ChapterPDF Available

Chapter 9. Coffee Beans and Processing

December 2015

DOI:10.1016/B978-0-12-409517-5.00009-7

In book: Coffee in Health and Disease Prevention (pp.73-81)

Authors:

Maik Kleinwaechter

Technische Universität Braunschweig

Dirk Selmar

Technische Universität Braunschweig

The mode of coffee processing strongly influences the quality of green coffee and thereby establishes the characteristic differences in the flavor of wet and dry processed coffees. These variations are due to differences in the metabolic processes occurring within the vital coffee seeds during the course of processing. Biochemical and molecular biological studies revealed that germination is initiated during postharvest treatment, and stress metabolism is induced in the coffee beans, especially while drying. The detected differences in the time courses and amplitudes of metabolic processes are responsible for the distinct substantial composition characteristic of differently processed green coffees, and the peculiarities of wet and dry process coffees are established. Accordingly, it should be possible to modify the substantial composition of green coffee by deliberately changing the processing conditions, thereby improving its quality and health promoting effects.

Quantification of germination-related processes during wet and dry processing. Coffee beans resulting from model processing under definite conditions were used for the quantification of common markers for germination. ICL expression was quantified by competitive RT-PCR using an internal standard, the abundance of β-tubulin was determined by immunostaining of Western blots, and the amount of C4 nuclei was estimated by flow cytometry. The data are compiled from earlier publications. (Adapted from Bytof, G. et al. 2007. Annals of Botany, 100:61–66; Selmar, D. and Bytof, G. 2007. Green coffee is alive! A review on the metabolic processes taking place in coffee beans during processing and their implication for modern coffee research. 21ème Colloque Scientifique International sur le Café, Montpellier, France, pp. 423–433.)

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Green coffee beans are viable and metabolically active. A coffee bean was sliced in thin disks and incubated in triphenyltetrazoliumchloride (TTC) solution as described by Selmar et al. (2008). After the viable and metabolic active cells have imported the TTC, it is reduced and a red dye is formed.

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Loss of dry matter during the course of green coffee processing. Coffee beansstill surrounded by the parchment (=pergaminho)-had been taken directly out of a commercial processing facility, either directly after depulping or after 24 h of fermentation. For the "dried" variant, the parchment beans additionally were dried for 24 h. In the "fresh" trial, the parchment beans were taken directly out of the fruits before the dry weight was determined. For this, the beans were dried in a laboratory oven at 110°C for 36 h. Prior to weighing, the beans were hulled to remove the parchment. For each trial, five samples of 1000 seeds were analyzed. (Adapted from Bytof, G. 2003. Einfluss der Nacherntebehandlung auf die Qualitätsausprägung bei Arabica-Kaffee (Coffea arabica L.). Braunschweig, Technische Universität, Germany.)

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431

Metabolic Responses

of coffee beans duRing

pRocessing and theiR

iMpact on coffee flavoR*

DIRK SELMAR, MAIK KLEINWÄCHTER,

AND GERHARD BYTOF

* is chapter is dedicated to Böle Biehl and Hans Gerhard Maier, two outstanding

scientists and true cocoa and coﬀee lovers, who actually gave the initial spark for the

interdisciplinary investigations of coﬀee postharvest treatment.

Contents

12.1 Introduction 432

12.2 Coﬀee Flavor Varies with the Process 432

12.3 Process-Speciﬁc Material Diﬀerences in Green

CoﬀeeBeans 434

12.3.1 Caﬀeine, Chlorogenic Acids, and Trigonelline 434

12.3.2 Free Amino Acids, Proteins, Peptides, and

Biogenic Amines 435

12.3.3 Low and High Molecular Carbohydrates 437

12.3.4 Lipids 440

12.4 Coﬀee Seeds Represent Living OrganismsExhibiting

anActive Metabolism 441

12.5 Germination Physiology during Green Coﬀee Processing 446

12.6 Green Coﬀee Beans Experience Drought Stress While

Drying 451

12.7 Capabilities to Improve Quality byDeliberatelyInﬂuencing

the Metabolic Events in GreenCoﬀee 457

12.7.1 Intermediate Storage of Progressively Processed

Green Coﬀees 457

12.7.2 Quality Eﬀect of Barning 459

12.8 Conclusion 461

References 462

432 CoCoa and Coffee fermentations

12.1 Introduction

e ﬂavor of a cup of coﬀee is determined by many preharvest and

postharvest factors. e preharvest factors include the cultivar of the

coﬀee bean, how it is farmed, and the stage of maturity at harvest

(Chapters 10 and 11). e postharvest factors include the wet and dry

processes for removal of the pulp and mucilage, drying of the beans

(Chapters 10 and 11), and subsequent storage and transport. Within

the last few years, substantial progress has been made in the under-

standing of the metabolic processes occurring within coﬀee seeds dur-

ing the course of postharvest treatments. It was demonstrated that

seed germination is initiated during processing, and—especially while

drying—a stress metabolism is executed in the coﬀee beans. Moreover,

it was shown that the metabolic reactions involved are strongly deter-

mined by the mode of postharvest treatment, thereby speciﬁcally

inﬂuencing the coﬀee quality. ese insights led to a paradigm shift

in coﬀee research: today, the green coﬀee beans should no longer be

considered just as an inanimate commodity, but rather as viable organ-

isms, whose physiological capacity oﬀer potential for quality improve-

ment. is, however, requires the comprehensive knowledge of the

related metabolic processes, especially that of their time courses and

their successions. Applying modern techniques of plant biochemistry

and molecular biology combined with ﬁeld and model experiments

close to praxis, it becomes possible to resolve the extent and amplitude

of these processes, outlined in this chapter: the keys to endogenous

quality development of green coﬀee by its processing. is chapter

gives an overview of the metabolic responses of coﬀee beans during

postharvest processing and discusses their potential impacts on coﬀee

ﬂavor.

12.2 Coﬀee Flavor Varies with the Process

It is well known that the mode of coﬀee processing, by either the

wet method or the dry method, strongly inﬂuences and determines

the quality of the corresponding green coﬀees, thereby establishing

characteristic ﬂavor diﬀerences. Whereas coﬀee beverages prepared

from coﬀees obtained from the wet process (often described as washed

Arabicas) are characterized by their full aroma and pleasant acidity,

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metaboliC responses of Coffee beans

the corresponding dry-processed coﬀees typically exhibit a so-called

full body (Streuli 1974; Sivetz and Desrosier 1979a,b,c; Illy and Viani

1995, 2005; Mazzafera and Padilha-Purcino 2005).

In the past, a classical explanation for this phenomenon was given

on the basis of two facts: ﬁrst, that generally mature fruits (“ripe cof-

fee cherries”) are used for wet processing, whereas dry processing uses

fruits from all maturation stages; and second, that much greater dili-

gence was given in the course of wet processing (Illy and Viani 1995;

Njoroge 1998; Brando 2004). us, the maturation state of the coﬀee

cherries processed and their heterogeneity were thought to be of spe-

cial concern. ere is no doubt that the speciﬁc ﬂavor diﬀerences—at

least in part—could be due to such dissimilarities. However, according

to recent research, these aspects appear to play merely supporting roles

among others (Selmar et al. 2002, 2005; Selmar and Bytof 2006).

In this context, the most decisive insight was the ﬁnding that

after ﬁeld experiments in Brazil, the typical diﬀerences in cup qual-

ity between washed and unwashed Arabicas also occurred when

virtually identical and thoroughly sorted starting material was

used for either method of coﬀee processing (Selmar etal. 2002).

Corresponding hints had been published earlier (Chassevent etal.

1970; Mutiso 1971; Vincent et al. 1979; Balyaya and Cliﬀord

1995a,b; Guyot etal. 1995; Puerta-Quintero 1999), of which some

experiments had been done with Arabica, others with Robusta, and

one even also with the hybrid Arabusta. It should be noted, however,

that at least the two ﬁeld experiments with Arabica coﬀee (Mutiso

1971; Puerta-Quintero 1999) did probably not reﬂect optimum con-

ditions for both ways of processing, but only for the wet process.

Consequently, the corresponding results that ascribed better quality

to the wet process may be due to these imbalances. A fair compara-

ble judgment of cup quality can only be expected, if (1) comparable

starting material is used; and (2) the conditions chosen are suitable

to produce good-quality coﬀee with either way of processing.

In contrast, the conditions for the ﬁeld experiments undertaken in

Brazil mentioned above were most suitable for such a comparison,

because the chosen production site (fazenda) in Minas Gerais rou-

tinely produced both wet and dry process Arabica coﬀee, which would

meet any quality standard of either way. A thorough manual sorting

of the fruits provided a homogenous starting material of sound, fully

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434 CoCoa and Coffee fermentations

ripe coﬀee cherries, which were subsequently submitted to the dry and

wet procedure, respectively. e roasted coﬀees were assessed by expe-

rienced cuppers who testiﬁed that no undesired oﬀ-note had spoiled

the coﬀees—and still, the dry process coﬀee revealed its own speciﬁc

sensorial characteristics, as did the wet process coﬀee (Selmar et al.

2002). Similar experiments undertaken later with Robusta conﬁrmed

these results (Leloup etal. 2005). is means that process-immanent

factors should be responsible for the speciﬁc ﬂavor expressions of dif-

ferentially processed coﬀees. us, these factors must generate directly

or indirectly speciﬁc material diﬀerences, that is, diﬀerences in those

constituents of green coﬀee, which—during roasting—give rise to ﬂa-

vor components that in sum express the sensorial characteristic of a

dry or a wet process coﬀee.

Also a more global approach, using attenuated total reﬂection

Fourier transform infrared (ATR-FT-IR) spectroscopy, demon-

strated that apparently any substantial change in the postharvest

fermentation and drying procedure, respectively, inevitably aﬀects

the composition of the green beans and that of the resulting coﬀee

brews (Lyman et al. 2011).

12.3 Process-Speciﬁc Material Diﬀerences in Green Coﬀee Beans

As mentioned above, the well-known diﬀerences between dry and wet

process coﬀees are due to diﬀerences in the composition of certain

substances. In living organisms, the substance spectrum is inﬂuenced

directly by their metabolism. Consequently, when the inﬂuence of pro-

cess-immanent factors on the diﬀerent quality expression should be

investigated, the metabolic reactions occurring in green coﬀee beans

while processing have to be analyzed. e following chapter summa-

rizes corresponding biochemical and molecular biological ﬁndings

and results, which mostly have been elaborated in the past decade, and

which form the background for the novel plant biology-based insight

in the understanding of green coﬀee processing.

12.3.1 Caﬀeine, Chlorogenic Acids, and Trigonelline

While caﬀeine is barely aﬀected by roasting, the chlorogenic acids

undergo drastic changes (Purdon and McCamey 1987), which lead

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metaboliC responses of Coffee beans

to the generation of a wide range of coﬀee ﬂavor-determining com-

pounds (Cliﬀord 2007). Consequently, if the chlorogenic acid com-

position of the green coﬀee beans is also speciﬁcally aﬀected by the

way of processing, the chlorogenic acids and their roast products

might be involved in characterizing the distinct ﬂavor portfolios of

washed or unwashed coﬀees. Trigonelline is also severely aﬀected

by roasting (Viani and Horman 1974), its roast products being less

prominent as ﬂavor components, but quite important for human

nutrition and health (Stadler et al. 2002; Somoza et al. 2003;

Farah2009).

Balyaya and Cliﬀord investigated the eﬀects of wet and dry pro-

cessing on the contents of caﬀeine and chlorogenic acids of Arabica

and Robusta coﬀees (Balyaya and Cliﬀord 1995a,b). Whereas these

authors did not ﬁnd signiﬁcant diﬀerences with regard to caﬀeine,

Guyot and coworkers reported small losses of caﬀeine (3%) during

the soaking phase of the wet process as compared to the dry pro-

cess (Guyot etal. 1995). However, other workers conﬁrmed the ﬁrst

observation that caﬀeine remained unchanged (Leloup etal. 2005;

Duarte etal. 2010; Joët etal. 2010). In contrast, chlorogenic acids

were proved to be aﬀected by processing—not only with reference

to total chlorogenic acids but also to chlorogenic acid subgroups or

individual chlorogenic acids (Balyaya and Cliﬀord 1995a,b; Guyot

etal. 1995; Leloup etal. 2005; Duarte etal. 2009, Joët etal. 2010).

A strong inﬂuence by the maturation stage of the coﬀee fruits was

reported (Cliﬀord etal. 1986; Cliﬀord and Kazi 1987; De Menezes

and Cliﬀord 1988; de Menezes 1994). Consequently, at least part of

the observed diﬀerences might be related to the maturation state of

the coﬀee cherries used for wet and dry processing.

Whereas in an older work (Chassevent etal. 1970), no inﬂuence

of processing was found on the content of trigonelline in Robusta

green coﬀee beans, in a more recent work, trigonelline was found to

be reduced by wet treatment (Leloup etal. 2005).

12.3.2 Free Amino Acids, Proteins, Peptides, and Biogenic Amines

Owing to their signiﬁcance for the roast aroma and cup quality, free

amino acids of the green beans are assigned to be the most important

coﬀee constituents (Macrae 1985). Consequently, their determination

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436 CoCoa and Coffee fermentations

has been the subject of several early investigations (Underwood and

Deatherage 1952; Barbiroli 1965; Walter etal. 1970; Campos and

Rodrigues 1973; Pereira and Pereira 1973; Pokorny etal. 1974; Tressl

etal. 1983). Free amino acids represent around 5% of the nitrogen frac-

tion of green coﬀee beans and are degraded during roasting to a very

high degree (Flament 2001). e speciﬁc impact of processing on the

composition of free amino acids in green coﬀee beans was ﬁrst investi-

gated by Arnold and Ludwig (1996). ese authors analyzed samples

of well-deﬁned sources as well as material resulting from laboratory

model-processing experiments. Unfortunately, they were not able to

demonstrate a clear eﬀect of processing on individual or total free

amino acids. Looking more closely to the experimental conditions cho-

sen, one must concede that their eﬀort of mimicking the dry posthar-

vest process did not really reﬂect the conditions in the ﬁeld, but rather

corresponded to an intermediate way of processing, generally referred

to as “semidry processing”: instead of drying the coﬀee beans inside the

whole fruit, as the dry process demands, the beans were pulped manu-

ally and directly submitted to drying in a laboratory oven.

In contrast to this study, later investigations demonstrated speciﬁc

inﬂuences of processing on the amino acid contents (Bytof etal. 2005).

ese diﬀerences do not only concern the total free protein amino

acids (Selmar etal. 2002) but also individual compounds, for example,

glutamic acid and gamma-amino butyric acid (GABA) (Bytof etal.

2005). Speciﬁcally, during dry processing, in green coﬀee beans there

is a rapid decrease of glutamic acid, which is accompanied by a strong

rise in the content of GABA. e fresh, untreated coﬀee beans hardly

contain any GABA and furthermore, coﬀee beans properly processed

via the wet method exhibit only very small amounts of GABA. ese

investigations proved for the ﬁrst time that signiﬁcant material diﬀer-

ences occur in green coﬀee beans, which are solely to be attributed to

the mode of processing (Bytof etal. 2005). e physiological back-

ground of this phenomenon is outlined in the following section.

Apart from the free amino acids, the bound amino acids present

in proteins and peptides also take part in ﬂavor-giving reactions dur-

ing coﬀee roasting (Ludwig etal. 1995, 1998, 2000; Montavon etal.

2003a,b). Although considerable protease activities could be extracted

from coﬀee beans—even if stored over several months (Ludwig et

al. 2000)—no speciﬁc impact of processing on the proteins of green

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metaboliC responses of Coffee beans

coﬀee beans has been reported so far. However, there is one curi-

ous exception, the “herbivorous method of coﬀee processing.” e

corresponding Kopi Luwak from Indonesia is prepared from beans,

which had been eaten by the palm civet (Paradoxurus hermaphroditus)

as entire coﬀee cherries. e digested and excreted beans are manu-

ally collected, washed, dried, and ﬁnally roasted to produce one of

the most expensive coﬀees of the world. e digestion process seems

to have an eﬀect on the protein composition of the unroasted beans

(Marcone 2004) and also on the composition of free amino acids

(eis etal. 2008).

Bioactive amines of green coﬀee have been analyzed by several

authors, mostly rather in relation to oﬀ-ﬂavors (Amorim etal. 1977;

Casal etal. 2002a,b, 2004; Dias et al. 2012; Cirilo etal. 2003; Oliveira

etal. 2005; Vasconcelos et al. 2005, 2007; Silveira etal. 2009). Based

on their statistical evaluation, some authors proposed to use the con-

tents of biogenic amines for the discrimination between green coﬀees

subjected to diﬀerent postharvest processes (Casal et al. 2004).

12.3.3 Low and High Molecular Carbohydrates

Carbohydrates represent a further important group of coﬀee con-

stituents, and comprise approximately half of the dry matter of the

bean (Bradbury 2001). Both fractions, low and high molecular car-

bohydrates present in green coﬀee, undergo extensive changes during

roasting. e low molecular carbohydrates take part in caramelization,

and—together with amino acids—they produce important coﬀee ﬂa-

vor compounds via Maillard reactions (Flament 2001; Redgwell and

Fischer 2006). Moreover, they are precursors of various aliphatic acids

(Ginz etal. 2000), and thus contribute to the acidity of the coﬀee bev-

erage. Coﬀee polysaccharides are involved in the formation of mela-

noidins during coﬀee roasting, which contribute to the color and ﬂavor

of coﬀee brews and turned out to be physiologically active components

(Gniechwitz etal. 2008). Finally, coﬀee polysaccharides are consid-

ered to be responsible for various facets of the mouthfeeling of the

beverage, for example, viscosity and foam stability (Nunes etal. 1997;

Navarini etal. 2004a,b). Consequently, process-speciﬁc inﬂuences on

the composition of the carbohydrates could be the further cause for the

characteristic ﬂavor diﬀerences between dry and wet process coﬀees.

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438 CoCoa and Coffee fermentations

Like in many other plants, in coﬀee, the composition of low molec-

ular carbohydrates like sucrose, fructose, and glucose largely depends

on mechanisms already acting during seed development (Rogers etal.

1999; Geromel etal. 2006). As for sucrose, the contents in Robusta

and Arabica beans diﬀer signiﬁcantly from another, evidently owing

to genetically diﬀerent control of anabolic and catabolic enzyme

activities in the later stages of seed development (Privat etal. 2008).

ere is also evidence that environmental inﬂuences, like sun, shade,

and altitude have a decisive impact on the sugar composition of the

coﬀee beans (Guyot et al. 1996; Geromel etal. 2008). Further inﬂu-

ences on green coﬀee carbohydrates occur during postharvest treat-

ments as outlined below.

In some early reports, the loss of sugars from Arabica coﬀee

beans during the fermentation step of wet processing was described

(Wootton 1973). Later experiments with Robusta beans revealed that

dry-processing resulted in a lower content of fructose as compared to

its content in beans obtained by wet treatment (Guyot et al. 1995).

Moreover, a correlation between the content of sucrose and the mode

of treatment was claimed to be established. However, Bucheli et al.

(1996) disagreed with those results after not being able to detect any

inﬂuence of the postharvest treatment on the sucrose contents of the

coﬀee samples analyzed, neither for Arabica nor for Robusta.

A more recent study, using ﬁeld-experimentally processed green

Arabica coﬀees from Brazil, conﬁrmed that sucrose is not signiﬁ-

cantly aﬀected by the mode of processing (Knopp etal. 2006). In

the same manner, no speciﬁc impact of processing on the contents

of the oligosaccharides stachyose, raﬃnose, and rhamnose could be

detected. In contrast, the contents of fructose and glucose are strongly

inﬂuenced by the postharvest treatment. While unwashed Arabicas

(green coﬀees prepared by dry processing) contain relatively high

contents of fructose and glucose, the washed Arabicas (processed in

parallel by the wet method) exhibit markedly lower amounts of these

hexoses (Knopp etal. 2006). Additional model processing experi-

ments, including a comparison with the untreated, fresh coﬀee beans,

demonstrated that the glucose and fructose concentrations in the

dry-process beans remained as high as those in the untreated con-

trol or even increased signiﬁcantly. In contrast, in the wet process

coﬀee beans, the ﬁnal content of glucose and fructose was reduced

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439

metaboliC responses of Coffee beans

drastically (up to 80–90%) as compared to the initial amount present

in the freshly harvested beans (Knopp etal. 2006). ese ﬁndings

have been conﬁrmed by other groups for Arabica (Bonnländer etal.

2007b; Joët etal. 2010). Without naming individual compounds,

Leloup etal. (2005) reported a reduction of free monosaccharides in

the wet processing of Robusta coﬀees. Also, the contents of the minor

free sugars, arabinose and mannose, were shown to decline during the

wet model processing (Knopp et al. 2006).

Semiwashed green coﬀees contain an intermediate amount of

glucose and fructose (Knopp etal. 2006). ese coﬀees, which are

characterized by qualities that are also situated between washed and

unwashed coﬀees (Illy and Viani 1995), are prepared by directly dry-

ing the depulped beans. Later, the results concerning semiwashed

Arabicas were conﬁrmed by other authors (Bonnländer etal. 2007a,b).

is is where a clear statement to the alleged interconnection of

sugars and the ﬂavor characteristics of unwashed Arabica should be

made: ere is the picture drawn from repeated scientiﬁc evidence

that during the dry processing the beans content of sucrose and other

low molecular sugars is virtually unaﬀected (see above). is picture

clearly stands in contradiction to the wide spread opinion, during the

drying of the whole fruits, sugars would migrate from the fruit ﬂesh

through the parchment into the bean. Not only that the latter opinion

lacks any scientiﬁc evidence, one should ﬁnd it diﬃcult to imagine

suitable mechanisms or routes in an entity where the major transport

direction is the water diﬀusion from inside to the outside. e same

arguments apply when it comes to explain the intermediate ﬂavor of

semiwashed coﬀee beans: the wet process includes mechanisms for

reducing sugar contents in the beans, the dry process does not. Other

phenomena must be responsible for the stronger expression of “body”

in the brew derived from unwashed Arabica.

e polysaccharide contents of Arabica and Robusta coﬀee seeds

account for a similar percentage (approximately 55%) of the dry

weight of the bean and reveal a similar composition (Fischer etal.

2001); however, there are some hints for speciﬁc diﬀerences in their

structural features, and in the percentage of mannose and galactose of

the total polysaccharides (Fischer etal. 2002; Redgwell etal. 2002).

It is assumed again that speciﬁcally diﬀerent gene expression during

seed maturation might be responsible for that (Pré etal. 2008).

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440 CoCoa and Coffee fermentations

Like low molecular carbohydrates, the high molecular ones are

also aﬀected by coﬀee postharvest treatment: for Robusta coﬀee, the

amount of polysaccharides (i.e., cell wall polysaccharides (CWP)

like arabinogalactans and storage polysaccharides like mannans) was

reported to increase during the course of wet processing (Leloup

etal.2005). As we can assume that the amount of polymeric stor-

age carbohydrates will not increase during germination, this ﬁnding

seems to be contradictory. However, we have to consider that the

extractability of complex carbohydrates will increase during germina-

tion and this might explain the putative increase in their content. In

this manner, just recently, it was reported that postharvest treatments

in general enhance the extractability of the polysaccharides present in

coﬀee beans, demonstrating a signiﬁcant impact of the processing on

the high molecular carbohydrate fraction. is eﬀect was strongest

in the case of wet processing and less pronounced when the beans

had been processed by the dry or the semidry method (Tarzia et al.

2010). Likewise, a signiﬁcant increase in the overall content of CWP

during wet processing was reported (Joët etal. 2010). It was not ruled

out that the putative CWPs, which were determined by measuring

the defatted alcohol insoluble residue of coﬀee seed, comprise other

compounds that might account for the observed changes (Joët etal.

2010).

12.3.4 Lipids

Although the lipids of coﬀee beans are not involved in Maillard reac-

tions during roasting, they reveal an important inﬂuence on the ﬂa-

vor of the beverage, primarily because they solve hydrophobic ﬂavor

components and thus signiﬁcantly contribute to the mouthfeeling of

the product (Illy and Viani 1995, 2005). Undesired oxidations of tri-

glycerides or free fatty acids are considered to be the predominant

cause of green coﬀee staling during storage (Multon 1974; Nikolova-

Damyanova etal. 1998; Dussert etal. 2006; Speer and Kölling-Speer

2006). Examples of coﬀee ﬂavor compounds derived from the lipid

fraction of coﬀee are the linalool enantiomers (Bonnländer et al.

2006) and the rather unpleasant “woody” oﬀ-note represented by

trans-2-nonenal (Parliment etal. 1973; Bonnländer etal. 2007a,b), a

well-known indicator for old crop coﬀee.

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metaboliC responses of Coffee beans

It has been suggested to use the fatty acid proﬁle of green and of

roasted coﬀee samples as a coﬀee species or variety marker, which

may also provide information on the provenience background of a

coﬀee (Alves et al. 2003; Bertrand et al. 2008). However, recently, it

was shown that climatic and environmental factors have such a strong

impact on the fatty acid proﬁle that they prevent any deductions of

coﬀee origin by this parameter (Villarreal etal. 2009). On the other

hand, this ﬁnding gives reason to expect that the lipids will also be

diﬀerentially aﬀected by the speciﬁc environments of diﬀerent kinds

of processing. Yet, up to now, only few attempts have been made to

investigate the impact of processing on the composition of triglycer-

ides. In this context, just the inﬂuence of variations in the dry pro-

cessing was analyzed; unfortunately, no clear results were obtained

(Jham et al. 2001, 2003). More recently, Bonnländer et al. (2006)

demonstrated that the postharvest treatment of coﬀee inﬂuences the

enantiomeric ratio of linalool. Whereas wet processed coﬀees reveal

nearly a racemic mixture of linalool, in corresponding dry processed

coﬀees, an excess of S-(+)-linalool was detected (Bonnländer etal.

2006). Moreover, Joët et al. reported a signiﬁcant increase in the lipid

contents as a result of wet processing, however without giving a physi-

ological explanation (Joët et al. 2010).

In conclusion, it has to be stated that the composition of numerous

metabolites, particularly that of amino acids (Bytof etal. 2005), car-

bohydrates (Knopp etal. 2006), and chlorogenic acids (Balyaya and

Cliﬀord 1995a,b; Guyot etal. 1995; Leloup etal. 2005; Duarte etal.

2009), diﬀers signiﬁcantly in wet and dry processed green coﬀees—

even if entirely identical starting material was used. Table 12.1 gives

an overview of the green coﬀee components aﬀected by postharvest

treatment. Consequently, the mode and extent of metabolic processes

occurring within the living coﬀee seeds and thus the metabolic status

of these coﬀee beans must diﬀer, too.

12.4 Coﬀee Seeds Represent Living OrganismsExhibiting

an Active Metabolism

When plant fruits acquire maturity, the contained seeds are already

fully developed. However, in many cases, these seeds are not yet

able to germinate. A speciﬁc maturation drying may be required to

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442 CoCoa and Coffee fermentations

Table 12.1 Compilation of Green Coffee Constituents and the Effect of Coffee Postharvest Treatment

COMPONENT ARABICA AVERAGE CONTENT ROBUSTA AVERAGE CONTENT SPECIFIC EFFECT OF POSTHARVEST TREATMENT

Caffeine 1.0%(1)

0.6% for the mutant Laurina (2)

2.0%(1) Not affected(3−5)

Trigonelline 0.79–1.05%(6) 0.32–0.68%(6) Slight reduction during wet treatment with Robusta(3)

and Arabica(11)

Chlorogenic acids 5–7.5%(7) 7–10%(7) Changes in total chlorogenic acids changes in

individual chlorogenic acids and subgroups(3,8−12)

Free amino acids 0.27–0.48%(13)

GABA: 0.08%(14)

0.35–0.60%(13)

GABA: 0.06%(14)

Conversion of glutamic acid into γ-aminobutyric acid

(GABA) to various extents during dry processing(15)

Bioactive amines 0.007%(16)–0.011% (17) 0.002%(16) Spermine/spermidine/putrescine may be suitable for

the statistical distinction between wet and dry

process Arabicas(16)

Peptides and proteins Proteins: 8.7–9.6%(18)

Peptides: 0.34–0.95%(19)

Protein: 9.2–9.7%(18)

Peptides: 0.30–0.59%(19)

No speciﬁc impact of processing on proteins and

peptides(19)

Carbohydrates Total: 55.8%(20)

Sucrose: 6.25–8.3%(21)

Glucose: 0.01–0.45%(21)

Fructose: 0.02–0.4%(21)

Mannan: 22%(22)

Cellulose: 6.7–7.8%(22)

Arabinogalactan: 14%(22)

Total: 55.5%(20)

Sucrose: 1.3–4.9%(21)

Glucose: 0.01–0.5%(21)

Fructose: 0.2–0.55%(21)

Mannan: 22%(22)

Cellulose: 7.8–8.7%(22)

Arabinogalactan: 17%(22)

Sucrose not affected(5,23)

Fructose and glucose drastically reduced during wet

processing(23)

Polysaccharides are better extractable after wet

processing(5,24)

Lipids 15%(25) 10%(25) Increase of lipids during wet processing(5)

1: Ashihara etal. (2008); 2: Baumann etal. (1998); 3: Leloup etal. (2005); 4: Duarte etal. (2010); 5: Joët etal. (2010); 6: Stennart and Maier (1993); 7: Clifford (1985); 8:

Balyaya and Clifford (1995a,b); 9: Guyot etal. (1995); 10: Duarte etal. (2009); 11: Duarte etal. (2010); 12: Joët etal. (2010); 13: Arnold and Ludwig (1996); 14: Casal etal.

(2003); 15: Bytof etal. (2005); 16: Casal etal. (2004); 17: Oliveira etal. (2005); 18: Thaler and Gaigl (1962); 19: Ludwig etal. (2000); 20: Fischer etal. (2002); 21: Silwar

and Lüllmann (1988); 22: Bradbury (2001); 23: Knopp etal. (2006); 24: Tarzia etal. (2010); 25: Speer and Kölling-Speer (2006).

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443

metaboliC responses of Coffee beans

complete seed ripening, resulting in so-called dormant seeds, which

are characterized by their inability to germinate even after appropri-

ate reimbibition. Only when the dormancy is broken, for example, by

deep temperatures, the seeds are able to germinate. Seeds exhibit-

ing such an attitude are referred to as “orthodox seeds” (Roberts 1973;

Bewley and Black 1994). e biological signiﬁcance of this feature is

given by the fact that—especially in temperate climatic zones—the

seeds that are becoming mature in late summer are hindered to ger-

minate before next wintertime, and thus to prevent any dieback of the

tender seedlings in the cold time of the year. Yet, such seasonal regu-

lation is not required in tropical climates. Consequently, the seeds

of most tropical plants do not exhibit any maturation drying. ese

seeds, which characteristically reveal a relatively high water content,

are ready to germinate already in the mature fruits. However, in gen-

eral, any germination process within the fruits is prevented by various

active mechanisms, for example, by germination inhibitors, by phyto-

hormones like abscisic acid, or by a high osmotic potential of the pulp

(Bewley and Black 1994). erefore, as soon as such seeds are released

from the pulp, germination will proceed. Owing to their high water

content, such seeds could not be stored for a longer time period. As

any attempt to elongate this storage time (e.g., by drying the seeds)

usually fails, these seeds are not suitable for longer storing in seed

banks and were classiﬁed as “recalcitrant” (Roberts 1973; Bewley and

Black 1994).

Coﬀee seeds (Coﬀea arabica L.) reveal an intermediate behavior, sit-

uated between orthodox and recalcitrant seeds (Ellis et al. 1990, 1991;

Hong and Ellis 1990, 1992, 1995; Eira etal. 1999). Like recalcitrant

seeds, the water content of coﬀee seeds inside the ripe fruit is relatively

high (about 52% on fresh weight basis, Eira etal. 2006). Moreover, a

true maturation drying of coﬀee seeds is lacking, or it is not very pro-

nounced (de Castro and Marraccini 2006; Eira etal. 2006; Hinniger

etal. 2006). Coﬀee seeds are able to germinate already when iso-

lated from yellowish-green fruits at around 225 days after anthesis,

weeks before a fully red exocarp indicates fruit maturity (de Castro

and Marraccini 2006; Eira etal. 2006)—further properties charac-

teristic for recalcitrant seeds. However, in contrast to typical recalci-

trant seeds, and like orthodox seeds, coﬀee seeds can be dried down

to a water content of less than 10% without immediate loss of their

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444 CoCoa and Coffee fermentations

viability. In contrast to orthodox seeds, this drying will not prolong

the viability during storage; the opposite eﬀect is described for coﬀee

seeds: a prolongation of viability during storage is achieved by high

moisture content (Valio 1976; Van der Vossen 1980).

Owing to the features mentioned above, it can be deduced that

directly after the harvest of the coﬀee cherries the mature coﬀee seeds

must be metabolically active. is activity should be maintained dur-

ing the course of processing. is can be easily demonstrated by a

standard viability test, using triphenyltetrazolium chloride (TTC)

(Dias and da Silva 1986). When green coﬀee seeds, either wet or

dry processed, are incubated with TTC, the reaction is quite evident

(Figure 12.1). e deep red color points out the extensive viability

and metabolic activity: the cells take up the TTC and due to the

presence of active dehydrogenases and suﬃcient amounts of reduction

equivalents—preferentially NADH + H+—the compound is reduced

to yield the red-dyed formazan.

A further evidence for their metabolic activity is given by the ﬁnd-

ing that the dry weight of coﬀee seeds decreases during processing

(Figure 12.2). In ﬁeld experiments in Mexico, Gonzales-Rios and co-

workers (2007a) compared diﬀerent ways of coﬀee fermentation with

the mechanical way of mucilage removal, a treatment which omits

the fermentation step. e results also reﬂected a loss of dry mat-

ter in beans submitted to tank fermentation plus subsequent drying

as compared to beans submitted to mechanical washing plus drying:

Figure 12.1 Green coffee beans are viable and metabolically active. A coffee bean was sliced

in thin disks and incubated in triphenyltetrazoliumchloride (TTC) solution as described by Selmar

etal. (2008). After the viable and metabolic active cells have imported the TTC, it is reduced and a

red dye is formed.

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445

metaboliC responses of Coffee beans

the yield of the latter expressed in weight of green beans/weight of

cherries, was found out to be 15% higher. Indeed, to a certain extent,

the loss of biomass might be due to leaching of substances (Wootton

1973), for example, from injured seeds. However, as the decrease of

dry matter continues, throughout the drying on the patio, leaching

processes must be considered as a minor cause for the dry matter loss

during processing (Bytof 2003). e results from semiwashed coﬀee

beans (i.e., depulped and dried, see Figure 12.2), which did not expe-

rience tank fermentation were signiﬁcantly lower than those depulped

fermented, but not dried on the patio, yet, but on the same level as

those fully washed (i.e., depulped, fermented, and dried) suggesting

that the major loss of dry matter did not take place in the tank but

rather on the patio. Consequently, the major loss of biomass must be

due to the consumption of organic matter by metabolic processes, that

is, respiration and/or anaerobic fermentation. A decrease of pH in the

beans as recorded for both, Arabica and Robusta coﬀee tank fermen-

tation (Velmourougane 2013), may primarily suggest the occurrence

of lactic acid fermentation in the tissue of the endosperm. Although

experimental proof for this theory is missing, the conclusion is quite

160

140

Seed dry matter (mg)

120

100

FreshDepulped Depulped

and

fermented

Depulped

and dried

Depulped,

fermented,

and dried

Figure 12.2 Loss of dry matter during the course of green coffee processing. Coffee beans—

still surrounded by the parchment (=pergaminho)—had been taken directly out of a commercial

processing facility, either directly after depulping or after 24 h of fermentation. For the “dried” vari-

ant, the parchment beans additionally were dried for 24 h. In the “fresh” trial, the parchment beans

were taken directly out of the fruits before the dry weight was determined. For this, the beans were

dried in a laboratory oven at 110°C for 36 h. Prior to weighing, the beans were hulled to remove the

parchment. For each trial, ﬁve samples of 1000 seeds were analyzed. (Adapted from Bytof, G. 2003.

Einﬂuss der Nacherntebehandlung auf die Qualitätsausprägung bei Arabica-Kaffee (Coffea arabica

L.). Braunschweig, Technische Universität, Germany.)

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446 CoCoa and Coffee fermentations

self-evident. Such a proof could be expected, for example, from an

aseptic (to rule out microbial activity!) incubation of coﬀee seeds in a

Warburg manometer apparatus.

As mentioned above, the most convincing proof that coﬀee seeds

indeed are metabolically active while they are processed is given by

the fact that the composition of several substances undergoes changes

particularly distinctively in wet and dry processed green coﬀee beans.

Whereas the content of glucose and fructose remains relatively

unchanged throughout dry processing, it decreases by up to 90% in

wet processing (Knopp etal. 2006).

Moreover, the amino acid spectrum of the diﬀerentially processed

coﬀee beans diﬀers signiﬁcantly (Bytof etal. 2005). It turned out that

the overall content of amino acids reveal high individual variations: even

when the mature coﬀee cherries are harvested at the same plantation, but

several weeks later, the corresponding diﬀerences might be higher than

the diﬀerences between wet and dry processed beans (Selmar etal. 2005).

It was concluded that there must be a signiﬁcant metabolic involvement

of amino acids in mature coﬀee seeds, which obviously persists through-

out the course of processing and which results in the diﬀerences in amino

acid contents in wet and dry processed coﬀee beans.

Altogether, there should be no doubt that metabolic reactions occur

in coﬀee beans while being processed, their extent depending on the

outer conditions of the processing applied.

12.5 Germination Physiology during Green Coﬀee Processing

As outlined above, germination in mature coﬀee cherries is sup-

pressed by various active principles present in the fruit ﬂesh. As soon

as the pulp is removed, for example, in the course of wet processing,

germination will be initiated. Although these facts should be obvious,

their consequences for green coﬀee processing have been considered

only during the last 10 years.

ere are several approaches to analyze the onset of germination of

a plant seed and to quantify its progress. In the past, the most promi-

nent procedure was based on the determination of the activities of

germination-speciﬁc enzymes. However, such analyses are problem-

atic, since seeds like cocoa or coﬀee contain lots of phenolic com-

pounds that might inactivate the enzymes by tanning reactions. To

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447

metaboliC responses of Coffee beans

avoid this problem, the expression of germination-speciﬁc enzymes is

now analyzed.

One of the ﬁrst enzymes expressed during the germination of seeds

is the isocitrate lyase (ICL), the key enzyme of the glyoxylate cycle,

which is responsible for the conversion of fatty acids into carbohydrates

(Zhang etal. 1993; Reynolds and Smith 1995). Expression of ICL is

considered suitable to determine the transition from late embryogene-

sis to germination (Goldberg et al. 1989). Accordingly, ICL expression

in coﬀee seeds during the course of green coﬀee processing was ana-

lyzed (Bytof etal. 2007). However, after the overall expression of ICL

in coﬀee seeds during early germination turned out to be too low for a

reliable Northern blot evaluation (Selmar etal. 2006), the correspond-

ing expression studies were performed as a competitive RT-PCR using

an internal standard. ese analyses revealed that ICL is expressed in

coﬀee seeds during the course of wet processing as well as during dry

processing, and conﬁrmed the postulate that coﬀee seeds do germi-

nate during the course of processing. However, the corresponding time

frames of ICL expression are quite diﬀerent. In the course of wet pro-

cessing, maximum expression occurs within 2 days after the start of

the treatment (Figure12.3), whereas in dry processed coﬀee seeds the

highest transcription level of ICL was only detectable about 1 week

after the commencement of processing (Selmar et al. 2006; Bytof

etal. 2007). e extent of germination-related metabolism is evidently

reduced by the decrease of water in the coﬀee beans submitted to dry-

ing, retarding all metabolic reactions and virtually ceasing them as soon

as the ﬁnal residual water content in the coﬀee bean of about 11% is

achieved.

Since a reliable, monofactorial estimation of the onset and prog-

ress of germination is quite problematic—especially in recalcitrant and

intermediate seeds—additionally, the cell cycle activity in the embryo

was analyzed (Bytof etal. 2007). For this, the presence of β-tubulin

was quantiﬁed as well as the ratio of 4C to 2C nuclei,* which was mea-

sured by ﬂow cytometry as previously applied for the seeds of various

* Arabica coﬀee (Coﬀea arabica, L.) is already tetraploid (4C). Consequently, the tech-

nically correct term for a nucleus with doubled sets of chromosomes would actually

be 8C. However, in order to make things less complicated, we decided to maintain

the traditional nomenclature, where 2C stands for regular, vegetative cells and 4C

for cells containing a doubled set of chromosomes.

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448 CoCoa and Coffee fermentations

plant species (de Castro etal. 1995, 1998; Hilhorst etal. 1998; Jing

etal. 1999; Vázquez-Ramos and Sánchez 2003). e well-reviewed

tomato, Solanum lycopersicon, oﬀers itself as a comparison model for cof-

fee seed biology, from which many biological aspects might be trans-

ferred: Tomato and coﬀee belong to the sister families Solanaceae and

Rubiaceae; their seeds have many morphological features in common

(Pinto et al. 2007), and also extensively share a common gene repertoire

(Lin etal. 2005; Lefebvre-Pautigny etal. 2010). e results concerning

the cell cycle activity of the coﬀee embryo fully conﬁrm the data on the

ICL expression: in wet processed seeds, the highest β-tubulin accumu-

lation was detected 2 days after the start of treatment (Figure12.3),

corresponding to the ﬁrst day of drying of the parchment coﬀee. In

contrast, during dry processing, the strongest signals were detected 6

days after initiation of processing, albeit at comparable maximum lev-

els as in the wet processed seeds. e ﬂow cytometric data for coﬀee

embryos, extracted from diﬀerentially processed coﬀee seeds, revealed

a pattern similar to that derived from β-tubulin analysis (Figure 12.3).

Maximum 4C content was found in samples with the highest β-tubulin

accumulation. e rate of DNA replication (increase in the frequency

of 4C nuclei) was higher in embryos from wet processed beans com-

pared to those from dry processed ones. Nevertheless, the maximum

frequency of 4C nuclei was much lower in the dry processed beans.

is relative reduction in 4C nuclei content as compared to the levels

of β-tubulin might be related to a requirement for higher seed moisture

levels for initiation of DNA replication compared to the initiation of

β-tubulin accumulation, as previously observed in priming experiments

with tomato seeds (Groot etal. 1997).

Taken together, these data substantiate that germination processes

take place in coﬀee seeds during processing and that the time frame

of the related reactions diﬀers clearly in wet and dry processed beans.

is leads to the question if germination metabolism and possibly also

reserve mobilization may be responsible for the processing-related

material diﬀerences in coﬀee beans speciﬁed in Section 12.2—a ques-

tion that can only be answered on the basis of investigations of coﬀee

seed physiology.

Apart from the experiments mentioned above, there are not

much data on coﬀee germination that could be interpreted with

respect to their impact on metabolic processes of coﬀee beans under

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449

metaboliC responses of Coffee beans

processing. Indeed, there are many more other studies on coﬀee

seed germination. However, the majority of these studies and the

corresponding cognitions originate from experiments with coﬀee

seeds, which had already run through the whole sequence of wet

coﬀee processing, including drying, and then were reimbibed. As

it was already outlined above, germination is initiated after pulp-

ing and shut down by the drying procedure. Consequently, such

experiments with already wet-processed and reimbibed coﬀee seeds

would never reﬂect the whole episode of coﬀee germination sensu

stricto but at most a sequel of the interrupted metabolism, which

once came to halt with the drying of the coﬀee beans. e outcome

of those experiments would be rather irrelevant for an interpreta-

tion of coﬀee seed physiology during processing. With respect to

common nomenclature of seed physiologists, the seeds used in such

Wet processing

Amount of ICL-RNA

(pg/100 ng total-RNA)

Content of β-tubulin

(relative units)

Amount of

4C-nuclei (%)

3.0

2.0

1.0

0.0

6.0

4.0

2.0

0.0 0

FreshFresh

Ferm.Drying Drying

123457 01 2345 7910 146

ICL-expression

Abundance of β-tubulin

Changes in the 4C-nuclei-content

Dry processing

Figure 12.3 Quantiﬁcation of germination-related processes during wet and dry processing.

Coffee beans resulting from model processing under deﬁnite conditions were used for the quanti-

ﬁcation of common markers for germination. ICL expression was quantiﬁed by competitive RT-PCR

using an internal standard, the abundance of β-tubulin was determined by immunostaining of

Western blots, and the amount of C4 nuclei was estimated by ﬂow cytometry. The data are compiled

from earlier publications. (Adapted from Bytof, G. et al. 2007. Annals of Botany, 100:61–66; Selmar,

D. and Bytof, G. 2007. Green coffee is alive! A review on the metabolic processes taking place in

coffee beans during processing and their implication for modern coffee research. 21ème Colloque

Scientiﬁque International sur le Café, Montpellier, France, pp. 423–433.)

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450 CoCoa and Coffee fermentations

experiments would be classiﬁed as “primed seeds” (McDonald 2000;

Varier et al. 2010). We can only speculate that practical reasons may

be the key for such an experimental design. However, it is self-evi-

dent that no one can expect the same physiological events to occur

and time courses in germinating fresh seeds as compared to “primed

seeds” after reimbibition.

In the case of a so-called endosperm-limited germination, the

endosperm acts as a mechanical barrier for radicule protrusion.

Coﬀee seeds (Arabica as well as Robusta) exhibit a case of a so-

called endosperm-limited germination, meaning, a special part of

the tissue, the so-called endosperm-cap, acts as a mechanical bar-

rier for radicule protrusion (Pinto etal. 2007). After germination

is initiated, in such seeds, a decline in the mechanical resistance

of the micropylar endosperm is caused by the activity of various

hydrolases against the structures of the cell walls (Leubner-Metzger

2003; Nonogaki et al. 2007). In the case of coﬀee, correspond-

ing enzyme activities have been described by various groups: cel-

lulases, mannanases, mannosidases, and galactosidases (Petek and

Dong 1961; Courtois and Le Dizet 1966; Shadaksharaswamy and

Ramachandra 1968; Takaki and Dietrich 1979; Giorgini 1992;

Giorgini and Comoli 1996; Marraccini etal. 2001, 2005; da Silva et

al. 2005; Shen etal. 2008). Two distinct endo-ß-mannanase genes,

manA and manB, have been isolated and cloned from germinating

coﬀee beans (Marraccini et al. 2001). In coﬀee seeds, the endo-

ß-mannanase enzyme is also present in the endosperm cap, but

unlike in tomato the increase of its activity was strongly suppressed

by the plant hormone abscisic acid (ABA) (da Silva etal. 2005).

Surprisingly, in coﬀee, the expression of manA and manB and thus

the activity of endo-ß-mannanase is strongly inhibited by exogenous

application of the plant hormone gibberellic acid (GA3) (Marraccini

etal. 2001), while endogenous GAs nonetheless are required for

germination (da Silva etal. 2005). A suitable explanation for this

unusual response to phytohormones might be the involvement of

two distinct functions, that is, endosperm weakening and mobiliza-

tion of reserves (Nonogaki etal. 2007). Yet, the activities of these

enzymes do not sooner increase than approximately 4 days (96 h)

after imbibition (Marraccini etal. 2001, 2005; da Silva etal. 2005).

Consequently, these activities are associated with a phase occurring

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451

metaboliC responses of Coffee beans

rather too late during germination for playing a major role in cof-

fee processing: with a time frame of 12–90 h for tank fermentation

(Rolz etal. 1973), the wet phase of coﬀee processing seldomly lasts

this long—and in the case of dry processing, germination processes

are turned down already during the early germination phase (see

above). However, an involvement of cell wall hydrolyzing enzymes

in the degradation of galacto-mannans during processing cannot be

totally excluded.

Apart from the enzymatic activities so far mentioned, only one

more metabolic event has been described to occur in germinating cof-

fee seeds within the time frame of green coﬀee processing: the expres-

sion of oleosin (Simkin etal. 2006). e expression of at least one

of the oleosin genes signiﬁcantly increases within the ﬁrst days after

the onset of germination. Since one function of oleosins is to orga-

nize the lipid reserves of seeds in small, easily accessible structures

(Huang 1992), any changes of their amounts could have an impact

on the general metabolism of germinating seeds: owing to the much

higher surface of the seed oil bodies formed, lipase-mediated hydro-

lysis of triacylglycerols into free fatty acids is believed to be facilitated

(Huang 1996).

Based on all data mentioned so far, we conclude that our current

knowledge on the germination processes occurring during green

coﬀee processing and their signiﬁcance on coﬀee quality is limited.

Much more research is needed to elucidate this fascinating ﬁeld of

plant biology and its impact on the sensory proﬁle of coﬀee.

12.6 Green Coﬀee Beans Experience Drought Stress While Drying

No matter whether green coﬀee is processed by the wet or dry method,

in any case, the beans have to be dried in order to produce tradable,

standard green coﬀees. In the dry process, the drying of coﬀee beans

from initially 50% water content down to 12% takes place inside the

fruit. Owing to the water-rich fruit ﬂesh, the drying of the entire

fruit (approximately 70% water content) takes about 3 weeks. In con-

trast, drying of the pulped and fermented parchment coﬀees on the

patio only requires several days (Brando 2004). Coﬀee drying may be

performed in diﬀerent ways. In the case of the wet processed parch-

ment coﬀees, apart from the classical sun drying on a drying patio or

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452 CoCoa and Coffee fermentations

on drying racks, various mechanical dryers are in use (static dryers,

column dryers, round dryers, etc.; for review, see Brando 2004, and

Chapter 10). In contrast, mechanical drying of the entire fruits occurs

quite rarely, since dry processing is abundant especially in those areas,

where the climatic conditions favor sun drying. ere is no doubt

that the drying process has a marked impact on coﬀee quality, but

generally there are various practical and economical considerations,

which determine the mode and progression of drying on the planta-

tion. Consequently, coﬀee farmers have to ponder drying capacities,

energy costs, and coﬀee quality.

Although it is frequently mentioned in the literature that sun dry-

ing has a beneﬁcial eﬀect on green coﬀee quality (Wootton etal.

1968; Gibson 1973; Sivetz and Desrosier 1979a,b; Jham et al. 2001;

Pimenta and Vilella 2001), only little research work has been done to

assess the actual beneﬁt of sun drying or the pros and cons of either

drying system. Including the few studies that were done over 40 years

ago (Wootton etal. 1968; Gibson 1973), these considerations are

based mainly on traditional knowledge and ﬁeld experience, without

oﬀering a scientiﬁc background. In the older literature, the quality

diﬀerences between machine- and sun-dried coﬀees are explained by

various eﬀects of visible or UV light on the drying beans (Gibson

1973). However, in this context, it has to be conceded that the opaque,

hardly translucent parchment predominantly is impermeable for most

light qualities. us, there must be another explanation for the merits

of sun drying. Recent considerations refer to the fact that the major

diﬀerence between sun and machine drying is that machine drying

is performed continuously whereas sun drying follows a natural day-

and-night rhythm (Kleinwächter and Selmar 2010). Unfortunately,

up to now it could not be elucidated how the mode and extent of

the drying procedure indeed aﬀects the overall quality of green cof-

fee. According to the deductions mentioned above, the diﬀerent green

coﬀee qualities must be due to diﬀerences in the composition of sub-

stances present in the coﬀee beans. Yet, corresponding diﬀerences in

the physiological status of diﬀerentially dried seeds have been demon-

strated (Kleinwächter and Selmar 2010).

As stated above, during the wet process, the content of glu-

cose and fructose decreases (Knopp etal. 2006; Bonnländer et al.

2007a,b), and this mainly takes place during the ﬁrst day of drying

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453

metaboliC responses of Coffee beans

(Kleinwächter and Selmar 2010). us, the decrease in the sugar con-

tent cannot be due to leaching as proposed earlier (Wootton 1973),

but must be related to the metabolic events occurring during drying.

Further evidence for the occurrence of metabolic reactions during

drying has been presented by Bytof etal. (2005), who have shown

that the nonprotein amino acid, GABA, is produced in the course

of the drying process (see above). e extent of GABA accumula-

tion strongly depends on the drying conditions and is highest in dry

processed seeds (Bytof etal. 2005). In plants, GABA corresponds to

a stress metabolite, which is produced by enzymatic decarboxylation

of glutamic acid as a characteristic stress response. e key enzyme

is a glutamate decarboxylase that is rapidly activated by calcium/

calmodulin and, therefore, does not require the biogenetic route of

gene expression, which would otherwise be rather time consuming

(Satya Narayan and Nair 1990; Bown and Shelp 1997; Bouché and

Fromm 2004).

e presence of elevated amounts of GABA in green coﬀee had

already been reported in previous works (Walter etal. 1970; Pokorny

etal. 1974; Tressl etal. 1983; Trautwein and Erbersdobler 1989).

Similarly, Casal etal. (2002a,b, 2003) have already reported that

reduced amounts of glutamic acid seemed to be statistically associ-

ated with dry process green coﬀee beans. Although Trautwein (1987)

already recognized a reciprocal relation between the concentrations of

free glutamic acid and free GABA in commercial green coﬀee samples,

no suggestions for the biological cause of this phenomenon had been

made in the literature. Unfortunately, in many earlier investigations

of amino acids, the stress-induced GABA accumulation could not be

detected, apparently because the chosen methodology did not include

GABA (Arnold et al. 1994; Arnold and Ludwig 1996; Leloup etal.

2005). Applying a GC-MS method, just recently the signiﬁcance of

the free amino acid GABA was not only conﬁrmed for Arabica and

Robusta coﬀee but also for Liberica or Excelsa coﬀee (Coﬀea liberica

Hiern), however the impact of processing was only remotely discussed

(Lonzarich et al. 2013).

e hypothesis that the observed accumulation of GABA in green

coﬀees is due to stress-related reactions that occur during processing is

supported by further evidence. Just recently, the stress status of coﬀee

during the course of drying was investigated (Kramer et al. 2010). In

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454 CoCoa and Coffee fermentations

addition to the accumulation of GABA, the expression of dehydrins,

which are small proteins that play an important role in the desiccation

tolerance of plant cells, was also estimated. Apart from their relevance

for several types of stress conditions, dehydrins are also abundant in

late embryogenesis (Close 1997; Allagulova etal. 2003). Owing to

their great hydrophilicity and thermostability, it is assumed that dehy-

drins function as structure stabilizers with detergent- and chaperone-

like properties (Borovskii et al. 2000, 2002). As the occurrence of

dehydrins is related to drought stress and dehydration, they represent

reliable markers for these metabolic situations (Nylander etal. 2001;

Rodriguez et al. 2005; Samarah etal. 2006). More recently, several

groups investigated the expression or abundance of enzymes during

the drying of coﬀee beans, most of them generally associated with

stress, as there are superoxide dismutase, guaiacol peroxidise, gluta-

thion reductase (Rendón et al. 2013), peroxidases, esterases, and cata-

lase, plus the germination-associated endo-ß-mannanase (Santos et al.

2013).

e investigation of the metabolic processes in drying coﬀee

beans revealed that the corresponding situation is quite complex

(Kramer et al. 2010) and, like germination (see above), it repre-

sents an entire physiological syndrome where numerous reactions

seem to run in parallel and/or subsequently. is complexity is even

greater, because of the concomitance of the stress-related reactions

and the various germination-related processes, an issue that so far

has been observed for a number of plants (Crèvecoeur et al. 1976;

Hegarthy 1977; Hegarthy and Ross 1979, 1980/1981; Spyropoulos

and Lambiris 1980; Botha and Small 1985; de los Reyes etal. 2003).

In this context, it must be considered that all these metabolic reac-

tions will be slowed down when the water content is decreasing in

the course of drying, leading ﬁnally to a virtual shut oﬀ of the entire

metabolism due to water deﬁciency. Consequently, apart from the

various physiological up and down regulations of germination and

stress-related processes, the metabolism is also directly inﬂuenced by

the increasing water shortage. Accordingly, various phases of GABA

accumulation could be detected. e ﬁrst phase, which apparently is

due to either mechanical irritation of the coﬀee beans caused by the

depulping procedure and/or due to a lack of oxygen, mostly will not

be recognized, since the accumulated GABA is rapidly degraded

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455

metaboliC responses of Coffee beans

(Kramer et al. 2010) (see Figure 12.4). Yet, it is very likely that

this ﬁrst phase of GABA accumulation is related to the strong

decrease of the oxygen concentration, which generally occurs dur-

ing tank fermentation (wet process). Corresponding anoxia-induced

GABA accumulations are quite common and have been reported

for instance for radish leaves (Streeter and ompson 1972) and for

Expression of marker genes for

Germination

(ICL)

Drought stress (dehydrins,

in endosperm and embryo?)

Start of

germination

Induction of

drought stress

Accumulation of GABA related to

Anoxia/

injuries Germination

Stress

(in endosperm and embryo?)

DryingFermentation

Figure 12.4 Scheme of the temporal succession of some metabolic events during wet coffee

processing. Performance of germination was estimated by determining the expression of isoci-

trate lyase (ICL), that of drought stress by analyzing the expression of dehydrins. The apparent

phases of GABA accumulation are thought to be related to different metabolic events (accord-

ing to Kramer et al. 2010; Bytof et al. 2007; Selmar and Bytof 2007). (Adapted from Bytof, G.

et al. 2007. Annals of Botany, 100:61–66; Selmar, D. and Bytof, G. 2007. Green coffee is alive!

A review on the metabolic processes taking place in coffee beans during processing and their

implication for modern coffee research. 21ème Colloque Scientiﬁque International sur le Café,

Montpellier, France, pp. 423–433.) The ﬁrst apex of GABA accumulation during drying is corre-

lated with the expression of isocitrate lyase, and thus, with germination processes in the coffee

seeds. Subsequent peaks of GABA accumulation are correlated with the corresponding maxima

of dehydrin expression and are thought to be induced directly by drought stress in the endosperm

tissue and the embryo, respectively. GABA accumulation in fresh green coffees prior to the drying

procedure is attributed to the strong decrease in oxygen concentration during fermentation, but

might also be due to bruising in the course of the mechanical depulping process.

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456 CoCoa and Coffee fermentations

rice roots (Reggiani etal. 1988, 2000). e second phase seems to be

associated with the germination-associated metabolism that is going

on during coﬀee processing (see above). Similar germination-related

GABA accumulations have been described for various other plants,

for example, mustard seeds, Sinapis alba (Vanderwalle and Olsson

1983), chestnut seeds, Castanea sativa (Desmaison and Tixier 1986),

or lentil seeds, Lens spec. (Rozan etal. 2001). e third phase of

GABA enhancement apparently is related to drought stress and

parallels the dehydrin expression (Figure 12.4). Interestingly, the

stress-related expression of dehydrins as well as the GABA accu-

mulation follows a bimodal pattern. It was proposed that the two

phases of stress induction possibly are due to temporal diﬀerences

in stress induction in diﬀerent organs, that is, the endosperm and

the embryo (Kramer et al. 2010). In this context, it has to be con-

sidered that the embryo is more or less entirely embedded in the

endosperm. e drying process starts with the loss of water in the

outer parts of the endosperm before the water potential of the inte-

rior cells decreases. us, the embryo is embedded in a still relatively

moist milieu for a certain time during the drying process, and it will

maintain a higher water potential than the outer endosperm cells.

is eﬀect is probably promoted by the thick nature of the galacto-

mannan cell walls of the endosperm cells, indicating that the man-

nans not only have the obvious function as a potential carbohydrate

reserve for seedling development but may also serve as a kind of

hygroscopic buﬀer, slowing down the otherwise too rapid and pos-

sibly deadly desiccation of the embryo (Bewley 1997). Accordingly,

when drought-stress-related reactions may already be induced in the

outer parts of the coﬀee endosperm, the coﬀee embryo may not suf-

fer drought stress for a longer time yet. Concerning the endosperm,

evidence from electronic microscopy demonstrated that the extent

of stress, reﬂected by a loss of cell membrane integrity, was par-

ticularly dependent on the rigor and temperature applied during the

drying of the coﬀee beans (Borém et al. 2007; Saath et al. 2010).

Summarizing, we can conclude that indeed stress-related processes

are taking place in the coﬀee beans while drying (drought stress) and

possibly also during fermentation (hypoxia). Moreover, we assume

numerous other metabolic processes running in parallel and/or sub-

sequent to each other, resulting in a very complex metabolic situation.

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457

metaboliC responses of Coffee beans

Much more research is needed to understand this phenomenon, and

to elucidate, how these processes inﬂuence the composition of aroma-

relevant compounds and how these insights could be used to modify

green coﬀee quality by deliberately changing the processing and dry-

ing conditions. Especially due to the high complexity outlined, a com-

prehensive analysis requires integrative research eﬀorts, combining the

wide range of all modern research methods. In particular, genomics and

metabolomics approaches would be promising if combined with realistic

model-processing or ﬁeld experiments. First omic approaches on coﬀee

seed physiology during processing produced very convincing support

for the occurrence of various and processing-speciﬁc metabolic events

(De Vos et al. 2007; Modonut etal. 2011). e elucidation of the causal

relationship between metabolic events and the occurrence of certain

aroma compounds is a great challenge for coﬀee research in the future.

12.7 Capabilities to Improve Quality by DeliberatelyInﬂuencing

the Metabolic Events in Green Coﬀee

It is self-evident that deliberate changes in the processing and drying

conditions inﬂuence green coﬀee quality. Generally, experienced pro-

cessors are cautious to stray oﬀ from standard procedures in order to

avoid any negative eﬀects such as an increased microbial spoilage or a

decline of quality. Only few processors are eager to favor new attempts

of endogenous quality improvement. Up to now, most approaches

have been applied exclusively on the basis of empirical acquirement

and practical experience of the green coﬀee producers.

12.7.1 Intermediate Storage of Progressively Processed Green Coﬀees

Based on the knowledge of the metabolic impacts of certain changes

in the processing procedure, it should be possible to predict their con-

sequences for coﬀee quality. Although we are still far away from a

comprehensive understanding of the complex metabolic events that

occur in coﬀee beans during processing, a plant biological-based

approach shall be highlighted here: namely, the introduction of an

intermediate moist storage phase of mechanically pulped green coﬀee

in order to achieve a quality equal to that of traditionally wet process

coﬀee.

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458 CoCoa and Coffee fermentations

Owing to aggravated environmental directives (i.e., wastewa-

ter management) and owing to an increasing cost pressure, greater

amounts of green coﬀee are being produced by new progressive meth-

ods, by which the mucilage is mechanically removed. Unfortunately,

the quality of the coﬀees produced by such progressive methods

does not reach the quality of those coﬀees produced by classical wet

processing—and the corresponding roast coﬀees or brews reveal dif-

ferent aroma proﬁles (Gonzales-Rios et al. 2007b; Steinhaus and

Schieberle 2007). From the viewpoint of plant biology—also when

applying the progressive methods—metabolic processes are unlocked

in coﬀee seeds as in wet processing. However, the time frames for

the corresponding reactions are diﬀerent (Figure 12.5). As outlined

previously, the removal of the pulp initiates germination-related pro-

cesses: the suppressed metabolism is unlocked. Over time, when the

parchment beans are dried and the water content decreases, addi-

tional stress-related reactions are induced (see above). As long as the

water content is suﬃcient (i.e., estimated 25%), the seeds remain

metabolically active and various reactions run in parallel or subse-

quently. However, as soon as a certain moisture threshold is under-

shot, the entire metabolism is virtually shut down. Accordingly, in the

case of classical wet processing, the related time span of metabolic

activity is about 2–4 days (Figure 12.5). Over the years know-how

and ﬁeld experiments have reﬂected that a controlled prolongation of

the moist treatment by applying a soaking step was beneﬁcial for cup

quality (Wootton 1966; Gonzales-Rios 2007a; Lambot et al. 2011).

By comparison when mucilage is removed mechanically and a fer-

mentation step is omitted, the corresponding phase is much shorter.

Consequently, the overall extent of the reactions taking place during

processing is much lower. If we assume that at least some of these

reactions are advantageous for the generation of relevant aroma pre-

cursors, it becomes obvious to extend this time span, that is, by intro-

ducing an interim storage phase under moist conditions (Selmar etal.

2005). As a consequence of such approach, the relevant reactions in

the coﬀee seeds should proceed, thereby improving the quality of the

green coﬀee. As the introduction of such intermediate storage phase

indeed increases the coﬀee quality signiﬁcantly (Selmar etal. 2005),

such approach represents a paragon of applying basic plant biological

knowledge to improve coﬀee quality.

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459

metaboliC responses of Coffee beans

12.7.2 Quality Eﬀect of Barning

After wet processing, the parchment coﬀee is generally kept in

storage facilities (tulhas) for a minimum period of 1 month. e

implementation of this process, also called barning, is an example

of the exploitation of positive eﬀects of a certain storage condition.

Depulping

Washing

and sorting

Unlock of

metabolism

Unlock of

metabolism

Unlock of

metabolism

Shutdown of

metabolism

Shutdown of

metabolism

Shutdown of

metabolism

2–6 h

1–4 h

20–36 h

24–48 h

20–36 h24–48 h

2–4 days

1–2 days

2–4 days

24–48 h

About 25%

Classical wet fermentation:

Mechanical mucilage removal (e.g. BECOLSUB):

Introduction of an intermediate storage phase:

Fermentation Drying to 12% water cont.

Drying to 12% water cont.

Drying to 12% water cont.Storage

Removal

of mucilage

Removal

of mucilage

Figure 12.5 Divergent time frames of metabolic activity in differentially processed coffee

beans. Owing to the removal of the pulp, germination processes are unlocked. When the water con-

tent decreases during the course of drying, additionally stress-related reactions are induced. As

long as the water content is sufﬁcient (i.e., estimated 25%), the seeds remain metabolically active.

However, as soon as certain moistness is undershot, all metabolic activities come to a standstill.

Whereas the related time span of metabolic activity is about 2–4 days in the case of classical wet

processing, due to the omission of a fermentation step, it is much shorter, when mucilage is removed

mechanically. An introduction of an interim storage phase enhances this time span and thereby lead-

ing to an improvement of green coffee quality.

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460 CoCoa and Coffee fermentations

It is based solely on the practical knowledge of coﬀee farmers, and

the scientiﬁc background still is scarce (Rendón et al. 2011). As a

matter of common knowledge, barning reduces the incidence of

certain undesired oﬀ-ﬂavors, such as the “peasy” oﬀ-ﬂavor, identi-

ﬁed as 2-isopropyl-3-methoxy pyrazine (Becker etal. 1988), or the

“hard”/“hardish”/“Rio” oﬀ-note of coﬀee, identiﬁed as 2,4,6-trichlo-

roanisole (Spadone etal. 1990). Although the eﬀects are frequently

reported in discussions with coﬀee producers, there are neither

systematic data available nor are there any scientiﬁc suggestions

for possible causes. In this context, another frequently mentioned

aspect, which also is based on the farmer’s experience, concerns the

viability of the coﬀee beans, which can be easily assessed with a

simple germination test or with the tetrazolium test (Rivas Vásquez

and Morillo 1961; Dias and da Silva 1986). e viability of the cof-

fee seeds is assumed to be directly linked to the potential cup quality

(Sivetz and Desrosier 1979a,b).

Corresponding scientiﬁc investigations have been performed only

recently. Selmar etal. (2008) demonstrated that the viability of coﬀee

beans stored in parchment (pergaminho) is extensively prolonged as

compared to the hulled coﬀee beans from the same batch and stored

under the same conditions (22°C, 63% relative humidity). In parallel,

signiﬁcant changes in low molecular sugars (e.g., glucose) and in free

amino acids (e.g., glutamine) had been detected (Selmar etal. 2008).

It was already demonstrated earlier that, under unfavorable storage

conditions, yielding a so-called old crop coﬀee, massive changes in

low molecular sugars occur (Pokorny etal. 1975; Bucheli etal. 1998;

Dussert etal. 2006). is decline of glucose in the green coﬀee beans

appears to be related to the relative humidity during storage (Dussert

etal. 2006). However, despite clear eﬀects on the viability, only minor

eﬀects on cup qualities had been detected. Nevertheless, there seems

to be a tendency for a sensorial beneﬁt of storing the green coﬀee

within the parchment (Selmar etal. 2008). In the recent time more

work has been done on this subject by several workgroups. Del Terra

et al. (2011) and Lambot et al. (2013) have conﬁrmed the close asso-

ciation of seed viability and resulting cup quality. Since the strongest

impact on green coﬀee quality was to be expected from the lipid frac-

tion (Dussert et al. 2006; Speer and Kölling-Speer 2006), it was only

consequent to look closer for free fatty acids and lipid peroxidations in

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461

metaboliC responses of Coffee beans

order to establish green coﬀee markers for ageing (Lambot et al. 2013,

Saath etal. 2013a; Rendón et al. 2014). Lambot et al. (2013) also

looked at the inﬂuence of temperature, relative humidity, and envi-

ronmental oxygen and found diverse impacts on sugars, free amino

acids, acetic acids, and hexanal. Furthermore, certain late embryo-

genesis abundant (LEA) proteins seem not only be involved in the

stress metabolism during green coﬀee drying (see above) but also

seem to be promising markers for the development of old crop coﬀee

(Saath et al. 2013b). Finally, the decrease of a particular chlorogenic

acid, the 5-caﬀeoylquinic acid was suggested as a putative marker for

ageing during green coﬀee storage, just recently (Rendón et al. 2014).

12.8 Conclusion

Based on modern biochemical and molecular biological analyses, it

was shown that various metabolic reactions occur in coﬀee beans dur-

ing processing. Apart from germination, a stress-related metabolism

also takes place. e time courses of these processes are signiﬁcantly

diﬀerent in wet and dry processed seeds, demonstrating that these

metabolic reactions are strongly inﬂuenced by the processing condi-

tions. ese diﬀerences in the time courses and the amplitudes of the

metabolic events determine the distinction in the substantial compo-

sition of diﬀerentially processed green coﬀees, thereby aﬀecting cof-

fee quality, and establishing the peculiarities of wet and dry process

coﬀees. Consequently, it should become possible to modify green

coﬀee quality by deliberately changing the processing conditions.

An intriguing example for such approach to modulate the metabolic

activity in green coﬀee beans is the introduction of an intermedi-

ate storage phase subsequent to mechanical demucilation in order to

enhance coﬀee quality.

However, the entire impacts and the overall consequences of the

endogenous metabolic processes taking place during coﬀee processing

are not yet known or even understood. In this context, it should be

mentioned that apart from these endogenous factors, many important

exogenous factors like microbial impacts on quality have to be con-

sidered (for review, see Chapter 11). Owing to the high complexity, a

comprehensive analysis of the metabolic eﬀects and their impacts on

the coﬀee quality and their modiﬁcation by external factors requires

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462 CoCoa and Coffee fermentations

further integrative research eﬀorts, combining a wide range of all

modern research methods, including the promising genomics and

metabolomics approaches. e elucidation of the causal relationship

between metabolic events and the occurrence of certain aroma com-

pounds is a great challenge for coﬀee research in the future.

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Article

Jan 1980

Seed germination under water stress, and seedling growth immediately after germination, have been investigated in a series of experiments. In the majority of species used in these experiments it was found that seedling growth immediately after germination could proceed at water potentials lower than those that permitted germination. This differential sensitivity to water stress could be reduced (i.e., the water potential of seed germination made more negative) by growth regulators and was completely removed by a combination of ethrel and kinetin in two of the species investigated, red clover and lucerne. Evidence suggests that the initiation of cell elongation is the process in germination most sensitive to water stress, and the work with growth regulators confirms that there are different effects on the initiation of germination and on subsequent growth. Differential sensitivity to moisture stress is suggested as a form of dormancy and we speculate on possible mechanisms that govern its control.

Coffee as a functional beverage

Article

Jan 2009

A. Farah

Individual chlorogenic acids and caffeine contents in commercial grades of wet and dry processed Indian green robusta coffee beans

Article

Jan 1995

Agronomic and Processing Factors Affecting Coffee Quality

Article

Sep 1998

J.M. Njoroge

Coffee quality is a parameter influenced by a wide range of factors. The criteria used to measure quality include raw bean size and shape as well as colour; roast bean appearance is measured by visual scoring. Cup quality is determined by organoleptic means. Samples are submitted to professional coffee tasters who assess body, acidity, flavour and general standard while off-flavours are distinguished. Owing to the role of quality in the pricing of coffee, it is important that coffee should be handled carefully at the farm and factory level. At all these levels, the recommended procedures of handling coffee should be adhered to, in order to obtain top quality coffee. Some of these procedures are discussed in the paper.

Impact of wet and dry process on green coffee composition and sensory characteristics

Article

Jan 2004

Chlorogenic acids. Coffee

Article

Jan 1985

Michael N Clifford

Small quantities of free quinic acid occur in green coffee beans. A greater quantity of quinic acid occurs as a series of esters (collectively) generally known as chlorogenic acids (CGA). Possibly the first report to describe these substances is attributable to Robiquet and Boutron.1 These workers, while searching for physiologically active substances in coffee, isolated from green coffee beans an acidic substance which gave a green pigment with ferric chloride. At that date this observation was interpreted as indicating the presence of a gallic acid residue. It is known now that a caffeic acid residue would have given a similar response.

Chapter 9. Coffee Beans and Processing

Abstract and Figures

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