Assessment of the Oxidative State of Thermally Treated Sunflower Oil After Regeneration with Molecular Sieves ^†

Abstract

Edible oils undergo undesirable changes over time or during thermal treatment due to enzymatic, microbial, and chemical processes, leading to spoilage. In this study, the oxidative state of sunflower oil was assessed by determining the peroxide value (PV), anisidine value (AV), and totox value (TV) using standard methods. The oil was heated at temperatures ranging from 110 to 190 °C for 10 and 30 min, also in the presence of molecular sieves (zeolite 4A, clinoptilolite, and bentonite). When using the synthetic molecular sieve zeolite 4A, a reduction in the totox value by 35.72% was observed. When natural molecular sieves were used, a reduction of 33.19% was recorded for clinoptilolite, while for bentonite, the reduction was 31.08%. Both natural and synthetic molecular sieves demonstrated a strong ability to regenerate thermally treated oils.

1. Introduction

Oils and fats are, in terms of their chemical composition, triacylglycerols, which are esters of the trihydroxy alcohol glycerol and fatty acids. Edible vegetable oils are products with a limited shelf life. Over time or during thermal treatment, edible oils undergo undesirable changes due to enzymatic and microbial processes, as well as chemical reactions, leading to spoilage. Oil spoilage results in the formation of various compounds that negatively affect organoleptic properties and reduce the nutritional value of the oil due to the loss of biologically active substances (essential fatty acids, provitamins, vitamins, etc.) and the formation of harmful substances (peroxides and polymers) [1]. Oxidation is the most important chemical reaction involved in oil degradation, although hydrolysis and polymerization reactions also occur. These reactions lead to the formation of fatty acid degradation products, such as lipid peroxides and hydroperoxides—highly reactive compounds that can further recombine with other fatty acids to form secondary products such as aldehydes, ketones, or dimerized and polymerized triglycerides. Many of these substances are volatile and, when accumulated in food, are responsible for an unpleasant odor and rancid taste. Additionally, certain compounds formed during frying can be toxic at specific concentrations [2]. After the heating process, in addition to oxidation products (hydroperoxides and their degradation products), thermal oxidation products are also formed, including dimers and polymers of fatty acids and triglycerides, oxy-polymers, and cyclic fatty acids, as well as other volatile and non-volatile compounds [3,4]. The term “oil regeneration” refers to the removal of oxidation products using natural and synthetic molecular sieves as adsorbents. Heating the oil leads to an increase in specific gravity, viscosity, free fatty acid content, saponification number, peroxide value, anisidine value, and a decrease in iodine value [5]. Therefore, an increase in both peroxide value (PV) and anisidine value (AV) can be considered as a measure of the degree of degradation of edible oils [6]. Determination of the anisidine value (AV) is an empirical test for assessing the oxidative state of oils and fats [7]. This test evaluates secondary oxidation products of unsaturated fatty acids, specifically the presence of aldehydes, particularly 2-akenals, which are formed after lipid peroxidation. Aldehydes are generally considered responsible for undesirable odors and flavors in fats and oils due to their low sensory detection threshold [7,8]. The anisidine value is often determined together with the peroxide value to calculate the total oxidation value, known as the totox value or totox number (TV) [7,8,9]. The peroxide value (PV) refers to the level of primary oxidation of fatty acids and indicates the amount of hydroperoxides as primary autoxidation products, expressed in mmol O₂ per 1 kg. Peroxides and hydroperoxides are the primary oxidation products, and their presence is primarily identified by calculating the peroxide value. The peroxide value is also widely used to determine the effectiveness of various antioxidants, as well as in rapid tests for evaluating the shelf life of oils [9].

Zeolites have excellent adsorption properties and are increasingly used for the removal of dissolved, predominantly organic matter [10,11,12]. Zeolite NaA, or 4A, has a framework formed by linking β-cages into a three-dimensional structure (LTA5) through double four-membered rings (D4R) [13]. The sodalite cage or β-cage in this zeolite contains 12 [SiO4]⁴⁻ and [AlO4]⁵⁻ tetrahedra interconnected by shared oxygen atoms. This requires 12 cations per cage to neutralize the negative charge of the Al tetrahedra, and upon complete dehydration, the cage can accommodate 27 water molecules. Within the NaA zeolite structure, there are two types of cavities: β-cages, which are located inside truncated octahedra with an entrance in the form of a six-membered ring, and α-cages, which are located in the center of the unit cell with an entrance in the form of an eight-membered ring (supercages). The free diameter of the central cavity, the α-cage, is 11.4 Å (1.14 nm), while the diameter of its entrance opening is about 4.2 Å (0.42 nm). The second group of cavities, the β-cages, is slightly smaller, with an internal diameter of 6.6 Å and a free entrance diameter of 2.2 Å [14,15].

The natural zeolite clinoptilolite belongs to the heulandite (HEU) group of zeolites [16,17]. The HEU-type structure contains small 4- and 5-membered rings, as well as larger 8- and 10-membered rings, which form the internal structure of micropores (channels). In HEU-type crystals, there are two different types of interconnected channels: one extends along the c-axis with eight- and ten-membered rings forming A- and B-type channels (3.3 Å × 4.6 Å and 3.0 Å × 7.6 Å, respectively), while the other channel system extends along the [1 0 2] direction and the a-axis with eight-membered rings forming C-type channels (2.6 Å × 4.7 Å) [18,19].

Clay minerals such as montmorillonite, bentonite, attapulgite, vermiculite, illite, sericite, and kaolinite are among the natural materials that have been studied as adsorbents. Bentonite has several advantages over other clay minerals due to its properties, such as high cation exchange capacity, large specific surface area, exceptional physical and chemical stability, and surface characteristics [20,21,22]. Bentonite primarily consists of montmorillonite, which is a 2:1-type aluminosilicate [23]. The inner layer consists of an octahedral layer sandwiched between two SiO₄ tetrahedral layers. Substitutions within the lattice structure, where trivalent aluminum replaces tetravalent silicon in the tetrahedral layer, as well as the substitution of lower-valence ions for trivalent aluminum in the octahedral layer, result in a net negative charge on the clay surface. This charge imbalance is compensated by exchangeable cations such as H⁺, Na⁺, or Ca²⁺ on the layer surfaces. In aqueous solutions, water molecules intercalate into the interlayer space of bentonite, causing mineral expansion. The chemical nature and porous structure of bentonite largely determine its adsorption capacity [24,25].

In this study, the oxidative state of sunflower oil was evaluated by determining the peroxide value (PV), anisidine value (AV), and totox value (TV). The oil was heated at 110 °C, 130 °C, 150 °C, 170 °C, and 190 °C for periods of 10 and 30 min. The procedure was repeated for regeneration using molecular sieves (zeolite 4A, clinoptilolite, and bentonite).

2. Materials and Methods

For the purposes of this study, we used commercially available sunflower oil produced in Bosnia and Herzegovina, as well as molecular sieves: zeolite type 4A (synthetic), clinoptilolite, and bentonite (natural zeolites). In the initial oil sample, we determined the peroxide value, anisidine value, and totox value at room temperature. The oil was then thermally treated at temperatures of 110 °C, 130 °C, 150 °C, 170 °C, and 190 °C. Oil regeneration was performed using natural and synthetic zeolites, and the peroxide value, anisidine value, and totox value were determined under the same conditions.

The peroxide value (PV) was determined using the iodometric titration method. The peroxide value is expressed in mmol/kg and is calculated using Equation (1):

$$PV \left({\displaystyle \frac{\mathrm{m}\mathrm{m}\mathrm{o}\mathrm{l}}{\mathrm{k}\mathrm{g}}}\right)={\displaystyle \frac{\left(V_1-V_0\right)·C}{m}}·500$$

(1)

where V₁—volume of sodium thiosulfate (Na₂S₂O₃) solution used for titrating the sample [mL]; V₀—volume of sodium thiosulfate (Na₂S₂O₃) solution used for the blank test [mL]; C—molar concentration of sodium thiosulfate (Na₂S₂O₃) solution [mol/L]; m—mass of the sample [g].

The anisidine value (AV) is a measure of secondary oxidation in fats and oils. It is used in oil quality analysis to assess the amount of aldehydes and other secondary oxidation products formed by the breakdown of peroxides. The method is based on the reaction of aldehydes with p-anisidine in an acidic medium, resulting in a colored product whose absorbance is measured spectrophotometrically at 350 nm.

Anisidine value (AV) is calculated using Equation (2):

$$AV=25·\left(\right(1.2·A-A_0)/m)$$

(2)

where A—absorbance of the solution with p—anisidine; A₀—absorbance of the solution before adding the reagent; m—mass of the oil sample in grams; 25—factor specific to the standardized method.

The totox value (TV) was calculated using Equation (3):

$$TOTOX=2·PV+AV$$

(3)

3. Results and Discussion

After determining the peroxide value, anisidine value, and totox value in the initial oil sample, the sunflower oil was thermally treated at the temperatures of 110 °C, 130 °C, 150 °C, 170 °C, and 190 °C for 10 min and 30 min, respectively (

Table 1. Peroxide, anisidine, and totox values of thermally treated oil before regeneration with molecular sieves (treatment times: 10 min and 30 min).

Time (min)	Temperature (°C)	Peroxide Value (PV)	Anisidine Value (AV)	Totox Value (TV)
Sunflower Oil	-	0.02	3.3	3.34
10	110	0.02	3.5	3.54
10	130	0.02	4.0	4.04
10	150	0.50	4.2	5.20
10	170	0.72	7.2	8.64
10	190	0.86	12.5	14.22
30	110	0.10	5.1	5.30
30	130	0.10	6.0	6.20
30	150	0.70	7.2	8.60
30	170	0.95	10.2	12.10
30	190	1.22	20.1	22.54

).

Based on the obtained data, we can conclude that, as the temperature of thermal treatment of sunflower oil increases, the values of the peroxide, anisidine, and totox values also increase. These values significantly depend on the temperature and duration of the thermal treatment. At a temperature of 110 °C and a treatment time of 10 min, the peroxide, anisidine, and totox values were 0.02 mmol/kg, 3.5 mmol/kg, and 3.54 mmol/kg, respectively. In contrast, at a temperature of 190 °C for the same duration of 10 min, these values increased to 0.86 mmol/kg, 12.5 mmol/kg, and 14.22 mmol/kg, respectively. Thus, we can observe that the totox value, as an indicator of the oxidation state of sunflower oil, increased by 325.7% (from 3.34 mmol/kg to 14.22 mmol/kg), when compared to untreated sunflower oil. During the 30 min thermal treatment of the oil, even more significant changes in the peroxide, anisidine, and totox values were observed. At a temperature of 190 °C, the recorded values were 1.22 mmol/kg, 20.1 mmol/kg, and 22.54 mmol/kg, respectively, reflecting an increase of 574.85% (from 3.34 mmol/kg to 22.54 mmol/kg) in totox value when compared to untreated sunflower oil.

The thermally treated oil was regenerated using molecular sieves (zeolite 4A, clinoptilolite, and bentonite), and the obtained results are presented in the tables (

Table 2. Peroxide, anisidine, and totox number of thermally treated oil after regeneration with zeolite 4A.

Time (min)	Temperature (°C)	Peroxide Value (PV)	Anisidine Value (AV)	Totox Value (TV)
Sunflower oil	-	0.02	3.3	3.34
10	110	0.02	3.0	3.04
10	130	0.02	3.0	3.04
10	150	0.40	3.2	4.00
10	170	0.60	4.7	5.90
10	190	0.72	7.7	9.14
30	110	0.05	3.5	3.60
30	130	0.05	3.7	3.80
30	150	0.50	4.0	5.00
30	170	0.35	4.7	5.40
30	190	0.74	11.0	12.48

,

Table 3. Peroxide, anisidine, and totox number of thermally treated oil after regeneration with clinoptilolite.

Time (min)	Temperature (°C)	Peroxide Value (PV)	Anisidine Value (AV)	Totox Value (TV)
Sunflower oil	-	0.02	3.3	3.34
10	110	0.02	3.2	3.24
10	130	0.02	3.2	3.24
10	150	0.32	3.5	4.14
10	170	0.50	5.0	6.00
10	190	0.70	8.1	9.50
30	110	0.05	3.5	3.60
30	130	0.05	3.7	4.10
30	150	0.50	4.0	5.20
30	170	0.35	4.7	6.20
30	190	0.74	11.0	13.40

and

Table 4. Peroxide, anisidine, and totox number of thermally treated oil after regeneration with bentonite.

Time (min)	Temperature (°C)	Peroxide Value (PV)	Anisidine Value (AV)	Totox Value (TV)
Sunflower oil	-	0.02	3.3	3.34
10	110	0.02	3.5	3.54
10	130	0.02	2.5	2.54
10	150	0.32	2.6	3.24
10	170	0.50	5.2	3.60
10	190	0.70	8.4	9.80
30	110	0.05	3.7	3.80
30	130	0.05	4.5	4.60
30	150	0.33	4.6	5.26
30	170	0.45	5.8	6.70
30	190	0.70	12.5	13.90

)

After the regeneration of thermally treated sunflower oil with molecular sieves, we can observe a decrease in peroxide, anisidine, and totox values in all oil samples. When using zeolite 4A (synthetic zeolite) for oil regeneration, at a temperature of 190 °C and a treatment time of 10 min, the totox value, as an indicator of the oil’s oxidative state, was 9.14 mmol/kg, which is approximately 35.72% lower compared to the totox value of the sunflower oil before regeneration with zeolite 4A, which was 14.22 mmol/kg. At a temperature of 190 °C and a treatment time of 30 min, the totox value of the oil sample after regeneration with zeolite 4A was 12.48 mmol/kg, which is approximately 44.63% lower compared to the totox value of the sunflower oil before regeneration. If we look at the results obtained with natural zeolites (clinoptilolite and bentonite), we obtain quite similar values. For the observed temperature of 190 °C and a treatment time of 10 min, the totox value during oil regeneration with clinoptilolite was 9.50 mmol/kg, which is approximately 33.19% lower compared to the value before regeneration with clinoptilolite. At a temperature of 190 °C and a time of 30 min, we can observe a decrease in the totox value to 13.40 mmol/kg, or approximately 40.55% lower. When using bentonite, the totox value was 9.80 mmol/kg at a temperature of 190 °C and a treatment time of 10 min, which is approximately 31.08% lower. At a temperature of 190 °C and a treatment time of 30 min, the totox value obtained was 13.90 mmol/kg, which is 38.33% lower compared to the value before the regeneration of thermally treated oil with bentonite. The greatest reduction in the totox value was observed when using synthetic zeolite 4A at a temperature of 190 °C. A comparison of the total oxidation number (totox value) at a temperature of 190 °C and treatment times of 10 and 30 min for different types of molecular sieves is shown in

Table 5. Totox value (TV) at a temperature of 190 °C for different types of molecular sieves (treatment times: 10 and 30 min).

Totox Value (TV)
Time (min)	Sunflower Oil	Zeolite 4A	Clinoptilolite	Bentonite
10	110	0.02	3.2	3.24
30	190	0.74	11.0	13.40

and Figure 1.

By extending the thermal treatment by 20 min at a temperature of 190 °C, the efficiency of removing sunflower oil oxidation products using molecular sieves also increased. Zeolite 4A proved to be the most efficient, as the percentage reduction of oxidation products increased by approximately 9% (from 35.72% to 44.63%, or from 22.54 mmol/kg to 12.48 mmol/kg). Clinoptilolite followed with an increase of about 8% (from 33.19% to 40.55%, or from 22.54 mmol/kg to 13.40 mmol/kg), and finally, bentonite showed an improvement of around 7% (from 31.08% to 38.33%). Therefore, when extending the thermal treatment to 30 min at a temperature of 190 °C, zeolite 4A exhibited the highest adsorption of sunflower oil oxidation products (9%), while bentonite showed the lowest (7%).

4. Conclusions

The conducted research represents a test of the oxidative stability of sunflower oil during thermal treatment within a temperature range of 110–190 °C for durations of 10 and 30 min. It was found that with increasing temperature, the peroxide, anisidine, and totox values also increased, clearly indicating the presence of oxidation processes due to the formation of peroxides, hydroperoxides, and carbonyl compounds. The regeneration process of the oils obtained in this way was carried out by adding molecular sieves zeolite 4A, clinoptilolite, and bentonite. It was found that the used adsorbents (molecular sieves) showed a pronounced ability to regenerate these oils. When using zeolite 4A (at 190 °C, 10 min), a reduction in the totox value of approximately 35.72% was observed compared to the value before regeneration with molecular sieves (from 14.22 mmol/kg to 9.14 mmol/kg). When natural molecular sieves, clinoptilolite and bentonite (at 190 °C, 10 min), were used, a reduction in the totox value of around 33.19% was observed with clinoptilolite (from 14.22 mmol/kg to 9.50 mmol/kg), while the use of bentonite (at 190 °C, 10 min) resulted in a reduction of the totox value by 31.08%. It was also observed that extending the thermal treatment time of the oil increases its oxidation products, while simultaneously allowing for their removal using selected adsorbents. Both natural and synthetic molecular sieves demonstrated a pronounced ability to regenerate thermally treated oils. Future research should be directed towards the regeneration of oils used during the frying process (one or multiple cycles), including both sunflower and palm oil. Additionally, research should focus on determining the quantity and type of molecular sieves, regeneration time, as well as the method of their removal, with the aim of assessing the potential for reuse of both the oil and the molecular sieves.