Kinetic and Molecular
Modeling of Selected Bio-hazardous By-products from High Temperature Cooking of
Goat Meat
J. K. Kibet*,
N. Rono, F. I. Okanga, C.
C. Kurgat
Department
of Chemistry, Egerton University, P.O Box 536 –20115,
Egerton, Kenya
*Corresponding
Author E-mail: jkibet@egerton.ac.ke
The environmental fate of
thermally released organic toxins including radical formation from high temperature
cooking has received mounting global concern due to the potential health
impacts associated with them. Therefore, this contribution investigates the
thermal degradation kinetics of pyrolysis products of
red meat under conditions representative of high temperature cooking. Evolution
of selected molecular toxins was monitored using an in-line Gas Chromatography
hyphenated to a mass spectrometer (GC-MS) in the temperature range 500 – 525
˚C. The primary focus is to propose a kinetic model for the thermal
destruction of bio-hazardous by-products; 2-(ethylthio)phenol,
indole and 2,3-dimethylhydroquinone within a
temperature region 723 and 798 K using pseudo-first order rate law. A reaction
time of 2.0 s was employed in line with the average residence time in
combustion systems. Nonetheless, for the formation kinetics of 2-(ethylthio)phenol), various cooking times were chosen.
Kinetic results showed that the destruction rate constants for indole, 2,3-dimethylhydroquinone, and 2-(ethylthio)phenol at 798 K were 1.19, 1.42, and 1.35 s-1
respectively. GC-MS results revealed the amount of 2-(ethylthio)phenol
evolved decreased with increase in the cooking time. The scission of the
phenyl-sulphur linkage in 2-(ethylthio)phenol
was determined using the density functional theory (DFT) and found to proceed
with an energy barrier of 319.31 kJmol-1. The band-gap energy for
2-(ethylthio)phenol was calculated using Chemissian and found to be 5.298 eV.
The kinetic behavior of combustion by-products from red meat is important in
understanding the formation of environmentally toxic free radicals from high
temperature cooking considered harmful to human health and natural ecosystems.
KEYWORDS: 2-(ethylthio)phenol, cooking time, rate of destruction,
toxicity.
INTRODUCTION:
The thermal
degradation of a complex organic matrix such as meat with the aim of obtaining
an array of reaction by-products in limited oxygen is termed pyrolysis1-3. Pyrolysis
reaction mechanisms are complex and therefore it is important to simplify input
factors and physical properties in order to simulate the largest possible
effect on the entire kinetic characteristics of high temperature cooking
procedures2,3. A kinetic
scheme of biomass pyrolysis involves the solution of
complex differential and integrated rate laws 4,5.
A single first order
decomposition model was employed to describe the thermal degradation of indole, 2,3-dimethylhydroquinone, and 2-(ethylthio)phenol in this work. We believe this an important
step in the study of kinetics from high temperature cooking and the formation
of intermediate by-products including environmentally persistent free radicals6. For simplicity, a consecutive first
order reaction characterized by rate constants 
and
has been suggested in which a global kinetic model7-9was employed to obtain the kinetic
parameters for the thermal destruction of molecular toxins in high temperature
cooking. Accordingly, pseudo-unimolecular reactions
in which the empirical rate of decomposition of the initial product is first
order and expressed by equation 1 is very useful in such kinetic studies.
(1)
where: Co and C
are respective concentrations of the
reactant at various residence times, while 
is the pseudo-unimolecular rate constant in the Arrhenius expression
presented by equation 2.
(2)
A is the pre-exponential factor (s-1), 
is the
activation energy (kJmol-1), R is the universal gas constant (8.314
JK-1mol-1), and T is the temperature in K. Despite
all the criticisms against the Arrhenius rate law, it remains the only kinetic
expression that can satisfactorily account for the temperature-dependent
behavior of even the most unconventional reactions including biomass pyrolysis3. The integrated form of the first order rate law (cf.
equation 3) is useful in calculating the rate constant for the pyrolysis behavior of biomass pyrolysis
such as meat at various reaction times. In this case a reaction time of 2.0
seconds will be considered.
(3)
The activation energy can be
determined from the Arrhenius plots (
vs.) which establishes a linear relationship between the pre-exponential
factor 
and the rate
constant as given by equation 4, where 
is the
y-intercept and is the slope.
(4)
The principal focus of this
study is to give a general kinetic account for the destruction kinetics of 2,3-dimethylhydroquinone, 2-(ethylthio)phenol, and indole, and
determine indirectly the concentration of intermediates, such as radicals especially
hydroxybenzyl radical which are usually tedious to
determine experimentally. The kinetics of 2,3-dimethylhydroquinone,
2-(ethylthio)phenol, and indole
destruction is based on high temperature regimes characteristic of high
temperature cooking procedures10,11.
More importantly, this work considered the gas-phase kinetics of 2,3-dimethylhydroquinone, 2-(ethylthio)phenol, and indole which may be fundamental towards understanding
the inhalation kinetics of molecular toxins in high temperature cooking
procedures. The information presented in this study is therefore important in
understanding the destruction kinetics of 2,3-dimethylhydroquinone,
2-(ethylthio)phenol, and indole
in high temperature regimes which may result in the formation of environmentally
toxic free radicals (ETFRs) usually considered detrimental to human health as
reported in previous works6,12-14.
Experimental protocol and Materials:
The heater (muffle furnace) was purchased from Thermo
Scientific Inc., USA while the quartz reactor was locally fabricated in our
laboratory by a glass-blower. Red meat (Goat meat) was purchased from retail
outlets and used without further treatment. Methanol (purity
) used to dissolve meat pyrolysate
was purchased from Sigma Aldrich Inc. (USA).
Sample preparation:
Goat meat (red meat) of 50±0.2 mg was accurately
weight and packed in a quartz reactor of dimensions: i.d. 1 cm x 2 cm
(volume 
1.6 cm3). The meat sample in the quartz reactor was
placed in an electrical heater furnace whose maximum heating temperature is
1000˚C. The meat sample was heated in flowing nitrogen (pyrolysis gas) and the pyrolysate
effluent was allowed to pass through a transfer column and collected in 10 mL methanol in a conical flask for a total pyrolysis time of 3 minutes and sampled into a 2 mL crimp top amber vials for GC-MS analysis. The gas flow
rate was designed to maintain a constant time of 2.0 s15,16. Combustion experiments were
conducted under conventional pyrolysis described
elsewhere14 and the evolution
of 2,3-dimethylhydroquinone, 2-(ethylthio)phenol, and indolewere
monitored between 300 and 525 ˚C.
GC-MS analysis of 2,3-dimethylhydroquinone, 2-(ethylthio)phenol, and indole:
Analysis of 2,3-dimethylhydroquinone,
2-(ethylthio)phenol,
and indole was carried out using an Agilent Technologies
7890A GC system connected to an Agilent Technologies 5975C inert XL Electron
Ionization/Chemical Ionization (EI/CI) with a triple axis mass selective
detector, using HP-5MS 5% phenyl methyl siloxane
capillary column (30 m x 250 µm x 0.25 µm). The temperature of the
injector port was set at 200 ˚C to vaporize the organic components for
GC-MS analysis. The carrier gas was ultra-high pure (UHP) helium (99.99%).
Temperature programming was applied at a heating rate of 15 ˚C for 10
minutes, holding for 1 minute at 200 ˚C, followed by a heating rate of 25
˚C for 4 minutes, and holding for 10 minutes at 300 ˚C. Electron
Impact ionization energy of 70 eV was used. To ensure that the right compound was detected,
standards were run through the GC-MS system and the peak shapes and retention
times compared with those of 2,3-dimethylhydroquinone, 2-(ethylthio)phenol, andindole
standards. The data was run through the NIST
library database as an additional tool to confirm the identity of compounds14. Experimental results were averaged
replicates of two or more data points.
Kinetic model development:
During the development of the kinetic model for the
destruction of molecular toxins (2,3-dimethylhydroquinone,
2-(ethylthio)phenol, indole)
from the thermal degradation of red meat, fundamental assumptions were taken
into account (Fig. 1): (i) the rate of formation of
molecular product is far much more than the rate of destruction, (ii) at the
peak of the curve, the rates of formation and destruction are approximately the
same, and (iii) as the temperature is increased, the rate of destruction
prevails the rate of formation. Based on these assumptions, it is possible to
calculate the apparent kinetic parameters for the destruction of pyrolysis reaction products from the temperature dependence
of their yields.
Fig. 1. The relationship between the formation rate (Rf) and destruction rate (Rd), where
Co is maximum concentration.
A simple single
step reaction mechanism during the thermal degradation of molecular toxins as
presented in equation (5) was considered. Although high temperature pyrolysis of meat is very complex, we believe simple models
based on relevant assumptions may yield reasonable results.
(5)
Conventionally, the differential rate laws for each
species; molecular compound, I (intermediate), and the final product are given by equations 6, 7, and
8 respectively.
(6)
(7)
(8)
If these equations are solved analytically, then the
integrated rate laws are given by equations 9 and 10.
(9)
Equations 10 and 11 give the respective concentrations
of the intermediate I and the product at any time t.
(10)
(11)
In order to simplify equation 11 further, we will
assume that step two (equation 5) is the rate determining step so that
and thus the term 
decays more rapidly than the term 
. Therefore equation 11 reduces to equation 12. This
assumption is valid based on previous studies documented in literature4,17.
(12)
Moreover, the rate of
formation of 2-(ethylthio)phenol from high
temperature cooking has been explored experimentally at modest cooking times
(10, 20, 30, 40, and 50 minutes). For every proposed cooking time, the
concentration of 2-(ethylthio)phenol was determined
using a GC-MS hyphenated to a mass selective detector (MSD).
RESULTS AND DISCUSSION:
Whereas the destruction
kinetics for 2,3-dimethylhydroquinone, 2-(ethylthio)phenol, and indole have
been explored in this study, only the formation kinetics for 2-(ethylthio)phenol, being a unique compound, have been
investigated. For formation kinetics of 2-(ethylthio)phenol,
residence times usually representative of real world cooking conditions (10,
20, 30, 40, and 50 minutes) has been explored. Consequently, a plot of ln as a function of cooking time yielded a straight line
with a slope of – 0.06426 (Figure 2) from which the formation rate constant of
2-(ethylthio)phenol (0.064 min-1) was calculated.
Fig. 2: The formation
kinetics of 2-(ethylthio)phenol from high temperature
cooking
The plot (Figure) is
consistent with first order reaction kinetics. The original amount of 2-(ethylthio)phenol was estimated from the y-intercept and established
to be 3.34 x 106 GC-Area counts. This value is remarkably close to
that obtained from experimental modeling ~ 2.6 x 106 GC-Area counts.
The decrease in 2-(ethylthio)phenol with cooking
times (Fig. 2) contradicts the expectation that the accumulation of molecular
products should increase with increase in cooking time. This is because longer
residence times may lead to possible secondary reactions which result in the
conversion of molecular products to other by-products. It is well documented in
literature that shorter residence times impede side reactions whereas longer
residence times lead to radical formation, recombination, and pyrosynthesis of new by-products14,15. Thus, these processes ultimately
decrease the yields of the parent compounds.
Product distribution of molecular toxins
The product distribution of 2,3-dimethylhydroquinone, 2-(ethylthio)phenol, and indole in
the temperature region 300-325˚C is presented in Fig. 3. Clearly,
2,3-dimethylhydroquinone and 2-(ethylthio)phenol were
evolved in high yields. All the molecular toxins explored in this investigation
reached a maximum at about 450 ˚C. This is consistent with literature data
which indicate that the concentration of most combustion by-products of biomass
matter peak between 400 and 500˚C14-16,18.
Indole, however; had the lowest concentration
compared to 2,3-dimethylhydroquinone and 2-(ethylthio)phenol. The molecular (indole,
2-(ethylthio)phenol, and 2,3-dimethylhydroquinone) toxins reported in this work decreased
sharply after 450˚C reaching respective GC-Area counts of 1.21 x 105,
1.51 x 105 and 1.88 x 105 at 525 ˚C. Phenol,
believed to be a by-product of both 2-(ethylthio)phenol
and 2,3-dimethylhydroquinone was also
investigated in this work (Figure 6, vide
infra) and found to reach a maximum concentration of about 8.39 x 105
at 450˚C. Interestingly, at 525˚C the concentration of phenol was
higher than that of the other molecular toxins ~ 2.22 x 105 GC-Area
counts.
Fig. 3: Product distribution
of selected molecular toxins from high temperature cooking
The destruction kinetics and energetics
of molecular toxins
The destruction energetics for selected organic toxins revealed that 2,3-dimethylhydroquinone, 2-(ethylthio)phenol, and indole
occurred with respective activation barriers 263.10, 283.38, and 315.76 kJmol-1
(Table 1). Clearly, the destruction of indole proceeds with not only a high activation energy
barrier but also a high collision frequency factor (Arrhenius constant, A).
This high energy barrier may be attributed to the 
interactions
which stabilize the indole molecule. Nevertheless,
the destruction energy barriers of 2,3-dimethylhydroquinone
and 2-(ethylthio)phenol are significantly low
but comparable with the destruction energies of other biomass components such
as cellulose3,19,20.
We note that the destruction
kinetics of biomass components from various materials may be different due to
the complex and heterogeneous composition of biomass matrices19,21. Thus the activation energies of
different species in the biomass may not necessarily be the same considering
the fact that species in an organic matrix may act as catalysts and ultimately
reduce or enhance the activation energy of a given compound in a complex
biomass material such as meat. However, in modeling the destruction kinetics of
bio-hazardous components, we are aware that the kinetic
characteristics of a given heterogeneous system such as meat matter may change
during the process of pyrolysis and so it is possible
that the complete reaction mechanism cannot be represented adequately by a
specific kinetic model3,19. Although we have assumed a linear
relationship between 
and
we note that not all reactions will necessarily obey
this relation. Therefore in order to estimate the Arrhenius dependent rate
constants consistent with experimental rate constants we apply the modified
Arrhenius rate law presented in equation 13.
(13)
Since the rate constant for a
given temperature has been determine experimentally and all the other
parameters are known, the value of n can be determined using equation 13. For
instance, the value of n at 723 K was determined and found to be -0.18, -0.18,
and 0.09 for the destruction of indole, 2-(ethylthio)phenol, and 2,3-dimethylhydroquinone
respectively. The same expression (equation 13) can be used to determine the
value of n at any temperature since the rate constant is temperature dependent.
Arrhenius plots for the
destruction of molecular toxins from the meat sample under investigation are
presented in Figs. 4 and 5, vide infra.
Fig. 4: Arrhenius Plots for
the kinetic destruction of 2,3-dimethylhydroquinone and 2-(ethylthio)phenol
The destruction rate constant
at 723 K for indole, 2-(ethylthio)phenol, and 2,3-dimethylhydroquinone were found
to be 0.006 s-1, 0.011 s-1, and 0.022 s-1
respectively. It is remarkable that the rate constants were in the approximate
ratios 1:2:4 implying that at 723 K, the rate of destruction for 2-(ethylthio)phenol is twice the rate of destruction of indole. Using the same argument, it was evident that the
destruction for 2,3-dimethylhydroquinone was four times the rate of destruction
of indole. Nevertheless, at the highest pyrolysis temperature (798 K), the corresponding rate
constants for the destruction of indole, 2-(ethylthio)phenol, and 2,3-dimethylhydroquinone were 1.19 s-1,
1.42 s-1, and 1.35 s-1.
Clearly, as the temperature is increased the rates of destruction are
not significantly different. The average rate constants for the destruction of
the compounds under investigation (indole, 2-(ethylthio)phenol, and 2,3-dimethylphenol) were respectively
0.94 s-1, 1.31 s-1, and 0.87 s-1 respectively.
This suggests that individual temperatures give a higher resolution on the rate
of destruction of a compound. In essence, the average rate constants give
little information on the destruction kinetics of a compound in a heterogeneous
system such as meat.
Fig. 5: The destruction
kinetics of indole at high temperatures
Table 1 below presents the
Arrhenius parameters from the destruction kinetics of molecular toxins under
investigation (Activation energies and Arrhenius factors). Whereas the
activation energies are comparably close, the pre-exponential factors (A) for
the compounds under study are significantly different. For instance, the ratio
of pre-exponential factors for 2,3-dimethylphenol, 2-(ethylthio)phenol,
indole were in the order, 1:27:3200. The ratios are
given in approximate whole numbers and it is clear the collision rate during
the thermal destruction of indole is very high in
comparison with the collision rate for the destruction of other molecular
toxins investigated in this work arguably because of the system in indole molecule.
Table
1. The Arrhenius parameters for the destruction of bio-hazardous combustion
by-products
|
Compound
|
Ea
(kJmol-1)
|
A
(s-1)
|
|
2,3-dimethylhydroquinone
|
263.10
|
4.11
x 1017
|
|
2-(ethylthio)phenol
|
283.38
|
1.10
x 1019
|
|
Indole
|
315.76
|
1.30
x 1021
|
In order to calculate the rate constant for the
formation of phenol resulting from the addition of an H radical to the
intermediate radical (hydroxybenzyl radical) in
scheme 1, vide infra, the
differential rate law, equation 12, is used. To be able to do this, critical
assumptions have to be considered. For instance, the major by-product from the
destruction of 2-(ethylthio)phenol must be phenol.
This assumption is acceptable if we take into consideration the reactive nature
of the H radical in the radical ‘pool’ in combustion systems relative to the
methyl radical14. The other
assumption is that o-cresol and ethanethiol are minor products. From an experimental
standpoint, this assumption is true because neither o-cresol (2-methylphenol) nor ethanethiol
were detected in the entire temperature range of meat pyrolysis
whereas significant amounts of phenol were detected, Figure 3, vide supra. The mechanistic considerations of these
assumptions are reported in scheme 1,
vide infra.
Clearly, by substituting the original concentration of
2-(ethylthio)phenol (2.60 x 106 GC-Area
counts) and the maximum concentration of the product, in this case, phenol
(8.39 x 105 GC-Area counts) into equation 12, vide supra, the value of 
was calculated and found to be 0.57 s-1.
This shows that the value of is ~2.3 times less than the average value of
. Secondly, since the rate constants and have been estimated, and the original value of 2-(ethylthio)phenol is known, then the concentration of the
intermediate, hydroxybenzyl radical, can be computed
from equation 10. Therefore, the concentration of hydroxybenzyl
radical was determined as 1.14 x 106 GC-Area counts. In doing these
calculations we have assumed the average rate constant for the destruction of 2-(ethylthio)phenol is more accurate than the destruction rate
constant at individual temperatures.
The sum of the concentrations of the intermediate and
the proposed final product (phenol) was estimated as
1.98 x 106 GC-Area counts. This shows that only ~ 76% of 2-(ethylthio)phenol has been converted to the final product
(phenol). In a complex matrix such as meat not all 2-(ethylthio)phenol
is converted to the intermediate and ultimately to the product. Some other side
reactions compete during combustion processes resulting to other by-products.
Nonetheless, a recovery of 76% is good enough. Although the pathways via 
and 
are competing
reactions, the rate constant was not possible to calculate because o-cresol was detected in insignificantly
low amounts in our experiments. The product distribution of phenol is presented
in Figure 6.
Figure 6: The product distribution of phenol from high
temperature cooking of red meat
Computational modeling and toxicity index of 2-(ethylthio)phenol
The bond dissociation energy via and the bond
formation energy via in scheme 1 vide supra, were estimated using the
density function theory framework at the B3LYP energy functional in conjunction
with 6-31G basis set22. The
scheme proposes a mechanistic pathway for the thermal degradation of 2-(ethylthio)phenol to the intermediate (hydroxybenzyl
radical) and eventually phenol. The bond dissociation energy for the scission
of phenyl-Sulphur bond (cf. scheme 1) was found
experimentally to be 283.38 kJmol-1 whereas the theoretical value
was found to be 319.31 kJmol-1, calculated using the most accurate
DFT analytical gradient22.
Considering the fact that the pyrolysis of meat is
highly heterogeneous as well as complex, we are confident that the experimental
result calculated from this study is satisfactory.
Scheme 1: Proposed mechanistic
destruction for 2-(ethylthio)phenol
Figure
7: The HOMO-LUMO band gap for 2-(ethylthio)phenol
determined using Chemissian
Despite continuing improvements in formulating new DFT
functionals with advanced predictive capabilities,
the B3LYP functional retains its comparative accuracy in general applications
to organic systems23,24. The application of Chemissian
software facilitated the construction of electron density contour maps and
molecular orbitals from which the band-gap between
the HOMO and the LUMO of 2-(ethylthio)phenol was
calculated and found to be -5.298 eV as reported24 in Figure 7. This high band-gap energy
shows that 2-(ethylthio)phenol molecule is relatively
stable.
The electron density contours
and molecular orbital for 2-(ethylthio)phenol are
presented in Figure 8 and 9 respectively.
Figure 8: Electron density
maps for 2-(ethylthio)phenol
Figure 9: 3-D molecular
orbital diagram showing electronic density for 2-(ethylthio)phenol
Conventionally, for organic
molecules the electron density is rich in the most electronegative atoms and
flow towards the less electronegative atoms. In the case of 2-(ethylthio)phenol the most positive electrostatic potentials
are around the hydrogen atoms while the most electronegative potentials are
around the electron rich benzene ring, oxygen, and to a less extent the sulphur atoms. Nonetheless, the electronegativity
of the neighboring molecules will have a significant bearing on the electron
distribution22,23. Therefore
studies on molecular orbitals and electron density
maps are critical towards understanding the reactivity and nucleophilicity
of molecules22.
The toxicity indices for 2-(ethylthio)phenol, indole, and
2,3-dimethylhydroquinone were estimated using HyperChem
computational platform and reported as log P = 0.26, log P=-0.24, and log P =
-0.15 respectively. The measure of toxicity is based on the octanol-water
partition coefficient25,26 and
shows that 2-(ethylthio)phenol is about 1.8, indole is 0.58, while 2,3-dimethylhydroquinone is 0.71 times more soluble in octanol
than in water hence these compounds are generally hydrophobic because of the
intense carbon content in their structures. Nevertheless, hydrophobic compounds
are known to cause toxicity by covalently bonding with lipids and other
biological structures to release harmful metabolites such as quinones27,28. Hydrophobic compounds have the
potential to cross biological barriers containing lipids, DNA, and microsomal cells causing serious cell injury and ultimately
cell impairment28,29.
CONCLUSION:
This work has provided a
basis for determining the existence of environmentally harmful intermediates
from a kinetic perspective and their environmental fate. Evidently, molecular
toxins investigated have exhibited various kinetic characteristics possibly
because of their heterogeneous kinetic behavior in meat. The concentration of
the intermediate radical (hydroxybenzyl radical) has
been estimated from kinetic modeling of 2-(ethylthio)phenol.
This is remarkable because the concentrations of intermediates in complex
reaction systems such as biomass are usually difficult to determine
experimentally. DFT calculations and experimental predictions for the energetics of 2-(ethylthio)phenol
were found to be very close despite the complex nature of meat pyrolysis. We
believe this work is critical in kinetically studying the fate of organic
toxins in the environment from high temperature combustion systems especially
high temperature cooking procedures.
ACKNOWLEDGEMENT:
The authors appreciate
partial funding from the Directorate of Research and Extension (R&E) at Egerton University.
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