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PAPER 
 
 
 
 
 

LIPID AUTOXIDATION OF FISH, LARD, CORN 
AND LINSEED OILS BY ISOTHERMAL CALORIMETRY 

 
 

 
 
 

N. HAMAN, M. BODNER, G. FERRENTINO and M. SCAMPICCHIO* 
Free University of Bolzano, Faculty of Science and Technology, Piazza Università, 1, 39100 Bolzano, Italy 

*Corresponding author: Tel.: +39 0471017210, Fax: +39 0471017009 
E-mail address: matteo.scampicchio@unibz.it 

 
 
 
 

ABSTRACT 
 
This work proposes an approach to characterize lipid oxidation of oils based on the 
measurement of isothermal calorimetry data, including information on the duration of the 
monomolecular, bimolecular and termination periods. The approach has been exploited 
with fish and lard oil samples at temperatures from 40 to 80°C and with corn and linseed 
oils at 80°C. The length of the monomolecular period was the most sensitive to the 
variation of temperature. Accordingly, the monomolecular period was used as index of 
oxidative stability of oils. Thus, the highest oxidative stability was observed for corn oil 
samples at 80°C (tmono = 2060 min), followed by linseed (tmono = 390 min), fish (tmono = 40 min) 
and lard oils (tmono = 30 min). The different stability of the samples was discussed with their 
fatty acids profile and antioxidant activity. The results confirmed that the content in 
natural antioxidants is the key responsible for the final oxidative stability of the samples. 
 
 
 
 
 

Keywords: autoxidation periods, fish oil, edible oils, isothermal calorimetry, oxidative stability 
 



	

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1. INTRODUCTION 
 
Lipid autoxidation is a radical chain mechanism that involves a reaction between 
unsaturated fatty acids and oxygen (JOHNSON and DECKER, 2015; TUORILA and 
CARDELLO, 2002). This reaction is of great importance for the food industry, as its 
occurrence is often associated with quality defects, like rancidity or nutritional loss. The 
mechanism of reaction is generally described in three main stages: an “initial pahse”, 
comprising the monomolecular phase of hydroperoxide formation. This is followed by the 
“propagation phase”, where radicals are formed with autocatalytic, monomolecular and 
bimolecular or branching reactions, resulting in the exponential increase of the rate of lipid 
autoxidation. Then, the process ends with a “termination phase”, where the rate of lipid 
reaction slow down and increase the formation of secondary oxidation products 
(GHNIMI, 2017). Between the first and second phase, the abrupt change in the rate of lipid 
autooxidation is often defined as the induction period (IP). This value is widespread used 
by food industry as index to characterize the oxidative stability of an oil sample 
(FRANKEL, 2014). 
The measurement of the induction period is generally obtained by differential scan 
calorimetry (DSC) (SHAHIDI and ZHONG, 2015; LITWINIENKO et al., 1999) or 
differential thermal analysis (DTA) (BACHA et al., 2015). However, the IP value obtained 
with these techniques is often difficult to interpret when the samples have a complex 
composition. One drawback of DSC is that, at low temperatures, the exothermic peak of 
oxidation is flat, and the IP value cannot be determined unambiguously. Moreover, when 
DSC is run at high temperatures, the oxidative behavior of foods become dependent on 
the temperature gradient used and also on the geometry of the samples (FOX and 
STACHOWIAK, 2007; KOUSKSOU et al., 2011). Furthermore, the analysis of oil samples 
that contain antioxidants at high temperatures may lead to misleading results when the 
antioxidants are volatile and can leave the sample before they can exert their protective 
action (HAMAMA and NAWAR, 1991). 
An alternative approach for the analysis of lipid autoxidation is offered by isothermal 
calorimetry (HAMAN et al., 2017a). This technique has some advantages respect DSC and 
DTA, as it can be used to monitor in real-time the rate of autoxidation of unsaturated fatty 
acids under mild temperatures. Its theoretical background is relatively simple and well-
described by Willson (WILLSON et al., 1995) and Hansen (HANSEN et al., 1990). Recent 
applications of isothermal calorimetry include the study of oxidation of lipid oxidation 
(DRIDI et al., 2016), the study of the radical scavenging activity of lipophilic antioxidants 
(HAMAN et al., 2017b), antioxidant and pro-oxidant activity of spent coffee extracts 
(HAMAN et al., 2018) and the study of shelf-life of foods (RIVA et al., 2001). However, in 
these studies, the calorimetric trace was mainly used to provide the cumulative enthalpy 
evolution of the sample, or the induction time. In such way, only a small fraction of the 
information contained in the calorimetric trace is used.  
For this reason, this work aims to propose a methodological approach that can extend the 
information normally achievable with calorimetric data, providing information about the 
duration of the monomolecular, bimolecular and termination phase of the lipid 
autoxidation process. For this purpose, fish and lard oil samples were analyzed by 
isothermal calorimetry at different temperatures (from 40 to 80°C), and the results 
compared with those obtained with linseed and corn oils at 80°C. The relevance and utility 
of this approach was discussed considering the fatty acid composition and the antioxidant 
capacity of the oils. 
 
 
 



	

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2. MATERIAL AND METHODS  
 
2.1. Materials 
 
Refined corn oil, fish oil, lard oil, cyclohexane (purity ≥ 99%), sodium thiosulphate (purity 
≥ 99%) and potassium iodide (purity ≥ 99%) were purchased from Sigma-Aldrich (Milan, 
Italy). Refined linseed oil (cold pressed) was from Sabo Italia s.r.l. (Firenze, Italy). 
Chloroform (stabilized with ethanol) was purchased from PanReac AppliChem (Milan, 
Italy). Acetic acid (purity ≥ 99.7%) was purchased from Fluka (Milan, Italy). 
 
2.2. Fatty acid profile by near infrared spectroscopy 
 
Fatty acids were determined by near infrared (NIR) spectrometer with a Multipurpose 
Analyzer (MPA) (Bruker Optik GmbH, Germany). Acquisition software was OPUS 7.5 
(Bruker Optik GmbH, Germany). The background spectrum consisted of empty cell at the 
same temperature of analysis. OPUS/QUANT package was used to quantify the 
composition of fatty acids (palmitic, stearic, oleic, linoleic and linolenic acid) in fish, lard, 
linseed and corn oils. The analysis was performed at the same conditions of the calibration 
kit (calibrations based on 8084 spectra) in the range of 12500-4000 cm−1 (resolution, 16 cm−1; 
sample, 64 scans) at 40°C. 
 
2.3. Isothermal calorimetry 
 
A micro-calorimeter (Thermal Activity Monitor, Model 421 TAM III, TA Instruments), 
equipped with 24 channels, was used to measure the heat rate. Each channel of the 
instrument is a twin calorimeter where the two units are positioned above each other. In 
isothermal mode, the oil in the thermostat was maintained at a constant temperature with 
an absolute accuracy of ±0.0005 °C (manufacturer’s data). The micro-calorimeters were 
equipped with built-in metal reference specimens having a heat capacity approximately 
equal to that of a vial.  
The heat generated or absorbed was continuously measured over time. Following the 
manufacturer’s instructions, each channel was calibrated prior to measurement using a 
gain calibration procedure with electric impulses. Ampoules were first lowered into the 
thermal equilibration position and left there for 15 min. Then, the ampoules were lowered 
into the measurement position and the heat flow rates recorded for up to 5 and 8 days at 
10-second intervals. The auto-oxidation reaction was monitored by calorimetry with close 
glass ampoules containing 100±3 mg of oil in isothermal conditions in the presence of air 
in the head space.  
 
2.4. DPPH assays for the determination of the antioxidant activity 
 
The antioxidant activity of the oils was evaluated using 1,1-diphenyl-2-picrylhydrazil 
(DPPH) as described by Brand Williams (BRAND-WILLIAMS et al., 1995) with small 
modifications. DPPH is a free radical that can react directly with most of the antioxidants 
and be captured by them (KOLEVA et al., 2002). The reduction of DPPH was measured by 
the decrease in absorbance at a characteristic wavelength (517 nm) and a determined time 
during the reaction (minimum 60 min). In the radical form (DPPH•) presented a violet 
color at 517 nm. With the addition of a reduced antioxidant (AH), its color changed from 
violet to yellow and consequently the absorption decreased as a result of the decrease of 
the DPPH concentration. Briefly, 10 mg of DPPH were dissolved in 250 mL of ethanol and 
sonicated for 1 min. A defined amount of oil was dissolved in 1 mL ethanol and sonicated 



	

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(DU-06, China) for 5 min. To perform the measurements, 1.9 mL of DPPH solution were 
transferred into the cuvettes and 100 µL of the extracts solutions were added. The samples 
were kept in dark for 1 h at room temperature (ca. 25 °C) and subsequently the absorbance 
was recorded at 517 nm with a spectrophotometer (Cary 100 Series UV-Vis 
Spectrophotometer, Agilent Technologies, Italy). The antioxidant capacity of the oils was 
determined as the IC50 - the concentration of antioxidant, which reduces the radical form 
(DPPH•) of 50%. A triplicate assay was performed for each sample.  
 
2.5. Determination of peroxide value 
 
The peroxide value (PV) of the samples was measured according to the AOAC Official 
Method 965.33 (HORWITZ, 2002). The method is based on an iodometric titration, which 
measures the iodine produced from the potassium iodide by the peroxides present in the 
oil. The measurements were performed in triplicate for each sample and PV values were 
expressed as meqO2 per Kg of oil. 
 
2.6. Statistical analysis 
 
Calorimetric results were reported as the mean of six independent experiments. All other 
measurements (NIR and DPPH) were performed in triplicate (n = 3). All the results are 
reported as mean, together with the relative standard deviation. To compare the means 
values, data were subjected to the analysis of variance (ANOVA) using Fisher’s least 
significance difference method with a 95 % confidence level.  
 
 
3. RESULTS AND DISCUSSION 
 
3.1 Oxidative stability based on monomolecular and bimolecular periods 
 
Figure 1 shows the heat-flow signal of a fish oil sample recorded at 40°C. At any point, the 
intensity of the heat-flow signal reflects the rate of the autoxidation process that is 
occurring to the sample, accordingly to the following general relationship (eq. 1) 
(WILLSON et al., 1995): 
 

heat flow = rate x enthalpy (1) 
 
At the beginning of the experiment, the signal is negligible (from the beginning to the 
point (a), Fig. 1). This corresponds to a slow autoxidation process. At about 25 h (point (a) 
of Fig. 1), the rate of heat production suddenly increases, at point (b), reaching a maximum 
heat flow value at point (c). After that, the signal fades down to a minimum steady state, 
at point (d). The final heat-flow signal has an intensity close to that recorded at the 
beginning of the experiment. 
Figure 1 also shows the changes in the apparent reaction order as a function of time. The 
apparent reaction order was determined at any time following an approach previously 
described by Haman (HAMAN et al., 2017a). Briefly, for a reaction A à B, where A 
represents unsaturated fatty acids and B is their oxidation products, the heat-flow signal 
(dq/dt) is equal to k·H·xn, where k is the rate constant, H is the reaction enthalpy, x is the 
number of moles of product formed at time t and n is the reaction order. Since x can be 
obtained, at any time, as the ratio between the heat (q) and the enthalpy (H), then this 
equation becomes (eq. 2): 



	

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𝑑𝑞
𝑑𝑡

= 𝑘 · 𝐻 ·
𝑞
𝐻

!
= 𝑘 · 𝐻!!! · 𝑞! 

 
The log-log plot of the heat flow with the heat leads a slope, which corresponds to the 
reaction order (n). Thus, by following the changes of n as a function of time, it is possible 
to identify three distinct phases of the autoxidation process. For fish oil, the first phase is 
characterized by a reaction order equals to ~ 0.5. This order corresponds to the 
monomolecular period, during which the reaction is catalyzed by monomolecular 
decomposition of the hydroperoxides (LOOH + LH → LO* + L* + H2O) (LABUZA et al., 
1969; GHIMNI et al., 2017). The duration of such phase lasts in about 25±0.3 h. 
Interestingly, this phase ends with the onset point of the calorimetric trace (also called 
induction point). This point is very informative since can be interpreted as the moment 
when a food become unacceptable for human consumption (LABUZA and DUGAN Jr, 
1971). After this point, the reaction order becomes close to 1. It is during this period that 
the rate of lipid autoxidation rises exponentially. In this exponential stage, the bimolecular 
decomposition of hydroperoxides is the dominant mechanism (LOOH + LOOH → LO* + 
LOO* + H2O). This process is known to follow the bimolecular period (LABUZA and 
DUGAN Jr, 1971). The length of such phase lasts in about 25±0.3 h (from point (a) to point 
(b) of Fig. 1).  
 

 
 
Figure 1. Calorimetric trace of fish oil in isothermal conditions at 40°C. Legend: (a) end of the 
monomolecular phase; (b) end of the bimolecular phase; (c) the maximum heat flow and (d) the end of the 
termination phase; Legend: m stands for monomolecular phase; b stands for bimolecular phase and t stands 
for termination phase. 
 
 
After this point, the reaction order, suddenly, declines towards negative values. Negative 
reaction orders correspond to the so-called “termination phase”, where the rate of the 
reaction is either inhibited by the high concentration of the reactants (i.e. radicals species) 
or by the absence of oxygen in the ampoule. In this stage, radical-radical combinations 
occur, and hydroperoxides start to exponentially decompose to secondary oxidation 
products, including aldehydes and ketones (GHINMI at al., 2017). In the experimental 



	

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setup used here, the termination phase occurred when the concentration of oxygen in the 
ampoule was limiting. This was verified in a control experiment, where, successively to 
the analysis of a fish oil sample, the ampoule was opened without removing the sample, 
fresh air was left to enter into the headspace of the ampoule, and the recording of the 
calorimetric signal re-started. In such way, we observed that a new exothermic peak could 
be observed. This result confirms that the termination phase was limited by oxygen and 
that further monomolecular and bimolecular process could occur provided that oxygen is 
available. Accordingly, the termination period of the observed autoxidation process in the 
ampoule was defined as the period occurring from point (b), where the reaction order 
starts decreasing, to point (d), where the reaction is likely terminated because of oxygen 
depletion in the headspace of the ampoule. At times above 80 h, the reaction was 
completed. The measure of the reaction order become unpredictable because of the heat 
flow values were nearly constant, with slopes equals nearly to zero. 
 
3.2. Oxidative stability of corn and linseed oil 
 
The same approach used before with fish oil was next applied to characterize the oxidative 
stability of different oil samples, including fish, lard, linseed and corn oils. All the sample 
showed similar initial peroxide values. The resulting heat flows were plotted as a function 
of time (Fig. 2).  
 

 
 
Figure 2. Heat flow vs. time curve plot of a) fish oil, b) lard oil c) linseed oil and d) corn oil, placed 
isothermally at 80°C into closed glass ampoule.  
 
 
Table 1 reports the duration of the monomolecular, bimolecular and termination step. By 
comparing the duration of the monomolecular period at 80°C, the highest oxidative 
stability was observed for the corn oil sample (2060 min), followed by linseed (390 min), 
fish (50 min) and lard (30 min) oil samples.  
 
 
 



	

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Table 1. Kinetic and thermodynamic parameters of fish, lard, corn and linseed oil. 
 

  Phase duration  

Oil Temperature (°C) 
Monomolecular 

(min) 
Bimolecular 

(min) 
Termination 

(min) 
ΔH 

(kJ/mol O2) 
Fish 40 2220±20   802±10 1780±35 43.1±0.1 
Fish 50 600±8   653±11 1102±24 59.3±0.4 
Fish 60 220±2 450±4 802±8 71.9±0.5 
Fish 70   90±4 311±2 648±6 84.0±0.6 
Fish 80   40±3 224±4 470±9 97.8±0.3 
Lard 40 2490±35 1519±25 5161±71 36.4±0.3 
Lard 50 550±2 190±3 3120±59 47.8±0.7 
Lard 60 200±3 121±3 2402±53 66.2±0.9 
Lard 70  600±3   60±2 1570±31 74.7±0.8 
Lard 80    30±4   104±18 1143±21 67.7±0.5 

Linseed 80  390±7 1102±18 1681±31 77.3±0.2 
Corn 80 2060±25 1121±11 1160±13 46.2±0.1 

 
 
Furthermore, Table 1 shows the changes of the monomolecular, bimolecular and 
termination durations as a function of temperature. For fish and lard oil samples, higher 
temperatures corresponded with faster reaction rates and, consequently, shorter duration 
of these periods. Moreover, the relationship between the duration of such phases follows 
the so-called Arrhenius function, where the logarithm of the duration of each phase leads 
a linear relationship when plotted as a function of the inverse of temperature (ln(t) vs 
1/T). For fish oil samples, the slopes of such linear relationships were 1.1·104 K, 3.6·103 K 
and 3.5·103 K, respectively, for the monomolecular, bimolecular and termination phase. 
Similar results were obtained for lard oil samples. The slopes were in this case equal to 
1.2·104 K, 3.7·103 K and 4.1·103 K, respectively, for the monomolecular, bimolecular and 
termination phase of lard oil samples. These values suggest that the most sensitive phase 
to the changes in temperature is the monomolecular phase. This information is important 
for food manufacturers, since it provides an evidence that the antioxidants needed to 
stabilize such oils are those with the highest capacity to retard or stabilize the 
monomolecular phase. 
Last experiments aimed to understand the reasons of the different oxidative stability 
observed between the oil samples. The ranking observed cold be explained considering 
either the quality of fatty acids composition (i.e. presence of mono- or poly-unsaturated 
fatty acids) or the antioxidant content of the samples. Table 2 reports the fatty acids profile 
and the antioxidant content of the oil samples. What is striking is that the ranking of 
oxidative stability of the oil samples obtained according to the duration of the 
monomolecular phase cannot be predicted simply by looking at the polyunsaturated fatty 
acid composition of the samples. Instead, the oxidative stability of the samples matches 
perfectly with the antioxidant capacity determined by the DPPH assay on the methanolic 
extracts of the samples. In practice, the oil samples with the highest oxidative stability (i.e. 
longer duration of the monomolecular phase, as determined by calorimetry) are also those 
with the highest antioxidant capacity (i.e. shorter value of the DPPH assay). 
 
 
 
 
 



	

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Table 2. Fatty acid composition, peroxide value and antioxidant capacity of fish, lard, corn and linseed oil. 
 

 Corn oil Linseed oil Fish oil Lard oil 
Fatty acids (%, w/w) 
Palmitic acid (16:0) 

 
 14.7±0.6b 

 
   6.3±0.5c 

 
 15.6±1.4b 

 
26.6±1.6a 

Stearic acid (18:0)    2.9±0.3b    4.2±0.6b    3.5±0.8b 10.7±0.9a 
Oleic acid (18:1)  30.8±1.8b  18.8±1.6c  11.1±0.9d 46.8±2.8a 
Linoleic acid (18:2) 48.4±1.7a  17.9±1.3b     2.5±0.5d   9.3±0.7c 
Linolenic acid (18:3)   1.1±0.2b  50.4±3.1a     1.5±0.3b   1.8±0.2b 
Total SFA 17.6±0.5b  11.1±0.7b   23.1±1.1b 37.3±1.5a 
Total PUFA 49.5±2.3b  68.3±3.9a   51.7±2.3b 11.1±0.6c 
Total MUFA 30.8±1.4b  19.2±1.8d   22.6±2.1c 49.6±2.4a 
Others   2.1±0.2a   1.4±0.4a     2.6±0.6a   1.9±0.2a 
Peroxide value (meqO2/kg oil)   2.9±1.2

b   1.6±0.5c     4.2±0.3a    5.4±1.7a 
DPPH (IC50) 70.3±1.5

d 83.9±2.7c 251.8±6.4a  98.9±3.4b 
 
Legend: SFA is saturated fatty acids; PUFA is polyunsaturated fatty acids; MUFA is monounsaturated fatty 
acids. 
Values with different letters in the same raw are significantly different (p < 0.05).  
Values are presented as mean±standard deviation (n = 3). 
 
 
4. CONCLUSIONS 
 
The results of the present study show the potential of isothermal calorimetry to monitor 
the oxidation processes of edible oils and exemplify a procedure to determine the 
monomolecular, bimolecular and termination periods according to the changes in the 
apparent reaction order of the overall process. In addition, we observed that the 
monomolecular phase was the most sensitive to the changes in temperature. This implied 
that the duration of the monomolecular phase is very informative to explain the oxidative 
stability of an oil sample. According to corn, linseed, lard and fish oil samples, their 
oxidative stability was not correlated with their qualitative composition in fatty acids. 
Instead, their oxidative stability was explained with their antioxidant capacity. On a more 
general perspective, this work demonstrated the potential of isothermal calorimetry to 
predict the oxidative stability of oil samples and characterizing the duration of the 
monomolecular, bimolecular and termination phase of the autoxidation process. 
 
 
ACKNOWLEDGEMENTS 
 
We are grateful to the Province of Bolzano for financial support (Landesregierung mittels Beschluss Nr. 1472, 07.10.2013). 
 
 
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Paper Received October 1, 2018  Accepted October 30, 2018