatography coupled to mass spectrometry (GCeMS). Data analysis and
identification of the compounds were performed using MSD Chemstation
software (Version G1701 EA E.02.00.493, Agilent Technologies, Switzerland)
and an NIST08 spectrum database. Chemical identification was made by
comparing the mass-spectra to the database, using the most intensive fragment
ion for quantification.
TABLE 14.2 Properties of the Two Compounds Used for the Freshness
Index Discussed Here
Compound Odor Volatility Reactivity
Dimethyl disulfide
(DMDS)
Sulfur, onion, garlic, burnt rubber Medium Medium
(ox.)
Methanethiol
(MeSH)
Sulfur, rotten egg, fish, cabbage,
garlic, cheesy
High Very high
(ox.)
Protecting the FlavorsdFreshness as a Key to Quality Chapter j14 337
Fig. 14.2 demonstrates the evolution of the GCeMS intensities in single
ion mode for dimethyl disulfide and methanethiol (left frame) and the cor-
responding freshness ratio DMDS/MeSH. One kilogram batches of washed
Ethiopian Limu, Grade 2, Arabica were roasted in a Probatino to a roast
degree 93 Pt (Colorette), corresponding to a medium roast degree. Sixty-five
grams roasted whole beans were packaged immediately after cooling in
plastic composite film with a thick aluminum layer. The bag did not contain
a degassing valve and was stored at 22C (room temperature) under inert
atmosphere for 3 weeks. At time zero and during each subsequent week, the
DMDS/MeSH ratio was determined for five bags. The results are plotted as
an average with 95% confidence intervals.
The results show a quick drop in the MeSH signal intensity. After only
1 week the intensity had already dropped to 25% of its initial value, and
to 10% of the initial value after just 3 weeks. We can also see an increase
in the DMDS signal by more than a factor of three. The right frame
shows the corresponding freshness index DMDS/MeSH for the same
storage period, where there is a pronounced increase during the 3 weeks
of storage.
Before applying this ratio to various storage conditions, it is important
to outline the underlying chemical processes that lead to the DMDS and
MeSH changes that were observed during storage. Fig. 14.3 summarizes
the major steps involved in the formation and degradation of DMDS and
MeSH.
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
0
50,000
1,00,000
1,50,000
2,00,000
2,50,000
0 1 2 3
I (Dimethyl disulfide) * g
I (Methanethiol) * g
Storage time / weeks
0
0.1
0.2
0.3
0.4
0.5
0.6
0 1 2 3
Ratio 1
Storage time / weeks
Ratio 1:
Dimethyl disulfide
Methanethiol
Dimethyl disulfide/Methanethiol
FIGURE 14.2 Demonstration of the freshness index dimethyl disulfide/methanethiol for whole
roasted beans stored at room temperature (22C) in tight packaging with an aluminum barrier (not
equipped with a CO
2
degassing valve).
338 The Craft and Science of Coffee
Methanethiol, also referred to as methyl mercaptan, is a known degradation
product of methinonine or its Strecker degradation product methional. It has an
unpleasant odor and a low threshold value of approximately 1 ppb (Devos
et al., 1990). As a strong nucleophile, it can easily be oxidized to dimethyl
disulfide, which has a sulfury odor. Such reactions may take place under
mild conditions in the presence of oxygen and transition metals, as shown for
2-furfurylthiol (FFT), one of the character impact compounds of coffee. The
dimerization of MeSH is due to oxidative instability, which can be accelerated
in the presence of radicals (Blank et al., 2002).
It should be noted, however, that the reaction products of thiol degradation
depend on moisture content as well. This means freshness in roast coffee, as
discussed throughout this chapter, cannot be simply extrapolated to freshness of
a coffee brew. In addition, reactions, initiated by Fenton chemistry, take place
in the brew, leading to many more degradation products. The coffee-like
smelling compound 2-furfurylthiol decomposes rapidly in the presence of
hydroperoxide radicals and transition metals such as of ferrous iron (Blank
et al., 2002). In a similar reaction system with methanethiol, methanesulfenic
acid (CH
3
SOH) has been proposed as an intermediate product during the for-
mation of DMDS and dimethyl trisulfide (DMTS) (Chin and Lindsay, 1994a).
However, its existence could not be substantiated, possibly because of the high
reactivity of sulfenic acids (Penn et al., 1978), which are known to easily
convert to thiosulfinate esters due to their dual electrophilic/nucleophilic
characteristics (Block and O’Connor, 1974). Alternatively, thiols may react
with phenolic compounds, as shown when FFT is trapped by oxidative coupling
to hydroxyhydroquinone in coffee brews (Mu
¨ller and Hofmann, 2007).
Freshness Ind
DMDS
ex
MeSH
=
methionine methional
dimethyl disulfide methanethiol
FIGURE 14.3 Reaction scheme underpinning the dimethyl disulfide/methanethiol freshness
index.
Protecting the FlavorsdFreshness as a Key to Quality Chapter j14 339
4. APPLICATIONS
4.1 Application 1: Whole Beans in Packaging With Valve
The first application of the freshness index examines the most common
method of storage for coffee. Roasted whole beans are stored in packaging
composed of a plastic composite film with a thick aluminum layer and
equipped with a CO
2
release valve. In this example, a medium roasted washed
Arabica was used. The coffee was stored at 22 and 50C.
Fig. 14.4 shows the evolution of the coffee during storage, seen in the
changes in the DMDS/MeSH freshness index. The two left frames represent
the evolutions of the two compounds methanethiol and dimethyl disulfide
(bottom) and the corresponding DMDS/MeSH ratio at 22C (top), over a 3-
week storage period. The two frames on the right correspond to storage at
50C, over 4 weeks. Two full replicates of the experiment were conducted, i.e.,
coffee was roasted twice and the storage experiments conducted separately
with the two separate roast batches.
As expected, we can see an increase of the DMDS/MeSH ratio with storage
time, irrespective of the temperature. The increase in the freshness index at
50C is approximately one order of magnitude higher than at 22C, indicating
an accelerated loss of freshness at elevated temperature. The bottom two
frames assist in the interpretation of the ratios by also showing the related
evolution of the two individual compounds DMDS and MeSH. At 22Cwe
can see a decrease in the MeSH content, whereas the DMDS content appears
to be essentially constant. We have tentatively interpreted this essential con-
stant value for DMDS as a steady-state concentration. Although DMDS is
being formed during storage at 22C, it is further reacting to compounds such
as DMTS and dimethyl sulfide (DMS). The result is an apparent constant
DMDS content over the 3-week storage period.
At 50C in contrast we can see a strong decrease in the DMDS during the
first week, before its content is stabilized. We have tentatively interpreted this
initial increase in DMDS to the higher rate of formation at elevated temper-
atures. Since DMDS is a relatively stable compound, it does initially accu-
mulate. Once the formation of new DMDS starts to decrease (due to a decrease
in its precursor, MeSH), the follow-up reactions of the DMDS to, for example,
DMTS and DMS start to establish a steady-state situation, i.e., the formation
of DMDS from oxidizing MeSH equals its degradation (further reaction). In
comparison to the situation at 22C, the steady-state content of DMDS is
higher at 50C. After 3 weeks, and once the pools of the MeSH precursors
have started to decrease, we observe a decrease in the DMDS content. To
elucidate these hypothetical processes, the reaction rate constants and the
temperature dependence of the reactions involved need to be experimentally
determined, and the underlying processes modeled.
340 The Craft and Science of Coffee
0
0.01
0.02
0.03
0.04
0.05
0.06
0123
DMDS / MeSH
Storage time / weeks
DMDS / MeSH
22 °C
0
1
2
3
4
5
01 23 4
DMDS / MeSH
Storage time / weeks
DMDS / MeSH
50 °C
0
1000
2000
3000
4000
5000
6000
7000
0
20'000
40'000
60'000
80'000
100'000
120'000
140'000
160'000
0123
I (dimethyl disulfide) * g
I (methanethiol) * g
Storage time / weeks
DMDS; MeSH
22 °C
0
2000
4000
6000
8000
10000
0
20'000
40'000
60'000
80'000
100'000
120'000
140'000
160'000
01234
I (methanethiol) * g
Storage time / weeks
I (dimethyl disulfide) * g
DMDS;MeSH
50 °C
dimethyl disulfide
dimethyl disulfide
methanethiol
methanethiol
FIGURE 14.4 Loss of freshness of whole beans, stored in bags with a valve. After each week, five samples were analyzed by gas chromatographyemass
spectrometry (five packs); each value is given as the mean value with a 95% confidence interval.
Protecting the FlavorsdFreshness as a Key to Quality Chapter j14 341
4.2 Application 2: Whole Beans in Packaging Without Valve
The second application is analogous to the first, with the exception that the
packaging was not equipped with a CO
2
release valve and the coffees were
heat sealed with variable oxygen contents in the pack. Storage was limited to
only 22C.
Fig. 14.5 shows the evolution of the DMDS/MeSH freshness index over a
period of 3 weeks for three independent experiments, with each charge cor-
responding to a different roasting batch. Although the three charges show
identical DMDS/MeSH freshness indices after 1 week storage, they start to
show distinctively different evolutions of the freshness ratios over the
following 2 weeks. The higher the oxygen content inside the packaging, the
faster the increase during weeks 2 and 3.
4.3 Application 3: Single Serve Capsules
Finally, the third application deals with the evolution of the freshness index for
commercial single serve capsule systems. Single serve coffee capsules avail-
able on the Swiss market from four different leading commercial brands,
labeled C1 to C4 (see Table 14.3), were analyzed (Gloss et al., 2014). The
capsules were stored at room temperature for up to 46 weeks. The evolution of
the freshness was monitored via the DMDS/MeSH freshness index and is
shown in Fig. 14.6.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
01 23
Ratio (DMDS / MeSH)
Storage time / weeks
Charge 1
Charge 3
Charge 2
FIGURE 14.5 Loss of freshness of whole beans, stored at different oxygen contents in bags
without valves.
342 The Craft and Science of Coffee
The three main findings are given below:
First, and most importantly, there is an obvious impact of the packaging
material. The two capsules C1 and C2, neither of which have any aluminum
layer, show the strongest increases in the freshness indices, indicative of a
reduction in methanethiol and loss of freshness over time. In contrast, the C4
capsule with a 100% aluminum body and aluminum cover shows hardly any
evolution in the freshness index over the 46 weeks of storage. We concluded,
therefore, that C4 preserves the freshness of coffee much more efficiently. C3
takes up an intermediate position, with respect to the evolution in the freshness
indices. This is in line with the fact that C3 has a PP body (no aluminum) and a
cover with only a thin aluminum layer. In addition, it is wrapped into a sec-
ondary aluminum packaging that significantly increases the actual headspace
and, therefore, the absolute amount of residual oxygen after packaging.
Clearly, the absence of aluminum has a strong impact on the reduction in
freshness indices for C1 and C2.
TABLE 14.3 Description of the Four Different Capsule
Systems, Labeled C1 to C4
Capsule Packaging Materials
C1 lBody: PP/EVOH/PP
lCover: PP/EVOH/PP; thickness: 0.1 mm
lBarrier properties integrated into capsule and cover
C2 lBody: PP/EVOH/PP
lCover: PP/EVOH/PP; thickness: 0.12 mm
lBarrier properties integrated into capsule and cover
lExtraction system (perforation points and aluminum
foil) and outlet for extract are integrated in cover-
material
C3 lBody: PP (injection molding without barrier-
properties)
lCover: Paper with aluminum coating; thickness:
0.03e0.05 mm
lSecondary packaging: Aluminum; each capsule is
individually packed; barrier properties integrated
into secondary packaging
C4 lBody: 99% aluminum, with thin coating of food-
grade shellac
lCover: Aluminum foil; thickness 0.03e0.05 mm
lBarrier properties integrated into capsule and cover
The second column describes the packaging materials for the cover and the body
of the capsule. C3 has in addition a secondary packaging. EVOH, ethylene vinyl
alcohol; PP, polypropylene.
Protecting the FlavorsdFreshness as a Key to Quality Chapter j14 343
Second, the starting value of the freshness indices varies between the four
different capsule systems. Although C1 already started with a high value, C4 had
the lowest freshness index. It is speculated that this is an indication of a certain
loss of aroma and freshness from processing prior to packaging into capsules.
Third, the consistency in the capsules appeared to differ greatly. Each
capsule was measured in five repetitions and the data are plotted as mean
values with a 68% confidence interval. The standard deviation revealed an
unexpected and interesting insight into the consistency of the coffee in the
various capsule systems. C4 showed the smallest confidence interval, which is
an expression of low capsule-to-capsule variability. In contrast, C1 and C2
show much greater variability between capsules. Particularly for single serve
capsules, where a range of capsules are offered by each brand, consistency is
an important quality criterion.
Fig. 14.6 gives a comparison of different capsule systems. In such a pre-
sentation the axis of the freshness ratio has been chosen such that it fits all four
systems. In Fig. 14.7, we show the evolution of the DMDS/MeSH index for
one specific aluminum capsule (Nespresso) over 52 weeks. Although the range
covered by the freshness index, over the year of storage is approx. 0.02e0.09
(in Fig. 14.6 the Nespresso C4 capsule covers the range 0.05e0.15), it still
demonstrates a loss of freshness, albeit over a much smaller range than the
other capsule systems.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
01020304050
Ratio (DMDS / MeSH)
time / weeks
C1
C2
C3
C4
FIGURE 14.6 The evolution of the dimethyl disulfide/methanethiol (DMDS/MeSH) freshness
index is plotted as a function of storage time for four different commercial single serve coffee
capsules, stored over a period of up to 46 weeks. The error bars correspond to the respective
standard deviations of the fivefold measurements.
344 The Craft and Science of Coffee
In conclusion, the three examples discussed in Section 4 demonstrate the
potential and sensitivity of the freshness index DMDS/MeSH for the moni-
toring of loss of freshness in whole beans and ground coffee. The dimerization
of MeSH is due to oxidative instability and DMDS is a suitable molecular
marker for freshness; however, the correlation with the overall coffee aroma
needs yet to be established.
Besides the DMDS/MeSH index, there are a series of other indices that
have been reported in the literature. However, most often the chemical pro-
cesses underlying the evolution of these other ratios are not well
understooddin contrast to the DMDS/MeSH index. We have chosen to
exclusively include the DMDS/MeSH ratio, because it is very sensitive, an
excellent early marker of loss of freshness and hence particularly well suited to
high-quality coffee applications.
5. ENSURING COFFEE FRESHNESS USING OPTIMAL
PACKAGING MATERIALS
As mentioned in the previous sections of this chapter, the freshness of R&G
coffee is significantly altered by oxidation. Therefore, limiting the access of
oxygen to the product is of key importance for guaranteeing freshness and
quality of specialty coffeedan area in which the packaging has a key role to
play.
The continuous objective of minimizing the environmental impact on food
products means that selecting optimal packaging to meet the protection needs
40'000
35'000
30'000
25'000
20'000
Response of individual compounds:
Ratio (DMDS / MeSH)
● DMDS
● MeSH
15'000
10'000
5'000
0
0.10
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
0 4 8 1216202428323640444852
Storage / Weeks
Dimethyldisulfid
Methanthiol x 0.1
(DMDS / MeSH)
Storage Temperature: 22 °C
FIGURE 14.7 Evolution of the freshness index for a single serve coffee capsule from Nespresso.
Protecting the FlavorsdFreshness as a Key to Quality Chapter j14 345
of the product is increasingly important. This process is essential to reduce
instances of under- and overpackaging, which can lead to either premature
product loss or unnecessary, overengineered packaging.
To optimize the packaging used for a given product, material properties
that correspond to the protection requirements of that product must be
specified. Therefore, the protection requirements of the product need to be
understood and, in the case of R&G coffee, a good understanding of the rate
at which it consumes oxygen together with the effect this oxygen con-
sumption has on quality is key. There are several notable studies that have
made progress toward establishing methods for determining the oxygen
consumption of coffee and the relationship this consumption has with coffee
quality.
In his 1997 thesis, Cardelli-Freire used two primary methods to determine
the oxygen consumption rate of R&G coffee. In the first, R&G coffee samples
were packaged in hermetic containers containing different initial oxygen
concentrations. The oxygen concentration within each container was measured
during the course of the study, which allowed the oxygen consumption rate to
be determined. Using this and the experimental results obtained by Radtke-
Granzer and Piringer (1981), a relationship between the oxygen consump-
tion rate and oxygen partial pressure in the headspace was found. This was a
first-order relationship for relatively high oxygen concentrations (around 5%),
and a half order relationship for lower oxygen levels (around 0.5%). The
second method used by Cardelli involved packing coffee samples in permeable
pouches. This approach was based on two main assumptions. The first was that
the permeation rate of oxygen through the packaging material is directly
proportional to the difference in oxygen partial pressure between the head-
space and the outside environment. The second assumption was that there is a
relationship between the oxygen consumption rate of a product and the oxygen
partial pressure in the headspace.
Using this approach, the amount of oxygen permeating into a package
decreases as the oxygen concentration in the headspace increases, along with
the increase in the oxygen consumption-rate of the product. An equilibrium
oxygen concentration will eventually be reached in the package at the point
where both mechanisms compensate for each other. The level of this equi-
librium oxygen concentration is directly related to the oxygen permeability of
the packaging material. The oxygen permeating through the packaging ma-
terial can therefore be used to calculate the oxygen consumption rate of the
packaged product.
Cardelli calculated an equilibrium oxygen partial pressure of approxi-
mately 100 mbar at 22C, when 20 g of roast and ground coffee were pack-
aged in an high-density polyethylene/low-density polyethylene laminate pouch
with a surface of 140 cm
2
, an oxygen permeability of 380 cc/m
2
day atm and a
headspace volume of 34 mL. This enabled an oxygen consumption rate of
3.7 10
7
g
O
2
/g
coffee
day mbar to be defined for ground coffee.
346 The Craft and Science of Coffee
Determining the oxygen consumption rate of a product provides one
indication of how a product evolves over time, however, this value has to be
put into perspective with the loss in quality of the product to be able to
determine the protection requirements that guarantee its freshness over its
complete shelf life.
Cardelli and Labuza (2001) evaluated the sensory profile of roasted and
ground coffee stored under different conditions. They showed that R&G coffee
was found to be of unacceptable quality after a total O
2
consumption of be-
tween 150 and 300 mg/g, depending on the water activity of the product.
More recently, Wyser et al. (to be published) have determined oxygen
consumption rates of R&G coffee stored under different conditions to enable
the optimal packaging material requirements to be specified for a portioned
coffee system. In this piece of work Wyser et al. used optical oxygen sensors
(Presens GmbH) to continuously monitor the oxygen partial pressure in her-
metic glass containers containing R&G coffee. Several samples were moni-
tored continuously after having been prepared with different initial headspace
oxygen levels. A first-order dependence assumption and experimental data
obtained at high oxygen partial pressure were then used to define a model to
predict headspace partial pressure as a function of time.
Fig. 14.8 shows how experimental data corresponds with that of the model
data at the same oxygen level and demonstrates that the first-order dependence
1000
800
600
Oxygen in headspace (10-6 g/g coffee)
400
200
0
02040
Linear model calculated from data at high
oxygen concentration
Measured at high oxygen concentration
Measured at low oxygen concentration
60
Time [days]
80 100 120 140
FIGURE 14.8 Headspace oxygen concentration as a function of time in a glass container for
roast and ground coffee stored in the dark at 23C. Experimental results at high (cream dots) and
low (brown dots) initial oxygen concentrations are shown to correspond and thus validate a model
(continuous line) based on a first-order dependence assumption.
Protecting the FlavorsdFreshness as a Key to Quality Chapter j14 347
assumption is valid. Based on these data, an oxygen consumption rate of
2.13 10
7
g
O
2
/g
coffee
$day$mbar was determined for the coffee tested.
Although the value differs from that found by Cardelli, both are of the same
order.
As a next step, Wyser et al. used the relationship between oxygen partial
pressure, oxygen permeation, and the oxygen consumption rate to calculate the
total oxygen consumed by R&G coffee as a function of packaging perme-
ability and initial oxygen concentration. Fig. 14.9 shows this relationship when
applied to a typical portioned coffee package containing 5.3 g of coffee with a
headspace of 10.3 mL after being stored for 1 year. Having established this
relationship, the maximum allowable oxygen uptake for coffee can be used to
specify packaging barrier properties required to achieve the desired shelf life
of the product, thus ensuring quality and maintaining freshness, while avoiding
overpackaging.
It should be noted that the results obtained by Cardelli were for mainstream
coffee. It is expected that the critical oxygen uptake value for specialty coffee,
where freshness is the key attribute, would be significantly lower, meaning the
requirements in terms of permeability and initial oxygen level are even more
demanding.
0.0010
0.0008
0.0006
0.0004
Oxygen consumed after 12 month [g/gcoffee]
0.0002
0.0000
15 10 500.000
Packaging permeability [cc/day/atm]
0.005
0.010
0.015
0.020
Initial oxygen concentration [%]
FIGURE 14.9 Predicted oxygen consumed of roast and ground coffee as a function of initial
oxygen concentration and package permeability after 1 year of storage.
348 The Craft and Science of Coffee
6. OUTLOOK
Although the concept of freshness is central to the high quality coffee business
in general and the specialty coffee movement in particular, a rational
description and method for quantitative measurement have long remained
elusive. The main motivation for this chapter was to put freshness on a more
rational and quantitative basis and to make it more tangible.
Two approaches for measuring freshness of roasted coffee were outlined:
The first refers to changes in the profile of volatile organic aroma com-
pounds of roasted coffee over time and is expressed as “freshness indices”; the
ratio of two specific aroma compounds. One particular freshness index was
discussed (DMDS/MeSH). Methanethiol is a well-known compound that is
found in freshly roasted coffee that decreases within days of roasting. The rate
of methanethiol decrease is strongly dependent on the damage caused by
contact with oxygen and storage temperature. We demonstrated that pack-
aging, whose role is to protect the product, can have a major impact on the
evolution of the freshness index. However, there is no “best” packaging that
meets all needs equally. Packaging should be adapted to the expected time
between roasting and consumption. For coffee that will be consumed within 1
or 2 weeks, packaging with high barrier properties, which in many cases
would have a higher environmental impact, might not be required. However,
coffee with extensive distribution channels and a global logistics chain be-
tween roasting, the supermarket and the consumer’s home requires packaging
materials with much higher barrier properties that would contribute to mini-
mizing coffee wastage by guaranteeing the quality of the coffee over its entire
shelf life. It should be noted that it is well known that, in most cases, the R&G
coffee has a higher overall environmental impact than its packaging.
The second approach is based on weight loss during storage, which is
linked to the degassing of roasted coffee (mainly loss of CO
2
). Up to 2% of the
weight of freshly roasted coffee is made up of entrapped gases, which are
released over time. Measuring the rate of weight loss indicates how much gas
was released during processing (e.g., grinding, heating) and how much was
released after it was freshly roasted.
Preserving the freshness of roasted coffee remains a central dogma of high-
quality espresso coffee. The current status of research indicates that the
following points should be taken into consideration to preserve freshness
during the desired shelf life of coffee:
1. Loss of freshness starts the minute roasting is complete. Hence, exposure to
oxygen and humidity during the time between roasting and packaging must
be avoided as much as possible.
2. The barrier properties of the packaging, together with the level of residual
oxygen after packaging represent the second most important factor to be
Protecting the FlavorsdFreshness as a Key to Quality Chapter j14 349
considered and adaptations need to be made to achieve the desired shelf
life.
3. Finally, the storage temperature further affects the evolution of the fresh-
ness index and the degassing kinetics.
Understanding is the basis for creativity. Once we know how to assess and
measure freshness we can also start to think beyond freshness. We wish to
facilitate and promote a more rational and fact-based discussion on the concept
of freshness. To further advance this concept, studies where the analytical data
are correlated to the sensory aspects in the cup are needed. At the same time,
this will open up the possibility to better assess new and creative concepts of
coffee preparation for one of the most aromatic products and to discover new
“sweet spots” and novel sensory experiences. What about cold brew, nitro
coffee, liquid coffeedcan these be considered part of the specialty coffee
movement? Questions like, (1) how long coffee should degas prior to grinding
and extraction and (2) whether coffee should be extracted immediately after
grinding or left to rest for some time, can be discussed in terms of facts and
numbers, as well as sensory aspects. Indeed, a better understanding of fresh-
ness will ultimately provide better assessment procedures in the world of high
quality coffee and open emerging and new avenues to explore methods of
coffee preparation and extraction methods.
ACKNOWLEDGMENT
We would like to acknowledge the SCAE for financial support. We also thank Alexia Glo
¨ss,
Marco Wellinger, Samo Smrke, and Barbara Edelmann for the fruitful discussions that they
contributed to.
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