Degradation and Migration in Olive Oil Packaged in Polyethylene Terephthalate under Thermal Treatment and Storage Conditions

Abstract

The research addresses challenges in food safety related to the migration of contaminants from plastics to food. It focused on the physicochemical and sensory degradation of olive oils packaged in polyethylene terephthalate (PET) and subjected to thermal exposure at 40 °C and 60 °C for several weeks and a subsequent 12 months of storage, as well as the stability and migration of compounds from the PET packaging itself. Olive oils (OO) from Spanish supermarkets (a mixture of refined and virgin, with commercial identifications of mild and intense) were selected and subjected to thermal treatments at 40 °C and 60 °C for 1, 2, and 3 weeks, followed by 12 months of storage. The treatments were conducted through the following two independent experiments: Experiment A focused on immediate analysis post-thermal treatment, while Experiment B included a 12-month storage period post-thermal treatment. The presence of antimony (Sb) was analyzed using acid digestion with nitric acid (HNO₃) and high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS), while the metals cadmium (Cd), copper (Cu), lead (Pb), and iron (Fe) were analyzed using inductively coupled plasma mass spectrometry (ICP-MS). The PET characterization was assessed using Fourier transform infrared spectroscopy in the mid-infrared range (FT-IR/MIR), carbonyl index, and differential scanning calorimetry (DSC). The results showed increases in the acidity index by 0.29%, the peroxide value by 25.92%, and the K₂₆₈ coefficient by 51.22% between the control sample and the most severe treatments, with more pronounced effects observed after 12 months. Sensory quality declined, with reduced intensity of the “fruity” attribute and increased presence of the “rancid” defect. PET degradation was reflected in an increase in the carbonyl index and greater structural amorphization. Fe was the predominant metal, and Sb concentration increased after thermal treatments. The lack of studies on the raw consumption of oils packaged in PET and the concerns about the migration of compounds from the packaging to the food highlight the relevance of this research. This study provides new insights into the effects of thermal exposure and storage on the migration of PET contaminants into oils, contributing to the development of strategies to ensure food safety and product quality.

1. Introduction

Food security (FS) has evolved significantly, with increasing focus on the regulation and control of food packaging, particularly in the use of materials like polyethylene terephthalate (PET). While PET has enhanced the durability and functionality of packaging, its use also raises important concerns about the potential migration of contaminants into food products. Understanding and addressing these risks is crucial to ensuring that modern packaging does not compromise FS [1,2].

Food contamination, both biological and chemical, remains a significant concern, causing foodborne illnesses due to microorganisms and chemical residues [3,4]. Food poisoning occurs from the ingestion of poorly prepared, improperly stored, or inadequately preserved foods and can be caused by physical, biological, and chemical contamination. Foods can become contaminated during their production, transport, storage, and preparation due to factors such as soil, water, air, and utensils. Therefore, food contamination is defined as the presence of extrinsic agents that compromise human safety and health [5,6].

These extrinsic agents can include heavy metals, such as cadmium, mercury, lead, and arsenic, which are highly toxic and non-biodegradable, accumulating in the environment and humans through air, water, and food and causing serious health issues. Environmental contamination by these metals, exacerbated by industrial and natural activities, negatively affects food security and public health, with effects ranging from damage to vital organs to cancer [5,7].

There is also the migration of substances from packaging to food, an increasing concern as certain toxic components can transfer and compromise food safety. Polyethylene terephthalate (PET), widely used in food packaging, can release minor compounds such as antimony, a catalyst used in its manufacture. The role of PET in the migration of heavy metals and other contaminants is critical, particularly given how factors such as temperature, contact time, and the nature of the food influence this process. This underscores the need for strict regulations to ensure FS and minimize the risks posed by packaging materials [8,9,10].

Within the current legislative framework on contaminants and heavy metals in foods and edible vegetable oils, such as olive oil, notable regulations include EU2023/915 [11], EU2020/1245 [12], and Codex Alimentarius [13], which outline the recommended maximum limits of these contaminants in food. However, there remains a certain laxity for many of these limits or even a lack of specific legislation, such as for cadmium, which lacks specific regulations governing its presence in foods.

Regarding vegetable oils, which are essential for the global diet and economy, the primary crops include soybean, rapeseed, and sunflower. Globally, palm oil is the most consumed, but in the European Union, olive oil stands out as a leader in production and consumption, representing approximately 67% of the global supply. In Spain, olive oil is also the most consumed, noted for its high market share, while PET remains the predominant packaging for edible vegetable oils, with 70% global usage [14,15].

Most studies on virgin olive oil (VOO) have focused on the evolution of quality parameters and shelf life over 12 months under various storage conditions, improving the understanding of the autooxidation process [16,17,18]. Antioxidant components, acidic composition, and antioxidant activity during prolonged storage have also been investigated [19,20], as well as the effect of storage at different temperatures and extraction conditions [21,22,23]. However, studies on the degradation of olive oils packaged in PET, derived from refining, and mixed with virgin fractions for commercial use as olive oil (OO) according to Regulation (EU) 2022/2104 [24] are limited for raw consumption. These have primarily focused on their use in frying and stability at high temperatures (170 °C to 185 °C) [25,26]. The existing literature focuses on the resistance of refined oil to oxidation under frying conditions, but there is little information on its behavior under thermal treatments and prolonged storage for raw consumption. It is necessary to investigate how these factors affect the quality and stability of refined olive oil to improve its preservation and use in culinary applications.

Current challenges, such as heavy metal contamination and the migration of substances from packaging, highlight the urgent need for more robust and specific regulations to protect public health. In this context, the following research will focus on analyzing the implications of contaminants in food, particularly in vegetable oils like olive oil, with the aim of identifying critical areas for improving food safety and consumer protection.

The quality of olive oil is affected by thermal exposure and prolonged storage, as reflected in physicochemical parameters such as the acidity index (AG), peroxide value (PV), and extinction coefficients, as well as in sensory analysis. Additionally, the degradation of PET containers, which are used for their transparency and resistance, is crucial for oil preservation. Exposure to high temperatures accelerates PET degradation, releasing compounds that alter the oil’s properties. This study proposes that thermal exposure at different temperature ranges and subsequent storage for 12 months could increase the mentioned physicochemical parameters and that PET degradation under thermal treatment would enhance the release of chemical compounds. Testing these hypotheses is essential to improve olive oil preservation strategies and better understand the impact of PET containers on the oil’s long-term quality.

2. Materials and Methods

2.1. Olive Oil Samples

For this study, only olive oils (OO) packaged in polyethylene terephthalate (PET) and available in supermarkets were selected as a commercial category according to Regulation (EU) 2022/2104 [24]. These oils are legitimately marketed in Spain with the labels “mild” and “intense”, terms referring to the proportion of virgin olive oil in the blend; “mild” contains 85–90% refined olive oil and 10–15% virgin olive oil, while “intense” contains 75–80% refined olive oil and 20–25% virgin olive oil. These products comply with the aforementioned regulation and Royal Decree 760/2021 [27] in Spain. In this study, the mild olive oil was coded as O04, and the intense olive oil was coded as O01. The selection of these oils was based on a previous study analyzing the most consumed oils worldwide [28], noting that many studies did not specify whether the oils were virgin, possibly because in many countries, extra virgin olive oil (EVOO) or virgin olive oil (VOO) is not marketed, but olive oil (OO) is. All containers were acquired respecting batch homogeneity and stored at room temperature (25 °C) and in darkness until the respective treatments began.

2.2. Olive Oil Treatments

The treatment of the samples was conducted through the following two independent experiments, which are detailed in

Table 1. Treatment conditions of mild olive oil (O04) and intense olive oil (O1).

Group	Temperature (°C)	Exposure Time (Weeks)	Storage (Months)
T0a *	0	0	0
T1	40	1	0
T2	40	2	0
T3	40	3	0
T4	60	1	0
T5	60	2	0
T6	60	3	0
T0b *	0	0	12
T7	40	1	12
T8	40	2	12
T9	40	3	12
T10	60	1	12
T11	60	2	12
T12	60	3	12

* T0a and T0b correspond to the control samples.

:

• Experiment A: This experiment focused on the physicochemical and organoleptic analysis of the oils and PET degradation (groups T0a to T6 in Table 1). The thermal study was carried out at temperatures of 40 °C and 60 °C for periods of 1, 2, and 3 weeks, respectively. After the thermal treatments, the samples were transferred to opaque glass jars and analyzed immediately;
• Experiment B: This experiment also involved the physicochemical and organoleptic analysis of the oils and PET degradation (groups T0b to T12 in Table 1). The methodology was similar to that of Experiment A; however, the oil samples were stored in darkness at room temperature (25 °C) for a period of 12 months following the thermal treatments. After this time, the samples were analyzed to assess their quality.

In both experiments, the treatments were applied in triplicate to the oil samples contained in their original PET packaging. The samples were kept in darkness throughout the process and rotated inside the oven (Memmert, model UM 600, Schwabach, Germany) on Mondays and Thursdays during the thermal treatments.

2.3. Olive Oil Physicochemical Parameters

The acidity index (AG) was determined according to the methodology described in ISO 660:2020, applicable to animal and vegetable oils and fats [29]. A 20 g sample was dissolved in a mixed solvent of ethanol and diethyl ether in a 1:1 ratio, and the free fatty acids present were titrated with a potassium hydroxide (0.1 M) solution. The titration was performed in the presence of phenolphthalein as an indicator. The acidity index (AG) was expressed as a percentage of oleic acid. All measurements were performed in triplicate. The obtained results were compared with the limits established by Regulation (EEC) 2568/91 (currently in force is Regulation (EU) 2022/2104) [24,30].

The peroxide value (PV) was expressed as milliequivalents of active oxygen per kilogram of oil and was determined according to the methodology described in ISO 3960:2007 [31]. A sample of 1.2–2.0 g of oil was dissolved in a mixture of chloroform and acetic acid (2:3 v/v). Then, 1 mL of saturated KI solution was added, and after agitation and maintaining in darkness for 5 min, 75 mL of deionized water was added. Finally, the sample was titrated with sodium thiosulfate and starch solution as an indicator. All measurements were performed in triplicate. The obtained results were compared with the limits established by Regulation (EEC) 2568/91 (currently in force is Regulation (EU) 2022/2104) [24,30].

The extinction coefficients K₂₃₂ and K₂₆₈ were calculated according to the analytical method described by Regulation (EEC) 2568/91 (currently in force is Regulation (EU) 2022/2104) [24,30], using spectrophotometric testing. A 100 mg sample of oil was dissolved in 25 mL of isooctane, and the extinction of the solution at the specified wavelengths was determined, compared with the pure solvent. Specific extinctions were calculated from the spectrophotometer readings using a quartz cell with a 1 cm optical path. All measurements were performed in triplicate.

The sensory analysis was conducted according to document COI/T.20/Doc. N°. 5 of 2007 [32]. The tasting panel consisted of 8 tasters (4 women and 4 men) aged between 25 and 45 years. They carried out the sensory evaluation and linked the flavor stimuli of the oils with a numerical scale (0–10) according to the standards [32]. Following the protocol for conducting the organoleptic test, with the necessary means established and the conditions indicated in the regulations clarified, the selected tasters smelled and tasted the sample in the tasting cup, analyzing perceptions through olfactory, gustatory, and retronasal pathways. On the standardized profile sheet according to the regulation, the tasters recorded and rated the presence and intensity of the aromas and flavors in each sample analyzed. In this analysis, positive perception was dedicated to the intensity of fruitiness, while negative perception was oriented toward identifying defects such as rancidity. However, commercial olive oil (OO) is not subjected to sensory analysis, as this type of evaluation is reserved for virgin olive oils (VOO) that have not undergone refining processes. Nevertheless, this testing procedure was included with the aim of establishing a methodology and observing trends associated with the chosen treatments. Moreover, since the final composition of commercial olive oil (OO) is organoleptically limited by the higher proportion of refined olive oil in the blend, only “fruity” intensity was considered a positive attribute, while “rancid” and any defects potentially related to PET were considered defects. In this context, only trends associated with the treatments were evaluated.

The determination of photosynthetic pigments, chlorophyll, and carotenoid content, was established according to the approach outlined by Puentes [33], which involves measuring the absorbance of olive oil dissolved in cyclohexane at the maximum wavelength of the predominant compound in the chlorophyll fraction (phaeophytin α) and the carotenoid fraction (luteolin). For the absorbance measurements, standard solutions with known concentrations were prepared. Specifically, a chlorophyll reference solution was made with a concentration of 10 µg/mL of phaeophytin α, and a carotenoid reference solution was made with a concentration of 5 µg/mL of luteolin. Following the procedure, 3.0 g of sample was weighed and dissolved with cyclohexane up to the mark in a 10 mL flask. The sample was stirred and homogenized and then measured in a spectrophotometer at 670 nm for chlorophylls and 470 nm for carotenoids.

The total chlorophyll and carotenoid content was obtained using the following equation:

$$\mathrm{C}\mathrm{p}{\displaystyle \frac{A\gamma \times Vf}{\mathsf{\epsilon }1\%\times ma}}\mathrm{10,000}$$

where

• Cp: pigment concentration (mg pigment/kg oil);
• Aγ: absorbance at 470 nm (carotenoids) or 670 nm (chlorophylls);
• ε1%: specific absorbance measurement of a 1% (w/v) solution. εi (phaeophytin α): 613 or εi (luteolin): 2000;
• ma: mass of oil sample (g);
• Vf: final volume of solution (mL).

2.4. PET Degradation

The determination of the influence of treatments on the spectral band behavior of PET containers was conducted using Fourier transform infrared spectroscopy with mid-infrared technology (FT-IR/MIR; Vertex 70, Bruker, Billerica, MA, USA). Spectra acquisition was performed with 32 scans/min, a resolution of 4 cm⁻¹, and a reading range between 400 and 4000 cm⁻¹. The sample preparation involved cutting fragments from the original containers, which were cleaned solely with water and soap and rinsed with distilled water. Once dried, both the internal and external faces were analyzed.

The study of the chemical and crystallographic behavior of PET containers after applying the treatments was carried out using differential scanning calorimetry (DSC; Mettler Toledo Model 822e, Greifensee, Switzerland). This determination allowed for the quantification of the crystallinity percentage of the PET container samples. The DSC analysis involved weighing approximately 5 mg of plastic sample in an aluminum crucible. It was subjected to heating with a ramp from 30 °C to 330 °C at a rate of 5 °C/min and a 3 h hold. The resulting thermograms were analyzed using STARe software Version 16.30 (Build 13243), and crystallinities were determined using the enthalpy of fusion for 100% crystalline polyesters (PET = 140 J/g) [34].

2.5. Determination of Heavy Metals

Just as the type of oil was carefully selected for this study, the most commonly analyzed metals worldwide were also chosen [28]. This selection was based on their relevance in previous studies, with the predominant metals being cadmium (Cd), lead (Pb), copper (Cu), and iron (Fe). Additionally, antimony (Sb) was considered due to its presence in the PET container as a catalyst used in the PET polymerization process. The determination of heavy metals Cd, Pb, Cu, and Fe was performed using inductively coupled plasma mass spectrometry (ICP-MS; Agilent Model 7900, Santa Clara, CA, USA). The calibration curve was prepared by diluting a multi-element standard solution with high-purity deionized water to obtain concentrations of 0.5 ppb, 1 ppb, 5 ppb, and 10 ppb for the calibration standards. The ISTD MIX 500 ppb multi-element standard containing internal standards 45-Sc, 72-Ge, 103-Rh, and 193-Ir used for the determination of the various metals studied was prepared. Subsequently, the prepared solutions were introduced into the ICP-MS equipment. Once the solutions from the different types of oils were ready for analysis, the blanks, calibration standards, and samples were placed in the automatic sampler. On the other hand, the determination of Sb was performed using high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS; Element XR, Thermo Fisher Scientific, Bremen, Germany). For this, the samples were digested by weighing 1 g into a PTFE digestion tube using an analytical balance. Then, 4 mL of HNO₃ + 4 mL of H₂O + 1 mL of H₂O₂ was added, and digestion was carried out in a microwave digestion oven at pressure. Once digestion was completed, the samples were diluted to a final volume of 25 mL with deionized water quality 1 (18.2 MΩ⋅cm). The digestion procedure followed the program detailed in

Table 2. Operating program for the microwave of mild olive oil (O04) and intense olive oil (O1).

Stage	Pwr Level a	Pw (%) b	Ramp Time	Temperature	Hold Time
1	800	100	5:00	50	3:00
2	800	100	5:00	120	3:00
3	800	100	5:00	150	3:00
4	1600	100	5:00	180	3:00

^a Pwr, indicates the power in watts. ^b Pw (%), shows the proportion of the maximum power being applied.

.

To ensure precision and robustness in the analysis, instrumental parameters and validation criteria were established.

Table 3. Detailed instrumental parameters and validation criteria for analytical method.

Instrumental and Validation Parameters	ICP-MS	HR-ICP-MS
LOD (µg/kg)	0.1	0.01
LOQ (µg/kg)	0.3	0.03
Precision (RSD, %)	3.5	2
Accuracy (% recovery)	95–105	98–102
Full procedure precision (RSD, %)	4	2.5

and

Table 4. Validation and precision results for analytical method (N = 4).

Metal	CRM (µg/kg)	Results (µg/kg)	RSD (%)	Precision (µg/kg)	Repeatability (%)
Cd	1	0.001	4	0.04	3.8
Pb	20	0.021	3.5	0.07	3.6
Cu	100	0.127	3	0.15	2.9
Fe	150	0.148	2.8	0.2	2.7
Sb	10	1.35	2	0.05	2.1

outline the validation criteria used to assess the accuracy and precision of the method, thereby ensuring the quality and consistency of the obtained results.

2.6. Statistical Analysis

A multifactorial analysis of variance (ANOVA) was conducted using SPSS software (IBM SPSS Statistics 20). This analysis aims to develop a statistical model to assess the impact of two or more categorical factors on a dependent variable. Tests were performed to identify significant differences between mean values and to examine potential interactions between the factors. The ANOVA table provides a decomposition of the variability of the measured parameter into contributions from the individual factors. Since Type III sums of squares were employed, the interactions between factors were assessed while controlling for the effects of the other factors. The p values indicate the statistical significance of each factor. Additionally, a post hoc analysis using Tukey’s HSD test was performed to determine which specific observations exhibited significant differences. Furthermore, a T-Student analysis was conducted for those analyses with a smaller number of groups to ensure robust statistical evaluation.

3. Results and Discussion

3.1. Physicochemical Parameters

In this section, the obtained results are detailed, and the influence of the treatments is interpreted with the aim of evaluating the potential loss and deterioration of the physicochemical and sensory parameters of the studied oils.

Table 5. Physicochemical parameters of mild olive oil (O04) and intense olive oil (O1).

O04	AG	PV	K232	K268	Chlorophylls	Carotenoids
T0a	0.28 ± 0.00	5.12 ± 0.22	2.73 ± 0.05	0.59 ± 0.00	0.33 ± 0.46	1.62 ± 0.21
T1	0.28 ± 0.01	4.91 ± 0.08	2.69 ± 0.06	0.58 ± 0.00	0.05 ± 0.08	1.25 ± 0.17
T2	0.27 ± 0.00	5.50 ± 0.08	2.92 ± 0.12	0.60 ± 0.02	1.14 ± 0.22	1.16 ± 0.11
T3	0.27 ± 0.01	5.44 ± 0.04	3.10 ± 0.72	0.59 ± 0.02	1.08 ± 0.31	0.68 ± 0.59
T4	0.28 ± 0.00	5.43 ± 0.01	2.95 ± 0.13	0.60 ± 0.02	0.98 ± 0.15	1.11 ± 0.02
T5	0.28 ± 0.01	5.65 ± 0.05	2.82 ± 0.09	0.60 ± 0.01	0.49 ± 0.69	1.00 ± 0.05
T6	0.28 ± 0.01	5.15 ± 0.27	2.67 ± 0.13	0.61 ± 0.01	0.93 ± 0.08	1.09 ± 0.03
T0b	0.14 ± 0.01	6.36 ± 0.06	2.39 ± 0.01	0.65 ± 0.01	2.20 ± 0.01	1.82 ± 0.03
T7	0.12 ± 0.02	5.19 ± 0.00	3.03 ± 0.04	0.66 ± 0.01	1.62 ± 0.15	1.63 ± 0.04
T8	0.13 ± 0.00	5.90 ± 0.72	2.52 ± 0.02	0.70 ± 0.03	2.12 ± 0.40	1.85 ± 0.25
T9	0.14 ± 0.00	7.12 ± 0.35	2.80 ± 0.45	0.62 ± 0.01	2.12 ± 0.38	1.81 ± 0.16
T10	0.13 ± 0.00	7.77 ± 0.27	2.91 ± 0.41	0.70 ± 0.05	1.96 ± 0.30	1.75 ± 0.21
T11	0.14 ± 0.00	7.05 ± 0.18	2.90 ± 0.39	0.73 ± 0.08	2.30 ± 0.79	1.85 ± 0.26
T12	0.14 ± 0.00	7.28 ± 0.92	2.82 ± 0.50	0.75 ± 0.00	2.67 ± 0.22	1.94 ± 0.14
O1	AG	PV	K232	K268	Chlorophylls	Carotenoids
T0a	0.23 ± 0.04	5.91 ± 0.03	2.39 ± 0.36	0.58 ± 0.00	7.51 ± 0.23	3.51 ± 0.37
T1	0.23 ± 0.01	6.79 ± 0.19	2.84 ± 0.15	0.57 ± 0.00	7.97 ± 0.98	3.77 ± 0.30
T2	0.23 ± 0.01	6.82 ± 0.29	2.76 ± 0.02	0.60 ± 0.01	6.45 ± 1.17	3.16 ± 0.48
T3	0.23 ± 0.01	6.58 ± 0.75	2.70 ± 0.29	0.63 ± 0.06	6.53 ± 0.54	3.39 ± 0.45
T4	0.23 ± 0.00	6.27 ± 0.49	2.79 ± 0.26	0.60 ± 0.02	6.41 ± 0.32	3.00 ± 0.13
T5	0.23 ± 0.01	6.40 ± 0.23	2.95 ± 0.27	0.61 ± 0.02	5.13 ± 0.36	3.34 ± 0.43
T6	0.24 ± 0.01	6.32 ± 0.33	2.75 ± 0.02	0.58 ± 0.00	6.11 ± 0.76	3.14 ± 0.33
T0b	0.30 ± 0.01	5.93 ± 0.04	2.68 ± 0.06	0.41 ± 0.01	3.01 ± 0.04	3.89 ± 0.05
T7	0.30 ± 0.00	7.21 ± 0.35	2.58 ± 0.33	0.42 ± 0.00	4.38 ± 0.00	3.79 ± 0.00
T8	0.29 ± 0.01	7.32 ± 0.89	2.54 ± 0.43	0.40 ± 0.01	3.59 ± 1.37	3.50 ± 0.06
T9	0.31 ± 0.01	7.42 ± 0.10	2.70 ± 0.37	0.43 ± 0.04	3.38 ± 1.54	3.72 ± 0.02
T10	0.30 ± 0.01	7.65 ± 0.84	2.69 ± 0.11	0.45 ± 0.02	5.10 ± 0.61	3.81 ± 0.07
T11	0.31 ± 0.01	7.36 ± 0.98	2.70 ± 0.45	0.43 ± 0.05	3.42 ± 1.92	3.75 ± 0.07
T12	0.31 ± 0.01	5.97 ± 0.91	2.47 ± 0.03	0.62 ± 0.03	3.39 ± 1.56	3.25 ± 0.01

presents the obtained results from the various analyses conducted.

3.1.1. Acidity Index (AG)

In Table 5, the treatments applied in Experiments A and B (referred to as A and B hereafter) did not show an increasing or decreasing trend in results concerning the treatments, nor did they exhibit a significant percentage change in O04 and O1 for A. However, for B, the O04 oil displayed either stable or decreasing values with the thermal treatments. In O1, the acidity values decreased or remained stable up to the T2 treatment, increasing only with more aggressive treatments. Similar to A, no considerable changes in acidity were observed following the treatments. Authors conducting similar treatments did not observe changes in acidity, suggesting that these treatments might have been insufficient to impact this parameter, unlike more severe treatments [35,36]. This could be due to the sample’s nature (low presence of free fatty acids) or the short exposure times not being enough to produce significant changes in acidity. Ultimately, both oils complied with the current legislation in A and B, which stipulates an acidity value of ≤1.0 [24,30].

3.1.2. Peroxide Value (PV)

Analyzing the results of A, it was observed that the PV values for O04 and O1 generally showed an increase with treatments compared to the control oil value (Table 5). Regarding the evolution of PV, it is noted that O04 reached a maximum value in the T5 treatment, while O1 reached a maximum in the T2 treatment. In B, a decrease in PV was observed in O04 for the 40 °C treatments of T7 y T8, followed by an increase with a maximum at T10. In O1, an increase in PV was observed across all treatments. Additionally, there was a maximum increase of 11.95% and 25.92%, respectively, for the T10 treatment. In this case, no clear trend of peroxide value reduction was observed in the 60 °C treatment.

There is no consensus in the literature on the evaluation of PV after thermal treatments. Some authors observed a significant decrease in PV with treatments similar to those established in this research [37,38]. Others observed a slight increase in the PV of extra virgin olive oil after 120 min of microwave heating at 170 °C, while a decrease in parameters was observed in virgin olive oil under the same conditions [39]. They also reported an increase in the PV of extra virgin olive oil after 8 min of microwave heating and suggested that the increase in PV is caused by the formation of hydroperoxides and the appearance of secondary oxidation products, which reduce the PV value [40]. In long-term tests, authors exposed samples to room temperature, both in the absence and presence of light, observing a slight increase in this index in samples treated in different containers [41].

Furthermore, no trend associated with storage was observed in any of the analyzed oils. However, a clearly upward trend of the thermal treatment effect at both 40 °C and 60 °C on PV in O04 and O1 of A can be observed. This indicates that the thermal effect increased the PV of the samples, as observed by other authors [42]. Finally, both O04 and O1 complied with current legislation in A and B, keeping values below the established limits of ≤15 [24,30] after the treatments.

3.1.3. Extinction Coefficients K₂₃₂ y K₂₆₈

In the results shown in Table 5 for A, there was an increase in the mean K₂₃₂ values for O04 and O1 at T3 treatment, with increases of 13.64% and 13.08%, respectively. This trend in the K₂₃₂ coefficient showed the following disparate changes at 60 °C: an increase in O1 and a decrease in O04. This could be explained by the higher concentration of virgin olive oil in O1 compared to O04, causing the primary oxidation compounds to continue increasing in the former and transforming into secondary compounds in the latter [33]. Regarding K₂₆₈, the values remained slightly higher than the control and stable with the treatments. These increases are due to the formation of oxidation products that explain the decrease in PV [43,44]. In B, O04 showed a slight increase compared to O1 in the most severe treatments for K₂₃₂, while O04 obtained higher results in all treatments than O1 for K₂₆₈. There was no constant trend of increasing or decreasing values after applying the treatments. However, at 60 °C in O04 and O1, there was an increase in the mean values between the control sample and the most extreme treatment for K₂₆₈ of 16.28% and 51.22%, with values of 0.65–0.75 and 0.41–0.62, respectively. Additionally, there was an increase in the mean values for K₂₃₂ in O04 at T12 treatment, with values higher than 20%, ranging from 2.91 to 3.18 compared to the control value (2.39). This explains the increase in the K₂₃₂ coefficient after thermal application to the samples as determined by other authors [37,44].

In K₂₆₈, both olive oils showed a notable increase after thermal treatment and one year of storage (Experiment B). This seems to indicate that prolonged storage produced the transformation of primary oxidation compounds, which were still present in the thermally treated and yet-to-be-stored oils, into secondary oxidation compounds related to this coefficient after the storage period [33]. This can be correlated with the decrease in the peroxide value (PV) in some cases. As observed by other authors, the increase in extreme conditions produced a decrease in primary oxidation compounds (PV) and an increase in secondary oxidation compounds (K₂₆₈) [33]. This may be due to the intrinsic properties of each type of oil, which allow them to transfer greater or lesser stability to the treatments.

The K₂₃₂ and K₂₆₈ extinction coefficients are critical indicators of the oxidation levels in oils, with K₂₃₂ reflecting the concentration of primary oxidation products and K₂₆₈ indicating secondary oxidation products. These coefficients are directly related to the oil’s quality and stability, and their changes can provide insights into the effects of PET degradation and thermal treatments on the oil [37]. Regarding compliance with legislation, the K₂₃₂ coefficient is not regulated for olive oils, while K₂₆₈ has a limit value of ≤0.90 [24,30]. Therefore, the effect of the treatments on both oils did not exceed this limit.

3.1.4. Sensory Analysis

The sensory analysis of the samples was carried out following the methodology established in Regulation (EEC) N°. 2568/91 (currently in force is Regulation (EU) 2022/2104) [24,30]. As indicated in the corresponding chapter of the analytical method, none of the studied oils are regulated by this regulation for organoleptic evaluation. Additionally, the process was simplified by selecting only the intensity of “fruity”, the presence of “rancid”, and the possibility of other defects reminiscent of the PET container. Figure 1 shows the obtained results from the medians of each descriptor for each of the studied olive oils. In this case, the intensity of the measured descriptors (on a scale of 0 to 10) is displayed on the y-axis, while the weeks of exposure (1, 2, and 3) are represented on the x-axis. The bars indicate the temperatures to which the samples were subjected (control sample, 0 °C; 40 °C; and 60 °C).

In A, the intensity of “fruity” in O04 and O1 was not significantly affected by the treatments. However, O04 showed a downward trend at temperatures of 40 °C and 60 °C. The median values compared to the control sample were 3.3–3.1 for 40 °C and 3.3–2.3 for 60 °C. O1 and did not present an increasing or decreasing trend for 40 °C but did so for 60 °C, with median values compared to the control sample of 5.1–4.0 at the end of the 3-week treatment. The “rancid” defect appeared from the beginning of the treatments in O04, with an increasing trend after treatment at 60 °C. The range of median values between the control sample and the most severe treatment (60 °C/3 weeks) was 0.6–2.4. In O1, this defect only appeared with the most severe treatment (60 °C/3 weeks) at an intensity of 2.0.

In B, the intensity of “fruity” in O04 and O1 was not significantly affected by the treatments. However, O04 showed a downward trend at temperatures of 40 °C and 60 °C. The median values compared to the control sample were 3.1–2.8 for 40 °C and 3.1–2.5 for 60 °C. O1 did not present an increasing or decreasing trend for 40 °C but did so for 60 °C, with median values compared to the control sample of 5.3–3.3 at the end of the 3-week treatment. The “rancid” defect appeared from the beginning of the treatments in O04, with an increasing trend after treatment at 60 °C. The range of median values between the control sample and the most severe treatment (60 °C/3 weeks) was 0.5–3.2. In O1, this defect only appeared with the most severe treatment (60 °C/3 weeks) at an intensity of 2.4.

The general behavior of the “fruity” intensity concerning the appearance of defects like “rancid” in all analyzed samples shows the same pattern of loss of sensory quality in favor of the appearance of such defects as the temperature and exposure treatments increase. This fact coincides with other authors, who demonstrate the loss of this quality due to the presence of temperature at any stage of the olive oil production or preservation process [33,45].

The sensory analysis results indicate changes in the “fruity” attribute and the “rancid” defect in olive oils subjected to thermal treatments. While the focus on these specific descriptors may limit the scope of the analysis, their selection was deliberate, as they are antagonistic attributes, allowing for a more precise assessment of oil degradation. The analysis was conducted by a trained panel and followed standardized methods to minimize biases. These observed changes highlight the impact of thermal conditions and packaging materials on the sensory properties of the oil, providing a detailed insight into how these factors affect product quality.

3.1.5. Photosynthetic Pigments

Considering the results presented in Table 5, in A, the chlorophyll content of O1 exhibited a more pronounced decreasing trend. These findings align with those reported by other researchers who have studied extra virgin olive oils, in which a reduction in photosynthetic pigments is observed with increasing temperature [33,37]. In B, a more subdued decrease was noted in relation to the temperature applied to the samples. However, in certain treatments, these results do not agree with those reported by the previously cited authors, which may be due to the low concentration of both chlorophylls and carotenoids present in O04 and O1. The oil analyzed by other authors is extra virgin olive oil, and the chlorophyll content is much higher [33,37]. Nonetheless, the only notable finding would be the decrease in carotenoids in O1 as the exposures and temperature increase.

3.1.6. Influence of Type of Oil, Temperature, Time Exposure, and Storage

The influence of the type of oil, temperature, exposure time, and storage is presented in

Table 6. Analysis of variance (ANOVA) and Tukey’s HSD post hoc test.

Parameters	Acidity P	PV P	K232 P	K268 P	Chlorophylls P	Carotenoids P
Main effects
Type of oil	0.000	0.000	0.055	0.000	0.000	0.000
Temperature	0.104	0.208	0.572	0.000	0.562	0.450
Time exposure	0.068	0.807	0.899	0.008	0.807	0.297
Storage	0.000	0.000	0.344	0.008	0.000	0.000
Interactions
Type of oil—Temperature	0.698	0.000	0.886	0.881	0.147	0.093
Type of oil—Time exposure	0.779	0.049	0.554	0.002	0.005	0.536
Type of oil—Storage	0.000	0.011	0.210	0.000	0.000	0.006
Temperature—Time exposure	0.610	0.004	0.588	0.012	0.508	0.299
Temperature—Storage	0.389	0.030	0.445	0.000	0.078	0.331
Time exposure—Storage	0.258	0.645	0.589	0.088	0.827	0.691
Post Hoc Tukey HSD
Type of oil
O04 vs. 01	−**	−**	+	+**	−**	−**
Temperature (°C)
0 vs. 40/60	−/−	−*/−**	−/−	−/−**	−/+	+*/+*
40 vs. 0/60	+/−	+*/−	+/−	+/−**	+/+	−*/+
60 vs. 0/40	+/+	+**/+	+/+	+**/+**	−/−	−*/−
Time exposure (weeks)
0 vs. 1/2/3	+/−/−	−*/−*/−*	−/−/−	−/−/−**	−/+/−	+/+/+*
1 vs. 0/2/3	−/−/−	+*/−/−	+/+/+	+/−/−*	+/+/+	−/+/+
2 vs. 0/1/3	+/+/−	+*/+/+	+/−/−	+/+/−	−/−/−	−/−/+
3 vs. 0/1/2	+/+/+	+*/+/−	+/−/+	+**/+*/+	+/−/+	−*/−/−
Storage (months)
0 vs. 12	+**	−**	+	+*	+	−**

The p values show the statistical significance of each factor. Interaction level 2: integration level “2”. For p < 0.005 the effects have a statistical significance over each parameter with a statistical confidence of 95.0%. −: Non-significant comparison with a negative mean difference. +: Non-significant comparison with a positive mean difference. −*: Significant comparison with a negative mean difference. +*: Significant comparison with a positive mean difference. −**: Highly significant comparison with a negative mean difference. +**: Highly significant comparison with a positive mean difference.

, in which a multifactorial statistical analysis (ANOVA) has been conducted. This analysis aims to study these variables and their relationship with the physicochemical parameters evaluated.

The multifactorial analysis of variance (ANOVA) revealed that the type of oil is the most influential factor in the variability of the quality parameters, showing significance in almost all parameters studied. Temperature and exposure time significantly impact only the K₂₆₈ parameter, while storage notably affects acidity, peroxide value (PV), chlorophylls, and carotenoids. Interactions between the type of oil and other factors, such as temperature, exposure time, and storage, were also significant, indicating that these factors do not operate independently. Post hoc Tukey’s tests identified specific significant differences between comparisons of oil type, temperature, exposure time, and storage, highlighting the need to consider these variables collectively. In summary, oil type and storage are the most critical factors affecting the quality parameters of the oil, while interactions between multiple factors also play an important role.

3.2. Analysis of PET Degradation

3.2.1. Fourier Transform Infrared Spectroscopy Results

In this analysis, PET samples from the olive oils subjected to different temperature treatments and storage times were used. The interpretation of the spectra results was carried out using the OPUS 8.7.31 software, and the absorption peaks of the characteristic FT-IR spectra of PET were described according to the various authors listed in

Table 7. Characteristic PET wavenumbers found in the literature.

Wavenumber (cm−1)	Functional Group Description	Reference
1714	Stretching of the C=O bond of the carbonyl group	[46,47,48,49]
1338	Stretching of the C–O bond and bonds of the ethylene glycol compound	[37,50]
1242	Associated with terephthalate (OOCC6H4–COO)	[37]
1096	Symmetric vibration of the ester bond and methylene group (CH2)	[51,52]
1017	Stretching of the C–H bond and aromatic rings	[53,54,55]
873	Associated with aromatic rings	[52,53,54,55,56,57,58]
724	C–H bond rocking movements	[58]

. This identification and description were common for all the olive oils analyzed in Experiments A and B.

In addition to the individual identification of the peaks described above, a set of absorption bands was observed that as a consequence of the thermal treatments applied to the samples, caused changes in the PET structure (

Table 8. Spectral bands of PET identified as susceptible to changes after the applied treatments.

Spectral Band (cm−1)	Functional Group Description	Reference
3200–2850	Symmetric and asymmetric stretching of the methylene group (CH₂)	[58]
1850–1600	Stretching of the C=O bond and appearance of the carbonyl group	[59]
1450–1350	Presence of alkenes	[59]
1300–1200	Stretching of the C(=O)–O bond and the presence of terephthalate	[50]
1095–1040	Symmetric and asymmetric stretching of the C–O–C bond and the presence of ether and ester groups	[52,60]
900–800	Stretching of the C–H bond and the presence of aromatic rings	[53,55]
780–650	Out-of-plane bending and the presence of aromatic rings	[58]

).

The PET samples were subjected to thermal treatments at 40 °C and 60 °C. During the thermal induction period, material degradation may have occurred, as one of the main causes is the presence of heat [61]. Generally, the synthesis of PET takes place at high temperatures, under vacuum conditions, and in the presence of effective catalytic conditions. As the temperature of PET increases, it degrades more easily [62].

Initially, the degradation process due to thermal action occurs in the cleavage of the ester bond chain, forming terminal carboxyl groups and a vinyl ester. The transesterification of the latter results in vinyl alcohol, which quickly converts into acetaldehyde. This process causes the polyester chain to regenerate, maintaining an average polymerization. As a result of this reaction, the terminal hydroxyl groups are replaced by terminal carboxyl groups, producing the same amount of acetaldehyde. Additionally, due to the generation of macroradical areas by hydrogen atom abstraction, thermally and photochemically unstable peroxide and hydroperoxide radicals may appear, leading to further degradation [63].

The absorption bands and the most notable peaks in the spectral range of 400–4000 cm⁻¹ described in Table 7 and Table 8 are consistent across all PET containers analyzed for O04 and O1, confirming that the containers are made of this material. The structure is represented only once for the olive oils in Figure 2, which corresponds to the superposition of spectra resulting from the treatments applied to O04 in Experiment A.

As observed by other authors, the degradation of PET occurs with the increase in temperature, resulting in a decrease in genuine functional groups [55] in favor of the appearance of new functional groups such as ketones, aldehydes, and alkenes, among others [58]. This degradation can be quantified through the carbonyl index, which is a widely used parameter to monitor degradation phenomena. In the case of PET, it allows for calculating the presence of carbonyl groups contained in the degradation products of acetaldehyde and carboxyl generated by this polymer. The carbonyl index is expressed using the following equation [64]:

$$\mathrm{C}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{o}\mathrm{n}\mathrm{y}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}={\displaystyle \frac{\mathrm{A}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{b}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}1715{\mathrm{c}\mathrm{m}}^{-1}}{\mathrm{A}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}974{\mathrm{c}\mathrm{m}}^{-1}}}$$

In this determination, the absorption band 1750–1700 cm⁻¹ is considered since, according to various authors, it is the region in which degradation products such as carbonyl are shown [46,47,48,49]. The most widely used peak is 1715 cm⁻¹, while 974 cm⁻¹ is the peak that does not undergo alterations in the degradation processes [65].

Table 9. Carbonyl index of PET containers of mild olive oil (O04) and intense olive oil (O1).

Group	O04	O1
T0a	2.14 ± 0.08	2.14 ± 0.07
T1	2.22 ± 0.05	2.16 ± 0.05
T2	2.23 ± 0.04	2.16 ± 0.07
T3	2.17 ± 0.08	2.16 ± 0.03
T4	2.16 ± 0.06	2.17 ± 0.36
T5	2.15 ± 0.05	2.15 ± 0.04
T6	2.27 ± 0.15	2.20 ± 0.09
t-Student (α = 0.05)	0.797	0.520
T0b	2.48 ± 0.21	2.49 ± 0.15
T7	2.52 ± 0.26	2.54 ± 0.23
T8	2.56 ± 0.26	2.57 ± 0.19
T9	2.63 ± 0.16	2.59 ± 0.18
T10	2.65 ± 0.17	2.64 ± 0.26
T11	2.67 ± 0.25	2.63 ± 0.27
T12	2.69 ± 0.27	2.67 ± 0.22
t-Student (α = 0.05)	0.249	0.250

shows the obtained results for the carbonyl index for O04 and O1 after exposure according to Experiments A and B.

In A for the PET O04 samples, there were no significant differences after applying the treatments, with the most extreme mean values found between the control sample and the most severe treatment (Table 9). An analysis of the results showed that there was no clear trend in the increase in the carbonyl index compared to the control sample; however, in the most severe treatment, this index increased by 5.87%. Slight changes were observed in the evolution of the carbonyl index after the application of the treatments. In this case, the influence of temperature on the increase in the carbonyl index occurred starting from T1. Here, the carbonyl index began to rise at the beginning of the less severe treatments, then stabilized and finally increased.

For the PET O1 samples, there were significant differences after applying the treatments, with the most extreme mean values found between the control sample and the most severe treatment (T6; Table 9). The analysis of the results showed a slightly increasing trend in the carbonyl index compared to the control sample in all phases of the applied treatments, with the most severe treatment (T6) showing the highest percentage increase at 2.55%. An evolution in the increase in the carbonyl index was observed from the beginning to the end of the treatments. Likewise, the influence of temperature on the increase in the carbonyl index occurred starting from the first phase of the T1 treatment, in which the carbonyl index began to rise slowly.

Generally, PET containers of both oils showed an increase in the carbonyl index compared to the control sample in some phases of the applied treatments. PET O04 also showed stages of stabilization or a slight decrease in the carbonyl index, coinciding with the T3 and T5 treatments; however, this “valley” was always above the result obtained for the control sample. In this regard, the observations described agree with those described by other authors, which would demonstrate that the degradation is in an initiation stage [55,66,67,68]. Furthermore, it can be observed that PET O04 has the highest carbonyl index during all stages of the process compared to PET O1, including the control sample, except for the T4 treatment, in which a decrease occurs. This fact could indicate better quality in the material, as it is more resistant to the onset of degradation.

In B for the PET O04 samples, there were no significant differences after applying the treatments, with the most extreme mean values between the control sample and the most severe treatment (T12; Table 9). An analysis of the results of this study showed a growing trend in the carbonyl index compared to the control sample in all phases of the applied treatments, with the most severe treatment (T12) having the highest percentage increase at 8.33%. Notable changes were observed in the evolution of the carbonyl index after applying the treatments. The influence of temperature on the increase in the carbonyl index occurred from the first phase of the treatments (T7), in which the carbonyl index began to grow more evidently.

Generally, an increase in the carbonyl index compared to the control sample was observed in all phases of the applied treatments for PET O04 and PET O1. On this occasion, the carbonyl index was higher in the initial phases of the treatments in PET O1, while PET O04 increased the carbonyl index in the later phases. This fact indicates that the plastic used in the O1 samples is more resistant to high temperatures, making it a higher-quality material.

Finally, comparing the obtained results in A and B, the storage period was key to increasing this index, being higher in both PET O04 and PET O1 in B.

3.2.2. Differential Scanning Calorimetry Analysis

The PET samples from O04 and O1 subjected to temperature treatments and storage time were used. The interpretation of the results was carried out using DSC STARe software, Version 16.30 (Build 13243). The analysis focused exclusively on the plastics of the containers subjected to A treatments for a period of 3 weeks. This is because, through DSC analysis, the thermal parameters of plastic can be identified, which are mainly altered by processes involving temperature changes. Therefore, the variations that may arise in the plastics as a result of B would be analogous to those observed in this section.

Table 10. Thermal properties of PET in mild (O04) and intense olive oils (O1).

O04	Tg 1	Tm 2	Tc 3	ΔHm 4	ΔHc 5	Wc 6
T0a	79.44 ± 0.01	264.06 ± 0.85	252.22 ± 0.81	24.84 ± 0.08	19.92 ± 0.06	6.85
T3	83.44 ± 0.01	263.72 ± 0.85	252.52 ± 0.80	21.71 ± 0.07	20.05 ± 0.06	6.89
T6	94.44 ± 0.01	257.19 ± 0.83	252.29 ± 0.76	26.28 ± 0.08	22.59 ± 0.07	7.76
t-Student (α = 0.05) T0a vs. T3	0.00	0.65	0.67	0.00	0.07
t-Student (α = 0.05) T0a vs. T6	0.00	0.00	0.92	0.00	0.00
t-Student (α = 0.05) T3 vs. T6	0.00	0.00	0.75	0.00	0.00
O1	Tg	Tm	Tc	ΔHm	ΔHc	Wc
T0a	79.61 ± 0.10	255.93 ± 0.82	249.79 ± 0.80	27.83 ± 0.09	26.42 ± 0.08	9.08
T3	82.43 ± 0.58	254.89 ± 0.82	251.35 ± 0.81	26.19 ± 0.08	20.78 ± 0.07	7.14
T6	91.44 ± 4.62	252.27 ± 0.81	252.71 ± 0.81	19.17 ± 0.06	17.53 ± 0.06	6.02
t-Student (α = 0.05) T0a vs. T3	0.01	0.20	0.08	0.00	0.00
t-Student (α = 0.05) T0a vs. T6	0.13	0.01	0.01	0.00	0.00
t-Student (α = 0.05) T3 vs. T6	0.38	0.02	0.11	0.00	0.00

¹ Tg: Glass transition temperature (°C). ² Tm: Melting temperature (°C). ³ Tc: Crystallization temperature (°C). ⁴ ΔHm: Enthalpy change in melting (J/g). ⁵ ΔHc: Enthalpy change in crystallization (J/g). ⁶ Wc: Crystallinity percentage (%).

presents the obtained results after conducting this analysis on the commercial PET of O04 and O1. The data revealed notable changes that were observed among the different samples. Specifically, an increase in the glass transition temperature (Tg) was recorded in both PETs as the treatments became more severe at 40 °C and 60 °C. This increase was notable in both cases, with increments of 5.04% and 18.88% for O04, and 3.54% and 14.86% for O1, respectively. Regarding melting temperatures (Tm) and crystallization temperatures (Tc), no differences, as marked in the case of Tg, were observed. Lastly, concerning the percentage of crystallinity (Wc), variations compared to the T0a were recorded in all cases, although they were not significant. Specifically, in the case of PET O04, Wc increased at T3 and T6 treatments, while in the case of PET O1, it increased after the T3 treatment and then decreased below T0a after T6 (Table 10).

The results of the Student’s t-test reveal significant variations between groups for specific parameters, particularly ΔHm and ΔHc, indicating substantial differences in these aspects. Parameters Tg and Tm also show significant differences in certain comparisons, while Tc exhibits fewer significant differences. These variations may reflect substantial differences attributable to the applied treatments.

Figure 3 presents the curves obtained during the second heating for PET O04 and PET O1. The first heating was discarded because it tends to reflect the thermal history of the material, which could introduce biases in the results. It is during the second heating that the influence of the thermomechanical history of PET is eliminated, revealing the true thermal behavior of the analyzed samples [69,70]. A shift in Tg toward an increase in temperature occurred in both treatments compared to the T0a, being more evident in the T6 treatment for both oils. These increases align with those obtained by other authors who studied similar thermal treatments, obtaining values close to 80 °C [70,71].

3.3. Heavy Metal Analysis

In a previous study, heavy metals and the most studied edible vegetable oils internationally were identified [28]. Based on this foundation, the most studied metals were selected, which in this case, were cadmium (Cd), lead (Pb), iron (Fe), and copper (Cu).

Table 11. Concentration of heavy metals in mild (O04) and intense (O1) olive oils after exposure according to Experiments A and B.

O04	Cd	Pb	Cu	Fe	Sb
T0a	0.001 ± 0.000	0.024 ± 0.002	0.107 ± 0.008	0.156 ± 0.021	1.422 ± 0.012
T6	0.001 ± 0.000	0.022 ± 0.002	0.134 ± 0.010	0.152 ± 0.027	0.885 ± 0.017
t-Student (α = 0.05)	0.005	0.236	0.007	0.676	0.000
T0b	0.001 ± 0.000	0.139 ± 0.011	0.126 ± 0.010	1.538 ± 0.041	1.075 ± 0.032
T12	0.003 ± 0.000	0.132 ± 0.010	0.118 ± 0.009	1.410 ± 0.096	1.317 ± 0.018
t-Student (α = 0.05)	0.000	0.358	0.291	0.167	0.011
O1	Cd	Pb	Cu	Fe	Sb
T0a	0.001 ± 0.000	0.020 ± 0.002	0.182 ± 0.014	0.197 ± 0.015	0.769 ± 0.060
T6	0.001 ± 0.000	0.017 ± 0.001	0.156 ± 0.012	0.392 ± 0.030	1.117 ± 0.087
t-Student (α = 0.05)	0.000	0.060	0.035	0.000	0.001
T0b	0.001 ± 0.000	0.202 ± 0.016	0.157 ± 0.012	1.800 ± 0.140	1.201 ± 0.094
T12	0.004 ± 0.000	0.145 ± 0.011	0.130 ± 0.010	1.551 ± 0.121	1.643 ± 0.128
t-Student (α = 0.05)	0.000	0.002	0.014	0.004	0.002

presents the obtained results in Experiments A and B for the two types of olive oil (OO) analyzed, O04 and O1.

In A, it was observed that cadmium (Cd) concentrations were below permissible levels in both T0a and T6. Copper (Cu) levels recorded in both oils, O1 and O04, were below the maximum recommended by Codex Alimentarius, which was set at 0.1 mg/kg. Both oils were under this threshold in both control sample (T0a) and treated sample (T6). Cu emerged as a predominant heavy metal, showing elevated levels among the analyzed metals, following antimony (Sb) and iron (Fe). Some studies suggest that recycled plastics, incorporated in small percentages into packaging, may contain trace amounts of Cu due to the use of copper catalysts in PET recycling processes [72]. Therefore, the presence of Cu could account for the elevated carbonyl index values in the O04 and O1 packaging, as this metal acts as a catalyst, accelerating PET degradation during treatments. Lead (Pb) levels were similar across both control sample (T0a) and treated sample (T6) for both oils. Fe was higher after treatments in oil O1 and remained constant in O04. Finally, Sb showed higher values only in O1 after treatments. The results obtained were below the limits specified by regulations for all heavy metals and oils studied. Previous studies have shown that thermal treatments on olive oils of various varieties increase antimony content as temperature and exposure time rise [25]. Although the trend was similar across the three varieties, Arbequina exhibited the highest Sb concentrations, exceeding the WHO limit of 20 ppb after three weeks at 60 °C. Temperature appears to be the most influential factor in migration, especially in the initial stages. Specifically, Picual oils showed the highest increases in Sb, followed by Arbequina and, finally, Hojiblanca. However, the study concluded that the specific characteristics of each oil variety (such as composition, physical properties, and surface tension) likely contributed to these results. The levels of all heavy metals were below the specific limits indicated in the regulations.

In B, Cd showed a higher concentration in the most severe treatment (T12) than in the control sample (T0b), both in O04 and O1. For Pb, Cu, and Fe, the results were lower in the most severe treatment (T12) compared to the control sample (T0b) in both oils, except for O04 and O1 in Sb, in which T12 was higher than T0b. It is important to note that the Sb levels obtained in the oils in this study are much lower than those in previous studies, which could be due to a substantial modification in the type of plastic being used or the type of catalyst, which is very satisfactory from a food safety perspective. The obtained results showed lower levels for all heavy metals and oils studied in relation to the regulations indicated above.

These findings coincide with the obtained results by the authors mentioned in the initial referenced study [28]. The obtained results for Cd are lower than those determined in olive oils from Iran and Cyprus, with values between 0.094 and 0.097 mg/kg [73] and 0.09 mg/kg [74], respectively. In the case of Cu, other works determine values close to 0.10 mg/kg and others that exceed it significantly for olive oils. For proximity, there is olive oil from Iran, with values between 0.091 and 0.098 mg/kg [73], while among the results that exceed this limit are the oils from Ukraine and Cyprus, with values of 0.355 mg/kg and between 1.02 and 3.81 mg/kg, respectively [74,75]. In the case of Fe, high values have been obtained in oils from Saudi Arabia in the range of 0.025–7.861 mg/kg [76]. Other studies related to Pb in olive oils give values in the range of 0.15–1.48 mg/kg in oils from Cyprus [74] or 1.321–7.249 mg/kg in oils from Pakistan [77].

After conducting the Student’s t-test, significant differences were found between various treatments and specific metals. However, it is important to note that, in the case of Cd, the results were below the instrument’s limit of detection (LOD) and limit of quantification (LOQ). Therefore, these findings should be interpreted with caution, considering the potential limitations in the sensitivity of the analysis.

3.4. Influence of Commercial PET Packaging on Olive Oil Properties

In the previous sections, it has been established that different treatments influenced the physicochemical and organoleptic properties of the analyzed oils. Additionally, analyses using FT-IR and DSC revealed that the PET containers used to hold the oils experienced deterioration due to thermal exposure and storage periods. The following presents some evidence of the influence of PET and its degradation on the modification of oil properties. The obtained results after exposure, according to Experiments A and B for O04 and O1 of the carbonyl index as indicators of plastic degradation and the sensory analysis as indicators of oil alteration, are shown in Figure 4 and Figure 5.

In these figures, it is observed that the carbonyl index increases with the treatments compared to the control sample in all cases. This could explain why compounds including the carbonyl functional group (C=O) mentioned earlier could transfer to the analyzed olive oils, causing undesirable flavors and aromas. In this case, the sensory analysis revealed a decrease in the “fruity” intensity in both oils and an increase in the “rancid” defect during the treatments for O04 and with the more severe treatments for O1, thereby showing the loss of quality and increased deterioration in both oils.

Furthermore, it was observed that under conditions of greater plastic deterioration, there is a greater diffusion of heavy metals, such as antimony (Sb), which is known to be present in it. In this context, PET O1 showed a reduction in the percentage of crystallinity (Wc) compared to the control sample after the treatments, with results of 9.08% and 6.02%, respectively (Table 10). O1 showed a higher concentration of Sb compared to the control sample after applying the treatments (Figure 5). Therefore, this observation could be explained by the fact that the applied treatments caused greater amorphization in the PET structure, which in turn, would facilitate the diffusion of compounds like Sb [78].

4. Conclusions

The physicochemical analysis of olive oils in Experiment A showed that thermal exposure did not significantly impact the acidity index or the conversion of hydroperoxides into secondary oxidation products. Fluctuations were observed in the extinction coefficients K₂₃₂ and K₂₆₈. In Experiment B, an increase in the peroxide value and the K₂₆₈ coefficient was detected during storage, indicating a higher presence of secondary oxidation products. Additionally, there was a reduction in photosynthetic pigments, a decrease in the “fruity” intensity, and an increase in the “rancid” defect with more severe treatments, although all results remained within regulatory limits.

In Experiment A, degradation of the PET showed initial fluctuations in the carbonyl index with different stabilization stages, while Experiment B exhibited a more linear increase in the carbonyl index, indicating more advanced degradation. The PET associated with the intense olive oil showed greater resistance to degradation in the early stages of storage, whereas the PET associated with the mild olive oil showed better resistance in the later stages. DSC analysis revealed a decrease in crystallinity (Wc) and alterations in glass transition (Tg), melting (Tm), and crystallization (Tc) temperatures, confirming PET degradation and suggesting possible effects on compound migration into the oil.

Heavy metal analysis showed levels within legal limits, although copper, used as a catalyst in PET recycling, could explain the increase in the carbonyl index and the decrease in Wc. Antimony, while present in higher concentrations in some treated samples, also remained within legal limits. These results suggest a potential relationship between PET degradation and compound migration into the oil, emphasizing the need for ongoing monitoring of PET stability and heavy metal levels to ensure food safety and maintain product quality.