The history of coffee is long and rich with the
early beginnings in Ethiopia and its introduction into Arabia and eventually
into Europe through Venice some time in the fifteenth century. There are a
number of excellent books on the history, cultivation and chemistry of
coffee(1,2). Visit the National Geographic Page on Coffee,
This discussion will focus on what is currently
known by modern analytical science techniques about the complex chemistry of the
volatile aroma and, more to the point for our application, higher molecular
weight components comprising the pigmentation or coloration of roasted and
brewed coffee.
We will limit this discussion to the two major
classes of green coffee beans known as Arabica and Robusta which vary in
caffeine content due to differences in the deriving plant and altitude-location
of cultivation. Typical roasting temperatures vary from ~190 °C to as high as
230 °C for times which are typically less than 15 minutes. In this limited space
we cannot do justice to the complex chemistry occurring during roasting and
brewing but can supply a simplified primer and encourage the interested reader
to delve further into the subject.
Table I: Composition of green and roasted
coffee (3).
Component
Green Coffee (%)
Roasted Coffee(%)
'cellulose'
36
37
Lignin
5.6
5.8
Fat
11.4
11.9
Ash
3.8
4.0
Sucrose
7.3
0.3
Chlorogenic acid
7.6
3.5
Protein
11.6
3.1
Table I lists a typical component breakdown of an
Arabica coffee as both the green bean and roasted. Clearly, the cellulosic
components (hemicellulose, celullose and lignin) are little effected by the
roasting process as are the ash (mineral) and fat components (fatty acids,
trigylcerides, waxs) since the roasting temperatures are low relative to their
decomposition temperatures. Whereas the sugars, organic acids and proteins are
dramatically reduced upon roasting. So it is apparent that the rich aroma and
pigmentation occurs because of chemistry occuring to and between these
components, the sugars, proteins and organic acids (chlorogenic acid). Indeed
non-enzymatic browning reactions, called Maillard reactions involve interactions
of amino groups of amino acids of proteins and other compounds and reducing
sugars to form glycosamines(2,4,5). These condensation reactions with subsequent
fission produces aliphatic and aromatic volatiles comprising the aroma. Much of
the distinctive aroma arises due to the presence of sulfur and oxygen bearing
aromatic (heterocyclic) compounds such as furans, furfurylthiol or
furfurylmethyl sulphides and a host of other similar compounds. As of 1985, some
660 separate compounds in the aroma of a roasted coffee have been identified by
gas chromotography and mass spectrometry.
As a result of the condensation reactions and
carmelization of the sugars, the heavier molecular weight components possess
varying degreees of extended conjugation which leads to the dark red-brown
pigmentation or coloration. These components which have a molecular weight
distribution from 5000-25,000 (1) or greater have energetically closely spaced
highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO) which
leads to a myriad of optical transitions spanning the uv-visible range into the
near infrared. As a result of the great multiplicity and heterogeneity of the
associated compounds the optical absorption spectrum of a brewed coffee is
smooth and monotonically decreasing from the far ultraviolet (uv) to the near
infrared wavelengths. It is believed that further condensation and oxidation
reactions occuring to these pigmentation-causing heavy molecular weight
components leads to continued increases in the optical absorption in the
red-near infrared regions of the spectra during the coffee's warming at ~
170-190 °F subsequent to brewing. This is shown in the plot below of the optical
absorption of fresh and stale coffee (1.5 hrs old). The stale coffee has
increased absorption in the blue and red parts of the spectrum. The region of
the LED probe is shown and the increase in the absorption at 680 nm is ~ 20 %
easily quantified to within < 1 % increments. The time dependence of the
photovoltage response of freshly brewed coffee is shown in Figure 3 below. The
point marked 42 is at the end of the brewing cycle. As can be observed the
absorption at 680 nm increases continuously with time so that by 30 minutes of
sitting on the warmer plate it is considerably darker. This increase in optical
darkness follows the loss of freshness and is correlated to it.
As a result of this insight into the effect of
oxidation on optical absorption, the use of a red or near infrared optical
monitor, such as the one discussed in this webpage is highly effective in
monitoring the degradation in real time and providing continuous feedback on the
"freshness"of the brewed coffee. Finally, a very simple freshness indicator can
be acheived by simply counting the time from some point after the brewing
process has begun. This elapsed time since brewing can be displayed and is a
simple, inexpensive indication of the freshness of the coffee. This is discussed
in detail in US 6,228,410, second embodiment.
Wavelength Dependance of Fresh and
Stale Coffee
Photovoltage Time Dependence of
Freshly Brewed Coffee
1) Coffee: Botany, Biochemistry and Production
of Beans and Beverage, Edited by M. N. Clifford and K. C. Willson, Croom
Helm Ltd. Beckenham, Kent UK, 1985.
2) Coffee Volume 1: Chemistry, Edited by
R. J. Clarke and R. Macrae, Elsevier Applied Science Publishers, London 1985.
3) Merritt, C., Jr., Robertson, D. H. and McAdoo,
D. J., Proc.4th Coll. ASIC, 1969, 144-8.
4) Baltes, W., Food Chemistry, 1982, 9,
59-75.
5) Nursten, H. E. Food Chemistry, 1981, 6,
263-77.
