Physicochemical Phenomena in the Roasting of Cocoa (Theobroma cacao L.)

Abstract

The quality of cocoa depends on both the origin of the cacao and the processing stages. The roasting process is critical because it develops the aroma and flavor, changing the beans’ chemical composition significantly by chemical reactions induced by thermal energy. Aspects have been identified as the main differences between bulk cocoa and fine cocoa, the effect of time and temperature on the formation of the flavor and aroma, and the differences between conductive heating in an oven, convective with airflow, and steam flow. Thermal energy initially causes drying, then non-enzymatic browning chemical reactions (Maillard reaction, Strecker degradation, oxidation of lipids, and polyphenols), which produce volatile and non-volatile chemical compounds related to the flavor and aroma of cocoa roasted. This review identified that the effect of the heating rate on the physicochemical conversion of cocoa is still unknown, and the process has not been evaluated in inert atmospheres, which could drastically influence the avoidance of oxidation reactions. The effect of particle size on the performance of product quality is still unknown. A more in-depth explanation of energy, mass, and chemical kinetic transfer phenomena in roasting is needed to allow a deep understanding of the effect of process parameters. In order to achieve the above challenges, experimentation and modeling under kinetic control (small-scale) are proposed to allow the evaluation of the effects of the process parameters and the development of new roasting technologies in favor of product quality. Therefore, this work seeks to encourage scientists to work under a non-traditional scheme and generate new knowledge.

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Vol.:(0123456789)
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Food Engineering Reviews
https://doi.org/10.1007/s12393-021-09301-z
Physicochemical Phenomena intheRoasting ofCocoa (Theobroma
cacao L.)
MyriamRojas1,2 · ArneHommes2 · HeroJanHeeres2 · FaridChejne1
Received: 30 June 2021 / Accepted: 14 November 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
The quality of cocoa depends on both the origin of the cacao and the processing stages. The roasting process is critical
because it develops the aroma and flavor, changing the beans’ chemical composition significantly by chemical reactions
induced by thermal energy. Aspects have been identified as the main differences between bulk cocoa and fine cocoa, the
effect of time and temperature on the formation of the flavor and aroma, and the differences between conductive heating
in an oven, convective with airflow, and steam flow. Thermal energy initially causes drying, then non-enzymatic browning
chemical reactions (Maillard reaction, Strecker degradation, oxidation of lipids, and polyphenols), which produce volatile
and non-volatile chemical compounds related to the flavor and aroma of cocoa roasted. This review identified that the effect
of the heating rate on the physicochemical conversion of cocoa is still unknown, and the process has not been evaluated in
inert atmospheres, which could drastically influence the avoidance of oxidation reactions. The effect of particle size on the
performance of product quality is still unknown. A more in-depth explanation of energy, mass, and chemical kinetic transfer
phenomena in roasting is needed to allow a deep understanding of the effect of process parameters. In order to achieve the
above challenges, experimentation and modeling under kinetic control (small-scale) are proposed to allow the evaluation of
the effects of the process parameters and the development of new roasting technologies in favor of product quality. Therefore,
this work seeks to encourage scientists to work under a non-traditional scheme and generate new knowledge.
Keywords Cacao types· Roasting parameters· Cocoa quality· Chemical conversion· Small-scale modeling
Introduction
Cocoa Industry
Cocoa and derivatives are one of the most popular food types
in the world due to their exquisite organoleptic properties
(i.e., flavor, aroma, texture) and high caloric and nutritional
value as it is rich in carbohydrates, fats, proteins, and min-
erals [118, 135]. It has even been claimed to offer health
benefits, such as antioxidant capacity, improving choles-
terol levels, lowering blood pressure, and antidepressant
and anti-stress effects [132]. However, the excessive con-
sumption of chocolate is generally considered unhealthy as
it can affect the nervous system, raise blood sugar levels,
and cause allergic reactions, constipation, and migraine [75].
Chocolate may be consumed in its pure form (i.e., white,
milk, or dark) or used in bakery and confectionery prod-
ucts, beverages, ice cream, and cosmetic applications such
as skin and hair care [141]. Cacao is the essential ingredi-
ent of chocolate and is responsible for its unique flavor and
texture [44]. Cocoa is derived from beans of the cacao tree
with the Latin name “Theobroma cacao,” meaning “food
of the gods” [84]. Cacao originates from the Amazon and
Orinoco rainforests in South America, where the tropical
climate with heavy rainfall and high temperatures creates
optimal growth conditions. In premodern Latin America,
cacao beans were considered so valuable that they were used
as a means of payment [83]. After the Europeans’ “discov-
ery” of the Americas, chocolate became famous worldwide,
which initiated cacao cultivation in other tropical areas (e.g.,
Africa, Asia, and Oceania).
* Farid Chejne
fchejne@unal.edu.co
1 Alliance forBiomass andSustainability Research –
ABISURE, Universidad Nacional de Colombia, Carrera 80
No 65-223 - Campus Robledo, Medellín, Colombia
2 Department ofChemical Engineering, Engineering
andTechnology Institute Groningen, Nijenborgh 4,
9747AGGroningen, TheNetherlands

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General Information
The total worldwide cacao production was 4,645,000 tons
in 2018: 75.9% from Africa (main producer is Côte d’Ivoire
with 43.06%), 17% from the Americas (main producer is
Ecuador with 6.03%), and 7.1% from Asia/Oceania (with
Indonesia that produces 5.8%) [65]. The processing of
cacao into chocolate (and derivatives thereof) is mainly
done in Germany, Belgium, and the Netherlands, with rev-
enues of 5.4, 2.9, and 2.3 billion euro, respectively [27].
Due to the increase of world consumption of about 91%
in the last 20years, the effects due to cocoa life cycle can-
not be neglected anymore. The growing demand has led to
the intensification of cocoa cultivation by expanding to new
lands, including forest lands, or by the use of more excessive
agrochemicals, which are not yet controlled [19]. However,
some Latin American farmers are applying agroforestry
systems to avoid excessive agrochemicals use and reduce
environmental impact [89]. These systems could represent
the challenge of future cocoa production for its ecological
and social benefits.
The global cocoa market size was valued at $44.35 Bn in
2019 and is projecting an increasing rate of 4.4% to reach
$61.34 Bn by 2027 Bianchi etal. [19], representing a posi-
tive economic impact for the industry and producers. But
it also means a need to exploit following the objectives of
sustainable development, with a more equitable distribu-
tion of profits, protection of the environment, and improve-
ment of the social conditions of the producing countries.
In cocoa production, the weakness is in the poor produc-
ing countries whose communities do not receive the fair
benefit. At the same time, the most promising strength for
the future is the production of high-value fine cocoa and
derived products.
Cacao Beans’ Origin
At the age of ca. 2years, the cacao tree produces a large
pod-shaped fruit (i.e., the cacao fruit; Fig.1a, b), which
changes color during its maturation (i.e., from green to
yellow, orange, and finally red or purple). Cacao beans are
covered by mucilage (i.e., pulp) inside pods (Fig.1c). Each
cacao bean consists of two cotyledons (the nib) of elliptical
shape (typical dimensions of 20–26mm length, 10–14mm
width, and 6–10mm thickness [16]) and a small embryo
plant, enclosed by the skin (the shell) (Fig.1d).
Cacao beans are typically distinguished between the bulk
and fine categories. The fine cacao beans come from the
“special” Criollo or Trinitario varieties, while the “ordi-
nary” bulk cacao beans come from the Forastero variety.
Ninety-five percent of the world cocoa market stems from
bulk beans. The other 5% are fine beans, mainly exported
by Bolivia, Ecuador, Nicaragua, Colombia, Costa Rica,
Peru, and Mexico [67]. However, the world demand for fine
cocoa increases due to its superior aroma and flavor [26].
Fine beans are associated with a high intrinsic quality due
to a more pleasant flavor and aroma and are the basis of the
luxurious “gourmet” chocolate. Bulk cacao is mainly used
to produce cocoa butter as it requires the addition of sugar
(milk/cream and flavor enhancers) to compensate for its bit-
ter taste to process it into high-volume common chocolate
products [26, 66]. For bulk beans, the purple or red color of
the interior is an indicator of good quality, as this is typically
associated with a high polyphenol content, which has good
antioxidant properties, but a bitter taste (which is compen-
sated for by adding lots of sugar). In fine cacao, the color
varies from white to light brown because of the low content
of polyphenol therein (Fig.1e) [133].
Cacao Beans' Composition
Freshly harvested cacao beans contain water, fats, carbo-
hydrates, proteins, vitamins, minerals (e.g., Fe, Cu, Mg,
Zn, Na, Ca, and P), active secondary metabolites (poly-
phenols), and methylxanthines: theobromine and caf-
feine [110]. Essential compounds are the reducing sug-
ars (e.g., sucrose, glucose, and fructose) and free amino
acids (e.g., leucine, alanine, phenylalanine, and tyrosine).
These are essential because they are the main precursors
in the formation of aroma compounds (i.e., during the
cacao bean processing) and give chocolate its character-
istic flavor. In fine cocoa, these precursors are typically
present in higher concentrations than in bulk cocoa; the
difference is about 12.5% less in the content of free amino
acids in bulk cocoa [133]. Other compounds contributing
to the characteristic cocoa taste/aroma are 3-methylbu-
tanoic acid, ethyl 2-methylbutanoate, and 2-phenyletha-
nol [46]. Polyphenol concentrations are typically higher
in bulk cocoa (86.1 ± 0.73mg g−1) than in fine cocoa
(66.0 ± 0.34mg g−1), which gives the prior a greater astrin-
gency (an unpleasant sensation of dry, puckering mouth-
feel) [133]. However, these compounds have antioxidant
properties that may benefit health [52]. The most abundant
polyphenols in bulk cocoa are proanthocyanins (ca. 58%),
catechins or flavan-3-ols (ca. 37%), and anthocyanins (ca.
4%) [8, 23]. The methylxanthines theobromine and caf-
feine increase motivation, alertness, and energy [142]. Both
polyphenols and methylxanthines contribute to the bitter
taste of chocolate [10]. The theobromine to caffeine weight
ratio is sometimes used to identify the cacao origin. This
ratio is typically 1–2 with 0.4–0.8 wt% caffeine for the Cri-
ollo species, whereas for the Forastero species, this ratio
is 5–14 with 0.1–0.25 wt% caffeine. Trinitario has values
between Criollo and Forastero [34, 134].

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It should be noted that the chemical composition, and
thus chocolate flavor/aroma, of cocoa products, strongly
depends on the cacao beans’ genotype (variety) [54], geo-
graphical origin, agroecological conditions during grow-
ing, and the post-harvest processing techniques applied
[62]. Therefore, an analysis of the influence of different
processing steps in processing cacao beans into choco-
late is necessary to evaluate how each step influences the
cocoa flavor/aroma. This work emphasizes the roasting
process.
From Cacao Beans toChocolate
Many processing steps are required to produce chocolate
and derivatives from cacao beans (Fig.2), which may
affect the final product characteristics. The mature cacao
fruits are harvested manually by removing the ripe pods
from the trees. These are then opened to remove the wet
beans classified/sorted (i.e., by pulp color and texture
and microbial contamination) and fermented before dry-
ing [86].
Fig. 1 Representation of (a) cacao tree, (b) cacao fruits, (c) cacao pods, and (d, e) bulk and fine cacao beans

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Fig. 2 Industrial processing steps from raw cacao to chocolate. (a) Precursor formation, (b) roasting, (c) flavor and aroma development, and (d)
texture improvement

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The fermentation is done by stacking the beans on a pile or
in wooden boxes (up to 210kg), which are protected from the
rain and sun. This process takes 5 to 7days at a temperature
between 48 and 51°C. To control the fermentation process’s
slight temperature increase, the beans are aerated manually
[126]. The cacao bean fermentation is a microbiological pro-
cess where microbiomes present in the beans (e.g., Lactoba-
cillus fermentumandAcetobacter pasteurianus) break down
sugars of the pulp (mucilage) to alcohol, lactic acid, acetic
acid, and mannitol [61]. Also, other (enzymatic) reactions
take place in the pulp and beans, where peptides and free
amino acids are produced through proteolysis from storage
proteins such as globulins [147]. Reducing sugars are formed
through hydrolysis of sucrose by invertase [114, 140]. The
endogenous formation of the precursors (i.e., in the beans) is
influenced by exogenous processes of the pulp, where formed
components may diffuse from the pulp into the cotyledon
where they are retained in the drying process [33, 40].
After fermentation, the beans are dried in the sun (e.g.,
by spreading them out on a floor or platform) until the mois-
ture content is reduced from ca. 55 wt% to 6.5–7 wt% [80].
Sometimes, improved methods such as direct solar drying or
hot air drying (i.e., tunnel dryer and hybrid forced convec-
tion and solar dryer) are adopted [4] to accelerate the process
and reduce microbial contamination in humid areas. Drying
of the beans causes them to contract and decrease in den-
sity, facilitating their transport and improving shelf life due
to the reduction microbial activity (water activity decreases
from 0.99 to 0.72) [28, 115]. During drying polyphenols and
some proteins degrade into free amino acids or denaturate
completely [7, 129]. Furthermore, theobromine and caffeine
contents typically decrease the drying process [35]. Addi-
tionally, drying causes the beans to shrink and increases the
bean porosity, mass, and heat transfer coefficient, whereas
the density decreases due to the drop in moisture content
[80]. Some ranges of crude protein content in dried beans
are from 15.2 to 22%, total fat 50.4 to 55.21%, ash 2.3 to
2.9%, and carbohydrates 21.0 to 24.9%; the minerals iden-
tified in mg/100g are Fe (1.2–2.2), Cu (8.8–17.3), Mg
(262.7–364), Zn (8.2–15.6), Na (2.0–3.0), Ca (143.5–170.8),
P (195.8–355), and K (2070.7–2557.9) [3, 5, 11].
In Latin America, fermentation and drying are mostly
done at the farms. Therefore, standardization is difficult to
achieve, causing a quality decrease in the Criollo varieties.
However, large-scale post-harvest centers are currently being
built in Colombia to standardize the post-harvest processes
of fine cocoa and preserve the quality [126]. Market quality
standards have been established for the dried cacao beans to
guarantee the quality of the chocolate [32] (Table1). Pro-
ducers supply the chocolate companies with the dried cacao
beans, and the processing of the chocolate begins with roast-
ing. The roasting of cocoa beans (see Fig.2b) is considered
the most critical step in chocolate processing because brown
color and volatile compounds (i.e., responsible for the flavor/
aroma) are generated. Also, roasting causes softening of the
material essential for subsequent processing [63, 76].
The roasted beans are winnowed, where the cocoa nibs are
separated from their shells (about 20% of the total weight)
using gravity and airflow, i.e., the air carries away the light
shell and the heavy nibs fall into the container [100]. Subse-
quently, the resulting cocoa nibs are ground with stone rollers
(i.e., refinement) to a paste known as cocoa mass or cocoa
liquor. The cocoa liquor is treated with alkali (i.e., alkaliza-
tion or Dutching) to improve the color and flavor and increase
dispersibility (see Fig.2c). It also reduces astringency by
complex polymerization of polyphenols, therewith decreas-
ing the bitterness and darkening of the cocoa [3, 51]. The
conching is the next step; this is a multi-day mix that contrib-
utes to the development of the final flavor and smooth texture,
generally performed at temperatures above 40°C (for dark
chocolate between 70 and 82°C) [3, 51]. Finally, the choco-
late product is molded into bars, after which it is packed and
ready to market. Large producers typically use continuous
conveyor belts, while small manufacturers do it manually.
Cocoa Products’ Characteristics
The final cocoa products’ quality indicators can vary accord-
ing to the consumer’s requirements. The sensorial percep-
tion in cocoa and chocolate is one of the most important
quality criteria in the industry and is related to the chemical
composition. Currently, both the sensory evaluation by the
panel of experts and the identification of the aromatic com-
pounds through a chromatographic analysis are applied. The
scores resulted define the quality profile in the Cocoa and
Chocolate Flavour Wheel. For cocoa roasted and chocolate,
105 sensory attributes have been selected. These attributes
Table 1 Chocolate and cocoa industry quality standards for dried
cacao beans
*w.b., wet basis
Parameter Range Reference
Moisture content 6.5–8.5% w.b. [70]
Bean count per 100g Standard beans: ≤ 100
Medium beans: 101–110
Small beans: 111–120
Very small beans: > 120
[71]
Cut tests Insect-damaged, insect-infested,
or insect-germinated beans
Light brown to dark brown,
violet, or mix of colors
Fissuring evaluation
Aroma (smoke and oily taints
putrid or hammy notes)
[69]
Maximum Cd content 0.5mg/kg (0.8mg/kg in special
cases)
[27]

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are grouped into eight flavor levels, which correspond to
the “Cocoa and Chocolate Flavour Wheel” shown in Fig.3.
Making fine chocolates and candies attributes as Fruity,
Floral, Sweet, and Cocoa is required, while Acidity, Bitter-
ness, Astringency, and off-flavors are unaccepted. That is, the
quality and value of chocolate are related to its unique flavor
and aroma profiles [131]. However, identifying quality is a
subjective issue related to the human senses and is measured
by a sensory panel of experts. For example, the sound of choc-
olate when it is breaking it is an indicator of quality, where the
high-quality has a clean, crisp, sharp snap when broken, while
low-quality chocolate bends rather than breaks cleanly, has a
dull sound when broken, or only crumbles [138].
High-quality or fine cocoa is characterized by a delicious
taste of cacao with beautiful fruit notes. The rare, complex
flavors of fine cocoa from Criollo varieties have an exclu-
sive mix of complex flavors and produce fine chocolates and
perfume applications [17]. Additionally, the cocoa industry
has sought to highlight the quality of cocoa globally through
competitions worldwide to identify where the best cocoa is
produced and improve its market.
In addition, the U.S. Food and Drug Administration
(FDA) has established standards of identity and quality
for cacao-derived products, where the ranges of ingredi-
ents (cocoa butter, sugar, dairy products) and additives
(flavorings and preservatives), the process applied, and
the microbiological and chemical state as well as the test-
ing methodologies standardized, e.g., chocolate liquor
contains not less than 50% and nor more than 60% wt. of
cacao fat [43].
Fig. 3 Sensorial attributes for cocoa and chocolate flavor wheel byJanuszewska etal [72]

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Cocoa Roasting Process
During the roasting processes, some undesired volatile
compounds (e.g., acetic acid and alcohols) are evaporated
and the moisture content is reduced to about 1% w.b. with
a water activity decrease from about 0.5 to about 0.2 [47,
115]. Moisture reduction minimizes the microbial activity
and increases the glass transition temperature (Tg, improving
the storage stability (i.e., less agglomeration of particles or
texture changes. Additionally, changes in the properties of
products such as texture; organoleptic, thermal, and rheo-
logical characteristics; shelf life; brown color; and functional
and physicochemical properties occur [24, 120, 121].
As such, cacao roasting phenomenology and essential
parameters have been researched extensively over the past
decades, but roasting is a complex process of a multicom-
ponent biomass; therefore, there are still unanswered ques-
tions and phenomena without deep explanations that are
identified in this work. In the roasting process, dried cacao
beans are heated from room temperature up to the roasting
temperature which is commonly from 110 to 160°C. From
34°C, the fat reaches its melting point and goes into a liq-
uid state within the beans’ structure [115]. When the beans
are further heated to temperatures above 100°C, moisture
is removed and low molecular weight compounds such as
alcohols and acetic acid (formed in the fermentation pro-
cess) are volatilized. Due to decreased humidity and water
activity (aw < 0.6), heating induces non-enzymatic brown-
ing reactions (NEB) [77]. NEB is mainly associated with
the Maillard reaction. However, other chemical reactions
also occur, such as Strecker degradation, lipid oxidation, and
polyphenol degradation [31]. The Maillard reaction begins
with the condensation of a reducing sugar and an amino
acid and after can take place through different routes such
as Strecker degradation, to generate aromatics such as pyra-
zines [48], aldehydes, esters, and ketones with a pleasant
aroma to chocolate [10], and dark melanoidins [108]. NEB
reactions are responsible for forming brown compounds such
as melanoidins that change the color to roasted and aromat-
ics that give it flavor and aroma.
The roaster’s technology retains the general basic design
of the past and improvements have mainly focused on heat-
ing systems such as infrared heating and infrared and con-
vection hybrids [2]. The effect of the main parameters of
temperature and time on variables related to quality might
be understood through modeling. This modeling must be
linked to the kinetics of chemical reactions; therefore, it will
make it possible to evaluate the effects of parameters not yet
studied to optimize the process in terms of final quality and
energy consumption.
Technologies oftheRoasting Process
Since the heating profile during cocoa roasting strongly
influences the product quality, specific roaster technologies
have been developed with high energy efficiency. The most
common equipment is heat conduction, hot air, rotary drum
roasters, or continuous roasters. The roaster technology
looks to achieve uniform heating of the beans, obtain more
homogeneous results, and avoid parts over-roasted or burned
(Rocha etal. 2017) [112].
Technologies include the three basic zones, the come
up (heating), constant targeted temperature, and the cool-
ing zone where the product is collected (Mohos 2017) [96].
Since ancient times, technologies have been developed
where new systems have been adopted, increasing heat trans-
fer capacity and efficiency. In the beginning, a metal plate
was used that contained the beans and was exposed directly
to the fire, while a person moved the beans with a wooden
cane (Mayer-Potschak and Kurz 1983) [191]. Today, there
is continuous and batch equipment, with capacities ranging
from grams (for the home) to thousands of kilograms per
day according to the industry’s requirements (see Table2
and Fig.4). High-capacity and high-performance roasters
Table 2 Roasting equipment is
commonly used for cocoa Roasting equipment Capacity (kg/h) Heating principle Reference
Sirocco or ball 10–480 Convection from the hot air
The beans are moved with a stirrer
[94]
Rotary drum All shapes and sizes
0.1–3000
Conduction from direct contact with
drum walls
Convection from the air flowing in
the drum
[37]
IR 0.4–46 Radiative heat Diedrich (USA)
Continuous 1–4000 Convection from hot air passes
through each permeable layer Bühler, Ger-
many
Royal Duyvis
Wiener, USA
Longer Food
Machinary,
China

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have been developed to process large volumes of bulk cocoa,
while small ones are used in small companies specialized in
fine chocolates. Given the cacao bean diversity and the trend
of the specific demand for differentiated chocolates, cocoa
roasting must specialize. Then, gourmet roasters have been
developed that allow a wide range of programmable param-
eters, such as time, temperature, airflow, steam injection,
and cooling time. In this way, it is possible to apply different
operating conditions depending on the type of raw cacao
and the characteristics of the product desired [58]. Addition-
ally, the heating systems have been improved using hybrid
systems (convection-infrared) and infrared (developed by
Diedrich, USA) as well as the use of microwaves as a future
and promising technology [128].
The cooling system that the roasters incorporate is
through airflow over the cocoa beans and stirring to cool
them quickly and avoid burning or over-roasting. In gen-
eral, the optimal operating parameters of temperature and
time depend on the type of roaster, raw material, and the
roasted cocoa’s characteristics, carried out by an operator
who makes decisions based on his long experience.
The evolution of roasters has not been drastic, the tech-
nologies keep the same principles, and no novel devel-
opments are known, e.g., there are no roasters that use
inert heating media such as N2 or CO2 or equipment that
minimizes the processing time by shredding the beans
into smaller particles. Later, emphasis will be placed on
roasting chemistry and the influence of process param-
eters on the formation or degradation of certain chemi-
cal compounds related to cocoa quality in the “Roasting
Chemistry” section, and in the “Mathematical Modeling to
Understand Thermochemical Processes” section, emphasis
will be placed on kinetics and modeling of the process.
Scope ofthis Review
Several reported reviews have been published on agricultural
topics (genotypes, harvest, and post-harvest), biochemistry
and chemistry (fermentation conditions, the formation of fla-
vor and aroma precursors) [10, 46], and health applications
(antioxidant capacity, extraction, and evaluation of molecules
with pharmaceutical potential) [6, 130], and also about social
areas due to the importance of this product in vulnerable
communities in developing countries [67, 86]. In this work,
the most relevant results of cocoa roasting and technologies
are grouped. It is critically evaluated how the parameters
(time and temperature) improve the quality of the product
related to flavor and aroma. The chemical compounds that
influence the aroma and their formation reaction networks
were identified. Besides, this document identified missing
explanations, e.g., the effect of some parameters, such as
heating rate, size particles, and reaction environment (i.e.,
air or inert atmosphere). Also, his work identifies the lack
of experimentation and modeling that allows knowing the
Fig. 4 Types of roasters for cocoa beans: (a) sirocco roaster designed
and patented by G.W. Barth, Ludwisburg (1900), (b) industrial drum
roaster up to 616lb/h by Diedrich (USA), (c) infrared roaster by Die-
drich (USA), (d) typical horizontal continuous bean roaster, and (e)
continuous vertical roaster by Royal Duyvis Wiener (USA)

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true nature of the process. Finally, a way towards investiga-
tion under kinetic control (small-scale) able to describe the
phenomenology present during the heating of cocoa was pro-
posed. In this way, it would be possible to identify variations
in liquid, solid, and gas fractions; temperature differentials;
volatilization; and the kinetics of chemical reactions that will
allow the development of new roasting technologies.
Roasting Chemistry
During cocoa roasting, the aroma, flavor, and texture charac-
teristics of the chocolate are developed. All the chemical pro-
cesses occurring during cocoa roasting are discussed in this
section. In the “Aroma and Flavor Profile Related to Volatile
Compounds” section, the relation between chemicals pre-
sent in chocolate to its aroma and flavor profile is described,
with particular attention to the differentiated characteristics
of grains of the Criollo genotype and the main chemical reac-
tions of roasting will be explained; in the “Maillard Reaction”
section, Maillard reaction; in the “Lipid Oxidation” section,
lipid oxidation; in the “Biogenic Amines’ Formation” sec-
tion, biogenic amines’ formation; and in the “Formation of
Carcinogenic Agents” section, formation of chemicals con-
sidered carcinogenic agents, and a table summary of the main
chemical reactions described above, and products generated
and parameters of the roasting of cocoa beans.
Aroma andFlavor Profile Related toVolatile
Compounds
The flavor and aroma profile of cocoa is directly related to
its chemical composition, and this determines its high or
low quality. Criollo cacao is of the highest quality and is
used in the manufacture of fine chocolates because of its
high aroma and flavor to chocolate, with floral, fruity, and
woody notes defined by aromatic compounds and low acid-
ity and astringency perceptions [25]. So, this variety is more
promising for future luxury applications (e.g., perfumes and
fine candies) of its unique chemical composition.
The perception of the flavor and aroma of cocoa and
chocolate comprises a mixture of a large amount of vola-
tiles generated during the roasting; around 600 compounds
have been identified [147], where there are alcohols, esters,
amines, furans, phenols, acids, furans, furanones, pyrans,
pyrones, pyrroles, pyridines, sulfur compounds, hydro-
carbons, lactones, quinolines, quinoxalines, thiazoles, and
oxazoles [10, 151]. With the purpose of showing the link
between the chemical compound and the sensory perception,
Table3 is made. This table shows the main volatile com-
pounds present in roasted cocoa and its sensory perception.
Among the groups of volatiles present in cocoa, the pyra-
zines are highlighted, because they contribute over 40% of
the aroma of cocoa powder and can be used as tracers for
the flavor of cocoa [20]. These pyrazines have been found in
greater numbers and higher concentrations in fine cocoa beans
from the Criollo variety [30, 117]. About 100 pyrazines have
been identified in cocoa aroma, though 2,3,5,6-tetramethyl-
pyrazine (TMP) has been reported to constitute about 90% of
the total pyrazines in cocoa beans [45]. Furthermore, the basic
aroma of roasted beans and chocolate is attributed to TMP and
TrMP (2,3,5-trimethylpyrazine), with TMP being considered
the most dominant [60,125].
In addition to pyrazines, the aroma of fine cacao is
defined by aldehydes and ketones [25]. According to Voigt
etal. [137], three aldehydes present in chocolate have a
strong chocolate aroma (2-methylpropanal, 2-methylbuta-
nal, and 3-methylbutanal), and the more important ketones
favorable for cocoa flavor and quality are 2-heptanone,
2-pentanone, 2-nonanone, acetophenone, and acetoin [15].
The total aroma of chocolate results from the contribution
of each volatile compound present in cocoa; therefore, the
variation of the concentration of these results in different
sensory profiles. The main desired flavors of the cocoa pro-
file are presented in Table3.
Considering the diverse chemical composition of
roasted cocoa and its relationship with quality is essential
to know the chemical reactions that govern the roasting
process, as well as the influence of the main parameters,
time and temperature, to look for process conditions that
favor the maximum production of desirable aromatics
and reduce undesirable characteristics such as astrin-
gency, acidity, and over-roasted. The above is challeng-
ing since to achieve a complete understanding of roasting
chemistry, it would be necessary to initially study the
kinetics of isolated principal compounds in model sys-
tems and then compare them with the cocoa matrix. The
above means a time-consuming job but would generate
invaluable knowledge for the cocoa industry.
The chemical reactions that govern the roasting process
are Maillard reaction, Strecker degradation, lipid oxidation,
and polyphenol’s degradation reactions.
Maillard Reaction
Maillard reaction is complex, comprising several stages and
pathways to generate volatile compounds and high molecu-
lar weight polymers (Fig.5a). Maillard reaction begins by
the condensation of amino groups with a reducing sugar to
produce Schiff bases and water [57, 74]. Then, a cascade
of chemical reactions that lead to the production of inter-
mediates, aromatics, and brown polymers as melanoidins
was continued [116]. The process depends on pH, tempera-
ture, type, availability, and concentration of reagents [87],
as presented in Table4. Heating accelerates the rate of the
Maillard reaction, increasing the formation of aromatic com-
pounds (showed in the previous Table3) of cocoa [57].

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Table 3 Volatile compounds present in cocoa with quantities (in mg/kg) within parentheses and their sensory perception properties
Sensory perception Compounds References
Aldehydes
 Sweet chocolate 3-Methylbutanal (2.69), 2‐methylbutanal (0.46), 2‐methylpropanal
(0.23), 5‐methyl‐2‐phenyl‐2‐hexenal (0.3), 4‐methyl‐2‐phenyl‐2‐
pentenal (0.055), 2‐phenyl‐2‐butenal (0.39), and vanillin (9.08)
[21, 25, 60, 88, 107, 113]
 Herbal, green n-Hexanal (0.135) [3, 60]
 Fatty, waxy, pungent Nonanal (0.063) [88]
 Bitter, grass, fruity Benzaldehyde (2.20) [60, 88]
 Fruity, floral Methyl p-tolyl ketone, 2‐phenylpropanal, and 2‐phenyl
acetaldehyde (0.33)
[21, 88, 109, 136]
Alcohols
 Fruity, herbal 2-Heptanol (1.19) and 1‐hexanol, 2‐hexanol [21, 60, 88]
 Fruity 2‐Methyl‐1‐butanol and 2-heptanone (0.48) [88, 136]
 Floral 1-Phenylethylethanol, 2‐phenylethanol (3.68), benzyl alcohol
(0.19), linalool (0.12), and 2,3-butanediol (2.68)
[60, 113, 136]
 Vegetal Trans‐3‐Hexen‐1‐ol and 2‐pentanol (0.19) [88, 136]
 Sweet chocolate 1,3-Butanediol (2.68) and 1‐propanol [113]
Esters
 Fruity Ethyl octanoate (0.3%), ethyl phenylacetate (0.4%), ethyl
acetate (0.31), isobutyl acetate (0.14), 2-phenylethylacetate
(1.5%), isoamyl acetate, ethyl butyrate, ethyl lactate, ethyl 2‐
methylbutanoate, ethyl valerate, ethyl hexanoate, ethyl decanoate,
ethyl laurate, and methyl salicylate
[12, 21, 88, 133, 136]
 Floral Benzyl acetate, methylphenyl acetate, ethylphenyl acetate, diethyl
succinate, and isoamyl benzoate
[113, 136]
 Sweet chocolate Methyl cinnamate and ethyl cinnamate [21, 109]
Ketones
 Floral Acetophenone (0.08) [60]
 Sweet, earthy 2‐Nonanone (4.35) [12, 60]
 Floral, herbal 2‐Hydroxy acetophenone [21]
 Fruity, floral 2-Phenylacetaldehyde (0.33), 2‐pentanone, and 2‐heptanone [21, 88]
Furans, furanones, pyrans, pyrones, and pyrroles
 Floral Linalool oxide (0.01), trans-linalool oxide, and 2‐furfuryl
propionate
[12, 21, 60]
 Sweet chocolate 5‐Methyl‐2‐furfural, 2‐acetylfuran, and 2‐acetylpyrrole [21, 113]
 Fruity, herbal, nutty 5‐(1‐Hydrohyethyl)‐2‐furanone and furaneol [21, 81]
 Nutty 3‐Hydroxy‐2‐methyl‐4‐pyrone, pyrrole‐2‐carboxaldehyde, pyrrole,
l-pantolactone, 2‐acetyl‐5‐methylfuran, and 2‐furfural
[21, 81, 136]
Acids
 Floral 2‐Methylpropionic acid, 3‐phenylpropionic acid, and cinnamic
acid
[21, 81]
Amines, amides, nitriles, purines
 Nutty, floral Benzonitrile and (2-phenylethyl)formamide [21, 136]
Pyrazines
 Cocoa, coffee, green, mocha, roast 2,3,5,6-Tetramethylpyrazine (6.35) [88]
 Cocoa, earth, must, potato, roast 2,3,5-Trimethylpyrazine (2.49) [88]
 Nutty, chocolate, cocoa, roasted nuts 3,5-Diethyl-2-methylpyrazine (0.34) and 3,5-diethyl-2-
methylpyrazine (0.019)
[25]
 Peanut butter, musty nutty 2‐Ethylpyrazine (0.03) [88]
 Cocoa, roasted nuts 2,6‐Dimethylpyrazine (0.17), and 2,5‐dimethylpyrazine (0.018), [60, 88]
 Caramel, cocoa 2,3‐Dimethylpyrazine (0.48) [88]
 Nutty, hazelnut, cereal 2,3‐Diethylpyrazine [21]

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An important pathway of Maillard is the Strecker deg-
radation, which leads to the initial formation of Strecker
aldehydes and, subsequently through condensation, oxi-
dation, and dehydration reactions, leads to pyrazines
and other aromatic heterocyclic compounds essential in
the flavor and aroma of cocoa roasted such as pyrroles,
pyridines, imidazoles, thiazoles, oxazoles, hydrocarbons,
ketones, esters, amines, and sulfur compounds [1113, 29,
143].
Table4 shows that the formation of pyrazines is gener-
ated from reducing sugars, amino acids, and peptides, the
contribution of peptides being greater and catalyzed by
bases; also, its formation is favored at a low water activity
and polyphenols can reduce pyrazines’ generation (entries
1 to 3), alkaline pH produces more amount of α-dicarbonyls
and less HMF and methylglyoxal (entry 4), formation of
other aromatics from same precursors as pyrazines (entry 5),
and the DKPs can be assigned to a single peptide precursor
(entry 6).
Lipid Oxidation
Cocoa beans have a high lipid content (32–53% by
weight), and their porosity increases after drying to
24.67% [80]. Lipids can react with oxygen and produce
highly reactive lipid carbonyls, which react with amino
groups to produce a cascade of chemical reactions similar
to the Maillard pathways and generate similar volatile
products as Fig.5b [144, 145]. In the reaction network of
Fig.5b, pathway a produces a new imine which is the pre-
cursor of the Strecker aldehyde and pathway b produces
Table 3 (continued)
Sensory perception Compounds References
 Nutty 2‐Ethyl‐6‐methylpyrazine (0.12), 2‐ethyl‐5‐methylpyrazine, 3‐
ethyl‐2,5‐dimethylpyrazine, and 2‐ethyl‐3,5‐dimethylpyrazine
[60, 109]
 Candy, sweet 2,3,5‐Trimethyl‐6‐ethylpyrazine (0.43) [88]
Fig. 5 (a) Simplified mechanism for flavor generation by the Maillard reaction by [102] with modifications, (b) amino acid degradation in the
presence of lipid carbonyls’ products of lipid oxidation [145], and (c) polyphenols’ degradation by oxidation [92]

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Table 4 Overview of chemical products and reactions during cocoa roasting
*↑ Increasing or formation, ↓ Decreasing or loss, aromatics (aldehydes, ketones, pyrazines, pyrroles, furanes, pyranes, and hydrocarbons), Glu (glucose), Fruc (fructose), HMF (5-hydroxyme-
thyl-furfural)
Entry Chemical product Precursors Chemical reaction Findings Additional observations References
1↑ Pyrazines (Pyr) Reducing sugars (RS)
Free amino acids (AA)
Peptides
Maillard and Strecker degrada-
tion
Formation is catalyzed by base
(model systems)
AAs are a nitrogen source
and do not provide carbon
skeletons
[143]
2 The contribution of peptides
is greater than AA (model
systems)
The generation of pyrazines was
enhanced at low aw (0.33)
[119]
3 Higher polyphenol concentration
reduced pyrazines formation
Polyphenol molecules may bind
part of the pyrazine formed
[95]
4↑ HMF
↑ N-ε-Carboxymethyllysine
(CML)
Reducing sugars (RS)
Free amino acids (AA)
Maillard and associated path-
ways
Alkaline cocoa had higher
concentrations of α-dicarbonyls
and less of HMF and
methylglyoxal
In slightly acidic conditions,
3-DG was further dehydrated
to form HMF while in alkaline
conditions its fragmentation
was predominant
[127,102]
5↑ Aromatics Reducing sugars (RS)
Amino acids (AA)
Maillard and Strecker degrada-
tion
Roasting activates reactions
between reducing sugars and
free amino acids or short-chain
peptides to generate aromatics
Reduction in the concentration
of free amino acids and
reducing sugars
[88]
6↑ 2,5-Diketopiperazines (DKP) Peptides Maillard and associated path-
ways
DKPs can be assigned to a single
peptide precursor
DKPs are essential for a
balanced cocoa bitter flavor
[9, 124]
7↓ Fatty acids
↓ Essential amino acids
Fats
Amino acids (AA)
Lipid oxidation Degradation of essential fatty
acids, amino acids
Decreasing nutritional value [38]
8↑ Melanoidins Sugars
Proteins
Amino acids
Lipids
Maillard, lipid oxidation, poly-
phenols’ degradation
Higher content of fat and
polyphenols increases the
formation of melanoidins
Glu and Fruc react with proteins
and amino acids to form
melanoidins
[103, 108]
9↑ Acrylamide Strecker degradation Reducing sugars
Amino acids
Aasparagine react with a
reducing sugar
Asparagine provides the
acrylamide skeleton
[42]

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α-keto acids and the pathway c leads to the formation of
biogenic amines and Strecker aldehydes. Additionally,
because of lipid carbonyl’s ability to decarboxylate and
deaminate amino acids, diverse studies have pointed to
lipid carbonyls as alternative compounds contributing
to acrylamide formation in fat-rich food products [53].
Also, by oxidation of lipids, nutritional compounds such
as essential fatty acids and amino acids are degraded
(Table4, entry 7). During heating, the remaining water
evaporates and leaves more porosity in the cocoa beans;
then, the oxygen comes into contact with the lipids and
causes oxidation reactions because always the process
is done in non-insulated roasters or using hot air as a
heating medium [115]. Thus, it has been determined that
roasting causes a decrease in fat content especially using
high temperatures and times [82]. Roasting causes a sig-
nificant increase in the primary oxidation state (peroxide
index) and secondary oxidation state of fats (Thiobarbi-
turic acid value) [38]. Therefore, it is essential to evaluate
inert atmospheres in the roaster chamber to identify dif-
ferences in the quality of the product. Above is a research
challenge that has not yet been done. That shows the need
for further research, e.g., evaluating the effect of CO2 and
other compounds released and accumulated in the roaster
chamber, which could influence chemical reactions in the
surface and porosity of the grains.
Polyphenols’ Degradation
Polyphenols are thermolabile components; therefore,
roasting temperature lowers the final concentration of
polyphenols by thermal degradation [1]. The thermal deg-
radation of polyphenols occurs by epimerization and oxi-
dation/autooxidation reactions. Usually, the epimerization
reactions occur from ( −)-epicatechin to ( −)-catechin, and
from ( +)-catechin to ( +)-epicatechin [79]. Figure5c
shows the general mechanism of polyphenol degradation
in cocoa given by heating in oxidizing environments.
Astringency is due to the presence of high concen-
trations of polyphenols common in bulk cocoa beans.
Various studies have been conducted to explain the deg-
radation of polyphenols during cocoa roasting looking
to preserve them because these have antioxidant activ-
ity[68,139, 150]. Then, in roasted cocoa, it is necessary
to obtain a balance of polyphenol content that gives it
antioxidant capacity but does not affect the taste of the
chocolate. In addition, the darkening due to the formation
of melanoidins is favored by a high initial content of fats
and polyphenols because they are products of oxidation
reactions of lipids and polyphenols as well as Maillard
(Table4, entry 8).
Biogenic Amines’ Formation
On the other hand, biogenic amines are non-volatile com-
pounds, without taste and aroma, with biological activity
on the central nervous system that can be risky if exces-
sive concentrations are consumed [14]. The main biogenic
amines found in cocoa and chocolate are 2-phenylethylamine,
tyramine, tryptamine, serotonin, dopamine, and histamine
[56]. The presence of these substances in high non-dangerous
quantities do not represent the negative effect on quality; on
contrast, this means cocoa can be the source of these compo-
nents that are of high value in the pharmaceutical industry.
These biogenic amines are related to lipid oxidation reactions
during heating in oxidizing atmospheres such as air (Fig.5b),
in the same way as some volatiles similar to the products of
the Maillard reaction (Strecker aldehydes).
Formation ofCarcinogenic Agents
However, the excellent quality of roasted cocoa can be affected
by the presence of furans, HMF, and acrylamide [149], which
have been classified as carcinogens and are controlled by the
WHO (World Health Organization). Both acrylamide and HMF
are products of the Maillard and lipid oxidation reactions; the
amino acid asparagine is the precursor of acrylamide formation
because it provides the basic molecular structure of acrylamide
(see Table4, entry 9) [15]. In this way, both aromatic com-
pounds that provide desirable flavor and aroma to cocoa and
undesirable components can also be generated during roasting.
However, roasting is a fundamental stage in the cocoa transfor-
mation that deserves further attention and research for optimiza-
tion purposes.
Roasting Process Parameters andConfiguration
The main parameters of the cocoa roasting process are
temperature and time; then, its influence on the formation
of chemical components that are relevant to the quality of
roasted cocoa is described below.
Considering that the process is divided into three basic
zones as Fig.6: (i) come up (heating), (ii) constant targeted
temperature (isothermal), and (iii) cooling. Roasting begins
with heating the beans from room temperature to the set
process temperature. The heating rate is slow due to conduc-
tive heat transport within large beans (Biot > 0.1) that offer
significant resistance to energy flow. Therefore, in equip-
ment that roasts large amounts of cacao and is heated from
the outside without adequate bean movement and bean gas
contact, this heating rate ends up being much slower because
it is a cluster of particles with bigger dimensions than an
individual bean. In the second step to maintain the process
temperature for a set time, time is between 5 and 120min,

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usually 10 to 60min, and temperature is between 110 and
160°C, usually 120 to 140°C [42, 68, 79, 106]. However,
high temperatures, 170 and 190°C, have also been studied,
resulting in good catechin content using short residence
times [122]. Finally, the third step is the cooling of beans,
which is intended to be quick to avoid overcooking and loss
of aroma, depending on the technology of the roaster.
Table5 shows the effect of temperature and time of roast-
ing on roasted cocoa’s main physicochemical changes found
in the literature. The rate of formation of pyrazines (entry 2)
is higher at high temperatures during the first 10min of the
process after the remaining concentration decreases [115,
151]. These results suggest that short roasting times < 15min
favor the maximum content of pyrazines in the cocoa beans,
which represents a good quality of aroma given by these
compounds. Most volatiles, including pyrazines, aldehydes,
alcohols, acids, esters, and ketones (entry 2), were gener-
ated and reached maximum concentrations within the first
15min of roasting [64]. In addition, high temperatures dur-
ing short times cause less generation of HMF (entry 3),
i.e., shorter times achieve lower thermal damage than low
temperature and long time [116]. Lower HMF contents are
positive factors of quality because they have cytotoxic effects
at high concentrations. However, it should be considered
that the volatile compounds mentioned above, measured
using solid-phase microextraction (SPME–GC–MS), were
generated during roasting and remained trapped within the
cocoa beans. So, when finding that the highest concentra-
tions are during the first minutes of heating, it is evident
that there is a lack of knowledge about any low molecular
weight compound lost by volatilization (the gas fraction) or
degradation when heating to longer residence times. Like-
wise, the distribution of these compounds within cocoa bean
is unknown since, due to the low conductivity, temperature
differentials can be generated and consequently difference in
the performance of chemical reactions. Therefore, it is rec-
ommended to do small-scale experimentation under kinetic
control to determine the rate of compound formation, and
process modeling to identify the evolution of thermal and
mass differentials in cocoa.
In the same way, as for volatile compounds, it has been
found that the generation of acrylamide (entry 4) tends to
decrease when using high temperatures (> 150°C); this is
because acrylamide is not thermally stable. Therefore, the use
of high temperatures is recommended to minimize the acryla-
mide concentration as a carcinogen and HMF, which affect the
quality of chocolate [42]. However, due to the risk of exces-
sive browning or thermal differentials when heated faster at
high temperatures, it is advisable to evaluate more efficient
heating technologies such as microwaves, IR, or hybrid sys-
tems, which could represent the future of roasters. On the
other hand, since current research has been done under typi-
cal conditions and conventional equipment, it is necessary to
leave this vision and dare to change conditions such as the use
of inert atmospheres and reduction of cocoa size to improve
heat transfer and track the chemical variation over time.
Roasting causes decreasing natural products with biologi-
cal activity such as theobromine, caffeine, and polyphenols
(entries 5 and 6). For example, roasting for 40min at 100
caused a decrease in total polyphenols of 9.7% while at 190°C
it decreased to 39.9% [123]. It is desirable to optimize roasting
conditions that maximize the desired compounds’ production
while preserving a desirable sensory profile. Nevertheless,
roasting increases the content of biogenic amines (entry 7),
resulting in Trinitario beans the highest final content [104].
The main biogenic amines found are 2-phenylethylamine,
Fig. 6 Illustrative diagram of
the stages of roasting cocoa in
a roaster

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tyramine, tryptamine, serotonin, dopamine, and histamine [78,
105]. Biogenic amines have physiological roles in humans,
acting as local hormones and neurotransmitters in low con-
centrations, but they can cause a range of toxicological effects
if are consumed in excess [130].
On the other hand, roasting also produces high molecular
weight polymers such as melanoidins (entries 9 and 10);
they are responsible for darkening and involved in sensory
properties (i.e., taste, flavor and texture) of foods [98] and
antioxidant activity [36]. Both the darkening and the forma-
tion of melanoidins are greater when using high tempera-
tures and longer times. Finally, in addition to the softening
of the cocoa beans, the heating causes the fat’s fusion and
mobility, which transports substances and obstructs empty
spaces, causing a diminution of surface area, specific inter-
nal volume, and the average pore size [115].
Consistent with the above discussion and table, there is evi-
dence that the analysis of the main physicochemical changes
caused by roasting, mainly the generation and volatilization of
aromatic compounds, is valuable but has not yet been explored
in depth. For example, the effect of heating rate, particle size,
and environment of reaction conditions (inert or oxidant) is
still missing, opening a world of exciting science possibilities
that deserve to be explored. For this purpose, an efficient tool
for analyzing and understanding the cocoa roasting process are
kinetics and phenomenological modeling explained in the next
chapter.
Mathematical Modeling toUnderstand
Thermochemical Processes
In order to improve the technological processes of cocoa
roasting and obtain a high-quality product in line with con-
sumer trends, it is essential to have a thorough knowledge
of the chemical, physical, and thermal phenomena in cocoa
beans during roasting. In this way, the kinetic analysis of the
formation of volatile compounds, the degradation of natu-
ral cocoa products, and the mathematical modeling of the
process (i.e., mass and energy balances of the system) are
handy tools for a deep understanding of the roasting. With
Table 5 Effect of the main roasting parameters on physicochemical changes in roasted cocoa
*HA (hot air), SHS (superheated steam), IR (infrared heating), CO (convection oven), C (conduction), RH (relative air humidity), ↑ (higher), ↓
(lower), and kg ff-dw (fat-free dry weight basis)
Entry Influence of roasting Experimental conditions Variety Effect References
1 Formation of volatile
compounds
HA vs SHS from 150 to 250°C
(15min) Forastero
(pH = 5.6)
↑ Rate < 15min (150°C)
↑ Rate < 10min (200°C)
[151]
[64]
2 Formation of pyrazines IR heating from 100 to 200°C
(15min) Criollo ↑ Content at 150°C
↓ Content at 100 and 200°C
[115]
3 Formation of HMF HA from 125 (74min) to
145°C (40min) Criollo ↓ Content at ↑ temperatures
(0.1–0.8g kg−1)
[116]
4 Formation of acrylamide HA from 110 to 160°C (15 to
40min) Forastero ↓ Content (> 150°C)
↑ Content (< 150°C)
[42]
5 Decreasing of methylxanthines C at 180°C (10min) Forastero ↓ Content of theobromine (28 to
70% lost)
↓ Content caffeine (around 60%
lost)
[146]
[41]
6 Polyphenols’ degradation CO from 100 to 190°C (10 to
40min) Trinitario ↓ Content at ↑ temperatures and
↑ time
9.17% lost (100°C) to 39.9%
lost (190°C)
[123]
[122]
7 Formation of biogenic amines HA from 110°C to 150°C (25
to 85min) with RH (0.3% to
5.0%)
Trinitario ↑ Rate at ↑ T and (18.67 to
33.46mg/kg ff-dw)
[104]
8 Decreasing of moisture (Xw)
and water activity (aw)
HA vs SHS from 150 to 250°C
(15min)
IR from 100 to 200°C (15min)
C at 110 to 170°C (5 to 65min)
Trinitario and
Criollo ↓ Rate at ↑ temperatures (~ 1%)
aw from 0.4 to 0.6 ↑ Maillard
reaction
[151]
[85]
[115]
9 Formation of melanoidins HA from 125 (74min) to
145°C (40min) Criollo ↑ Rate at ↑ temperatures [116]
10 Darkening HA from 125 (74min) to
145°C (40min) Criollo ↑ Rate at ↑ temperatures (Ea:
132kJ mol−1)
[116]
11 Diminution of surface area,
specific internal volume, and
the average pore size
IR from 100 to 200°C (15min) Criollo Melted fat and solidified after
heating accumulate in the pore
walls affecting microporosity
[115]

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these theoretical and experimental methodologies, it is pos-
sible to predict results accurately and reliably and optimize
the process.
Modeling allows drawing, interpreting, and understand-
ing a particular process’s phenomenology by applying
phenomenological laws of nature. With these models, it is
possible to identify and describe events that, through experi-
mentation, are not possible for any reason (e.g., times too
short or too long to track the phenomenon). In engineering
sciences, modeling is performed to predict results, design
and optimize processes, minimize the costs of experimen-
tation and validation, and characterize and generate knowl-
edge of any process. For example, with modeling, it would
be possible to understand the physicochemical and thermal
changes that occur within cocoa beans during heating, and it
would be possible to predict the evolution of phenomena that
cannot be observed experimentally, such as heat transport
through the solid, porosity, and gas generation over time,
among others.
Mathematical models can be empirically based if the
expressions between the different quantities that are mod-
eled (temperature, concentration, moment vs input quanti-
ties) are determined based on a statistical analysis of the
experimental results. In this sense, they are mathematical
expressions that allow predicting the magnitude at different
times, and conditions are valid for the equipment where the
experimentation was carried out; they cannot be considered
general models. Also, the models can have a combination of
respecting the balances of matter, energy, and moment that
include terms based on constitutive relations of transport
mechanism that can be determined experimentally and there-
fore are considered semi-empirical models. Finally, the mod-
els can be phenomenological in nature, including balance
equations and theoretical models for constitutive equations
or relationships. Also, the models can be made at differ-
ent scales macroscopic (bulk and particle) and microscopic
(molecular); both are a useful emerging tool to analyze the
thermochemical process as roasting.
Some mathematical models describe the evolution over
time of variables such as density, momentum, and energy
at the macroscopic level. At this macro-scale, the variation
in time and physical space of these variables can be fol-
lowed due to the macroscopic movement of the continuous
system and the transport phenomena that occur within the
system due to the microscopic world. In this way, both
empirical models and Fick, Fourier, Newton, and Ohm’s
laws can be applied.
In heterogeneous systems such as cocoa and other bio-
masses (see Fig.7c), it is necessary to consider the volume
variation and the interactions between the present phases;
this applies to processes as drying, roasting, torrefaction
pyrolysis, combustion, and gasification. Also, the models
must consider the different scales, which can be understood,
taking into account the concept of volume fraction (εj) for
each j-phase. In this way, the cocoa roasting process can be
modeled on different scales (Fig.7d). The bulk scale (i.e.,
several beans or in a roaster), then on a smaller scale, is the
particle models. A particle is like a small, heterogeneous
cocoa bean that preserves all the phases present in the origi-
nal system (a group of cells or an individual cell). Going on
a smaller scale, there is modeling at the molecular level that
could explain specific phenomena such as forming a certain
compound, the mechanisms, and interactions present at the
micro-scale.
Depending on the particle size and heating rate, both dry-
ing and devolatilization processes may occur simultaneously
inside the particle, and thus, one could expect wet biomass,
dry biomass, and char at the same time. This is observed in
case of large particles, within which large temperature gradi-
ents are present, but for very small particles, such gradients
are unlikely, and in such cases, only one of these phases can
be found inside the particle. For example, [55] developed
and validated a torrefaction model of large biomass particles
by coupling the material balance and energy to a kinetic
model in two steps, drying and devolatilization. This model
predicted the variation of gas, water, and solid fractions over
time, as well as variations in pressure and density within the
particle.
Alean etal. [7] considered a thin layer of spherical cocoa
beans (bulk scale), dried by hot air at temperature “T,” abso-
lute gas moisture, with velocity “V” and the phases gas “g”
and solid “g.” The material balance considered the convec-
tion, diffusion of water, and degradation of polyphenols (
Y
)
(Eq.1). The energy balance in the gas is considered the
convection and phase change (Eq.2) and the conduction in
the solid (Eq.3).
where the bed porosity
𝜀
, i.e., the void fraction between
beans and intra-beans. It is determined from the volume
fraction
𝜌s
occupied by the solid. Besides, Borda-Yepes etal.
[22] proposed a drying model with volume variation
(
1
V
𝜕V
𝜕t
)
.
This consideration of the volume variation is a great contri-
bution because it is helpful to apply in cocoa roasting, where
(1)
𝜀𝜌
g
𝜕Y
𝜕t
=−𝜀𝜌gvg
𝜕Y
𝜕x
+
𝜕
𝜕x(
𝜌gDeff
𝜕Y
𝜕x)
+𝜌sm
���
w
(2)
𝜀𝜌
gCp,g
𝜕T
g
𝜕t
=−𝜌gvgCp,g
𝜕T
g
𝜕x
−Ua
(
Tg−Ts
)
−m���
w
[
Cv
(
Tg−Ts
)
+𝜆
]
(3)
(
1−𝜀)𝜌sCp,s
𝜕Ts
𝜕
t
=−𝜕
𝜕
x
Keff
𝜕Ts
𝜕
x
+Ua

Tg−Ts

−m���
w[𝜆
]

Food Engineering Reviews
1 3
the swelling of the beans occurs during heating due to the
strong internal pressures of gasses generated (Fig.7c).
Particle Mathematical Modeling
At the particle level, the mass and energy transfer phenom-
ena especially alter the behavior of the released products
(distribution and quantity). During the heating, many gase-
ous species are released, which, depending on the type of
biomass and operating conditions of the reactors, can favor
the production of tars, the loss of valuable aromatics, or
heat release.
Many authors have established that the main factors
affecting the particles’ mass transfer processes are size
and pressure. Niksiar and Rahimi [101] considered that
for small particles (< 900μm, there is a kinetic control
scale; therefore, the restrictions on the transfer of mass
and energy are negligible. In these cases, it is enough to
use zero-dimension models to present the conversion dur-
ing the reaction. In VRM models (Fig.8), it is assumed
that the particle is consumed uniformly throughout the
volume at constant volume expressed as
d𝛼
dt
=k(1−𝛼
)
,
where k is the kinetic coefficient of the reaction and α is
the conversion.
The decreasing nucleus models (SUCM and SUPM)
describe a particle dense and non-porous one, where the
reaction occurs at an interface, which moves to the center.
SUCM is associated with the ash production, where the
ashes cover the unreacted nucleus of the particle. In this
model, the size of the nucleus decreases, but the total vol-
ume of the particle remains constant. The above models
can be represented by the same previous mathematical
expression with an order of
(
2
3
)
.
Fig. 7 Illustrative scheme of (a) types of systems single and multiphase and (b) heterogeneous system at different scales and different phases
useful for the mathematical modeling of thermochemical processes

Food Engineering Reviews
1 3
The progressive models (PMSP and PMSC) are exten-
sions of the previous models, where the reaction occurs in
the entire volume, but intermediate conversion states appear.
The RPM (Ramdom Pore Model) has normally been used
to represent progressive models. The RPM describes the
behavior of a porous particle, whose internal surface area
changes depending on the conversion as (Eq.6)
where φ is a parameter associated with the internal structure
of the porous. A high value of
𝜑
means that the initial poros-
ity is small and that any change in the internal surface area
greatly affects the degree of conversion.
Solid–Gas Reaction Models
Gas–solid reaction systems are useful to know the
dynamics of gas interactions with the solid surface. Thus,
in gas–solid reactions, the diffusivity and the kinetics
depend largely on the solid’s microstructure, which can
vary due to the chemical reaction and sintering, among
others. So, grain size distribution and sintering affect the
(4)
d𝛼
dt
=k(1−𝛼)⋅
[
1−𝜑log(1−𝛼)
]
1∕
2
microstructure’s development during solid–gas reactions,
a problem of interest in classical grain models [49, 50].
To study this effect, a model based on population bal-
ances was developed by [90], to consider the distribu-
tion of grain size inside the pellet and at the same time
consider sintering through the terms of death and birth
in the population balance. In this way, as the chemical
reaction proceeds, the initial grain radius (r0) increases
or decreases (rp) depending on the molar volume of the
solid product and the sintering process. In contrast, the
unreacted core (rc) radius decreases (see Fig.9). It is con-
sidered that the particle is composed of a large number
of grains with an initial radius distribution (see Fig.9b);
then, each grain changes from size due to chemical reac-
tion and sintering.
The material balance for the gas is based on the reaction
system:
(gas)+bB(solid)
→
cC(solid)+dD(gas)
only with
the diffusive term:
J
j=
1
R2
𝜕
𝜕R
(
DeR2𝜕C
𝜕R
)
, where C is the
molar density of the specie j,
De
is the effective diffusion, R
is the spatial variable, and the generation term is related to
the chemical reactions (
r
) (Eq.5).
Fig. 8 Type of models based on the conversion of the particle: vol-
ume reaction model (VRM), shrinking unreacted particle model
(SUPM), shrinking unreacted core model (SUCM), progressive
model with shrinking reacting particle (PMSP), and progressive
model with shrinking reacting core (PMSC) [148]

Food Engineering Reviews
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On the other hand, other authors have developed models
to understand and predict the behavior of the small-scale
pyrolysis process, so Montoya [97] developed a mathemati-
cal model in fast pyrolysis (1200°C s−1) considering that
phases solid, liquid, and gaseous coexist in the particle
(equations not shown). Likewise, this model considered the
consumption and formation of species (reagents and prod-
ucts) and bubble dynamics generation with a population
balance.
Modeling oftheCocoa Roasting Process
Cocoa roasting modeling is complex because it is neces-
sary to include some quality attributes as dependent vari-
ables [18]; these variables are subjective to the perception
of human senses so it is difficult to predict them. However,
these variables are correlated with roasted cocoa’s chemical
composition, so a model able to predict the most relevant
chemical compounds is a current challenge for engineer-
ing. On the modeling of cocoa roasting, there is little bib-
liographic information; an approach towards kinetics was
primarily found where both semi-empirical and empirical
models have been used all of them on bulk scale.
The main phenomenon that has been studied is the loss
of water. In this way, the kinetics of moisture loss follows
an exponential decay very similar to the drying processes;
such behavior was similar for all the models shown Table6.
(5)
𝜀𝜕C
𝜕t
=
1
R
2
𝜕
𝜕R(
DeR2
𝜕C
𝜕R)
−r
���
That is, the mass transfer process is dominated by internal
diffusion [73, 99]. The loss of moisture resulted up to a final
value between 1.9 and 2.3% d.b. [116]. Furthermore, water
loss depends strictly on the temperature used with an activa-
tion energy of 59.6 ± 0.8kJ mol−1, which indicates that it is
a coupled process where the phenomena of mass and energy
transfer are closely related.
Likewise, the kinetics of HMF generation by the expo-
nential model [116], the formation of 2,5-diketopiperazines
(DKPs) was correlated positively with their peptide precur-
sor and was analyzed using a solid-state reaction of zero-
order (sigmoidal) [9].
Darkening has been evaluated with first- and zero-order
models finding activation energy of 59 ± 8kJ mol−1 that is
similar to that of the oxidation of polyphenols [68]; there-
fore, it was concluded that non-enzymatic browning during
roasting is initially induced by the oxidation of polyphe-
nols [116]. The dependence of melanoidin formation on the
roasting temperature was also determined by the asymptotic
model, and the degradation kinetics of polyphenols during
the roasting of cocoa beans was evaluated with the Weibull
probabilistic cumulative model [68]. All the model equations
are in Table6.
On the cocoa roasting process, there are several specific
points where the modeling, on a smaller scale, is necessary
to explain this process’s phenomenology better. Although
it is known that there are important differences directly
related to factors such as temperature and process time,
there are no studies conducted where the influence of the
heating rate is analyzed; therefore, its effect on the final
Fig. 9 Graphic illustration of (a) grain size change and (b–f) evolution of the change in the size of each grain due to chemical reaction and sin-
tering [90]

Food Engineering Reviews
1 3
chemical composition and volatilization is still unknown
and physical properties.
The heating rate can determine the residence time of
the process; the efficiency of heat transfer will also depend
directly on being a thermal gradient which can be gener-
ated and consequently, some areas over-roasted and other
partially raw areas if very large particles as cacao beans
are used. So, another important factor that has not been
explained is the effect of particle size. Considering that in
the thermochemical processes, there are coupled phenomena
of mass and energy transfer, so the particle size can deter-
mine these; therefore, it is necessary to study this factor.
Thus, particle scale modeling would be a helpful tool to
analyze the cocoa roasting process.
Table 6 Kinetics and phenomenological models used for the cocoa roasting process
Type Phenomena Model Equation Reference
Kinetics Moisture loss Newton
MR =exp−kt
[59,39, 116]
Page
MR =exp−
kt
n
Two-term
MR =exp−kt +bexpgt
b, k, g (constants), MR (moisture
ratio)
Weibull
MR =
1
−exp−(t∕𝛽)𝛼
𝛼
,
𝛽
(shape and rate parameters)
HMF formation Exponential
Ct
=C
0
+k
0
exp
k
t
k0 (pre-exponential factor, kt (rate
of formation)
[116]
Formation of 2,5-diketopiperazines
(DKPs)
Zero order and solid-state reaction
P=A0+kt
P (DKPs concentration), A0 (initial
concentration), k (rate constant)
[9]
Malanoidins generation Asymptotic
C
t=C0
t
1
K
in
+t
C
e−
C
0
kin
(
initial rate melanoidin
formation), C0, Ce, and Ct
(initial, equilibrium and time t
concentrations)
[116]
Darkening First order
L
∗=
1
kLt+1
L∗
0
L∗
0
,
L∗(
initial and time t lightness),
kL, (reaction rate)
Zero order
h◦=h◦+kht
h◦
0
,
h◦
(hue angle value),
kh
(reaction rate)
Polyphenols’ degradation Weibull probabilistic cumulative
C
o
−C
t
Co
−C
e
=1−exp−(t∕𝛽)
𝛼
C0, Ce, Ct (initial, equilibrium and
time t concentrations),
𝛼
(shape
parameter),
𝛽
(rate parameter)
[68]
Phenomenological Effective diffusivity of water (De) Arrhenius equation
De=
D
o
exp−
E
RT
Do (diffusivity constant), E
(activation energy), R (universal
gas constant)
[5913]
Diffusion of moisture from inside
the bean to the surface
Fick’s second law
MR
=
6
𝜋2
∞

n=1
1
n2exp
−

Den2𝜋2t
r2
0


MR (initial, equilibrium and
time t moisture), De (effective
diffusion), r (radius), n ( integer
number)
[39, 116]
Thermal diffusivity (αe) Fourier law
TR
=8
𝜋2
∞
∑
n=1
1
2n−1exp(2n−1)
𝜋
2
4L2
t

TR
=
T
i
−T
s
T0−Ts
TR, T0, Ts (temperature ratios,
initial, and surrounding)
[9359]

Food Engineering Reviews
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Mathematical modeling of thermochemical processes from
biomass for energy applications is more advanced than ther-
mochemical processes for food and flavoring applications.
These latest applications include quality attributes and vari-
ous variables to make them viable for human consumption,
making them a great challenge for the engineering sciences.
Concluding Remarks
The quality of chocolate and cocoa derivatives is condi-
tioned by the origin of the cacao beans and the processing
stages during their transformation; therefore, the traceability
of these products is decisive to guarantee a uniform final
quality as well as knowing the origin of the raw material.
The roasting process has been evaluated on a bulk scale
(beans) using traditional hot air toasters and ovens, while
the small-scale study has been neglected, and consequently,
the explanations of all the phenomenology present during
heating are still missing.
The characteristics of quality highly appreciated in the
cocoa and chocolate market are related to the chemical com-
position (i.e., aromatic components generated in roasting).
However, the effect of variables such as heating rate, particle
size, and reactor atmosphere (i.e., oxidant vs inert gasses)
is still unknown.
The literature showed that the researchers identified the
chemical compounds present in roasted cocoa, that is, the
chemical compounds of the solid fraction, while the chemi-
cal compounds released during heating in the gas fraction
are still unknown. Consequently, there is a potential for
processing cocoa that is not evaluated, representing a great
economic value in the industry.
Acknowledgements The authors would like to acknowledge the
University of Groningen, and theAlliance for Biomass and Sustain-
ability Research–ABISURE“Universidad Nacional de Colombia”Her-
mescode53024 for the support and financing of this work. Farid Chejne
wishes to thank project “Strategy of transformation of the Colombian
energy sector in the horizon 2030” funded by call 788 of Minciencias Sci-
entific Ecosystem. Myriam Rojas wishes to thank to “Fundación CeiBA”
for the forgivable education grant for Doctorate in Engineering—Energy
Systems and H. Vallejo for the illustrations.
Funding The first author received funding from “Fundación CeiBA”
under a forgivable education grant for Doctorate in Engineering and
support from the University of Groningen and “Universidad Nacional
de Colombia.” The research leading to these results received funding
from the project “Strategy of transformation of the Colombian energy
sector in the horizon 2030” funded by call 788 of Minciencias Scien-
tific Ecosystem.
Data Availability Data and code are not shared.
Declarations
Ethics Approval Ethics approval was not required for this research.
Consent to Participate Not applicable.
Consent for Publication All permissions for this document are avail-
able.
Conflict of Interest The authors declare nocompeting interests.
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