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The mathematical prediction model for the oxidative stability of vegetable oils by the main fatty acids composition and thermogravimetric analysis
Accepted Manuscript The mathematical prediction model for the oxidative stability of vegetable oils by the main fatty acids composition and thermogravimetric analysis Jinwei Li, Jian Liu, Xuyuan Sun, Yuanfa Liu PII:
S0023-6438(18)30417-1
DOI:
10.1016/j.lwt.2018.05.003
Reference:
YFSTL 7113
To appear in:
LWT - Food Science and Technology
Received Date: 30 January 2018 Revised Date:
19 April 2018
Accepted Date: 1 May 2018
Please cite this article as: Li, J., Liu, J., Sun, X., Liu, Y., The mathematical prediction model for the oxidative stability of vegetable oils by the main fatty acids composition and thermogravimetric analysis, LWT - Food Science and Technology (2018), doi: 10.1016/j.lwt.2018.05.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT The mathematical prediction model for the oxidative stability of vegetable oils by the main
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fatty acids composition and thermogravimetric analysis
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Jinwei Li, Jian Liu, Xuyuan Sun, Yuanfa Liu*
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State Key Laboratory of Food Science and Technology, School of Food Science and Technology,
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Jiangnan University, 1800 Lihu Avenue, Wuxi, Jiangsu Province 214122, People’s Republic of
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China
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*Corresponding author: Yuanfa Liu
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Phone: 0510-85876799; Fax: 0510-85876799; E-mail: [email protected] 4
ACCEPTED MANUSCRIPT Abstract
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Thermal oxidation stabilities of four kinds of vegetable oils (palm oil, rapeseed oil, sunflower oil,
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linseed oil), their characteristic fatty acids (FA, palmitic acid, oleic acid, linoleic acid and
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linolenic acid) and corresponding fatty acid methyl esters (FAME, methyl palmitate, methyl
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oleate, methyl linoleate and methyl linolenate) were quantified by the parameter of onset
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temperature (Ton) of the thermal oxidation. The parameter Ton was obtained by the
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non-isothermal pattern of TGA method in the oxygen atmosphere at different heating rates (1, 5,
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7.5, 10, 15, 20 °C/min). The results showed that the stability order of four kinds of vegetable oils
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was: palm oil > rapeseed oil > sunflower oil > linseed oil. For FAs, palmitic acid and oleic acid
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were more stable than linoleic acid and linolenic acid. And the same situation was found in
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FAMEs because of the difference in the unsaturation degree. Furthermore, based on the
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composition of four most essential fatty acids in the oils, a mathematical model based on the
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stability of FAs and FAMEs was found to predict the stability of vegetable oils. The
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mathematical predictions of oil oxidative stability by FAME system were more accurate with
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lower average deviation below 2%.
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Keywords: Vegetable oils, Fatty acid (FA), Fatty acid methyl ester (FAME), Oxidative stability,
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Thermogravimetric analysis (TGA)
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1. Introduction
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Oils and fats play a critical role in the human diet because of the high calorific value and
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essential fatty acids for the development of human tissues. However, oils and fats start
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decomposing after being isolated from their natural status. Many processing operations in the
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food industry, especially heat treatment, may lead to remarkable changes in the colors, the 5
ACCEPTED MANUSCRIPT flavors and the ingredient in vegetable oils. However, besides all the processing operations, the
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storage lipid oxidation is also one of the main factors that can cause the losses of nutrition,
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deterioration of oil quality, the generation and accumulation of some unpleasant flavors and even
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toxic and harmful compounds. Oxidative stability is a significant qualitative index, which refers
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to the resistance of lipids to the oxygen and temperature in these processes. And it is closely
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related to the triacylglycerol and fatty acid composition of oils and lipids, as well as some natural
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and added minor antioxidant components and numerous external factors (Cheng et al., 2015;
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Kerrihard, Nagy, Craft, Beggio, & Pegg, 2015; Sabolová et al., 2017). Oxidation stability in the
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storage and processing operations varies for different oils. In fact, it’s an essential reference for
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the determination of the applicability of fats and oils in food processing and their shelf life (Choe
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& Min, 2006). The application of most commercial vegetable oils is in some degree limited by
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their short shelf life. Vegetable oils with high stability could be more competitive in extending
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the shelf life of frying oils and fried foods, as well as in some other special purposes (Merrill,
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Pike, Ogden, & Dunn, 2008). A good understanding of lipid oxidation stability of different
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vegetable oils and some new kinds of lipids can improve our abilities to make the best use of oils
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and lipids in food systems and minimize the generation and accumulation of undesirable
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breakdown products.
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Extensive studies on lipid oxidation stability have developed various effective methods and
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spurred series of findings. Some methods are used to determine the oxidation stability by
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measuring the concentration of primary and secondary oxidation products, including Rancimat,
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the oven test, gas chromatography and titration (Cordella, Tekye, Rutledge, & Leardi, 2012;
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Symoniuk, Ratusz, & Krygier, 2017). Some are based on the changes of some physical quantities 6
ACCEPTED MANUSCRIPT (oxygen pressure, weight, heat flowing) related to the thermal oxidation process (Sabolová et al.,
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2017; Symoniuk et al., 2017). Other methods are focus on the measurement of antioxidant
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activity (Choe & Min, 2006; Laguerre, Lecomte, & Villeneuve, 2007). Furthermore, some classic
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methods of determining the oxidative stability, such as Rancimat, ESR, DSC, and TGA, were
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compared in many research works (Jain & Sharma, 2010; Velasco, Andersen, & Skibsted, 2004).
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All proven methods could reveal the oxidation stability in a certain way with its own
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advantages and disadvantages. Among them thermal analysis techniques have been proved a
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suitable and effective methods with low expense and high accuracy (Borugadda & Goud, 2014;
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Dantas et al., 2011). For example, TGA method is very suitable to reflect the stability in the
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perspective of Ton and activation energy (Ea) calculated from Arrhenius Kinetics because it can
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continuously monitor the mass loss under a programmed temperature process. However, even for
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the extraordinary TGA method, the determination of the oxidative stability is relatively time
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consuming or complex. Besides, most of the researches focused on the stability comparison of
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different oils, the evaluation and calculation methods, the comparison of different methods. Even
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though a close relationship between oil oxidative stability and their FA composition was
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demonstrated in dozens of studies, a few researches characterized the oxidation stability of
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vegetable oils on the basis of their main fatty acids composition.
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The main objectives of this study were: 1) to evaluate the thermal oxidation stability of
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different vegetable oils, foremost fatty acids and corresponding methyl esters with TGA method,
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2) to establish a mathematical model to predict the oxidative stability of vegetable oils on the
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basis of main FA composition and the stability of main FAs as well as FAMEs. Thus, the
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oxidative stability properties of the common vegetable oils and even some new kinds of 7
ACCEPTED MANUSCRIPT vegetable oils could be calculated and predicted by this model without conducting some
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traditional detection methods which were time consuming and complicated.
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2. Materials and methods
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2.1 Materials
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Four types of vegetable oils (palm oil, rapeseed oil, sunflower oil, linseed oil) were directly
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obtained from Yihai Kerry (Wilmar International) without any antioxidant added. Four kinds of
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fatty acids (palmitic acid, oleic acid, linoleic acid and linolenic acid) and their esters (methyl
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palmitate, methyl oleate, methyl linoleate and methyl linolenate) were obtained from J&K
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Chemicals. All were of 95-99% purity and stored in the dark to minimize the oxidation process.
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All required chemicals and solvents were of Analar or high-performance liquid chromatography
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grade.
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Since the oxidation stability depends not only on triacylglycerol and fatty acid composition of
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oils but also some natural and added minor antioxidant components in the oils. The natural
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antioxidant components such as tocopherols and sterols were removed from the oils in our
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research with the purification method of Lee and Min (Lee & Min, 1990).
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2.3 Composition analysis of vegetable oils
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Fatty acid composition was analyzed using AOCS method to prepare fatty acids methyl esters,
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followed by gas chromatographic (Shimadzu GC-2010, Japan) detection (Badings & Jong, 1988).
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A TR-FAME column of 260M154P (60m length × 0.25mm I.D., 0.25 µm film thickness) was
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used in the GC detection process.
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2.4 TGA analysis and mathematical prediction model establishment 8
ACCEPTED MANUSCRIPT The thermal oxidation stability of different samples was determined by thermogravimetric
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analysis (TGA) method. The non-isothermal analysis was performed in thermogravimetric
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analyzer model TG/SDTA851e (METTLER TOLEDO). Experiments for each sample were
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conducted in a 70 µL ceramic crucible at different heating rates of 1, 5, 7.5, 10, 15, 20 °C/min.
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The samples went through a temperature range of 50-620°C with a constant oxygen flow rate of
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30 ml/min at atmospheric pressure (A. G. D. Santos et al., 2014). In order to ensure the full
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contact between the samples in open ceramic pans and the purge gas as well as avoiding the
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temperature gradient, sample weight for each test was around 4 mg. The TGA machine was
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calibrated to insure the accuracy of temperature controlling and reading before the experiments.
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In addition, all experiments were performed 3 times for repeatability.
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TGA method were regarded as an effective method specializing in determining the thermal
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oxidation stability by monitoring the mass loss under a programmed temperature process. The
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specific principles and derivation process of Ton parameter from TGA method were shown in Fig.
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1. The thermal behavior of palm oil in oxygen atmosphere at the heating rate of 5 °C/min were
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present in Fig. 1, including the typical TGA mass loss curve and the differential thermal
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gravimetric (DTG) curve. The DTG curves were the first derivative form of typical TGA mass
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loss curves as function of heating temperature. It referred to the weight reduction rate and mostly
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was used to analyze the processes and differentiate stages of the reaction. From the DTG curve
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of palm oil, three oxidation stages with peaks were observed at the temperature of 246.43 °C,
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320.91 °C, and 464.23 °C respectively. The first stage, starting at around 220 °C and finishing at
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approximately 280 °C, was considered the most significant as it represented the initial process of
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triglyceride degradation (K. A. Santos et al., 2017). In this stage, peroxides generated and
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ACCEPTED MANUSCRIPT accumulated due to the reaction with oxygen (dos Santos Politi, de Matos, & Sales, 2013). Thus,
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this was the most important step to determine the edible oils thermal stability (J. C. O. Santos et
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al., 2004). The most vital parameter of TGA kinetic method is the initial weight loss temperature
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of the first step, also called onset temperature (Ton). According to some previous researches, the
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parameter Ton was widely used to evaluate the oxidation stability and it was calculated by the
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thermal gravimetric curves (Jain & Sharma, 2012; Nik, Ani, & Masjuki, 2005; Santander et al.,
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2012). With the differential thermo-gravimetric curve, a tangent line on the maximum point of
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mass loss rate can be determined accurately. The intersection point of the tangent line and the
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baseline was described as Ton (Fig. 1). The Ton parameter obtained by this method was used to
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reflect the oxidative stability of the sample. Since thermal oxidation stability was evaluated by
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the onset temperature, higher Ton means greater tolerance to temperature as well as better thermal
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stability.
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In order to set up the most reliable mathematical prediction model, the stability data with the
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highest accuracy which were determined by the TGA method of 5 °C/min and 1 °C/min were
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used. Moreover, 4 main fatty acids would be selected to represent the complete FA composition
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of each oil in order to reflect the relationship between the stability of oils and their FA
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composition. The reorganized proportion of 4 main fatty acids were used to calculate the
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coefficient of the prediction equation.
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3. Results and discussion
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3.1 Fatty acid composition of the oils
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In order to find a widely applicable law for the oxidation stability of common vegetable oils,
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palm oil, rapeseed oil, sunflower oil and linseed oil were investigated in this research. These oils 10
ACCEPTED MANUSCRIPT showed a huge diversity in composition, therefore, the commonly used vegetable oils in the
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market were well represented by these four oils. Fatty acid composition of palm oil, rapeseed oil,
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sunflower oil and linseed oil was presented in Table 1. As can be seen, palmitic acid, oleic acid,
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linoleic acid, linolenic acid are the main components in these oils as well as in many other
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vegetable oils according to some researches (Li et al., 2013; Tudorachi & Mustata, 2015).
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Considering the fact that oil oxidation stability was closely related to their fatty acids
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composition, palmitic acid, oleic acid, linoleic acid, linolenic acid and their esters were chosen to
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be regarded as the simple and basic systems when predicting the oxidation stability of different
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vegetable oils (Kerrihard et al., 2015).
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3.2 Thermal oxidation stability of fatty acids, their esters and vegetable oils
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In order to precisely determine and compare the stability of different samples, the Ton of the
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first thermal decomposition event was used. Ton of main fatty acids and their esters were present
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in Fig. 2 and Fig. 3. Since a higher Ton of the sample means better thermal oxidation stability, the
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same stability order for 4 kinds of fatty acids was obtained under different heating rate: palmitic
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acid > oleic acid > linoleic acid > linolenic acid. Palmitic acid and oleic acid showed higher
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onset temperatures, which meant they retained better oxidation stability than linoleic acid and
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linoleic acid. And the same stability results were found in different FAMEs that methyl palmitate
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and methyl oleate were more stable than methyl linoleate and methyl linolenate under the same
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condition of atmosphere and temperature. This result agreed with G. Litwinienko’s research in
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which the comparison of stability of unsaturated fatty acids and their esters was conducted by
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DSC method (Litwinienko, 2001). The stability of different FAs and FAMEs was mainly due to
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their chemical structure. Compositional features of fatty acid esters that influence the thermal
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ACCEPTED MANUSCRIPT oxidation properties include chain length, degree of unsaturation and branching of the chain. As
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is well known, oxidation mostly takes place in the double bonds, therefore, fatty acid esters of
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high unsaturation degree are more prone to oxidation (Knothe 2005, Moser 2009). Consequently,
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unsaturated methyl esters, unsaturated fatty acids (or vegetable oils for that matter) are less stable
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than those with saturated carbon chains (Nik et al., 2005; Refaat, 2009).
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The Ton of different oils were shown in Fig. 4. The Ton parameter of palm oil was above
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225 °C, while it was only around 185 °C for linseed oil. And Ton of different oils at different
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heating rates showed the same regulation and trends. Therefore, under the same experimental
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conditions, the initial mass loss temperature difference can be used to compare the oxidation
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stability of various oils. And the following order for oxidation stability was found: palm oil >
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rapeseed oil > sunflower oil > linseed oil, which showed a good agreement with previous
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researches (Cordella et al., 2012; Qi et al., 2016). The results of oxidation stability of these oils
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were due to the fact that FA composition and chemical structure play a significant role in thermal
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stability, which has been demonstrated by Monika Sabolova who studied the relationship
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between the composition of fats and oils and their oxidative stability by the Oxipres apparatus
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(Sabolová et al., 2017). The content of saturated fatty acids in palm oil was above 30% (Table 1).
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For rapeseed oil and sunflower oil, the proportion of unsaturated fatty acids was more than 65%.
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And linolenic acid has the largest share of more than 41% in the fatty acid composition of linseed
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oil. Vegetable oils with high proportion of unsaturated fatty acids were usually less stable than
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those with saturated fatty acids (Micić et al., 2015; Nik et al., 2005). Thus palm oil and rapeseed
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oil showed better stability than sunflower oil and linseed oil.
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When comparing thermal decomposition profile at the heating rates of 1, 5, 7.5, 10, 15, 12
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Under different heating rates, the Ton obtained from TGA method raised from 219.87 °C to
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267.71 °C, 192.55 °C to 227.91 °C and 191.99 °C to 225.72 °C for palm oil, palmitic acid and
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methyl palmitate, respectively. Under the heating rates of 1 °C/min and 20 °C/min, a temperature
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difference of around 30 °C between the onset temperatures was found. The same results have
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been reported by many researchers (Gao, Chen, Zeng, Xu, & Wang, 2017; Micić et al., 2015; A.
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G. D. Santos et al., 2014). There was an agreement that the temperature difference might result
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from the less detailed weight loss and oxidation reaction process under a faster heating rate as
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20 °C/min. Considering the results obtained by this study and some previous researches, it can be
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concluded that the stability parameter can be determined by the experiments under different
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heating rates by TGA. Oxidative stability of all samples can be compared under the same
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experimental conditions. Moreover, the lower heating rate would lead to a more detailed mass
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loss process and thus a Ton parameter closer to the real value could be obtained. While the Ton
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parameter of the method of 1 °C/min and 5 °C/min was pretty close, with the temperature gap of
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merely 2-4 °C. Thus relatively low heating rates as 5 °C/min and 1 °C/min were both competent
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to get results precise enough. And the mathematical prediction model would be established on
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the basis of databases under the heating rate of 5 °C/min and 1 °C/min.
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3.3 Mathematical prediction of oxidative stability of vegetable oils based on fatty acid
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composition
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The fatty acid composition varies in different oils. In order to reveal the relation between the
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stability of oils and their fatty acids, the main fatty acids (palmitic acid, oleic acid, linoleic acid
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and linolenic acid) in Table 1 were selected to represent the fatty acid composition of each oil. 13
ACCEPTED MANUSCRIPT The minor fatty acids were not considered because of their low content and ignorable
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contribution to the oxidation stability of vegetable oils. A renormalization method was used to
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describe the proportion of the main fatty acids in different oils. And the detail reorganized
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proportion of 4 main fatty acids were present in Table 2. The absolute content of palmitic acid,
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oleic acid, linoleic acid and linolenic acid in palm oil were 29.29%, 33.56%, 8.12% and 0.11%
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(Table 1), and the proportion became 41.22%, 47.22%, 11.2% and 0.14% (Table 2) after the
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normalized reorganization. The same situation could be found for other oils. And these
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reorganized proportions of main fatty acids, which represented their contribution to the oxidative
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stability of oils, would be used in the model establishment of oxidative stability prediction.
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3.3.1 Prediction of vegetable oil oxidative stability by FA system
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By comparing the parameter Ton (Fig. 2) and the main fatty acid composition of different oils
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(Table 2), some regulations can be found. Compared with rapeseed oil and sunflower oil, palm
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oil was more prone to thermal oxidation because of higher proportion of unsaturated fatty acids.
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The palmitic acid and oleic acid content were almost the same in sunflower oil and linseed oil
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while the content of linolenic acid proportion was much higher in linseed oil, and it turned out
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sunflower oil showed better oxidation stability than linseed oil. These results demonstrated the
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existing conclusion that oils with fatty acids of higher unsaturation degree had poorer oxidative
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stability (Micić et al., 2015, Qi et al., 2016). The content of main FAs plays an important and
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direct role in the oxidation stability of different samples. Based on the FA composition and the
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Ton parameter under the heating rate of 5 °C/min and 1 °C/min, an association between the
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oxidation stability of vegetable oils and their main fatty acids were found, which can be
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described by the equation below:
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ACCEPTED MANUSCRIPT T=φ Where Ai stands for the reorganized proportion of 4 kinds of fatty acids in oils, Xi is the exact
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Ton for each fatty acid (i = 1, 2, 3, 4). φ is a coefficient related to the heating rates, and it was the
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same (φ=1.13) for the method of 5 °C/min and 1 °C/min because of the close Ton results. T refers
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to the predicted onset temperature for each kind of oil.
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The prediction of oil stability with high accuracy usually needs the FA composition as well as
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the content of various oil components which contribute to the stability (Hidalgo 2002,
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Castelo-Branco et al., 2016). In this work, after the removal of natural antioxidants and minor
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compounds, the stability prediction model could be directly described by the succinct equation
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based on the FA composition only. With a given vegetable oil and its FA composition, and the Ton
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in Fig. 2, a specific onset temperature for the oil can be calculated by this empirical equation.
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The results and estimation deviation of the prediction for each oil by the FA stability parameter
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under the heating rate of 5 °C/min and 1 °C/min were given in Table 3. It turned out the
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estimation equation was an accurate and effective means with the prediction deviation all below
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6% for 4 kinds of vegetable oils. The average deviation of the prediction for all vegetable oils
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was respectively 2.45% and 2.40% for the method of 5 °C/min and 1 °C/min. Thus the oxidation
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stability could be predicted by the FA composition and the stability of main fatty acids in the oil.
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These results are in good agreement with what Adrian L. Kerrihard has reported where they also
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studied the relationship between the FA composition and the oxidative stability using a
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mathematical modeling method. According to the findings of this study, the exact concentrations
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of monounsaturated, diunsaturated and tri-unsaturated fatty acids together showed a strong
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correlation (R2=0.915) with the stability of the different vegetable oils in their model (Kerrihard
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ACCEPTED MANUSCRIPT et al., 2015). Similar studies have been previously published in which a multiple linear
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regression model were established to demonstrate that saturated, monounsaturated and
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polyunsaturated FA together accounted for 67% in stability evaluation (Redondo-Cuevas et al.,
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2018).
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3.3.2 Prediction of vegetable oil oxidative stability by FAME system
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Generally, all kinds of vegetable oils are made up of glycerides, a kind of ester formed by the
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reaction of fatty acids and glycerol. The prediction by corresponding FAMEs was taken into
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consideration in this part. The same method was used to derive the equation as following, from
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which the stability parameters of different vegetable oils can be calculated through the Ton of
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main FAMEs:
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T=λ
Where Ai stands for the reorganized proportion of 4 kinds of fatty acids in vegetable oil, Xi is
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the exact Ton for each corresponding fatty acid methyl ester of the main fatty acids (i = 1, 2, 3, 4).
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And λ is a coefficient related to different heating rates, it was also the same (λ=1.18) for the
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method of 5 °C/min and 1 °C/min. T refers to the predicted onset temperature for the given oil.
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With the reorganized FA composition in Table 2 and the Ton of different FAMEs in Fig. 3, the
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Ton for the oils were calculated by another empirical equation with a different coefficient λ.
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Many researches revealed that the stability of vegetable oils was related to their FA composition
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and the unsaturation degree (Sabolová et al., 2017, Evangelos 2018). However, the work in this
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research moved forward to focus on the stability prediction of different vegetable oils on the
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basis of FA composition. Moreover, the results and deviation of the prediction for each oil by the
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stability parameter of FAME system under the heating rate of 5 °C/min and 1 °C/min were given
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ACCEPTED MANUSCRIPT in Table 4. Results showed that the Ton of 4 kinds of vegetable oils could be calculated by the
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equation model with the prediction deviation all below 3%. The average deviation of the
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prediction for all vegetable oils was respectively 1.81% and 1.75% for the method of 5 °C/min
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and 1 °C/min. It was much lower than the prediction deviation from the system of FA (Table 3).
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Compared to the mathematical prediction model established based on the stability data of main
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FAs, oxidation stability could be predicted by the stability of main FAMEs with higher accuracy
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under different heating rates. Considering the basic composition and structure of vegetable oils
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and FAMEs, this may be due to the higher similarity between these 2 systems. Besides, the
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model based on the stability database from TGA method at lower heating rate showed better
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prediction reliability. This might be due to the lower average deviation in the stability
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quantification process by TGA method at lower heating rate, as described in section 3.2.
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These results provide a comprehensive quantification of the relationship between fatty acid
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composition of vegetable oils and oxidation stability. With a given vegetable oil and its FA
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composition, the oxidation stability reliably reflected by the Ton parameter can be derived by the
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empirical equation. Both the equations based on the stability data of main FAs and their FAMEs
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gave a good prediction result about the oxidation stability of different oils. And the equation
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based on the stability of FAME system under lower heating rate will be more precise.
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4. Conclusion
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In the present study, the oxidative stability of vegetable oils, FAs, and FAMEs were well
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estimated by the parameter of Ton derived from TGA carves by the non-isothermal pattern
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method. It turned out that the oxidative stability order was: palm oil > rapeseed oil > sunflower
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oil > linseed oil. Reason for these results is related to the lower quantity of unsaturated fatty 17
ACCEPTED MANUSCRIPT acids in palm oil and rapeseed oil. For the FA and FAME systems, fatty acids and their esters
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with higher unsaturation degree showed poorer oxidation stability because the unsaturated bond
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of higher unsaturation degree need lower activation energy to initial the oxidation and that means
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a lower Ton. Thus the TGA method used in this research is a pretty suitable way to evaluate the
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oxidative stability of FAs, FAMEs, and vegetable oils. Besides, the experiments under lower
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heating rates provide results with higher accuracy.
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Furthermore, a mathematical equation model based on the 4 foremost fatty acids composition
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was found to predict the stability of vegetable oils through the Ton of both FA and FAME systems.
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Under the TGA method of 5 °C/min, the average deviations for the prediction model based on
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the stability database of FA and FAME were 2.45% and 1.81% respectively. In addition, for the
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TGA method of 1 °C/min, the average deviations of the prediction were respectively 2.40% and
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1.75%. The mathematical predictions from the perspective of FAMEs stability were more
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accurate than FAs stability data. Moreover, the mathematical equation model based on the
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database of lower heating rates showed higher accuracy. In the practical application, it’s strongly
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recommended that the mathematical prediction model based on the stability data of FAMEs
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under the heating rate of 1 °C/min should be used to predict the oxidative stability of a specific
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vegetable oil.
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AUTHOR INFORMATION
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Corresponding author
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* Phone: 0510-85876799; Fax: 0510-85876799; E-mail: [email protected]
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Notes
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The authors declare no competing financial interest.
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ACCEPTED MANUSCRIPT Acknowledgments
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This study was funded by the Natural Science Foundation of China (31571878), Northern
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jiangsu province science and technology projects (BN2016137), Jiangsu province science &
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technology (BE2016635) and the Fundamental Research Funds for the Central Universities
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(JUSRP51501).
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Figure and Table Captions
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Figure 1
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The specific principles and derivation process of the Ton from TGA method
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Figure 2
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Fig. 2 Ton of palmitic acid, oleic acid, linoleic acid and linolenic acid from TGA under different
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heating rates.
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Figure 3
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Fig. 3 Ton of methyl palmitate, methyl oleate, methyl linoleate and methyl linolenate from TGA
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under different heating rates.
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Figure 4
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Fig. 4 Ton of palm oil, rapeseed oil, sunflower oil and linseed oil from TGA under different
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Table 1
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Chemical composition of palm oil, rapeseed oil, sunflower oil, and linseed oil (%)
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Table 2
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Reorganized proportion of 4 main FAs (palmitic acid, oleic acid, linoleic acid and linolenic acid)
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in palm oil, rapeseed oil, sunflower oil, and linseed oil
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Table 3
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Ton from the mathematical prediction model based on FA system under 5 oC/min and 1 oC/min
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Table 4
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Ton from the mathematical prediction model based on FAME system under 5 oC/min and 1
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ACCEPTED MANUSCRIPT Table 1 Chemical composition of palm oil, rapeseed oil, sunflower oil, and linseed oil (%) Palm oil
Rapeseed oil
Sunflower oil
Linseed oil
Miristic (C14:0)
0.43±0.01
--
--
--
Palmitic (C16:0)
29.29±0.68
2.69±0.01
3.89±0.02
3.12±0.01
Stearic (C18:0)
3.59±0.12
1.51±0.01
3.25±0.02
2.76±0.02
Oleic (C18:1)
33.56±1.02
46.29±1.21
13.62±0.34
13.15±0.28
Linoleic (C18:2)
8.12±0.11
16.22±0.41
51.87±1.52
11.96±0.38
Linolenic (C18:3n6)
--
0.51±0.03
--
0.24±0.04
Linolenic (C18:3n3)
0.11±0.01
5.15±0.62
0.22±0.03
41.02±1.18
Arachidic(C20:0)
0.34±0.01
0.54±0.08
0.21±0.01
0.1±0.01
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Fatty acid
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ACCEPTED MANUSCRIPT Table 2 Reorganized proportion of 4 main FAs (palmitic acid, oleic acid, linoleic acid and linolenic acid) in palm oil, rapeseed oil, sunflower oil, and linseed oil Linoleate acid
Linoleate acid
41.22 4.09 5.43 4.49
47.22 70.39 19.03 18.94
11.42 24.67 72.46 17.23
0.14 0.85 3.08 59.34
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Oleic acid
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Palm oil Rapeseed oil Sunflower oil Linseed oil
Palmitate acid
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ACCEPTED MANUSCRIPT Table 3 Ton from the mathematical prediction model based on FA system under 5 oC/min and 1 oC/min
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5 C/min
1 oC/min
Model Reliability
Palm oil
Rapeseed oil
Sunflower oil
Linseed oil
Ton by experiment (°C)
225.32
209.38
197.43
185.82
Ton by prediction (°C)
213.58
207.14
198.12
191.73
Prediction deviation (%) Ton by experiment (°C)
5.21 219.87
1.07 206.41
0.35 194.45
3.18 182.11
Ton by prediction (°C)
209.83 4.57
203.62 1.35
193.81 0.33
188.21 3.35
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Heating Rates
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ACCEPTED MANUSCRIPT Table 4 Ton from the mathematical prediction model based on FAME system under 5 oC/min and 1 oC/min
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5 C/min
1 oC/min
Model Reliability
Palm oil
Rapeseed oil
Sunflower oil
Linseed oil
Ton by experiment (°C) Ton by prediction (°C) Prediction deviation (%) Ton by experiment (°C)
225.32 219.99 2.37 219.87
209.38 211.4 0.97 206.41
197.43 195.58 0.94 194.45
185.82 191.30 2.95 182.11
Ton by prediction (°C)
216.02 1.75
207.23 0.40
191.15 1.70
187.82 3.13
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Prediction deviation (%)
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Fig. 1 The specific principles and derivation process of the Ton from TGA method.
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Fig. 2 Ton of palmitic acid, oleic acid, linoleic acid and linolenic acid from TGA under
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Fig. 3 Ton of methyl palmitate, methyl oleate, methyl linoleate and methyl linolenate
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Fig. 4 Ton of palm oil, rapeseed oil, sunflower oil and linseed oil from TGA under
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ACCEPTED MANUSCRIPT Method validation of oxidative stability of FAs, FAMEs and oils by TGA was verified. A database of stability parameters of four main FAs and FAMEs was established.
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The database and model based on FAMEs were suggested in practical
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S0023-6438(18)30417-1
DOI:
10.1016/j.lwt.2018.05.003
Reference:
YFSTL 7113
To appear in:
LWT - Food Science and Technology
Received Date: 30 January 2018 Revised Date:
19 April 2018
Accepted Date: 1 May 2018
Please cite this article as: Li, J., Liu, J., Sun, X., Liu, Y., The mathematical prediction model for the oxidative stability of vegetable oils by the main fatty acids composition and thermogravimetric analysis, LWT - Food Science and Technology (2018), doi: 10.1016/j.lwt.2018.05.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT The mathematical prediction model for the oxidative stability of vegetable oils by the main
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fatty acids composition and thermogravimetric analysis
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Jinwei Li, Jian Liu, Xuyuan Sun, Yuanfa Liu*
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State Key Laboratory of Food Science and Technology, School of Food Science and Technology,
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Jiangnan University, 1800 Lihu Avenue, Wuxi, Jiangsu Province 214122, People’s Republic of
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China
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*Corresponding author: Yuanfa Liu
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Phone: 0510-85876799; Fax: 0510-85876799; E-mail: [email protected] 4
ACCEPTED MANUSCRIPT Abstract
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Thermal oxidation stabilities of four kinds of vegetable oils (palm oil, rapeseed oil, sunflower oil,
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linseed oil), their characteristic fatty acids (FA, palmitic acid, oleic acid, linoleic acid and
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linolenic acid) and corresponding fatty acid methyl esters (FAME, methyl palmitate, methyl
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oleate, methyl linoleate and methyl linolenate) were quantified by the parameter of onset
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temperature (Ton) of the thermal oxidation. The parameter Ton was obtained by the
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non-isothermal pattern of TGA method in the oxygen atmosphere at different heating rates (1, 5,
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7.5, 10, 15, 20 °C/min). The results showed that the stability order of four kinds of vegetable oils
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was: palm oil > rapeseed oil > sunflower oil > linseed oil. For FAs, palmitic acid and oleic acid
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were more stable than linoleic acid and linolenic acid. And the same situation was found in
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FAMEs because of the difference in the unsaturation degree. Furthermore, based on the
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composition of four most essential fatty acids in the oils, a mathematical model based on the
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stability of FAs and FAMEs was found to predict the stability of vegetable oils. The
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mathematical predictions of oil oxidative stability by FAME system were more accurate with
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lower average deviation below 2%.
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Keywords: Vegetable oils, Fatty acid (FA), Fatty acid methyl ester (FAME), Oxidative stability,
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Thermogravimetric analysis (TGA)
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1. Introduction
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Oils and fats play a critical role in the human diet because of the high calorific value and
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essential fatty acids for the development of human tissues. However, oils and fats start
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decomposing after being isolated from their natural status. Many processing operations in the
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food industry, especially heat treatment, may lead to remarkable changes in the colors, the 5
ACCEPTED MANUSCRIPT flavors and the ingredient in vegetable oils. However, besides all the processing operations, the
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storage lipid oxidation is also one of the main factors that can cause the losses of nutrition,
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deterioration of oil quality, the generation and accumulation of some unpleasant flavors and even
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toxic and harmful compounds. Oxidative stability is a significant qualitative index, which refers
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to the resistance of lipids to the oxygen and temperature in these processes. And it is closely
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related to the triacylglycerol and fatty acid composition of oils and lipids, as well as some natural
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and added minor antioxidant components and numerous external factors (Cheng et al., 2015;
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Kerrihard, Nagy, Craft, Beggio, & Pegg, 2015; Sabolová et al., 2017). Oxidation stability in the
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storage and processing operations varies for different oils. In fact, it’s an essential reference for
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the determination of the applicability of fats and oils in food processing and their shelf life (Choe
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& Min, 2006). The application of most commercial vegetable oils is in some degree limited by
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their short shelf life. Vegetable oils with high stability could be more competitive in extending
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the shelf life of frying oils and fried foods, as well as in some other special purposes (Merrill,
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Pike, Ogden, & Dunn, 2008). A good understanding of lipid oxidation stability of different
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vegetable oils and some new kinds of lipids can improve our abilities to make the best use of oils
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and lipids in food systems and minimize the generation and accumulation of undesirable
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breakdown products.
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Extensive studies on lipid oxidation stability have developed various effective methods and
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spurred series of findings. Some methods are used to determine the oxidation stability by
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measuring the concentration of primary and secondary oxidation products, including Rancimat,
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the oven test, gas chromatography and titration (Cordella, Tekye, Rutledge, & Leardi, 2012;
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Symoniuk, Ratusz, & Krygier, 2017). Some are based on the changes of some physical quantities 6
ACCEPTED MANUSCRIPT (oxygen pressure, weight, heat flowing) related to the thermal oxidation process (Sabolová et al.,
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2017; Symoniuk et al., 2017). Other methods are focus on the measurement of antioxidant
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activity (Choe & Min, 2006; Laguerre, Lecomte, & Villeneuve, 2007). Furthermore, some classic
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methods of determining the oxidative stability, such as Rancimat, ESR, DSC, and TGA, were
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compared in many research works (Jain & Sharma, 2010; Velasco, Andersen, & Skibsted, 2004).
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All proven methods could reveal the oxidation stability in a certain way with its own
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advantages and disadvantages. Among them thermal analysis techniques have been proved a
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suitable and effective methods with low expense and high accuracy (Borugadda & Goud, 2014;
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Dantas et al., 2011). For example, TGA method is very suitable to reflect the stability in the
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perspective of Ton and activation energy (Ea) calculated from Arrhenius Kinetics because it can
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continuously monitor the mass loss under a programmed temperature process. However, even for
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the extraordinary TGA method, the determination of the oxidative stability is relatively time
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consuming or complex. Besides, most of the researches focused on the stability comparison of
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different oils, the evaluation and calculation methods, the comparison of different methods. Even
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though a close relationship between oil oxidative stability and their FA composition was
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demonstrated in dozens of studies, a few researches characterized the oxidation stability of
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vegetable oils on the basis of their main fatty acids composition.
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The main objectives of this study were: 1) to evaluate the thermal oxidation stability of
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different vegetable oils, foremost fatty acids and corresponding methyl esters with TGA method,
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2) to establish a mathematical model to predict the oxidative stability of vegetable oils on the
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basis of main FA composition and the stability of main FAs as well as FAMEs. Thus, the
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oxidative stability properties of the common vegetable oils and even some new kinds of 7
ACCEPTED MANUSCRIPT vegetable oils could be calculated and predicted by this model without conducting some
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traditional detection methods which were time consuming and complicated.
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2. Materials and methods
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2.1 Materials
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Four types of vegetable oils (palm oil, rapeseed oil, sunflower oil, linseed oil) were directly
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obtained from Yihai Kerry (Wilmar International) without any antioxidant added. Four kinds of
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fatty acids (palmitic acid, oleic acid, linoleic acid and linolenic acid) and their esters (methyl
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palmitate, methyl oleate, methyl linoleate and methyl linolenate) were obtained from J&K
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Chemicals. All were of 95-99% purity and stored in the dark to minimize the oxidation process.
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All required chemicals and solvents were of Analar or high-performance liquid chromatography
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grade.
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Since the oxidation stability depends not only on triacylglycerol and fatty acid composition of
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oils but also some natural and added minor antioxidant components in the oils. The natural
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antioxidant components such as tocopherols and sterols were removed from the oils in our
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research with the purification method of Lee and Min (Lee & Min, 1990).
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2.3 Composition analysis of vegetable oils
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Fatty acid composition was analyzed using AOCS method to prepare fatty acids methyl esters,
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followed by gas chromatographic (Shimadzu GC-2010, Japan) detection (Badings & Jong, 1988).
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A TR-FAME column of 260M154P (60m length × 0.25mm I.D., 0.25 µm film thickness) was
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used in the GC detection process.
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2.4 TGA analysis and mathematical prediction model establishment 8
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analysis (TGA) method. The non-isothermal analysis was performed in thermogravimetric
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analyzer model TG/SDTA851e (METTLER TOLEDO). Experiments for each sample were
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conducted in a 70 µL ceramic crucible at different heating rates of 1, 5, 7.5, 10, 15, 20 °C/min.
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The samples went through a temperature range of 50-620°C with a constant oxygen flow rate of
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30 ml/min at atmospheric pressure (A. G. D. Santos et al., 2014). In order to ensure the full
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contact between the samples in open ceramic pans and the purge gas as well as avoiding the
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temperature gradient, sample weight for each test was around 4 mg. The TGA machine was
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calibrated to insure the accuracy of temperature controlling and reading before the experiments.
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In addition, all experiments were performed 3 times for repeatability.
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TGA method were regarded as an effective method specializing in determining the thermal
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oxidation stability by monitoring the mass loss under a programmed temperature process. The
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specific principles and derivation process of Ton parameter from TGA method were shown in Fig.
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1. The thermal behavior of palm oil in oxygen atmosphere at the heating rate of 5 °C/min were
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present in Fig. 1, including the typical TGA mass loss curve and the differential thermal
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gravimetric (DTG) curve. The DTG curves were the first derivative form of typical TGA mass
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loss curves as function of heating temperature. It referred to the weight reduction rate and mostly
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was used to analyze the processes and differentiate stages of the reaction. From the DTG curve
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of palm oil, three oxidation stages with peaks were observed at the temperature of 246.43 °C,
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320.91 °C, and 464.23 °C respectively. The first stage, starting at around 220 °C and finishing at
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approximately 280 °C, was considered the most significant as it represented the initial process of
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triglyceride degradation (K. A. Santos et al., 2017). In this stage, peroxides generated and
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ACCEPTED MANUSCRIPT accumulated due to the reaction with oxygen (dos Santos Politi, de Matos, & Sales, 2013). Thus,
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this was the most important step to determine the edible oils thermal stability (J. C. O. Santos et
135
al., 2004). The most vital parameter of TGA kinetic method is the initial weight loss temperature
136
of the first step, also called onset temperature (Ton). According to some previous researches, the
137
parameter Ton was widely used to evaluate the oxidation stability and it was calculated by the
138
thermal gravimetric curves (Jain & Sharma, 2012; Nik, Ani, & Masjuki, 2005; Santander et al.,
139
2012). With the differential thermo-gravimetric curve, a tangent line on the maximum point of
140
mass loss rate can be determined accurately. The intersection point of the tangent line and the
141
baseline was described as Ton (Fig. 1). The Ton parameter obtained by this method was used to
142
reflect the oxidative stability of the sample. Since thermal oxidation stability was evaluated by
143
the onset temperature, higher Ton means greater tolerance to temperature as well as better thermal
144
stability.
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In order to set up the most reliable mathematical prediction model, the stability data with the
146
highest accuracy which were determined by the TGA method of 5 °C/min and 1 °C/min were
147
used. Moreover, 4 main fatty acids would be selected to represent the complete FA composition
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of each oil in order to reflect the relationship between the stability of oils and their FA
149
composition. The reorganized proportion of 4 main fatty acids were used to calculate the
150
coefficient of the prediction equation.
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3. Results and discussion
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3.1 Fatty acid composition of the oils
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In order to find a widely applicable law for the oxidation stability of common vegetable oils,
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palm oil, rapeseed oil, sunflower oil and linseed oil were investigated in this research. These oils 10
ACCEPTED MANUSCRIPT showed a huge diversity in composition, therefore, the commonly used vegetable oils in the
156
market were well represented by these four oils. Fatty acid composition of palm oil, rapeseed oil,
157
sunflower oil and linseed oil was presented in Table 1. As can be seen, palmitic acid, oleic acid,
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linoleic acid, linolenic acid are the main components in these oils as well as in many other
159
vegetable oils according to some researches (Li et al., 2013; Tudorachi & Mustata, 2015).
160
Considering the fact that oil oxidation stability was closely related to their fatty acids
161
composition, palmitic acid, oleic acid, linoleic acid, linolenic acid and their esters were chosen to
162
be regarded as the simple and basic systems when predicting the oxidation stability of different
163
vegetable oils (Kerrihard et al., 2015).
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3.2 Thermal oxidation stability of fatty acids, their esters and vegetable oils
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In order to precisely determine and compare the stability of different samples, the Ton of the
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first thermal decomposition event was used. Ton of main fatty acids and their esters were present
167
in Fig. 2 and Fig. 3. Since a higher Ton of the sample means better thermal oxidation stability, the
168
same stability order for 4 kinds of fatty acids was obtained under different heating rate: palmitic
169
acid > oleic acid > linoleic acid > linolenic acid. Palmitic acid and oleic acid showed higher
170
onset temperatures, which meant they retained better oxidation stability than linoleic acid and
171
linoleic acid. And the same stability results were found in different FAMEs that methyl palmitate
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and methyl oleate were more stable than methyl linoleate and methyl linolenate under the same
173
condition of atmosphere and temperature. This result agreed with G. Litwinienko’s research in
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which the comparison of stability of unsaturated fatty acids and their esters was conducted by
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DSC method (Litwinienko, 2001). The stability of different FAs and FAMEs was mainly due to
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their chemical structure. Compositional features of fatty acid esters that influence the thermal
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ACCEPTED MANUSCRIPT oxidation properties include chain length, degree of unsaturation and branching of the chain. As
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is well known, oxidation mostly takes place in the double bonds, therefore, fatty acid esters of
179
high unsaturation degree are more prone to oxidation (Knothe 2005, Moser 2009). Consequently,
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unsaturated methyl esters, unsaturated fatty acids (or vegetable oils for that matter) are less stable
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than those with saturated carbon chains (Nik et al., 2005; Refaat, 2009).
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The Ton of different oils were shown in Fig. 4. The Ton parameter of palm oil was above
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225 °C, while it was only around 185 °C for linseed oil. And Ton of different oils at different
184
heating rates showed the same regulation and trends. Therefore, under the same experimental
185
conditions, the initial mass loss temperature difference can be used to compare the oxidation
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stability of various oils. And the following order for oxidation stability was found: palm oil >
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rapeseed oil > sunflower oil > linseed oil, which showed a good agreement with previous
188
researches (Cordella et al., 2012; Qi et al., 2016). The results of oxidation stability of these oils
189
were due to the fact that FA composition and chemical structure play a significant role in thermal
190
stability, which has been demonstrated by Monika Sabolova who studied the relationship
191
between the composition of fats and oils and their oxidative stability by the Oxipres apparatus
192
(Sabolová et al., 2017). The content of saturated fatty acids in palm oil was above 30% (Table 1).
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For rapeseed oil and sunflower oil, the proportion of unsaturated fatty acids was more than 65%.
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And linolenic acid has the largest share of more than 41% in the fatty acid composition of linseed
195
oil. Vegetable oils with high proportion of unsaturated fatty acids were usually less stable than
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those with saturated fatty acids (Micić et al., 2015; Nik et al., 2005). Thus palm oil and rapeseed
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oil showed better stability than sunflower oil and linseed oil.
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When comparing thermal decomposition profile at the heating rates of 1, 5, 7.5, 10, 15, 12
ACCEPTED MANUSCRIPT 20 °C/min, it was observed that degradation rate increased with the increase of heating rate.
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Under different heating rates, the Ton obtained from TGA method raised from 219.87 °C to
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267.71 °C, 192.55 °C to 227.91 °C and 191.99 °C to 225.72 °C for palm oil, palmitic acid and
202
methyl palmitate, respectively. Under the heating rates of 1 °C/min and 20 °C/min, a temperature
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difference of around 30 °C between the onset temperatures was found. The same results have
204
been reported by many researchers (Gao, Chen, Zeng, Xu, & Wang, 2017; Micić et al., 2015; A.
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G. D. Santos et al., 2014). There was an agreement that the temperature difference might result
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from the less detailed weight loss and oxidation reaction process under a faster heating rate as
207
20 °C/min. Considering the results obtained by this study and some previous researches, it can be
208
concluded that the stability parameter can be determined by the experiments under different
209
heating rates by TGA. Oxidative stability of all samples can be compared under the same
210
experimental conditions. Moreover, the lower heating rate would lead to a more detailed mass
211
loss process and thus a Ton parameter closer to the real value could be obtained. While the Ton
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parameter of the method of 1 °C/min and 5 °C/min was pretty close, with the temperature gap of
213
merely 2-4 °C. Thus relatively low heating rates as 5 °C/min and 1 °C/min were both competent
214
to get results precise enough. And the mathematical prediction model would be established on
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the basis of databases under the heating rate of 5 °C/min and 1 °C/min.
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3.3 Mathematical prediction of oxidative stability of vegetable oils based on fatty acid
217
composition
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The fatty acid composition varies in different oils. In order to reveal the relation between the
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stability of oils and their fatty acids, the main fatty acids (palmitic acid, oleic acid, linoleic acid
220
and linolenic acid) in Table 1 were selected to represent the fatty acid composition of each oil. 13
ACCEPTED MANUSCRIPT The minor fatty acids were not considered because of their low content and ignorable
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contribution to the oxidation stability of vegetable oils. A renormalization method was used to
223
describe the proportion of the main fatty acids in different oils. And the detail reorganized
224
proportion of 4 main fatty acids were present in Table 2. The absolute content of palmitic acid,
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oleic acid, linoleic acid and linolenic acid in palm oil were 29.29%, 33.56%, 8.12% and 0.11%
226
(Table 1), and the proportion became 41.22%, 47.22%, 11.2% and 0.14% (Table 2) after the
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normalized reorganization. The same situation could be found for other oils. And these
228
reorganized proportions of main fatty acids, which represented their contribution to the oxidative
229
stability of oils, would be used in the model establishment of oxidative stability prediction.
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3.3.1 Prediction of vegetable oil oxidative stability by FA system
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By comparing the parameter Ton (Fig. 2) and the main fatty acid composition of different oils
232
(Table 2), some regulations can be found. Compared with rapeseed oil and sunflower oil, palm
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oil was more prone to thermal oxidation because of higher proportion of unsaturated fatty acids.
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The palmitic acid and oleic acid content were almost the same in sunflower oil and linseed oil
235
while the content of linolenic acid proportion was much higher in linseed oil, and it turned out
236
sunflower oil showed better oxidation stability than linseed oil. These results demonstrated the
237
existing conclusion that oils with fatty acids of higher unsaturation degree had poorer oxidative
238
stability (Micić et al., 2015, Qi et al., 2016). The content of main FAs plays an important and
239
direct role in the oxidation stability of different samples. Based on the FA composition and the
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Ton parameter under the heating rate of 5 °C/min and 1 °C/min, an association between the
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oxidation stability of vegetable oils and their main fatty acids were found, which can be
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described by the equation below:
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14
ACCEPTED MANUSCRIPT T=φ Where Ai stands for the reorganized proportion of 4 kinds of fatty acids in oils, Xi is the exact
244
Ton for each fatty acid (i = 1, 2, 3, 4). φ is a coefficient related to the heating rates, and it was the
245
same (φ=1.13) for the method of 5 °C/min and 1 °C/min because of the close Ton results. T refers
246
to the predicted onset temperature for each kind of oil.
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The prediction of oil stability with high accuracy usually needs the FA composition as well as
248
the content of various oil components which contribute to the stability (Hidalgo 2002,
249
Castelo-Branco et al., 2016). In this work, after the removal of natural antioxidants and minor
250
compounds, the stability prediction model could be directly described by the succinct equation
251
based on the FA composition only. With a given vegetable oil and its FA composition, and the Ton
252
in Fig. 2, a specific onset temperature for the oil can be calculated by this empirical equation.
253
The results and estimation deviation of the prediction for each oil by the FA stability parameter
254
under the heating rate of 5 °C/min and 1 °C/min were given in Table 3. It turned out the
255
estimation equation was an accurate and effective means with the prediction deviation all below
256
6% for 4 kinds of vegetable oils. The average deviation of the prediction for all vegetable oils
257
was respectively 2.45% and 2.40% for the method of 5 °C/min and 1 °C/min. Thus the oxidation
258
stability could be predicted by the FA composition and the stability of main fatty acids in the oil.
259
These results are in good agreement with what Adrian L. Kerrihard has reported where they also
260
studied the relationship between the FA composition and the oxidative stability using a
261
mathematical modeling method. According to the findings of this study, the exact concentrations
262
of monounsaturated, diunsaturated and tri-unsaturated fatty acids together showed a strong
263
correlation (R2=0.915) with the stability of the different vegetable oils in their model (Kerrihard
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15
ACCEPTED MANUSCRIPT et al., 2015). Similar studies have been previously published in which a multiple linear
265
regression model were established to demonstrate that saturated, monounsaturated and
266
polyunsaturated FA together accounted for 67% in stability evaluation (Redondo-Cuevas et al.,
267
2018).
268
3.3.2 Prediction of vegetable oil oxidative stability by FAME system
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Generally, all kinds of vegetable oils are made up of glycerides, a kind of ester formed by the
270
reaction of fatty acids and glycerol. The prediction by corresponding FAMEs was taken into
271
consideration in this part. The same method was used to derive the equation as following, from
272
which the stability parameters of different vegetable oils can be calculated through the Ton of
273
main FAMEs:
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T=λ
Where Ai stands for the reorganized proportion of 4 kinds of fatty acids in vegetable oil, Xi is
275
the exact Ton for each corresponding fatty acid methyl ester of the main fatty acids (i = 1, 2, 3, 4).
276
And λ is a coefficient related to different heating rates, it was also the same (λ=1.18) for the
277
method of 5 °C/min and 1 °C/min. T refers to the predicted onset temperature for the given oil.
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With the reorganized FA composition in Table 2 and the Ton of different FAMEs in Fig. 3, the
279
Ton for the oils were calculated by another empirical equation with a different coefficient λ.
280
Many researches revealed that the stability of vegetable oils was related to their FA composition
281
and the unsaturation degree (Sabolová et al., 2017, Evangelos 2018). However, the work in this
282
research moved forward to focus on the stability prediction of different vegetable oils on the
283
basis of FA composition. Moreover, the results and deviation of the prediction for each oil by the
284
stability parameter of FAME system under the heating rate of 5 °C/min and 1 °C/min were given
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ACCEPTED MANUSCRIPT in Table 4. Results showed that the Ton of 4 kinds of vegetable oils could be calculated by the
286
equation model with the prediction deviation all below 3%. The average deviation of the
287
prediction for all vegetable oils was respectively 1.81% and 1.75% for the method of 5 °C/min
288
and 1 °C/min. It was much lower than the prediction deviation from the system of FA (Table 3).
289
Compared to the mathematical prediction model established based on the stability data of main
290
FAs, oxidation stability could be predicted by the stability of main FAMEs with higher accuracy
291
under different heating rates. Considering the basic composition and structure of vegetable oils
292
and FAMEs, this may be due to the higher similarity between these 2 systems. Besides, the
293
model based on the stability database from TGA method at lower heating rate showed better
294
prediction reliability. This might be due to the lower average deviation in the stability
295
quantification process by TGA method at lower heating rate, as described in section 3.2.
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These results provide a comprehensive quantification of the relationship between fatty acid
297
composition of vegetable oils and oxidation stability. With a given vegetable oil and its FA
298
composition, the oxidation stability reliably reflected by the Ton parameter can be derived by the
299
empirical equation. Both the equations based on the stability data of main FAs and their FAMEs
300
gave a good prediction result about the oxidation stability of different oils. And the equation
301
based on the stability of FAME system under lower heating rate will be more precise.
302
4. Conclusion
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In the present study, the oxidative stability of vegetable oils, FAs, and FAMEs were well
304
estimated by the parameter of Ton derived from TGA carves by the non-isothermal pattern
305
method. It turned out that the oxidative stability order was: palm oil > rapeseed oil > sunflower
306
oil > linseed oil. Reason for these results is related to the lower quantity of unsaturated fatty 17
ACCEPTED MANUSCRIPT acids in palm oil and rapeseed oil. For the FA and FAME systems, fatty acids and their esters
308
with higher unsaturation degree showed poorer oxidation stability because the unsaturated bond
309
of higher unsaturation degree need lower activation energy to initial the oxidation and that means
310
a lower Ton. Thus the TGA method used in this research is a pretty suitable way to evaluate the
311
oxidative stability of FAs, FAMEs, and vegetable oils. Besides, the experiments under lower
312
heating rates provide results with higher accuracy.
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Furthermore, a mathematical equation model based on the 4 foremost fatty acids composition
314
was found to predict the stability of vegetable oils through the Ton of both FA and FAME systems.
315
Under the TGA method of 5 °C/min, the average deviations for the prediction model based on
316
the stability database of FA and FAME were 2.45% and 1.81% respectively. In addition, for the
317
TGA method of 1 °C/min, the average deviations of the prediction were respectively 2.40% and
318
1.75%. The mathematical predictions from the perspective of FAMEs stability were more
319
accurate than FAs stability data. Moreover, the mathematical equation model based on the
320
database of lower heating rates showed higher accuracy. In the practical application, it’s strongly
321
recommended that the mathematical prediction model based on the stability data of FAMEs
322
under the heating rate of 1 °C/min should be used to predict the oxidative stability of a specific
323
vegetable oil.
324
AUTHOR INFORMATION
325
Corresponding author
326
* Phone: 0510-85876799; Fax: 0510-85876799; E-mail: [email protected]
327
Notes
328
The authors declare no competing financial interest.
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ACCEPTED MANUSCRIPT Acknowledgments
330
This study was funded by the Natural Science Foundation of China (31571878), Northern
331
jiangsu province science and technology projects (BN2016137), Jiangsu province science &
332
technology (BE2016635) and the Fundamental Research Funds for the Central Universities
333
(JUSRP51501).
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vegetable oils by monitoring the tendency to radical formation. A comparison of electron spin
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resonance spectroscopy with the Rancimat method and differential scanning calorimetry. Food
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Chemistry, 85(4), 623-632. doi: 10.1016/j.foodchem.2003.07.020
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Figure and Table Captions
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Figure 1
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The specific principles and derivation process of the Ton from TGA method
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Figure 2
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Fig. 2 Ton of palmitic acid, oleic acid, linoleic acid and linolenic acid from TGA under different
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heating rates.
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Figure 3
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Fig. 3 Ton of methyl palmitate, methyl oleate, methyl linoleate and methyl linolenate from TGA
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under different heating rates.
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Figure 4
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Fig. 4 Ton of palm oil, rapeseed oil, sunflower oil and linseed oil from TGA under different
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ACCEPTED MANUSCRIPT heating rates.
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Table 1
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Chemical composition of palm oil, rapeseed oil, sunflower oil, and linseed oil (%)
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Table 2
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Reorganized proportion of 4 main FAs (palmitic acid, oleic acid, linoleic acid and linolenic acid)
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in palm oil, rapeseed oil, sunflower oil, and linseed oil
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Table 3
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Ton from the mathematical prediction model based on FA system under 5 oC/min and 1 oC/min
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Table 4
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Ton from the mathematical prediction model based on FAME system under 5 oC/min and 1
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ACCEPTED MANUSCRIPT Table 1 Chemical composition of palm oil, rapeseed oil, sunflower oil, and linseed oil (%) Palm oil
Rapeseed oil
Sunflower oil
Linseed oil
Miristic (C14:0)
0.43±0.01
--
--
--
Palmitic (C16:0)
29.29±0.68
2.69±0.01
3.89±0.02
3.12±0.01
Stearic (C18:0)
3.59±0.12
1.51±0.01
3.25±0.02
2.76±0.02
Oleic (C18:1)
33.56±1.02
46.29±1.21
13.62±0.34
13.15±0.28
Linoleic (C18:2)
8.12±0.11
16.22±0.41
51.87±1.52
11.96±0.38
Linolenic (C18:3n6)
--
0.51±0.03
--
0.24±0.04
Linolenic (C18:3n3)
0.11±0.01
5.15±0.62
0.22±0.03
41.02±1.18
Arachidic(C20:0)
0.34±0.01
0.54±0.08
0.21±0.01
0.1±0.01
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Fatty acid
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ACCEPTED MANUSCRIPT Table 2 Reorganized proportion of 4 main FAs (palmitic acid, oleic acid, linoleic acid and linolenic acid) in palm oil, rapeseed oil, sunflower oil, and linseed oil Linoleate acid
Linoleate acid
41.22 4.09 5.43 4.49
47.22 70.39 19.03 18.94
11.42 24.67 72.46 17.23
0.14 0.85 3.08 59.34
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Oleic acid
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Palm oil Rapeseed oil Sunflower oil Linseed oil
Palmitate acid
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ACCEPTED MANUSCRIPT Table 3 Ton from the mathematical prediction model based on FA system under 5 oC/min and 1 oC/min
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5 C/min
1 oC/min
Model Reliability
Palm oil
Rapeseed oil
Sunflower oil
Linseed oil
Ton by experiment (°C)
225.32
209.38
197.43
185.82
Ton by prediction (°C)
213.58
207.14
198.12
191.73
Prediction deviation (%) Ton by experiment (°C)
5.21 219.87
1.07 206.41
0.35 194.45
3.18 182.11
Ton by prediction (°C)
209.83 4.57
203.62 1.35
193.81 0.33
188.21 3.35
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Prediction deviation (%)
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Heating Rates
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ACCEPTED MANUSCRIPT Table 4 Ton from the mathematical prediction model based on FAME system under 5 oC/min and 1 oC/min
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5 C/min
1 oC/min
Model Reliability
Palm oil
Rapeseed oil
Sunflower oil
Linseed oil
Ton by experiment (°C) Ton by prediction (°C) Prediction deviation (%) Ton by experiment (°C)
225.32 219.99 2.37 219.87
209.38 211.4 0.97 206.41
197.43 195.58 0.94 194.45
185.82 191.30 2.95 182.11
Ton by prediction (°C)
216.02 1.75
207.23 0.40
191.15 1.70
187.82 3.13
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Prediction deviation (%)
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Heating Rates
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Fig. 1 The specific principles and derivation process of the Ton from TGA method.
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Fig. 2 Ton of palmitic acid, oleic acid, linoleic acid and linolenic acid from TGA under
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Fig. 3 Ton of methyl palmitate, methyl oleate, methyl linoleate and methyl linolenate
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from TGA under different heating rates.
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Fig. 4 Ton of palm oil, rapeseed oil, sunflower oil and linseed oil from TGA under
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different heating rates.
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ACCEPTED MANUSCRIPT Method validation of oxidative stability of FAs, FAMEs and oils by TGA was verified. A database of stability parameters of four main FAs and FAMEs was established.
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A prediction model for stability of oils based on FA & FAME system was established.
The database and model based on FAMEs were suggested in practical
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application.
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