What Determines the Color of Brewed Coffee?

One of the most driven coffee professionals I’ve had the honor of meeting is Reza Kosar. In addition to being the first Iranian to become a licensed Q Grader, Reza is a talented coffee roaster with an insatiable curiosity in coffee science. He recently posed an interesting question on his Instagram account: what determines the color of brewed coffee? Which raises a second question, what effect does color have on flavor? We chimed in with what we thought was the right answer, but it turns out Reza had taken a deep dive into the subject and has graciously offered to share his research with us. 

Roast

The roasting process leads to the formation of the characteristic coffee aroma and dark-colored
compounds. These compounds are formed mainly as the result of the Maillard reactions
that take place between carbohydrates/degraded carbohydrates and proteins. These dark-colored
compounds are referred to as melanoidins, and they make up to 25% of the dry matter in coffee brew.
Melanoidins are generally defined as brown, nitrogenous macromolecular compounds that absorb light
at 405-420 nm. Their chemical structure is extremely complex and still largely unknown due to the fact
that many green coffee components might play a role in their formation. Bekedam et al. (2008), showed
that next to Maillard reactions, incorporation of chlorogenic acid plays an important role in coffee brew
melanoidins formation as well. Melanoidins are not only of interest because of their contribution to the
color of coffee brew but also for their flavor-binding properties, antioxidative capacity, and reactivity in
brew.

Melanoidins are the main determinants of the color of the coffee we know (coffee is actually a seed of a
fruit and is “green”) but that’s not all. For melanoidins to form, precursors play a significant role, and as
mentioned before their chemical structure is extremely complex and still largely unknown due to the
fact that many green coffee components might play a role in their formation. However, according to
Bekedam (2008), three main proposals for the structure of melanoidins have been put forward till now.
First, melanoidins were reported to be low molecular weight colored substances which crosslink with
free amino groups in proteins (Arg, Lys) leading to high molecular weight colored structures (Hofmann,
1998). Second, it was proposed that melanoidins are macromolecular structures of repeating units that
are built up from polycondensated furan-like and pyrrole-like structures (Tressl, 1998). Third, it was
reported that the melanoidin skeleton is mainly built up from sugar degradation products which are
formed in the early stages of the Maillard reaction, polymerized and linked by amino compounds
(Cämmerer et al. 2002; Cämmerer and Kroh, 1995).

Green

The color of the exocarp at the beginning of fruit development is green due to the presence of
chloroplasts (chlorophyll) which then disappear as the fruit matures. However, color upon maturation
depends on coffee variety, but is most commonly red or yellow as we know. Red skin color comes from
anthocyanin pigments, while yellow skin color is attributed to lutein (xanthophyll). In plants, the
xanthophyll lutein is the most abundant carotenoid. Carotenoids, also called tetraterpenoids, are yellow,
orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and
fungi. The first studies of red and yellow cultivars were lead in 1938 by Krug; He discovered that a
dominant pair of genes determine the color of the coffee fruit and he named it “Xanthocarpa”. A
dominant homozygous “XcXc” gives the red color to the cherries. These genes lead the fruit’s
metabolism to produce red-colored substances like anthocyanins (cyanine, pelogonin, and peonin). In
yellow(ish) coffee fruits the genes are homozygous recessive “xcxc”, and the predominance of
leucoanthocyanins neutralizes anthocyanins, which allows the production of yellow flavones such as
luteolin and apegenine.

Anthocyanins belong to a parent class of molecules called flavonoids synthesized via
the phenylpropanoid pathway. They are phenolic compounds which can be found throughout the plant kingdom; occur in all tissues of higher plants, including leaves, stems, roots, flowers, and fruits. They are
odorless and moderately astringent. We used to originally and incorrectly classify phenolic compounds
as tannins, according to Lingle (2011), although there are tannins in coffee but they are not the main
ones. Condensed tannins are the main phenolic compounds in coffee pulp and skin, however, in the
seed, phenolic compounds are present predominantly as a family of esters formed between certain
hydroxycinnamic acids and quinic acid, collectively known as chlorogenic acids (CGA) (Clifford, 1985).
Chlorogenic acids (CGA) and related compounds are the main components of the phenolic fraction of
green coffee beans, reaching levels up to 14 % (dry matter basis, depending on species and varieties). It
is worth mentioning that basically chlorogenic acid is a pseudo tannin, which is a low molecular weight
compound associated with other compounds. So talking about tannins in coffee (astringency, color
absorbance, etc.) might not be wrong, but also it’s not accurate. According to Bekedam et
al. (2008), incorporation of chlorogenic acid plays an important role in coffee brew melanoidins
formation.

I believe next to chlorogenic acids, anthocyanins also could play a significant role in the formation of
coffee brew melanoidins. To my knowledge, there is no study to prove this argument and I’m not a
biochemist to say that. Yet the answer might lie in the processing method: we already know that there
are anthocyanins in coffee pulp and skin. And empirically we observe the color differences between
different processing methods (say wet vs. dry). In a study in 2007, Prata investigated fresh coffee husks,
comprised of outer skin and pulp, as potential sources of anthocyanins for applications as natural food
colorants.

“In the wet process, the coffee husks are removed prior to drying, while still fresh, and their color is
rapidly degraded by the action of enzymes (peroxidases and polyphenoloxidases) liberated by the
damaged cells of the outer skin and pulp during the de-hulling process, or by other oxidizing agents,
such as oxygen. Thus, large amounts of natural colorants are wasted in this process (Prata, 2007).”
Given that these pigments (anthocyanins) are water soluble, could it be the reason why wet-processed
coffees tend to have more clarity and be lighter in color? While dry-processed coffees are darker in
color with less clarity? To me, it’s like as if phenolic compounds or to be more specific in my argument,
anthocyanins are bleached in the wet process and somehow leach/percolate in the dry process.
Therefore, the amount (concentration) present at the time of the reactions during roasting
could lead to creating low to high molecular weight melanoidins.

Brew

Coffee extraction is an essential process that determines the coffee brew characteristics. During the
process, water-soluble components including chlorogenic acids, caffeine, nicotinic acid, soluble
melanoidins, and volatile hydrophilic compounds are extracted (Farah, 2012). The polyphasic nature of
the beverage and the volatile and non-volatile compound concentration determine the physiochemical
and sensory properties of different coffee beverages (Illy and Viani, 2005). The brewing step allows
extracting the odorants and taste molecules from the roast and ground coffee into the consumer’s cup.
There are various variables and parameters that play a significant role in the process of coffee brewing
that can alter the final product. Coffee brewing is a solid-liquid extraction wherein the process parameters have a significant impact on the extraction kinetics of the different chemical compounds
present in roasted coffee (Cordoba et al., 2019). The main chemical reactions that occur during roasting
which generate important aroma volatiles include Maillard reactions, phenolic acid and carotenoid
degradation (Holscher & Steinhart, 1992; Reineccius, 1995; Tressl, 1980); Strecker degradation;
breakdown of sulfur amino acids, hydroxy-amino acids, proline and hydroxyproline; degradation of
trigonelline, chlorogenic acids and quinic acid, pigments, and lipids; as well as reactions between other
intermediate products (Buffo & Cardelli-Freire, 2004; Ribeiro, Augusto, Salva, Thomaziello, & Ferreira,
2009).
As you can see, it’s a very complicated pathway and so many chemical compounds and reactions are
involved. However, we do know that melanoidins give coffee its distinctive color (although we may not
know the exact structure of them) and that chemical composition of the green coffee is a must for
chemical reactions to happen.

So, here I’d like to offer the following classification of determinants of coffee brew color:

  • Those that play a role as precursors in green coffee state
  • Those that play a role in formation of melanoidins in roasting
  • Those that play a role in representing what have been already formed

About Reza Kosar

Reza Kosar is head of roasting and QC at Vulcan Coffee Roastery in Doha, Qatar. He holds an M.A. in linguistics and has been a licensed Q Grader since 2016.
 

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