A Comparative Study on Formation of Polar Components, Fatty Acids

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Food and Beverage Chemistry/Biochemistry

A comparative study on formation of polar components, fatty acids and sterols during frying of refined olive pomace oil pure and its blend coconut oil Ibtissem Ben Hammouda, Mehdi Triki, Bertrand Matthäus, and Mohamed Bouaziz J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b05163 • Publication Date (Web): 10 Mar 2018 Downloaded from http://pubs.acs.org on March 11, 2018

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Journal of Agricultural and Food Chemistry

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A comparative study on formation of polar components, fatty acids and

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sterols during frying of refined olive pomace oil pure and its blend coconut oil

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Ibtissem Ben Hammouda 1, Mehdi Triki 2, Bertrand Matthäus 3, Mohamed Bouaziz *, 1, 2

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Laboratoire d’Électrochimie et Environnement, École Nationale d’Ingénieurs de Sfax, Université de Sfax, B.P. 1173, 3038 Sfax, Tunisia

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Institut Supérieur de Biotechnologie de Sfax, Université de Sfax, B.P. 1175, 3038, Sfax, Tunisia

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Max Rubner-Institut (MRI), Department for Safety and Quality of Cereals, Working Group for Lipid Research, Schützenberg 12, D-32756 Detmold, Germany

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* Corresponding author:

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Prof. Mohamed BOUAZIZ, Laboratoire d’Électrochimie et Environnement, Institut Supérieur de

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Biotechnologie de Sfax, BP « 1175 » 3038, Université de Sfax, Tunisia.

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Tel: +216 98 667 581 / Fax: +216 74 674 364.

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E-mail: [email protected]

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.

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1 ACS Paragon Plus Environment


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ABSTRACT

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The frying performance of pure refined olive-pomace oil (ROPO) and blended with refined

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coconut oil (RCO) (80:20) was compared during a frying operation of French fries at 180 °C.

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Blending polyunsaturated oils with highly saturated or monounsaturated oils has been studied

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extensively, however in literature there is no study has been reported so far on blending ROPO

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(rich in monounsaturated fatty acids) with RCO (rich in saturated fatty acids) to formulate new

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frying oils. At the end of the frying process, the blend of ROPO/RCO exhibited a higher

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chemical stability than the pure ROPO based on total polar compounds (TPC), and polymers.

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The rate of TPC formation was achieved 23.3% and 30.6% for the blend and the pure oil,

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respectively. Trans and free fatty acids content, as well as anisidine value were also observed to

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be the highest in the pure ROPO. This study evaluated the frying performance in the search for

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appropriate frying oils to deliver healthy fried products with optimized nutritional qualities.

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Keywords Deep-fat frying, Fourier-transformed near-infrared spectroscopy, refined oils, fatty

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acids composition.

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INTRODUCTION

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Deep-frying at a high temperature ≃180 °C is one of the most common methods used for food

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preparation1-3. During this phase, the presence of oxygen, the moisture content of the food, the

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high oil temperature and the leaching of components from the food result in formation of a high

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variety of products with different polarity, stability and molecular weight. These components

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include TAG dimers, polymers, oxidized TAGs, and volatile compounds, all of which contribute

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to degradation of the frying oil1-4. From a chemical point of view, many reactions can occur. The

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latter are very complex and actually not comprehensively understood. From these reactions, we

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can name oxidation, hydrolysis, isomerisation, polymerisation, and cyclisation1-4. As a matter of

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fact, various chemical compounds are formed by following these reactions, which affect sensory,

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functional, and nutritional qualities of the frying oil 5. The oil quality is of great importance for

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food quality. It is essential to assess oil quality to avoid the use of deteriorated oil, which may

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have harmful consequences. This includes the fume generated during food frying which

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generates detrimental compounds, such as polycyclic aromatic hydrocarbons 6. Thus, to avoid

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such inconvenience, some guidelines have been established in many countries 6.

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Vegetable oil used for frying must have good flavor and oxidative stability in order to achieve

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good shelf life for the fried product. Besides, oil performance and behavior during first day of

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frying does not represent its performance over a prolonged period such as utilized during typical

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institutional frying. Today, oils with a high content of favorable monounsaturated fatty acids like

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oleic acid and a low level of polyunsaturated fatty acids such as linoleic or linolenic acids are

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preferred to combine health aspects with a desirable oxidative stability

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worldwide demand for the production of oil blends which take into account gastronomical

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advantages to consumer awareness, better thermal stability, nutritional benefits, alternatives to

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pure vegetable oils, and ability to tailor the desired properties. It is therefore very important to 3 ACS Paragon Plus Environment

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. However, there is a


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explore the changes in the frying behavior of the blended oil, when used as a medium to fry the

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foods consisting of various amounts of moisture. Fatty acids composition is the main

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determining factor for lipid oxidation during frying operations1.

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Blending the polyunsaturated oils with more saturated or monounsaturated oils are a potential

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solution to improve oil stability. Ben Hammouda et al.4 demonstrated that blends of

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monounsaturated oil like refined olive pomace oil (ROPO) with refined oil rich in saturated fatty

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acids like refined palm oil (RPO), revealed better frying performance 4. Indeed, Zribi et al1,

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demonstrated that blends of refined olive oil (ROO) (monounsaturated oil) with refined palm oil

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(RPO), exhibited the highest chemical stability during the frying process, compared with

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polyunsaturated oil as refined soybean oil (RSO) blended with a refined palm oil (RPO) 1.

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Mishra et al.9 reported that a blend of high oleic acid canola oil and palm olein showed higher

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oxidative stability, less free fatty acids and polar compounds formation and higher heat stability

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than the one of palm olein alone9. It is considered to be good frying/cooking oil due to its high

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smoke point and delicate flavor10. Oil stability can be assessed by monitoring the

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physicochemical changes occurring during heating/frying the oil11. Actually, the determination

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of total polar compounds (TPC) and the determination of polymers values provide the best

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methods recommended for evaluating frying degradation .Thus, they are the most commonly

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used method in order to evaluate oil quality12. Moreover, most European countries have

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established regulations that limit the degradation of used frying fats and oils for human

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consumption, with a maximum tolerated content of polar compounds is estimated as being 25%

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or 16% polymers13.

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At high temperatures, during the frying process, oil resistance to oxidation depends mainly on

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the fatty acids composition and antioxidant content of the oil14. The oxidation reaction can be


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inhibited by antioxidants that are naturally present in the oils, such as tocopherols and

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tocotrienols.

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During frying, antioxidant power is shown in refined olive and seed oils, by the presence of

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tocopherols, tocotrienols, phenolic compounds and phytosterols 15. Phytosterols oxidation during

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the heating process is facilitated by many factors, for instance temperature, light, oxygen, metal

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ions, free radical initiators, and the formation of many compounds such as 5β, 6β-epoxides, and

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triols from β-sitosterol, campesterol, and stigmasterol16.

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On the other hand, olive-pomace oil was used to build an ideal frying oil mixture with other

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oils4. If virgin olive oil has been well studied, refined olive-pomace oil has not been thoroughly

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evaluated for frying. Better frying quality of virgin olive oils has been presented as a well

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established phenomenon17; however frying studies dealing with refined olive-pomace oil (edible

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pomace oil) (ROPO) are still limited. In Turkey, olive-pomace oil has just become edible oil in

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2007 by officially employing the Codex Standard18.

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Besides, coconut oil which is one of the widely used cooking oil in many countries is very a

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special oil having natural pleasant taste of coconut. 19. The highly saturated coconut oil is unique

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as one of the richest sources of medium chain fatty acids (MCFA) even though several

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controversies exist because of its effect on Low density lipoprotein (LDL) .The suitability of

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coconut oil as frying medium is well explored20. However, coconut oil lacks in ω-3 and ω-6 fatty

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acids and oil soluble vitamins, which needs to be supplemented through blending or other means.

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Therefore, the present study aims to demonstrate the frying performance of refined olive-pomace

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oil blended with coconut oil during frying at 180°C. The different samples were examined for

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total polar compounds (TPC), dimeric and polymeric triacylglycerols (DPTG), anisidine value



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(AnV), iodine value (IV), acid value (AV), monomeric oxidized TAGs (OTG), trans fatty acid

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(TFA), color value and fatty acid methyl esters (FAMEs) composition as well as phytosterols.

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Materials and Methods

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Chemicals and reagents

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All solvents used were of analytical grade: n-hexane, n-heptane, diethyl ether, ethanol, and

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methanol were acquired from Merck (Darmstadt, Germany).

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β-sitosterol, campesterol, and stigmasterol were obtained from Aldrich (Munich, Germany).

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Oil samples

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The studied oils, refined olive-pomace oil (ROPO) and refined coconut oil (RCO), were obtained

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from the National Oil Office of Sfax, Tunisia. The studied oil blend (ROPO/RCO) was prepared

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in the volume ratio of 80:20, after comparison of oxidative stability with other proportions using

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the Rancimat method3.

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Frying Process

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Spunta potatoes variety from Tunisia were peeled, cut approximately in uniform pieces (5 cm

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long and 0.5 cm thick), washed, and wiped before the frying experiments. The latter were

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conducted in the same manner as the actual household cooking process. For deep-frying, an

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electric common domestic fryer type was used. It is equipped with a thermostat and supplied

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with an inert cross-linked steel wire mesh that allows the food to be dipped into the oil without

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coming in contact with the fryer’s inner surface. In every frying session, 200 g of potatoes was

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deep-fried for 9 min at 180°C in 2.7 L of refined oil blend, without replenishment.

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The fryer’s lid was closed when the cooling started. After cooling (≈30 min), the frying process

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was repeated 60 times using new potatoes in the same oil. The samples of the fresh refined oils


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together with those sampled after each ten successive deep-frying sessions were stored in sealed

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dark glass bottles at -20 C° until examination21.

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FT-(NIR) spectroscopy

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Total polar compounds (TPC), di- and polymerized triacylglycerols (DPTG), anisidine value

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(AnV), Monomeric oxidized TAGs (OTG),iodine value (IV), acid value (AV), color value and

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trans fatty acid (TFA) have been determined by FT-NIR transmission measurement22 according

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to standard method DGF C-VI 2123. FT-NIR analysis of the iodine value (IV) was done

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according to AOCS‐Method Cd 1e‐0124.

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Briefly, oil samples were filled in 8-mm disposable vials. All spectra were recorded at 50 °C

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after a thermal preconditioning for 10 min in a separate thermoblock to avoid turbid solutions.

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Spectra were obtained in transmission mode from 12,500 to 4,000 cm−1. Each spectrum was

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time-averaged based on 32 scans at a resolution of 8 cm−1 using Bruker MPA FT-NIR

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spectrometer (Bruker Optik GmbH, Ettlingen, Germany), equipped with OPUS software version

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7.2.

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The NIR calibration of each parameter has been set up with a big number of samples (N=1200)

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of used frying fats collected from several fast food restaurants, bakeries, caterers or industrial

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producers by the food inspection authorities. Their compositions were very different: solid, semi-

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solid, liquid, containing low and high levels of mono-, polyunsaturated and saturated and trans-

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isomerized fatty acids. The Iodine values vary from 16 to 150. The validation of all methods was

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executed with independent samples.

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The calibration and validation sets represent the whole concentration ranges of each analyte from

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the lowest to the highest values which can occur in practical life. High calibration accuracy was

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obtained for all NIR determination.



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For the TPC, the coefficient of determination R2 was 97.5 with an RMSECV (root mean square

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error of cross validation) of 1.5% over a range from 5 to 41%. Also polymerized TAG was

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predicted successfully with R2=97.6 and RMSECV=1.2% (0-39%). The AnV was measured over

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a broad range of 7 to 91 with R2=94.3 and RMSECV=5.8%. For FFA covering a wide range

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from 0% to 9.6% the R2 value was 95.0% with an RMSECV of 0.18%.

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Fatty acids composition

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The fatty acid methyl esters (FAMEs) were determined according to the method adopted by the

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International Olive Council (IOC)25. These compounds were analyzed as described in our

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previous research work 2,4. Briefly, the GC analysis of FAMEs was performed on an AutoSystem

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gas chromatograph equipped with a flame ionization detector (FID) (HP 6890 N, Agilent, Palo

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Alto, CA, USA). The column used was a capillary Agilent CPSil88 (length = 50 m, i.d. = 0.25

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mm, and film thickness = 0.20 µm), and the analysis conditions were as follows: The initial

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column temperature was set at 165 °C for 25 min, then raised at a gradient of 5 °C/min to 195

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°C; the temperature of the injector and detector was set at 250 °C; helium was used as the carrier

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gas at a flow rate of 1 mL/min and 1:100 split ratio; and the injection volume was 1 µL. The

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identification of these compounds was performed through a comparison of their retention times

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versus pure standards analyzed under the same conditions. They were quantified according to

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their percentage area, obtained by the integration of the peaks. The results were expressed as the

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percentages of individual fatty acids in the lipid fraction2,4.

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Sterol Determination

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The sterols composition of the oils was determined following ISO/FIDS 12228:1999 (E)26.

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Briefly, 250 mg of oil was saponified with a solution of ethanolic potassium hydroxide by

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boiling under reflux. The unsaponifiable matter was isolated by solid-phase extraction on an


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aluminum oxide column (Merck, Darmstadt, Germany) on which fatty acid anions were retained

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while sterols passed through. The sterol fraction was separated from unsaponifiable matter by

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thin-layer chromatograph (Merck, Darmstadt, Germany), re-extracted from the TLC material,

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and afterwards, the composition of the sterol fraction was determined by GLC using betulin as

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the internal standard. The compounds were separated on a SE 54 CB (Macherey-Nagel, Düren,

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Germany; 50 m long, 0.32 mm ID, 0.25 µm film thickness). Further parameters were as follows:

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hydrogen as carrier gas, split ratio 1:20, injection and detection temperature adjusted to 320°C,

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temperature program, from 245°C to 260°C at 5°C/minutes. Peaks were identified either by

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standard compounds (β-sitosterol, campesterol, stigmasterol), by a mixture of sterols isolated

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from rapeseed oil (brassicasterol) or by a mixture of sterols isolated from sunflower oil (∆7-

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avenasterol, ∆7-stigmasterol, and ∆7-campesterol). All other sterols were identified by GC-MS

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for the first time and afterwards by comparison of the retention time.

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Determination of oxidative stability by Rancimat method

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The oil samples (3.6 g) were heated in the Rancimat equipment at 120 °C (Metrohm Ltd.,

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Herisau, Swiss), with a continuous air flow of 20L/h passing through the samples. The

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conductivity cells were filled with 60 mL of deionized water. The time needed for the

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appearance of a sudden water conductivity rise, caused by the adsorption of volatiles derived

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from oil oxidation, was registered as the induction time in hours27.

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Statistical analysis

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The results were expressed as mean _standard deviation (SD) of three measurements for the

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analytical determination. The significant differences between the values of all parameters were

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determined at P < 0.05 according to the one-way ANOVA: post hoc comparisons (Student

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Newman–Keuls test). This statistical analysis was performed using SPSS Statistics 17.0 for



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Windows. The principal component analysis (PCA) was applied to the data set of all analyses

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performed for fresh oil blends and those obtained after the deep-frying sessions.

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PCA plots were performed using XLSTAT software for Windows (v.2014.1.08, Addinsoft, New

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York, NY).

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Results and Discussion

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Total Polar Compounds

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Measuring the content of total polar compounds (TPC) gives a reliable parameter to evaluate the

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quality of used frying oils. Indeed, it offers the most accurate assessment of the thermo-oxidative

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degradation of frying oils measuring directly all the degraded substances present in the oil, and

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gives information of the total amount of newly formed compounds which have higher polarity

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than that of triacylglycerols1. Frying oil is suspected to contain most of the harmful substances

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that are not volatile. These substances formed during frying are usable to assess the stability of

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the frying oil. Many countries have established regulatory limits for TPC in frying oils and

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considered 25% as limits28.

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As shown in Figure 1a, the TPC increased with the number of frying operations. This result is in

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agreement with that reported by Casal et al., showing that TPC increased with the number of

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frying operations in all oils29.

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At fresh conditions, the percentages of TPC recorded 7.7% in ROPO/RCO and 7.3% in pure

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ROPO (figure 1a).

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In order to compare the rates of TPC increment between the pure and blended oils during frying,

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the raise percentages were calculated for initial and the end of frying sessions. Therefore, the

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fastest increment was found (about 76.1%) for the pure refined olive pomace oil, from 7.3 to

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30.6% while the lowest was detected (about 66.8%) for the blend ROPO/RCO from 7.7 to 23.3%


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after 60 successive deep-frying sessions at 180°C. The low TPC level can be explained by

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considering that ROPO/RCO blend is rich in monounsaturated fatty acids (MUFAs), especially

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oleic acid and also in saturated fatty acids (SFAs). Similar findings were also found by Omar et

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al.30, who suggested that the oil blends rich in SFA have the lowest TPC throughout frying and

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thus are able to resist more to thermo-oxidation30. In a related study, Zribi et al2, proved that the

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high stability of refined olive oil (ROO) to changes in TAGs that occurred during the frying

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experiments, can be explained by considering that this oil is rich in MUFAs, especially in oleic

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acid2.

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The pure ROPO reached the discarding range of TPC content during the frying process, but its

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blends did not. Assuming that the limit of acceptance for the TPC content is 25% 28, the frying

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sessions required to reach this limit was considered as a measure of frying stability. The pure

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ROPO showed a frying stability significantly lower (40-50 sessions) than those of its blends (60

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sessions of frying). As consequent, the blended oil showed the excellent performance and was

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still in good quality in comparison to the pure ROPO.

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Di- and Polymerized Triacylglycerols

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During frying and as a consequence of prolonged oxidation, a continuous formation of high

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molecular weight degradation products, mainly di- and polymer triacylglycerols (DPTG), were

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observed. These components remain in the oil and are used as a reliable indication of oil

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quality31. Limits in different countries have been set between 10 and 16 %, indicating that the

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used frying oil is no longer suitable for human consumption and should be discarded31.

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The changes in the DPTG of the studied oils after 60 sessions during deep frying at 180° C are

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shown in Figure 1b. Indeed, the percentage of DPTG increased in all different samples with the

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number of frying experiments, ranging from 3.8 % to11.9% for the pure ROPO and from 4.3% to



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8.3% for the blend ROPO/RCO after 10 and 40 sessions, respectively. Whereas after 50 and 60

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sessions, the content of DPTG increased faster and the highest levels was observed for the pure

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ROPO with 14.2 % and 16.5%, respectively and the lowest one for the blend ROPO/RCO was

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10.2% and 12.0%, respectively (Figure 1b) .

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These results showed that the blend oil ROPO/RCO showed still good quality after 60 sessions

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of frying, whereas, the pure ROPO should be discarded after 40 sessions based on the limit fixed

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by different countries31. In brief, according to our previous results (figure 1a and 1b); TPC and

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DPTG are in compliance. Their content increased gradually with frying sessions which are

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similar with previous findings2,

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weight substances by polymerization occurring at elevated frying temperature1.

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Monomeric oxidized TAGs (OTG)

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OTG are the primary products formed during PUFA oxidation and they react and are degraded

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further to form a host of secondary compounds, including oligomers and a host of free radicals.

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Quantification of monomeric oxidized triacylglycerols (OTG) is reported as a good measure for

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early and advanced stages of oxidation32.

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They are characterized by the presence of extra oxygen in at least one of the fatty acyl groups of

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the molecule, which are formed through autoxidation reactions. Thus, they are final stable

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products resulting from breakdown or decomposition of primary oxidation compounds

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(hydroperoxides). OTG include short –chain fatty acyl and short –chain n-oxo- fatty acyl groups,

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as well as different oxygenated groups such as hydroxyl, keto, and epoxy33. Their quantification

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is useful to detect both primary and secondary oxidation products and is of great utility for

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oxidative stability evaluation34.

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. This increase is due to the formation of higher molecular


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The changes in the OTG of the studied oils after 60 sessions of deep-frying at 180°C are shown

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in Figure 1c.

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For the fresh ROPO/RCO blend, the amount of OTG was at 3.3 % and 4.1% for the pure ROPO

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As matter of fact a gradually continuous increase of OTG during the frying experiment was

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showed. Indeed, the content of OTG was observed to be higher in pure ROPO at the end of

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frying with 10.8%, as compared to ROPO/RCO blend with 8.3% (Figure 1c). This change may

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be attributed to a higher percentage of UFA's in the pure ROPO (78.9 %) in comparison with a

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lower percentage of UFA's in the ROPO/RCO blend (70.1 %) (Table 1). This in agreement with

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Martin et al.

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degree was higher.

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In fact, there is a growing interest in monomeric oxidized triacylglycerols from a nutritional

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point of view because of their high absorbability and their occurrence in used frying fats at non-

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negligible levels.

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Fatty acids composition

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Assessment of changes in fatty acid composition can be used to monitor thermo-oxidative

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degradation occurring during deep-frying.

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Table 1 shows the fatty acid compositions of pure refined olive pomace oil (ROPO) and blended

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with refined cocount oil (ROPO/RCO) before and after frying.

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The most predominant fatty acids for the fresh oils (pure ROPO and ROPO/RCO) were palmitic

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(15.5-19.6%), oleic (56.9-54.1%) which has been described as a reducer of cardiovascular risk

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by reducing blood lipids, mainly cholesterol2, 35 and linoleic (20.8-15.1%) acids, respectively.

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The fresh ROPO and ROPO/RCO (80:20) contained a higher amount of monounsaturated fatty

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acids (MUFAs) with 59.0 % and 56.0%, respectively. Whereas, the higher content of SFAs were

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who has shown that the amounts of oxTGM increased as the oil unsaturation



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observed in the blended ROPO/RCO (28.4%) and the highest amount of PUFAs was found in the

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fresh ROPO (21.8 %).

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Generally, PUFAs are more prone to oxidation than MUFAs and SFAs. High percentages of

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PUFAs in oil lead to high levels of conjugated dienes and trienes formed during frying36.

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During different sessions of frying, the data showed that the amount of PUFAs decreased

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gradually and significantly (p< 0.05) during repeated deep-frying cycles in both oils and

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consequently a relative increase in the percentages of SFAs. In fact, an important and a

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significant (p< 0.05) decrease in the PUFAs content was observed for the pure ROPO, such as

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the linoleic and linolenic acids ranging between 17.0% and 0.64% after 60 sessions of frying

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from its initial values, varying between 20.8% and 1.0%, respectively. These two fatty acids are

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the main compounds affected by the various chemical reactions occurring during frying37.

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It is well-known that oils rich in SFAs are characterized by high thermal stability38. As listed in

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Table 1, the highest percentage of SFAs was observed in the blend ROPO/RCO with 29.9 %

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compared to the pure ROPO with 21.1% at the end of frying.

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On the other hand, the ratio of C18:2/C16:0 is used to indicate the degree of oxidative

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deterioration of frying oil1, 4. Palmitic acid is more stable against oxidation, whereas linoleic acid

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is more susceptible. Indeed, the highest decrease in C18:2/C16:0 was observed for pure ROPO

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(from 1.3 to 1.0 (about 36%)) while a lower decrease was found for the blend (ROPO/RCO)

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(from 0.8 to 0.6 (about 24%)) after 60 sessions of deep-frying. The pure ROPO has higher UFAs

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percentages (80.9%); hence its oxidative stability was lower when compared to the blend

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(ROPO/RCO) (71.6%).

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Acid Value


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Free fatty acids (FFA) are formed during hydrolysis of exposed triglycerides and as degradation

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products from oxidized triglycerides (OTG during frying process). Both FFA and acid value did

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not exceed the limit of 1 % and 2 mg KOH/1 g of oil, respectively, as established in some

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European countries4, 39.

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Changes in acid value content of oil during frying are shown in Figure 1d. Initially and after 10

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sessions of frying, the acid values vary between 0.5 mg and 0.7 mg KOH/ g for the pure ROPO.

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On the other side, it was detected at 0.05 mg KOH/g oil for the blend of ROPO/RCO. During the

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frying experiment, the acid value increased with the frying sessions. Similar results reported by

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previous studies on frying oils, have shown that FFA content increased during deep-frying4, 40.

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As given in figure 1d, a significant increase in acid values for pure ROPO was observed after 50

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sessions from 0.5 mg/g to 1.4 mg/g while in the ROPO/RCO blend the increase was from

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0.05mg/g to 0.4 mg/g.

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At the end of the frying experiment, a continuous increase in the acid value was monitored. It

337

was observed to be higher in pure ROPO but did not exceed the limit and it recorded only 1.6 mg

338

KOH/g, as compared to ROPO/RCO blend (80:20) with 0.6 mg KOH/ g. This increase may be

339

due to a higher percentage of UFA's in the pure ROPO in comparison with a lower percentage of

340

UFA's in the blend oil (table1) and may be attributed to the moisture content of the fried product

341

that accelerates the hydrolysis of triacylglycerols to form a combination of mono-and -

342

diacylglycerols, glycerol and FFAs41. In fact, the low acid value of the blend ROPO/RCO

343

suggests that it had a high inherent oxidative stability as compared to the pure ROPO.

344

Anisidine Value (AV)



345

During deep-fat frying, a huge number of volatile and nonvolatile components are formed as a

346

result of the thermal decomposition of hydroperoxides generating a number of secondary

347

oxidation products, with carbonyl compounds being the most prominent.

348

Determination of the p-anisidine value (p-AV) provides a good indication of the level of

349

nonvolatile aldehydes in the oil which were formed as secondary degradation products

350

(principally 2-alkenals and 2, 4-alkadienals) in oils. In addition, this parameter provides also the

351

level of oxidative deterioration occurring in the frying oil1, 2. Changes of the p-AnV in oil during

352

frying are shown in Figure 1e.

353

A significant higher increase in p-AnV are shown for pure ROPO from 20.8 to 123.5 in

354

comparison to ROPO/RCO blend increasing from 41.7 to 97.3 after 60 successive deep-frying

355

sessions. This may be due to the decomposition of less stable primary oxidative products

356

(hydroperoxides) to aldehydic compounds42. The highest values were found for pure ROPO,

357

indicating extensive degradation of oxidized UFAs.

358

These results have confirmed that pure ROPO is more susceptible to oxidation at high frying

359

temperature than the blend oil. This could be due to the presence of a high concentration of

360

PUFAs, especially in pure ROPO (Table 1), being related with the formation of oxidation

361

products. This also can explain the higher TPC observed in these samples.

362

Similar results reported by Zribi et al.2 and Ben Hammouda et al.4; have shown that vegetable oil

363

blend rich in PUFA presents less resistance to oxidation under frying conditions when compared

364

to others oil blends containing lower amounts of PUFAs2, 4.

365

In brief, a lower p-anisidine value in oil indicates a lower oxidation level in that oil, as

366

consequent, the ROPO/RCO blend suggests that it is most stable during frying as compared to

367

the pure ROPO.


Page 16 of 34

Page 17 of 34


368

Iodine value

369

Iodine value (IV) is an index of the unsaturation, which is the most important analytical

370

characteristic of oil. The higher the iodine value was the quicker the oil tends to be oxidized,

371

particularly during deep-fat frying43.

372

Changes in IV of the pure ROPO and the ROPO/RCO blend during deep-frying are presented in

373

Figure 1f. At fresh conditions, the maximum IV was observed in pure ROPO with 87.4 and 75.7

374

in the ROPO/RCO blend due to their relatively high content of UFAs with 80.9 and 71.6,

375

respectively (Table1).

376

The data revealed that there was a decrease in IV of the studied oil blend after 60 sessions of

377

frying. The decrease in IV with frying time could be attributed to the changes in fatty acids

378

observed with frying duration and to double bonds destruction by oxidation and

379

polymerization4,36.

380

In fact, the highest decrease in IV is shown in the pure ROPO which recorded from 87.4 to 78.8

381

and the lowest is observed for the ROPO/RCO blend which varied from 75.7 to 70.30 after 60

382

successive deep-frying sessions (Figure 1f). The low decrease in IV recorded for the ROPO/RCO

383

blend can be explained by its richness in MUFA and its relatively high percentage of SFA1.

384

Color changes

385

Changes in color index of the pure ROPO and the ROPO/RCO blend during deep-frying are

386

presented in figure 1g. The color measurement of frying oil over the process could be a tool to

387

assess the oil quality44.

388

Thus, the color values of fresh pure ROPO and ROPO/RCO blend were 6.1 and 5.8, respectively.

389

During frying process, a continuous increase in the color index was monitored. Consequently,

390

color darkening increased with heating time. In fact, the highest color changes were observed in



391

pure ROPO from 6.1 to 7.4 after 60 sessions of frying with increment about 17.6 %, while the

392

most stable color was found in ROPO/RCO blend from 5.8 to 6.5 with increment about 10.77 %.

393

This may be due to a lower degree of degradation for the blend oil than the pure oil. In fact, an

394

increase of the Gardner number is a visual indication of the deterioration of frying oil samples. It

395

is caused by the oxidation and the formation of nonvolatile decomposition and oxidation

396

products which are due to the thermal oxidation and polymerization of the unsaturated fatty acids

397

in fat44. According to the last arguments and our results which demonstrate that the highest

398

amounts of unsaturated fatty acids were observed in pure ROPO with 78.9 % while the lowest in

399

ROPO /RCO blend with 70.1 %. It can be concluded that the blend oil, has the most stable color

400

during deep frying because it undergoes less degradation compared to the pure oil. These results

401

are in agreement with those reported by Farhoosh et al.45, showing that the color index reflects

402

overall chemical degradation and polymerization44.

403

Changes in Trans-fatty acid compositions

404

The data on changes in Trans FAs of both oil samples during deep frying are presented in the

405

Figure 1h.

406

At fresh conditions, the content of Trans FAs recorded 1.1% and 1.2% for the pure ROPO and

407

the ROPO/RCO blend, respectively. The formation of Trans FAs during food frying is closely

408

related to the temperature and use-time of fat/oil46. At the end of frying at 180°C, an increase in

409

formation of Trans FAs was observed for both oil samples. This is in agreement with Zribi et al2,

410

who demonstrate that high temperature (190°C) can increase the amount of trans-isomers during

411

deep-frying 2.

412

In fact, the highest amount of Trans FAs was detected for the pure ROPO with 1.6 %, while the

413

lowest was observed for the ROPO/RCO with 1.3 % (Figure 1 h). This change can be explained


Page 18 of 34

Page 19 of 34


414

to a higher content of UFAs and PUFAs in the pure ROPO as compared to the blended oil.

415

Similar results reported by Zribi et al.1, showing that a high increase of the trans-PUFA

416

percentages is detected in polyunsaturated oil like RSO and a very small increase of these

417

compounds is identified in monounsaturated oil like ROO during repeated pan and deep-frying1.

418

Sterol content

419

As minor components existing in oils and fats, phytosterols have many benefits for human

420

health. Phytosterols are the major constituents of unsaponifiables present in edible oils47. They

421

are triterpenes, structurally different from cholesterols only in the side chain configuration. In

422

vegetable oils, phytosterols are the dominant class of minor components and occur primarily as

423

free sterols or steryl fatty acid esters. The contents of sterols in vegetable oils vary, even of those

424

of the same vegetable origin. Concentrations ranging from 0 to 206 mg/100 g of individual

425

sterols have been found in different vegetable oils48. As far as oxidation is concerned, from the

426

various sterols, cholesterol has been extensively studied. So far, more than 60 oxidation products

427

of cholesterol have been identified in foodstuffs. β-sitosterol is the predominant and most widely

428

distributed phytosterols.

429

During frying of foods, sterols oxidation occurs mainly following the autoxidation pathway. Oils

430

and fats heat stability depends not only on their fatty acid composition but also on the presence

431

of non-glyceridic constituents such as phytosterols. These compounds are closely related to the

432

chemical structure of cholesterol, a sterol of animal origin. The formation of phytosterols

433

oxidation products have been studied both in model heating systems and under actual deep-

434

frying conditions. The stability of phytosterols depends on the sterol structure, mainly on the

435

unsaturation of the ring, temperature and composition of matrix49.



436

The amount of sterols in the oils ranged between 2139 mg/kg for the pure ROPO, and

437

ROPO/RCO recorded 1795 mg/kg before frying (Table 2). After 60 deep-frying sessions, the

438

total content of phytosterols underwent a decrease to 2057 mg/kg and 1716 mg/kg, respectively.

439

This decrease could be caused by different factors, like polymerization, degradation, or

440

oxidation49.

441

The main sterol of the studied oil samples was β-sitosterol, followed by ∆-5-avenasterol,

442

campesterol and stigmasterol. For pure ROPO at fresh conditions, β-sitosterol was recorded 1734

443

mg/ kg, and for the blend ROPO/RCO an amount of 1441 mg/ kg was found. After 60 sessions

444

of frying, the amount of ß-sitosterol underwent a significantly decrease (p< 0.05) to 1663 mg/kg

445

and 1369 mg/kg, respectively. Before frying, campesterol was found as 70.5 and 76.5 mg/kg for

446

pure ROPO and blended with RCO, respectively .After 60 sessions the amounts decreased

447

significantly (p< 0.05) to 69.0 and 75.0 mg/kg for both oils, respectively. In fresh pure ROPO

448

oil, a remarkably high amount of ∆5-avenasterol was observed, with 121.6 mg/kg, while in the

449

blend ROPO/RCO with 101.0 mg/kg of the total sterols were found. During deep-frying the total

450

amount of ∆5-avenasterol underwent a gradually and significantly (p< 0.05) decrease from

451

111.6 to 109.2 and 108.9 mg/kg, after 20, 40 and 60 sessions, and from 91.5 to 89.1 and 88.8

452

mg/kg for pure ROPO and blends, respectively. The highest decrease of ∆5-avenasterol depends

453

probably on the occurrence of two double bonds in a molecule49. ∆5-avenasterol is known to act

454

as an antioxidant and as an antipolymerization agent in frying1, 50. Sterols with an ethylidene

455

group in the side chain are most effective as antioxidants. These results are in good harmony

456

with those of Zribi et al.

457

antioxidants51.

458

Oxidative stability

1

Moreover, a synergistic effect of the sterols can occur with other


Page 20 of 34

Page 21 of 34


459

As shown in table 1 and according to our previous studies, the blended ROPO/RCO presents the

460

greatest oxidative stability after 60 sessions of frying compared to the pure oil27.

461

Chemometric analysis

462

PCA is applied to reduce the redundant information in data and to group the correlated responses

463

into principal components.

464

The PCA plot (1st principal component vs 2nd principal component) as presented in Figure 2,

465

explains 96.55% of the total system variance. Each of the first and second principal components

466

explains a variance of 67.14% (PC1) and 29.41% (PC2), respectively. In the present study, PCA

467

was applied to the dataset of all analyses performed for oil samples, pure ROPO and ROPO/RCO

468

blends, after deep frying at 180° C. Twenty four variables (1: C14:0; 2: C16:0; 3:C16:1; 4:

469

C17:0; 5:C17:1; 6:C18:0; 7:C18:1; 8: C18:2; 9: C18:3; 10: C20:0; 11:C20:1; 12: Σ SFA;13: Σ

470

MUFA;14: Σ PUFA;15: Σ UFA;16: C18:2/C16:0; 17: AV;18: TPC;19: DPTG; 20: p-AnV; 21:

471

color; 22: trans FA; 23: IV;24: OTG) were selected and presented in the loading plot (Figure 2a)

472

and fourteen observations shown in the score plot (Figure 2b). In fact, it is evident that while

473

PC1 clearly separates ROPO/RCO blend from pure ROPO, PC2 distinguishes each frying

474

session. Moreover, these principal components have confirmed that the quality of the pure

475

ROPO and the ROPO/RCO blend diminishes during the deep-frying process, specifically for the

476

pure ROPO as revealed by Figure 2b. PC1 clearly separates the most stable frying oil ROPO

477

/RCO from the pure ROPO which is less stable.

478

Indeed, the fresh oil ROPO pure at t = 0 is highly and negatively related to PC2. However, the oil

479

samples of pure ROPO from session twenty to sixty are highly and positively related to PC1,

480

which are mainly correlated with the significant increase in the trans-UFA (23), TPC (19), DPTG



481

(20), p-AnV (21) and they are also correlated with the significant decrease in PUFA (15) such as

482

linoleic acid (9) and linolenic acid (10) percentages.

483

In addition, the oil blends ROPO/RCO at T=0 and after 60 frying sessions are highly and

484

negatively related to PC1. As observed in Figure 2a, they are characterized by the highest

485

percentages of SFA (12) especially in palmitic acid (2), which is more stable against oxidation4

486

and stearic acid (6).

487

In conclusion, the results of this research work clearly indicated that the frying performance of

488

ROPO is significantly improved by blending with RCO. This study has shown that the blend

489

ROPO/RCO (80:20) is an excellent mixture for frying operations and revealed a great resistance

490

to oxidative deterioration after 60 successive sessions of frying as compared to ROPO pure.

491

Thus, the frying process and the higher degree of control over changing oil chemistry that may

492

occur in the food should be studied in depth in order to obtain better food and fat quality.

493 494 495

ABBREVIATIONS USED

496

AnV, anisidine value ; AV, acid value ; DPTG, dimeric and polymeric triacylglycerols; FFA,

497

free fatty acids; FT-NIR, Fourier-transformed near-infrared; IV, iodine value; MUFA,

498

monounsaturated fatty acids; PUFA, polyunsaturated fatty acids; ROPO, refined olive-pomace

499

oil; RCO, refined coconut oil ; SFA, saturated fatty acids; TAG, triacylglycerols; TPC, total

500

polar compounds; Trans FA, trans fatty acid; UFA ,unsaturated fatty acids.

501

Notes

502

The authors declare no competing financial interest.

503


Page 22 of 34

Page 23 of 34


504

ACKNOWLEDGMENTS

505

The authors thank the Ministry of Higher Education Scientific Research, Tunisia, for the support

506

of this research work. The authors are also grateful to Dr. Christian Gertz from Maxfry GmbH,

507

Germany for carrying out the FT-NIR analysis.

508



509

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refining on these constituents: A review. J.Am.Oil.Chem.Soc .2013,90, 923–932. (51)

Azhari, S.;Xu, Y.S.; Jiang, Q.X.; Xia, W.S.Physicochemical properties and

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chemical composition of Seinat (Cucumis melo var. tibish) seed oil and its antioxidant activity.

644

Grasas Aceites .2014, 65, e008 .

645 646 647

FIGURE CAPTIONS

648 649

Figure 1 Changes in composition of total polar compounds (TPC) (a), dimeric and polymeric

650

triacylglycerols (DPTG) (b), monomeric oxidized TAGs (OTG) (c), acid value (AV) (d) ,

651

anisidine value (AnV) (e) , iodine value (IV) (f) , color value (g) and Trans Fatty acids (TFA)

652

(h) of refined olive-pomace /refined coconut oil blend before and after 60 successive deep-frying

653

sessions at 180°C.

654 655 656

Figure 2 Principal Component analysis applied to all samples analyzed during successive deep-

657

frying sessions (a: loading plot; b: score plot).


Page 28 of 34

Page 29 of 34


658

Tables

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Table 1.Fatty acids composition of ROPO (100:0) and refined ROPO/RCO (80:20) blend before and after deep-frying sessions.

Fatty Acid (%)

ROPO at t0

ROPO after 20 D-Fs

ROPO after 40 D-Fs

ROPO after 60 D-Fs

ROPO/RCO at t0

ROPO/RCO after 20 D-Fs



C6:0

-

C8:0

-

-

-

-

0.06±0.00A

0.04±0.00B

0.03±0.00C

0.03±0.00C

-

-

-

0.62±0.01A

0.50±0.01B

0.50±0.02B

0.46±0.02C

C10:0

-

-

-

-

0.36±0.01A

0.35±0.00A

0.32±0.00B

0.31±0.01B

C12:0

-

-

-

-

2.50±0.04B

2.55±0.02A

2.56±0.03A

2.59±0.01A

C14:0

0.02±0.00Bb

0.03±0.00Ab

0.03±0.00Ab

0.03±0.00Ab

1.09±0.01Da

1.11±0.01Ca

1.14±0.01Ba

1.16±0.01Aa

C16:0

15.54±0.18Cb

15.60±0.11Cb

15.99±0.10Bb

17.24±0.10Ab

19.60±0.15Da

19.94±0.10Ca

20.33±0.07Ba

20.87±0.12Aa

C16:1

1.69±0.02Da

1.71±0.01Ca

1.74±0.02Ba

1.77±0.02Ab

1.51±0.01Db

1.52±0.01Cb

1.55±0.01Bb

1.59±0.01Aa

C17:0

0.06±0.00Ba

0.07±0.00Aa

0.07±0.00Aa

0.07±0.00Aa

0.05±0.00Cb

0.06±0.00Bb

0.07±0.00Aa

0.07±0.00Aa

C17:1

0.08±0.00Ba

0.08±0.00Ba

0.08±0.00Ba

0.09±0.00Aa

0.06±0.00Bb

0.07±0.00Ab

0.07±0.00Ab

0.07±0.00Ab

C18:0

2.92±0.02Db

3.01±0.01Cb

3.11±0.02Bb

3.21±0.02Ab

3.65±0.04Da

3.73±0.01Ca

3.80±0.02Ba

3.89±0.01Aa

C18:1

56.94±0.31Da

57.72±0.32Ca

58.53±0.30Ba

59.13±0.35Aa

54.13±0.35Db

54.40±0.26Cb

54.44±0.25Bb

54.84±0.34Ab

C18:2

20.79±0.20Aa

19.63±0.20Ba

18.27±0.20Ca

16.95±0.23Da

15.07±0.08Ab

14.52±0.07Bb

13.82±0.13Cb

12.94±0.16Db

C18:3

1.04±0.01Aa

0.90±0.00Ba

0.77±0.00Ca

0.64±0.00Da

0.54±0.01Ab

0.51±0.02Bb

0.46±0.02Cb

0.40±0.01Db

C20:0

0.52±0.00Da

0.53±0.00Ca

0.55±0.00Ba

0.56±0.00Aa

0.45±0.01Cb

0.47±0.01Bb

0.48±0.01Ab

0.49±0.00Ab

C20:1

0.33±0.00Aa

0.33±0.00Aa

0.33±0.00Aa

0.32±0.00Ba

0.26±0.00Cb

0.29±0.01Bb

0.30±0.01Ab

0.30±0.01Ab

Σ SFA

19.13±0.20Db

19.62±0.12Cb

20.29±0.12Bb

21.11±0.12Ab

28.42±0.28Da

28.66±0.16Ca

29.40±0.16Ba

29.86±0.18Aa

Σ MUFA

59.04±0.33Da

59.85±0.33Ca

60.67±0.32Ba

61.30±0.37Aa

55.97±0.36Cb

56.31±0.28Bb

56.32±0.27Bb

56.79±0.36Ab

Σ PUFA

21.83±0.21Aa

20.53±0.20Ba

19.04±0.20Ca

17.59±0.23Da

15.61±0.09Ab

15.03±0.09Bb

14.27±0.15Cb

13.34±0.17Db

Σ UFA

80.87±0.54Aa

80.38±0.53Ba

79.71±0.52Ca

78.89±0.60Da

71.58±0.45Ab

71.34±0.37Bb

70.59±0.42Cb

70.13±0.53Db

C18:2/C16:0

1.33

1.23

1.11

0.98

0.77

0.73

0.68

0.62

660

2.73 ± 0.03Ab 0.59 ± 0.02Bb 0.33 ± 0.02Cb 0.25 ± 0.01Db 3.82 ± 0.04Aa 1.91 ± 0.03Ba 1.80 ± 0.03Ca 0.97 ± 0.02 Da ROPO: refined olive-pomace oil; RCO: refined coconut oil. D-Fs, deep fryings; IT, induction time; SFA: saturated fatty acids; MUFA: monounsaturated fatty

661

acids; PUFA: polyunsaturated fatty acids; UFA: unsaturated fatty acids. Each value represents the mean of three determinations (n = 3) ± standard deviation.

IT



Page 30 of 34

662

Different upper case letters (A–D) within the same row indicate significant differences (p< 0.05) for the same refined oil blend. Different lower case letters (a and

663

b) within the same column indicate significant differences (p

A Comparative Study on Formation of Polar Components, Fatty Acids

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