Thermal decomposition of some chemical compounds used as food preservatives and kinetic parameters of this process

Article abstract of DOI:10.1023/A:1010194501649

Thermal decomposition processes of selected chemicals used as food preservatives such as sodium formate, sodium propionate, sodium nitrates(V and III) and sodium sulphate(IV) were examined by the derivatographic method. Based on the curves obtained, the number of decomposition stages and characteristic temperatures of these compounds have been found. Mass decrements calculated from TG curves ranged from 28.9% for sodium formate to 77.8% for sodium nitrate(V), while sodium sulphate showed a mass increment of 5.6%. Kinetic parameters such as activation energy (E_a), frequency factor (A) and reaction order (n) were calculated from TG, DTG and T curves. Sodium formate shows the highest values of E_a and A which amount to 171.7 kJ mol^-1 and 5.8·10¹⁴ s^-1, respectively, while the lowest ones, E_a = 28.2 kJ mol^-1 and A = 3.65·10² s^-1 belong to sodium nitrate(V).

Full text of DOI:10.1023/A:1010194501649

Journal of Thermal Analysis and Calorimetry, Vol. 60 (2000) 305–312
THERMAL DECOMPOSITION OF SOME CHEMICAL
COMPOUNDS USED AS FOOD PRESERVATIVES AND
KINETIC PARAMETERS OF THIS PROCESS
J. Mas³owska and M. Wiêd³ocha
Institute of General Food Chemistry, Technical University of £ódŸ, £ódŸ, Poland
Abstract
Thermal decomposition processes of selected chemicals used as food preservatives such as sodium
formate, sodium propionate, sodium nitrates(V and III) and sodium sulphate(IV) were examined by
the derivatographic method. Based on the curves obtained, the number of decomposition stages and
characteristic temperatures of these compounds have been found. Mass decrements calculated from
TG curves ranged from 28.9% for sodium formate to 77.8% for sodium nitrate(V), while sodium sul-
phate showed a mass increment of 5.6%. Kinetic parameters such as activation energy (Ea), fre-
quency factor (A) and reaction order (n) were calculated from TG, DTG and T curves. Sodium
formate shows the highest values of Ea and A which amount to 171.7 kJ mol–1 and 5.8·1014 s–1, re-
spectively, while the lowest ones, Ea=28.2 kJ mol–1 and A=3.65·102 s–1 belong to sodium nitrate(V).
Keywords: food preservatives, kinetic parameters, thermal decomposition
Introduction
Food preservatives are chemicals that prevent food from decaying, prolonging food
life with relatively low doses not exceeding 0.2% and sometimes even considerably
lower. They show no or almost no effects on the flavour of food being preserved [1].
Food preservatives function is either to kill microorganisms or inhibit their growth,
which prevents undesirable processes such as fermentation, mildew growing or rot-
ting. However, not all antiseptics may be used in food due to their toxicity. There are
only ten various chemicals permissible as food preservatives in Poland [2], such as:
1. Benzoic acid (E 210) and sodium benzoate (E 211);
2. p-Hydroxybenzoic acid ethyl ester (E 214) and p-hydroxybenzoic acid ethyl
ester sodium salt (E 215);
3. p-Hydroxybenzoic acid propyl ester (E 216) and p-hydroxybenzoic acid
propyl ester sodium salt (E 217);
4. Formic acid (E 236), sodium formate (E 237) and calcium formate (E 238);
5. Nisin (E 234);
6. Propionic acid (E 280), sodium propionate (E 281) and calcium propionate
(E 262);
1418–2874/2000/ $ 5.00
Akadémiai Kiadó, Budapest
© 2000 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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MAS£OWSKA, WIÊD£OCHA: FOOD PRESERVATIVES
7. Sodium sulphate (IV) (E 221);
8. Sodium sorbate (E 201);
9. Sodium nitrate (V) (E 251), potassium nitrate (V) (E 252);
10. Sodium nitrate (III) (E 250).
Among them, compounds such as sodium sulphate (IV), sodium sorbate and so-
dium benzoate have found the most common use.
Aim of the present study was to examine the thermal decomposition of selected
food preservatives at high temperatures and to determine the kinetic parameters of
these processes.
Materials and methods
Following compounds were used for the study: sodium benzoate (POCh S. A. –
Gliwice), sodium propionate (Fluka), sodium formate (Fluka), sodium nitrate (V, III)
and sodium sulphate (IV) (POCh S. A. – Gliwice).
Thermal curves were recorded by means of a Paulik-Paulik-Erdey 1000/1500
Derivatograph of MOM (Hungary). Weighed portions of the sample under investiga-
tion were from 200 to 600 mg depending on the density. Samples were heated up to
1273 K under static atmosphere of air a rate of 10 deg min–1 in corundum crucibles,
using α-Al2O3 as reference.
Based on the TG, DTG and T curves obtained, the following kinetic parameters
of first step thermal decomposition were determined: activation energy (Ea), fre-
quency factor (A) and reaction order (n). The values of the kinetic parameters are one
of the most important characteristics in thermal analysis. In accurate determine con-
ditions experiment the values of the kinetic parameters are characteristic for particu-
lar chemicals [3]. The calculations were carried out, assuming that deviations from
the Arrhenius equation can be correlated linearly only in the initial phase decomposi-
tion of the examined samples [4–7].
The Arrhenius equation in logarithmic form (Eq. (1)) was solved graphically:
log(dm/dt)=logA+nlogc–Ea /RT
The value of c was calculated from Eq. (2)
c=(∆m∞ –∆m)/∆m∞
(1)
(2)
where ∆m – maximum mass loss at the given stape. In order to calculate the activa-
¥
tion energy Ea, the plot function log(dm/dt)=f(1/T) was made as shown in Fig. 1.
Knowing the value of Ea, function (Eq. (3)) was plotted:
1/T (tgα )+log(dm/dt)= f (logc)
(3)
as shown in Fig. 2. The slope of the straight line obtained corresponds to the order of
reaction n (n=tgβ). The value pointed out by the straight line on the axis of ordinates
is equal to logA.
J. Therm. Anal. Cal., 60, 2000

MAS£OWSKA, WIÊD£OCHA: FOOD PRESERVATIVES
307
Fig. 1 Graphic dependence log(dm/dt)=f(1/T) applied for appointing activation energy
Ea of I step thermal decomposition sodium formate
Fig. 2 Graphic dependence log(dm/dt)+1/T loga=f(logc) applied for appointing fre-
quency factor (A) and reaction order (n) of I step thermal decomposition sodium
formate
Results and discussion
Figure 3 shows groups of derivatographic curves obtained for sodium formate
(Fig. 3a), sodium benzoate (Fig. 3b) and sodium propionate (Fig. 3c).
Sodium formate HCOONa
From the course of the TG, DTG and DTA curves (Fig. 3a) it can be seen that
HCOONa is decomposed in three stages. The decomposition begins at a temperature
of 510 K, melting of sample endothermic peak in the DTA curve without mass decre-
ment. The first stage of decomposition takes place in the temperature range 543–733,
most rapidly at 703 K. The mass decrement at this stage is 10.7%. In this temperature
range the DTA curve indicates an endothermic reaction (700 K) which suddenly turns
J. Therm. Anal. Cal., 60, 2000

308
MAS£OWSKA, WIÊD£OCHA: FOOD PRESERVATIVES
Fig. 3 Thermal curves of samples: a – sodium formate m0=0.200 g, TG=200; b – so-
dium benzoate m0=0.200 g, TG=200; c – sodium propionate m0=0.208 g,
TG=200
to an exothermic one at 720 K. The second decomposition stage proceeds the most
rapidly at 843 K, with mass decrement being 5.6%. The small exothermic peak in the
DTA curve corresponds to the second stage. The small endothermic peak in the DTA
curve appears at a temperature 1123 K it is the melting point Na2CO3 formed in the
crucible. The thermal decomposition of sodium formate is a complex process . Based
on the TG, DTG, DTA curves and earlier studies [8–10] the following reactions are
possible:
I stage
2HCOONa 543–733 →K (COONa)2 +H2 (↑)
2HCOONa 543–733 →K Na2CO3 +CO(↑)+H2 (↑)
(COONa)2 733–848 →K Na2CO3 +CO(↑)
Na2CO3 above 923 →K Na2O+CO2 (↑)
(4)
(5)
(6)
(7)
II stage
III stage
Sodium benzoate C6H5COONa
Figure 3b shows derivatographic curves obtained for sodium benzoate. Thermal de-
composition C6H5COONa takes place in two stages. It begins at a temperature of
728 K melting of the sample (endotermic peak in the DTA curve). The first stage of
thermal decomposition takes place at 783 K (peak in the DTG curve). Mass decre-
ment at this stage is 35.8%. The second mass loss observed at 1103 K, it is smaller
than the first – 12.3% of its mass. By reaching the temperature 1153 K a small endo-
thermic peak appears in the DTA curve the melting point Na2CO3. Total mass decre-
ment of the sodium benzoate sample on heating within the temperature range from
293 to 1273 K amounts to 48.1%. Based on derivatographic curves and earlier ther-
J. Therm. Anal. Cal., 60, 2000

MAS£OWSKA, WIÊD£OCHA: FOOD PRESERVATIVES
309
mal analyses [11–12] decomposition of sodium benzoate involves the following
non-stoichiometric reactions:
I stage
C6 H5COONa 748–848 →K Na2CO3 +H2 (↑)+CO(↑)
Na2CO3 above 948 →K Na2O+CO2 (↑)
(8)
II stage
(9)
Sodium propionate C2H5COONa
The thermal analysis curves of this compound are shown in Fig. 3c. The first endo-
thermic peak in the DTA curve at temperature 553 K corresponds to the melting of
the sample. The first decomposition stage, between 673 and 843 K is endothermic.
The mass loss 31.7% corresponds to the loss of H2O and CO. In the final stage tem-
perature after melting Na2CO3 (endothermic peak DTA in 1123 K); partially conver-
sion Na2CO3 to Na2O. Based on the derivatographic curves [13] the following reac-
tions are possible:
I stage
2CH3CH2COONa 673–843 →K Na2CO3 +4H2O+5CO(↑)
(10)
+4 0
2
II stage
Na2CO3 above 970 →K Na2O+CO2 (↑)
(11)
Figure 4 shows groups of derivatographic curves obtained for sodium nitrate (V)
(Fig. 4a), sodium nitrate (III) (Fig. 4b) and sodium sulphate (IV) (Fig. 4c).
Sodium nitrate (V) NaNO3
The compound is thermally decomposed within one stage (Fig. 4a). The decomposi-
tion begins at a temperature of 593 K melting of sample (endothermic peak in the
DTA curves) without mass decrement. Thermal decomposition in the temperature
Fig. 4 Thermal curves of samples: a – sodium nitrate(V) m0=0.200 g, TG=200; b – so-
dium nitrate(III) m0=0.200 g, TG=200; c – sodium sulphate(IV) m0=0.200 g,
TG=200
J. Therm. Anal. Cal., 60, 2000

Table 1 Date based on thermoanalytical curves of thermal decomposition process of food preservatives
I stage
Tf/K
II stage
Tf/K
III stage
Name of
compounds
All mass
loss/%
Ti/K
543
748
Tm/K
703
∆mI /%
10.7
Ti/K
733
948
Tm/K
843
∆mII/%
5.6
Ti/K
923
–
Tm/K
1173
–
Tf/K
–
∆mIII/%
12.6
–
HCOONa
733
848
848
28.9
48.1
C6H5COONa
783
35.8
1103
1253
12.3
–
CH3CH2COON
a
673
783
843
31.7
–
–
–
14.6
–
–
–
–
46.3
NaNO3
NaNO2
Na2SO3
793
963
573
11.43 1243
77.8
73.3
–5.6
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
77.8
73.3
–5.6
1173
773
1263
823
Ti – initial temperature
Tm – peak temperature
Tf – final temperature

MAS£OWSKA, WIÊD£OCHA: FOOD PRESERVATIVES
311
range 793–1243 K corresponds to the loss of NO. By reaching the final measurement
temperature 1273 K, the samples loses 77.8%. Thermal decomposition NaNO3 ac-
cording to the following reaction [14–15]:
2NaNO3 793–1243 →K Na2O+2NO(↑ )+1.5O2 (↑)
(12)
Sodium nitrate (III) NaNO2
The decomposition processes of NaNO2 and NaNO3 are similar (Fig. 4b) within one
stage. It starts with melting at a temperature 553 K (endothermic peak DTA). Mass
decrement of the sample begins at 963, proceeds violently at 1173 and is terminated
at 1243 K. For NaNO2 the decomposition temperatures are slightly lower than for
NaNO3, with its mass decrement amounting to 73.3%.
The DTA curve indicates an endothermic reaction (970 K) which suddenly turns
to an exothermic one at 1200 K. While decomposition NaNO2 the following reaction
is possible:
2NaNO2 973–1243 →K Na2O+2NO(↑ )+0.5O2 (↑)
(13)
Sodium sulphate (IV) Na2SO3
Figure 4c shows the thermoanalytical curves obtained for Na2SO3. A different behav-
iour was observed during the heating Na2SO3 at 773 K which shows a gain in mass by
5.6%. This is due to the oxidation of Na2SO3 to Na2SO4 with air oxygen at elevated
temperature. The following reaction is possible [16–17]
Na2SO3 573–823 →K Na2SO4
(14)
The values of characteristic temperatures and mass decrements for all investiga-
tion compounds are given in Table 1.
Kinetic parameters of the first stage of the examined compounds decomposition
were calculated for the initial phase of that stage where the function log(dm/dt)=
f(1/T) is a straight line. The kinetic parameter values found for the compounds exam-
ined are listed in Table 2.
Table 2 Kinetic parameters of I stage thermal decomposition some food preservatives
Kinetic parameters
Name of compound
Activation energy
Ea/kJ mol–1
Frequently factor
A / s–1
Order of reaction
n
HCOONa
C6H5COONa
CH3CH2COONa
NaNO3
171.7
161.2
58.8
5.88·1014
1.22·1012
7.97·104
3.65·102
5.64·104
1.33·10–1
1.92·10–1
4.90·10–1
6.68·10–1
3.20·10–1
28.2
NaNO2
52.0
J. Therm. Anal. Cal., 60, 2000

312
MAS£OWSKA, WIÊD£OCHA: FOOD PRESERVATIVES
The value of Ea for sodium formate HCOONa is 171 kJ mol–1, the frequency fac-
tor A=5.88·1014 s–1, and the reaction order n=1.33·10–1. The kinetic parameters for so-
dium benzoate show lower values, namely: Ea=161 kJ mol-1, A=1.22·1012 s–1 and
n=1.92·10–1. Sodium nitrate(V) shows the lowest activation energy Ea=28 kJ mol–1,
with its A and n values amounting to 3.65·1012 s–1 and 6.68·10–1, respectively. The ki-
netic parameters for the remaining compounds under investigation assume intermedi-
ate values as listed in Table 2.
Conclusions
The compounds used as preservatives in food industry are of very different chemical
character and therefore their thermal decomposition proceeds differently. Among six
examined food preservatives in this study, only sodium formate HCOONa decom-
poses in three stages, its decomposition begins at the lowest temperature and it loses
the least of its mass. The decomposition of sodium nitrate(III) NaNO2 begins at the
highest temperature, which indicates that it has the highest thermal stability among
the examined food preservatives. The highest mass decrement due to thermal decom-
position is shown by sodium nitrate(V).
The results obtained indicate that the derivatographic method may be used to exam-
ine the thermal decomposition of food preservatives. This method allows one to monitor
the processes taking place on increasing the temperature of samples under testing.
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