Chlorogenic Acids (CGA)
No explanation of organic acids would be complete without a
thorough discussion of chlorogenic acids (CGA). As seen from
Figure 1, CGA accounts for the majority of the organic acid
concentration in coffee, accounting for six to seven percent for
arabica and up to 10 percent in robusta on a dry basis. Although
it may not appear to be much, the relative content of CGA
compared to that of caffeine is seven to eight times higher. And
in a typical eight-ounce cup of coffee, CGA represents roughly
two ounces, or an estimated 30 percent by volume.
During roasting, CGA plays an important role in the
development of coffee flavors. Almost half of the CGA content
is decomposed in a medium roast, whereas French roasts
can exhibit up to an 80 percent loss. The portion that does
decompose is used in the production of quinic acid and flavor
precursors.
It should be mentioned that chlorogenic acid does not
refer to a single compound, but rather a family of more than
six different isomers of the acids, each with different flavor
attributes. Without getting too technical, there are basically two
families of these acids; mono-caffeoyl and di-caffeoyl. While
mono-caffeoyl acids readily decompose during roasting, those
of the di-caffeoyl family remain almost unchanged and have
been reported to impart a metallic-bitter taste. It’s no surprise
then that robustas, which exhibit a similar metallic taste,
contain a larger concentration of these acids than their arabica
counterparts. It has even been suggested that due to their
undesirable taste, CGAs have effectively been used by plants
for protection from animal and insect infestation. Perhaps
those pesty bugs do know a thing or two about specialty-grade
coffee!
At just past the second crack, these di-caffeoyl acids
decompose and the metallic-bitter taste slowly diminishes,
though a few savvy chemists at soluble factories have effectively
been able to slightly alter the levels of these acids through steam
treatment of lower quality green beans. Who would have ever
thought of such a thing? I guess necessity is truly the mother
of invention.