Photooxidation Effect in Liquid Lipid Matrices: Answers from an

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The photooxidation effect in liquid lipid matrices: Answers from an innovative FTIR spectroscopy strategy with ‘mesh cell’ incubation Noelia Tena, Ramon Aparicio, and Diego Luis Garcia-Gonzalez J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b05981 • Publication Date (Web): 10 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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Journal of Agricultural and Food Chemistry

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The photooxidation effect in liquid lipid matrices: Answers from

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an innovative FTIR spectroscopy strategy with ‘mesh cell’

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incubation

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Noelia Tena*, Ramón Aparicio, Diego L. García-González

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Instituto de la Grasa (CSIC), Ctra. de Utrera, km. 1, Campus Universitario Pablo de

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Olavide - building 46, 41013 - Sevilla, Spain

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*corresponding author: [email protected]; Tel: +34 954 61 15 50; Fax: +34 954 61

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1 ACS Paragon Plus Environment


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ABSTRACT

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Developing new approaches to evaluate the stability of edible oils under

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moderate conditions is highly demanded today to avoid accelerated experiments that are

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not well correlated with actual shelf life. In particular, low intensity of visible light

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(photooxidation) needs to be integrated in stability studies, together with mild

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temperature. Thus, in this work a strategy based on ‘mesh cell’-FTIR to monitor

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chemical changes in lipid matrices as consequence of a combination of light and mild

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heating was applied. The results were compared with those obtained for the stability of

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triolein used as a molecular model. The study showed that the moderate light intensity

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(400 lx) at low temperature (23ºC) has an early effect on the degradation of lipid

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matrices that is not observed when they are stored at 35ºC in absence of light. Thus,

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results proved that exposition to light (400 lx) pointed out to be more relevant than mild

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heating (35ºC) in monounsaturated lipid matrices, while polyunsaturated lipid matrices

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were more sensitive to mild heating.

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Keywords: FTIR spectroscopy; mesh cell; photooxidation; edible oils; triolein;

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combination of mild storage conditions.

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1. INTRODUCTION

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Currently, there is a marked demand from retailers and producers for analytical

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tools to exert a major control on their products and to ensure that they comply with the

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labelling regulation during their whole shelf life. At the same time, consumers are

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demanding better quality of food products. Thus, they are paying more attention to the

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‘best before’ date of fresh foods.

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The prediction of the stability of fresh foods during their shelf life depends on

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the complex interplay among several compositional variables, together with packaging

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and environmental factors (moderate light and temperature). Fat food products, in

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particular, are unstable because the chemical composition of their lipid fraction is

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sensitive to oxidation. Thus, liquid lipid matrices, such as edible oils, are in continuous

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evolution of their chemical composition because their fatty acid composition is

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susceptible to be easily oxidised. The oxidative stability of oils depends on the ratio

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between polyunsaturated and monounsaturated fatty acids composition. It has been

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estimated that the relative rate of autoxidation of oleate:linoleate:linolenate is 1:40–

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50:100 in regards to oxygen uptake.1,2 However, the effect of pigments, peroxides,

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phenols, and other minor constituents also play an important role in the oxidation

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process. Some of these compounds are removed during the refining process. However,

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others remain in the oil having an influence in the kinetics of the degradation. Thus, the

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degradation of the oil depends on the remained chemical compounds and the type of

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energy being exposed to the oil (heat, light or both).

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Considering how energy affects the rate of oxidation, it can be stated that the

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shelf life of edible oils is greatly affected by variables that depend on the packer (type of

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container, volume, etc.), and the distributor and storekeeper (temperature, light, etc.).



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These variables are not always suitable and perfectly controlled. Thus, they may affect

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the kinetics of the oxidation process and modify the actual ‘best before date’ with

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respect to the date declared on the label. The immediate consequence of fails in oil

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handling is the decrease of sensory quality of the product and the consequent consumer

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rejection, thereby causing an image problem and economical loses. For that reason,

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maintaining freshness along the food chain is of great concern to oil producers.3,4

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Furthermore, gaining knowledge about the alteration processes of different lipid

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matrices under real storage conditions is also of interest for other food producers since

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edible oils are frequently used in the elaboration of foods that undergo a quality loss

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over time influenced by their lipid fraction.

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Different studies have been focused in the stability of the oils under accelerated

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conditions, mainly focused on high temperatures that are close to 100ºC.5 These studies

84

are based on the oil stability index (OSI) determined by Rancimat and the active oxygen

85

method (AOM).5-8 Although research with these methods have yielded relevant

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knowledge that has helped in the understanding of oxidation mechanisms, the acquired

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information is speculative and it does not have a satisfactory correlation with the actual

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oxidation process that slowly occurs at room temperature during the oil storage. Thus,

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the degradation of oils is subjected to a different kinetic and degradation rate when they

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are stored at low temperature (

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exposition to light > mild heating) is similar in ROO and EVOO matrices. In these oils,



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light also showed to be a factor that was more relevant than only mild heating. However,

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this order was different in SO matrix (combination of mild heating and light > mild

312

heating > exposition to light). Thus, this oil showed to be highly sensitive to oxidation

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when it was heated at 35ºC. Thus, the maximum peak height of hydroperoxide band in

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this oil was 0.60 and 0.34 when applying mild heating and exposition to light

315

respectively. The ratio between these two values resulted in 1.8, which revealed the

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importance of mild heating in this lipid matrix.

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The band assigned to alcohols (~3535 cm-1) was also showed in the cumulative

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spectra displayed in Figure 3. This band was clearly observable in SO when applying a

319

combination of mild heating and light. The maximum peak heights of this band in all of

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the samples followed the same order than that described above for hydroperoxide band

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(Table S1 of the Supporting Information). Thus, these results also prove the relatively

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higher influence of exposition to light compared with mild heating, excepting again for

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SO.

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The fact that SO was more sensitive to mild heating than to light can be

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explained by its fatty acid composition. Table 1 shows the composition in percentage of

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fatty acids of the samples studied in this work. SO is rich in polyunsaturated fatty acid

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(58.66%), the linoleic acid being the most abundant fatty acid (58.46%), while the rest

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of oils were characterized by a major amount of monounsaturated fatty acids (>81.92%).

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In order to compare better the stability between matrices, the evolution of the

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intensity of the band assigned to hydroperoxides (~3430 cm-1) was plotted against the

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time of incubation (360 h). Figure 4 shows the trend lines of the hydroperoxides band

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for the four matrices grouped according to the three storage conditions. Figure 4-A

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shows that all the lipid matrices were quite stable when they were subjected to mild


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heating (35 ºC). Only SO and OOO underwent an increment of hydroperoxides under

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this condition after 150 and 170 hours respectively. This increment can be explained by

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the absence of protective minor compounds, and in the case of SO, by its fatty acid

337

composition as described above.

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Regarding the experience carried out with light (400 lx and 23 ºC), Figure 4-B

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shows that all the lipid matrices presented an increment of hydroperoxide band after the

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first 50 hours, which evidences the higher effect of photooxidation compared to mild

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temperature (35 ºC). However, the speed of hydroperoxide formation is not the same in

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all of the matrices. At the beginning of the incubation, OOO sample presented the

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fastest formation of hydroperoxide (Figure 4-B), again explained by the lack of

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protection of minor compounds. On the other hand, the SO underwent an acceleration in

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the formation of hydroperoxide after 250 hours, being the most deteriorated sample at

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the end of the experiment.

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Finally, when samples are subjected to the combined action of light and mild

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heating (Figure 4-C), the formation of hydroperoxides also was accelerated in greater

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extent compared with the other storage conditions, overall in SO and OOO. Regarding

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ROO and EVOO, they were the matrices that showed higher stability even when this

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more drastic storage conditions was applied. In this case, ROO and EVOO matrices

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showed a similar trend in the formation of hydroperoxides, but with slight differences in

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their kinetics over time (Figure 4-C).

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In general terms, Figure 4 shows that different conditions affect not only the

355

final values of the band assigned to hydroperoxides at the end of the experiment, but

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also the speed in their rising over time. Thus, the relative influence of light and

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temperature was studied in terms of speed in the formation of hydroperoxides by means



358

of the slopes of trend lines (Table S1 of the Supporting Information) and the ratios

359

between different conditions. Figure 5 presents the values of the slopes determined with

360

the following equation: ℎ () =

∆

3430 ∆

361

Thus, the effect of light (400 lx and 23ºC) in OOO, measured as slope of the

362

trend line for hydroperoxide band, is 4 times stronger than mild heating (35 ºC). Thus

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this high slope for OOO prove that monounsaturated triacylglycerols, principally

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glyceryl trioleate, are more resistant to moderate temperature than to photooxidation

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when no minor compounds are present. On the contrary, mild heating was the most

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remarkable factor accelerating the formation of hydroperoxides in SO matrix. The

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slopes for this oil (Table S1 of the Supporting Information) show a double effect of mild

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heating in this oil compared with light in regards to hydroperoxide formation. Finally, in

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the case of ROO and EVOO, the slopes when exposing the samples to mild heating

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condition were almost null, while exposition to light produced a slope slightly lower

371

than OOO. Therefore, it could be concluded that also in terms of oxidation speed, the

372

effect of light was more significant for monounsaturated lipid matrices (OOO, ROO,

373

EVOO) than for polyunsaturated lipid matrices (SO), the latter being more sensitive to

374

mild heating.

375

This study pointed out that the effect of mild light intensity on the stability of

376

different lipid matrices may be more remarkable than the application of mild

377

temperature in some cases. In the case of mild temperatures, the degradation rate

378

measured as the intensity of hydroperoxide FTIR band seems to be strongly related to

379

the fatty acid composition and the presence of minor compounds. Thus, the SO matrix

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of linoleic sunflower oil was presented as the most unstable sample when it was


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exposed to 35ºC in dark. By contrast, SO oil exposed to light (400 lx at 23 ºC) was 1.8

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times more stable than the same oil subjected to mild heating (kept in dark at 35 ºC). In

383

the case of OOO (triolein), ROO and EVOO (refined and extra-virgin olive oils),

384

exposition to light showed a higher effect in the formation of hydroperoxides compared

385

with mild temperature.

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All these results obtained with samples of different production sources (refined

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vs. virgin, sunflower seeds vs. olives), highlight the importance of determining the

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stability of the lipid matrices including multiple factors (light in addition to mild

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heating) and considering mild values of intensity. Thus, the combination of these two

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factors, light (400 lx) and mild heating (35 ºC), was presented as the best way to predict

391

the stability of the lipid matrices because this combination produced the highest

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differences in the spectral bands when different oils were compared (Table S1 of the

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Supporting Information). The in-house system designed for combining different

394

oxidation factors in a controlled way (Figure 1) proved to be a useful tool to simulate

395

the real conditions of storage of fat foods. Considering this combination of factors (both

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light and temperature), and the speed of the oxidation process measured as slope (Figure

397

5), the matrix that was most stable under these conditions was EVOO, followed by

398

ROO, OOO and SO. The three last oils were 1.14, 2.0 and 4.57 times more unstable

399

than EVOO respectively in terms of FTIR band of hydroperoxides.

400 401

Supporting Information

402

Details of spectra features (Table S1) as affected by each one of the three

403

conditions studied: mild heating, exposition to light and a combination of light and mild



404

heating; spectra of triolein (OOO) before and after purification and spectrum of extra

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virgin olive oil (VOO) (Figure S1).

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Funding

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Financial support by Fundación General CSIC (Programa ComFuturo) is

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acknowledged. This research was also funded by AGL2015‐69320‐R project (Spanish

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State Secretariat for Research, Development and Innovation).

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Oxidation of lipids in food. Polish journal of food and nutrition sciences, 2004,

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Morales, M.T. Predicting extra virgin olive oil freshness during storage by

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and Long-Term Room Temperature Storage. Journal of Agricultural and Food

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Chemistry, 2007, 55(20), 8231–8236.



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on the Oxidative Stability of Extra Virgin Olive Oil. Journal of Agricultural and

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Food Chemistry, 2006, 54, 529-535.

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Sordini, B.; Servili. M. Effect of light exposure on the quality of extra virgin

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Compounds during Long-Term Storage (18 Months) at Temperatures of 5−50

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°C. Journal of Agricultural and Food Chemistry, 2015, 63, 6779−6786.

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Evolution of major and minor components and oxidation indices of virgin olive

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oil during 21 months storage at room temperature. Food Chemistry, 2007, 100,

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(14) Li Y, García-González DL, Yu X, van de Voort FR. Determination of free fatty

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acids in edible oils with the use of a variable filter array IR spectrometer.

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two-dimensional correlation techniques. Journal of Agricultural and Food

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Chemistry, 2008, 56, 1532–1537.

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(16) Yu X, van de Voort FR, Sedman J. Determination of peroxide value of edible oils

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Talanta, 2007, 74, 241–246.

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(18) van de Voort FR, Ghetler A, García-González DL, Li Y. Perspectives on

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analysis: evolution and integration of analytical methodologies. Food Analytical

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Methods, 2008, 1, 153–163.

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Food Chemistry, 2009, 57, 9997–10003.

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lipid oxidative processes by Fourier transform infrared spectroscopy. Applied

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Acid Methyl Esters by Gas Chromatography, Madrid, Spain, 2015.

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LLC, Boca Raton: FL, USA, 2002; pp. 128-156.

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Compounds for olive oil. In: Handbook of olive oil: Analysis and properties,

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2nd ed., Aparicio, R., Harwood, J. L. Eds.; Springer: New York, 2013; pp. 261–

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(31) Tena, N.; Aparicio-Ruiz, R.; García-González, D. L. Use of polar and nonpolar

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fractions as additional information sources for studying thermoxidized virgin

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(32) García-González, D. L.; Baeten, V.; Fernández Pierna, J. A.; Tena, N. Infrared,

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Raman, and Fluorescence Spectroscopies: Methodologies and Applications. In:

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J. L. Eds.; Springer: New York, 2013; pp. 335-393.

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FIGURE CAPTIONS

547 548

Figure 1. In-house system designed to study the simultaneous effect of mild heating and

549

exposition to light in the oxidation process of lipid matrices.

550 551

Figure 2. Spectral changes of different lipid matrices when they are incubated in mesh

552

cells under conditions combining exposition to light and mild heating (400 lx and 35 ºC

553

for 360h).

554

Note: Triolein (OOO), refined olive oil (ROO), extra virgin olive oil (EVOO) and

555

sunflower oil (SO). The scale of spectral regions that evolve during incubation has been

556

maximized and marked with an arrow, which indicates the direction of the evolution of

557

the band with time. The assignments of the spectral region are also indicated.

558 559

Figure 3. Spectral regions assigned to hydroxyl groups obtained for the cumulative

560

variance spectra computed from the spectra of triolein (OOO), refined olive oil (ROO),

561

extra virgin olive oil (EVOO) and sunflower oil (SO) in mesh cells for a total period of

562

360 hours (~15 days) under mild heating (kept in dark at 35 ºC), exposition to light (400

563

lx and 23 ºC) and combined action of light and mild heating (400 lx and 35 ºC).

564

Note: The spectral bands that show changes during all the experiments are marked by an

565

arrow. The arrow indicates the tendency of the changes with the incubation time. The

566

chemical assignments of the spectral bands that present evolution are identified.

567 568

Figure 4. Time course trends of peak height for the spectral band at 3430 cm-1 assigned

569

to hydroperoxides during 360 hours in the spectra of different lipid matrices when they


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are stored in mesh cells subjected to three conditions: A, mild heating (kept in dark at

571

35 ºC); B, exposition to light (400 lx and 23 ºC); C, a combined action of light and mild

572

heating (400 lx and 35 ºC).

573

Note: Triolein (OOO), refined olive oil (ROO), extra virgin olive oil (EVOO) and

574

sunflower oil (SO). The Y-axes have been set at the same value (0.35) to facilitate the

575

comparison of the trends. Some samples (OOO, SO) reached this value before 360

576

hours.

577 578

Figure 5. Speed of hydroperoxides formation determine by the trend lines slope of the

579

peak height of the spectral band at 3430 cm-1 during 360 hours in the spectra of different

580

lipid matrices loaded in mesh cells subjected to mild heating (kept in dark at 35 ºC),

581

exposition to light (400 lx and 23 ºC) and a combined action of light and mild heating

582

(400 lx and 35 ºC). Note: Triolein (OOO), refined olive oil (ROO), extra virgin olive oil

583

(EVOO) and sunflower oil (SO).

584



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Table 1. Fatty acid composition (percentage) of selected samples.

Samples

OOO

ROO

EVOO

SO

∑ SFA

8.74

15.46

14.86

11.19

∑ MUFA

84.71

76.79

81.92

30.10

∑ PUFA

8.76

7.71

3.20

58.66

∑ UFA

93.47

84.50

85.12

88.76

C18:1

66.69

75.32

80.56

29.78

C18:2

6.49

7.06

2.59

58.46

C18:3

0.30

0.59

0.61

0.05

Note: OOO, Triolein; ROO, Refined olive oil; EVOO, Extra virgin olive oil; SO, Sunflower oil; ∑SFA, Sum of saturated fatty acids; ∑MUFA, Sum of monounsaturated fatty acids; ∑PUFA, Sum of polyunsaturated fatty acids.


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FIGURE 1



FIGURE 2


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FIGURE 3



FIGURE 4


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FIGURE 5



Table of Contents Graphics (TOC)


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Supporting Information Table S1: Minimum (min. peak height) and maximum (max. peak height) values of peak height of the bands assigned to alcohols (~3535 cm-1), hydroperoxides (~3430 cm1 ), unsaturated carbonyl compounds (~1640 cm-1), conjugated trans isomers (~987 cm-1) and isolated trans isomers (~967 cm-1); increment of peak height (∆ height), standard deviation (SD) and slopes of the trend lines obtained by plotting peak height vs incubation time for the four lipid matrices when they are stored in mesh cell subjected to a combined action of light and mild heating (400 lx and 35 ºC), exposition to light (400 lx and 23 ºC) and mild heating (kept in dark at 35 ºC) during 360h respectively. Note:a slope calculated in the range where the trend was linear; triolein (OOO), extra virgin olive oil (EVOO), refined olive oil (ROO) and sunflower oil (SO).



Lipid matrix/ storage condition

Min. peak height Max. peak height ∆ height Alcohols (~3535 cm-1) OOO/ 400 lx and 35 ºC 0.05 0.25 0.20 OOO/ 400 lx and 23 ºC 0.05 0.12 0.07 OOO/kept in dark at 35 ºC 0.05 0.08 0.03 ROO/400 lx and 35 ºC 0.06 0.16 0.10 ROO/400 lx and 23 ºC 0.07 0.11 0.04 ROO/kept in dark at 35 ºC 0.05 0.06 0.01 EVOO/400 lx and 35ºC 0.04 0.11 0.07 EVOO/400 lx and 23 ºC 0.04 0.07 0.02 EVOO/kept in dark at 35 ºC 0.03 0.04 0.01 SO/400 lx and 35ºC 0.05 0.93 0.88 SO/400 lx and 23ºC 0.05 0.14 0.10 SO/kept in dark at 35ºC 0.04 0.36 0.32 Hydroperoxide band (~3430 cm-1) OOO/ 400 lx and 35 ºC 0.05 0.52 0.48 OOO/ 400 lx and 23 ºC 0.03 0.29 0.26 OOO/kept in dark at 35 ºC 0.04 0.14 0.10 ROO/400 lx and 35 ºC 0.03 0.31 0.28 ROO/400 lx and 23 ºC 0.04 0.24 0.20 ROO/kept in dark at 35 ºC 0.03 0.04 0.01 EVOO/400 lx and 35ºC 0.03 0.29 0.26 EVOO/400 lx and 23 ºC 0.03 0.20 0.17 EVOO/kept in dark at 35 ºC 0.03 0.04 0.01 SO/400 lx and 35ºC 0.05 1.05 1.00 SO/400 lx and 23ºC 0.03 0.34 0.31 SO/kept in dark at 35ºC 0.04 0.60 0.56 C=C Stretching absorption of unsaturated carbonyl compound (~1640 cm-1) OOO/ 400 lx and 35 ºC 0.06 0.12 0.06 OOO/ 400 lx and 23 ºC 0.08 0.12 0.40 OOO/kept in dark at 35 ºC 0.06 0.08 0.02 ROO/400 lx and 35 ºC 0.06 0.11 0.05 ROO/400 lx and 23 ºC 0.07 0.07

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