Basic Chemical Reactions Occurring in the Roasting Process
by Carl Staub
Sourced from the SCAA Roast Color Classification System
developed by Agtron - SCAA in 1995
Many thermal and chemical reactions occur during the roasting
process: decarboxylation, dehydration of quinic acid moiety, fractionization,
isomerization, polymerization, and complex sugar reactions. The principal
thermally reactive components are monosaccharides and sucrose, chlorogenic
acids, free amino acids, and trigonelline. Both aravinose and calactose of
polysaccharides are splitoff and the basic sulfur containing and hydroxyamino
acids decompose. Carbohydrates both polymerize and degrade, liberating thermally
unstable monosaccharides decomposing 20-30% of the polysaccharides, depending on
the degree of roast.
Sucrose: Disaccharide of d-Glucosyl and d-Fructosyl Moieties
Sucrose is the principle sugar in coffee. The melting point
of pure crystalline sucrose is in the 320-392 degrees F with 370 degrees F most
commonly accepted. Degradation of dry sucrose can occur as low as 194 degrees F.
and begins with the cleavage of the glycosidic bond followed by condensation and
the formation of water. Between 338 and 392 degrees F, carmelization begins. It
is at this point that water and carbon dioxide fracture and out-gassing begins
causing the first mechanical crack. These are the chemical reactions, occurring
at approximately 356 degrees F, that are exothermic. Once carmelization begins,
it is very important that the coffee mass does not exotherm (lose heat) or the
coffee will taste "baked" in the cup. A possible explanation is that
exothermy of the charge mass interrupts long chain polymerization and allows
cross linking to other constituents. Both the actual melting point of sucrose
and the subsequent transformation, or carmelization, reaction are effected by
the presence of water, ammonia, and proteinatious substances. Dark roasts
represent a higher degree of sugar carmelization than light roasts. The degree
of carmelization is an excellent and high resolution method for classifying
roasts.
Cellulose: A Long Linear Polymer of Anhydroglucose Units
Cellulose is the principle fiber of the cell wall of coffee.
It is partially ordered (crystalline) and partially disordered (amorphous). The
amorphous regions are highly accessible and react readily, but the crystalline
regions with close packing and hydrogen bonding may be completely inaccessible.
Native cellulose, or cellulose 1, is converted to polymorphs cellulose III and
cellulose IV when exposed to heat. Coffees structure is a well developed matrix
enhancing the mass uniformity and aiding in the even propagation of heat during
roasting. Cellulose exists in coffee imbedded in lignocellulose (an amorphous
matrix of hemicellulose and lignin containing cellulose), making up the matrix
cell walls. Hemicellusloses are polysaccharides of branched sugars and uronic
acids. Lignin is of special note because it is a highly polymerized aromatic.
Severe damage occurs to the cell walls of the matrix at distributed temperatures
above 446 degrees F and bean surface temperatures over 536 degrees F The actual
temperature values will change due to varying levels of other constituents.
Second crack, associated with darker roasts, is the fracturing of this matrix,
possibly associated with the volatilization of lignin and other aromatics. Under
controlled roasting conditions, the bean environment temperature should never
exceed 536 degrees F. A wider safety margin would be achieved by limiting the
maximum environment temperature to 520 degrees F. These temperature limits
minimize damage to the cell matrix and enhances cup complexity, roasting yield,
and product shelf life.
Trigonelline: A Nitrogenous Base Found in Coffee
Trigonelline is 100% soluble in water and therefore will end
up in the cup. Trigonelline is probably the most significant constituent
contributing to excessive bitterness. At bean temperatures of 445 degrees F,
approximately 85% of the trigonelline will be degraded. This bean temperature
represents a moderately dark roast. For lighter roasts there will be more
trigonelline, hence bitterness, but also less sugar carmelization. Caramelized
sugar is less sweet in the cup than noncaramelized sugar, so when properly
roasted these two constituents form an interesting compliment to each other.
Trigonelline melts in it's pure crystalline form at 424 degrees F Degradation of
trigonelline begins at approximately 378 degrees F.. The degradation of
trigonelline is one of the key constituent control flags for determining the
best reaction ratio.
Quinic Acid: Member of the Carboxylic Acids Group
Quinic Acid melts in pure crystalline form at 325 degrees E,
well below the temperatures associated with the roasting environment. Quinic
Acid is water soluble and imparts a slightly sour (not unfavorably as in
fermented beans) and sharp quality, which adds to the character and complexity
of the cup. Surprisingly, it adds cleanness to the finish of the cup as well. it
is a stable compound at roasting temperatures.
Nicotinic Acid: Member of the Carboxylic Acid Group
Nicotinic Acid melts in pure crystalline form at 457 degrees
F. Naturally occurring Nicotinic Acid is bound to the polysaccharide cellulose
structure. Nicotinic Acid is also derived in soluble form during roasting.
Higher levels of Nicotinic Acid for any given degree of roast are associated
with better cup quality. Since it is I 00% soluble, it will end up in the cup.
Nicotinic Acid contributes to favorable acidity and clean finish. It's
derivation rate is one of the key constituent control flags for determining the
best reaction ratio temperature and chemistry propagation rates. Additionally,
the interaction of melted Nicotenic Acid with other constituents contributes
significantly to the intensity associated with darker roasts.
Environment Temperature
The temperature of the roasting environment determines the
specific types of chemical reactions that occur. There is a window of
temperatures that produce favorable reactions for the ideal cup characteristics.
Temperature values outside of this window have a negative effect on
quintessential cup quality. Even within the window values, different
temperatures will change the character of the cup, giving the roaster the
latitude to develop a personality or style desired, or to tame the rough
signature of certain coffees while still optimizing relative quality. System
Energy: At any given environment temperature, the amount of energy (BTU) and the
roasting system's transfer efficiency will determine the rate at which the
specific chemistrywilloccur. Higher levels of both energy andt ransfer
efficiency will cause the reactions to progress more quickly. There is a window
of reaction rates that will optimize cup quality. This is called the Best
Reaction Ratio, or BRR.
Best Reaction Ratio (BRR)
The best cup characteristic are produced when the ratio of
the degradation of trigonelline to the derivation of Nicotinic Acid remains
linear. The control model of this reaction ratio is a time/temperature/energy
relationship. The environment temperature (ET) establishes the pyrolysis region
for the desired chemical reactions while the energy value (BTU) and system
transfer efficiency (STE) determines the rate of reaction propagation and
linearity of Nicotinic Acid derivation to degradation of trigonelline. Because
green bean density varies dramatically, under any given ET / BTU / STE format,
the reaction distribution will vary. it takes longer to obtain comparable
uniformity for a higher density bean. Monitoring the bean temperature offers a
good method of approximating the reaction distribution during this phase of the
roasting. The ideal environmental temperature, ET, for best reaction ratio, BRR,
is from -401-424 degrees F, with 405 degrees F as the default value. The BTU
required is determined by the systems transfer efficiency, or ability to impart
the energy to the charge mass.
Maximum Environment Temperature (MET)
Establishing the thermal environment protocol for the ideal
roast is a balancing act. While it is desirable to maintain the BRR temperature
and energy levels until the target reactions are achieved, the BRR temperature
is well above the carmelization temperature of sucrose. Because many roasting
systems exhibit thermal hysterysis using simple temperature regulating schemes,
care must be taken not to allow the coffee mass to exotherm. Additionally,
limiting the maximum environment temperature, MET, is also important. As
previously mentioned, maintaining structural integrity of the cellulose matrix
is of great importance. Lower temperatures will reduce surface evaporation of
constituents minimizing the capillary action that draws constituents to the
surface where they would be volatilized. Hydraulic action, a function of
internal pressure which is directly related to bean temperature, is already at
work. By limiting the maximum temperature, losses will be minimized and the
essence of coffee retained. Consequently, the MET should not exceed 520 degrees
F. This roasting system bases the MET value on the actual final bean, or drop
temperature, which correlates to the degree of roast.
Last Updated:
April 05, 2008
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