Evaluation of Oxidative Stability of Compound Oils under Accelerated Storage Conditions

ABSTRACT

The oxidative stability of linseed (L), cotton (A), and coconut (C) oils, as well as of linseed:cotton (LA), linseed:coconut (LC), and linseed:cotton:coconut (LAC) compound oils was evaluated under accelerated storage at 60°C/20 days. Coconut oil showed to be rather stable, mainly due to low levels of peroxides, conjugated dienes, ρ-anisidine, and long induction period. In addition, along with cotton oil, it improved the stability of linseed oil in the formulation of LAC compound oil. As to fatty acid profile, the compound oils showed to be composed mainly by unsaturated fatty acids. Cotton and coconut oils presented higher retention of total phytosterols, 78.87 and 76.16%, respectively, after 20 days of storage, when compared to linseed oil. The highest retention of total tocopherols at the end of storage was observed in LA (90.81%). In relation to antioxidant activity, by the DPPH method, with the increase in storage time, a reduction in the antioxidant substances of linseed, LC, and LAC oils was observed. Through the FRAP method, oscillations were observed, especially in linseed and compound oils. Although the oils were degraded over time, it was possible to verify that cotton and coconut oils contributed to increase the stability of linseed oil, which, in turn, raised the levels of coconut oil bioactive compounds.

Keywords:
linseed; cotton; coconut; lipid oxidation; vegetable oils

INTRODUCTION

Vegetable oils are widely consumed all over the world, being of great importance in human diet. These oils are sources of energy and essential fatty acids, carry liposoluble vitamins, and participate in the formation of steroid hormones^1,2. In addition, they contribute to increased palatability, providing pleasant taste, aroma, and texture to foods³.

However, vegetable oils are rather susceptible to lipid oxidation, due to their composition, especially the presence of unsaturated molecules, which can undergo degradation reactions, leading to nutritional losses and formation of compounds that are toxic to human health. Such degradation, although relatively slow, may occur during heating and/or storage of the final product, affecting its shelf life⁴.

It is possible to evaluate the stability of oils by their storage under accelerated storage conditions, in which periodic analyses are performed to monitor chemical, physical, or sensorial changes. The Schaal Oven Test is one of the most widely used methods⁵. This test makes it possible to know the oil shelf life, since the results provided have a good correlation with the evaluation carried out in storage at room temperature⁶.

In order to avoid oxidation problems and increase stability in the oils, it is necessary to eliminate traces of metals such as iron, copper, and chromium. Additionally, it is important to prevent contact with oxygen and high temperatures, as well as to eliminate the pro-oxidants and block the formation of free radicals through antioxidants, which, in small amounts, act to inhibit or retard the oxidation process of lipids⁷.

The formulation of compound oils has emerged as an alternative to increase stability, in addition to raising the content of vegetable oil bioactive compounds. These oils are products obtained from the mixture of oils from two or more plant species, which have been studied as an economical way to modify fatty acid composition and physicochemical characteristics².

The investigation of compound oils, which are formulated by the combination of vegetable oils, is a field of emerging research that has not yet been explored. In this context, the objective of this study was to evaluate the oxidative stability and antioxidant activity of compound oils formulated with linseed, cotton, and coconut oils stored at 60°C for 20 days.

MATERIAL AND METHODS

Oils

Linseed (L) oil was extracted by cold continuous pressing (< 60°C) in Scott Tech Equipamentos industries, located in Vinhedo, São Paulo, Brazil. The seeds were previously submitted to a dryer (model SMR 610-G, Scott Tech, Vinhedo, SP, Brazil) with a rotary system and LPG gas at 50°C for 25 minutes, to reduce moisture. Subsequently, they were taken to a vegetable oil extractor (model ERT 60 III, Scott Tech, Vinhedo, SP, Brazil) with a system of tubular radial extraction. Then, the oil went through filter press (model FP 240-N2, Scott Tech, Vinhedo, SP, Brazil), was bottled in amber bottles, and kept under freezing temperature (-18°C) until analysis.

Cotton (A) and coconut (C) refined oils from Triângulo Alimentos industries, located in Itápolis, São Paulo, Brazil, were used to formulate the compound oils: linseed and cotton (LA), 1:1 (v:v); linseed and coconut (LC), 1:1 (v:v); and linseed, cotton, and coconut (LAC), 2:1:1 (v:v:v).

Accelerated storage test

The accelerated storage test was performed at 60ºC for 20 days, using beakers containing 30 mL of each oil with a surface/volume ratio of 0.3/cm. The samples were collected in 0, 10, and 20 days, inertized with nitrogen gas, and stored at -18°C until analysis.

Physicochemical properties

Physicochemical analysis were performed using the compound oils The following parameters were analysed: peroxide value, conjugated dienes, and ρ-anisidine indexes, according to the AOCS method⁸. The total oxidation value (Totox) was calculated by using the equation: Totox = 2 (peroxide index) + (ρ-anisidine value)⁹. Oxidative stability was performed using Rancimat (Model 743, Metrohm Ltd., Herisau, Switzerland) at 110°C, with 20 L/h air flow⁸.

In the sequence, the fatty acid profile was determined by means of gas chromatography of the esterified oils according to the AOCS method⁸. A gas chromatograph (Model 3900, Varian, Walnut Creek, CA, USA) was used with a flame ionization detector, split injection system, and fused silica capillary column (CP-Sil 88, Microsorb, Varian, Walnut Creek, CA, USA). The initial oven temperature was 90°C for 4 min, heated at 10°C/min up to 195°C, then maintained at the same temperature for 16 min. The temperatures used in the injector and the detector were 230 and 250ºC, respectively. Hydrogen was used as carrier gas. Fatty acids were identified according to their retention times, comparing them to the standard composed of 37 fatty acid methyl esters of C4:0 to C24:1, with purity between 99.1 and 99.9% (Supelco, Bellefonte, USA).

The phytosterol profile was determined by gas chromatography from the unsaponifiable matter. Saponification was performed according to Duchateau¹⁰. For the determination, a gas chromatograph (Model 2010 Plus-Shimadzu, Chiyoda-ku, Tokyo, Japan) was used with flame ionization detector, split injector and fused silica capillary column (Restek RTX 5, Shimadzu, Chiyoda-ku, Tokyo, Japan). The column temperature was maintained at 260°C for 35 min. The temperatures used in the injector and the detector were 280 and 320°C, respectively. Hydrogen was used as carrier gas. The quantification of each isomer was performed by internal standardization (cholestane-5α-3β-ol), based on peak areas, and expressed as mg/kg.

The analysis of tocopherols was conducted in a high performance liquid chromatograph (Model 210-263 Varian Inc., Walnut Creek, CA, USA), with fluorescence detector, silica packed stainless steel column (100 Si, Microsorb, Varian, Walnut Creek, CA, USA) with a 290 nm excitation wavelength and a 330 nm emission wavelength. The concentration values were calculated based on the peak excitation areas and expressed as values for each separate isomer. A standard curve of α-, β-, γ- and δ-tocopherols (Supelco, Bellefonte, USA) was prepared with a high purity level to express the tocopherol contents in mg/kg. Vitamin E was calculated according to the method described by McLaughlin and Weihrauch¹¹. The conversion values were: α-tocopherol x 1.0; β-tocopherol x 0.40; γ-tocopherol x 0.10; and δ-tocopherol x 0.01.

Phenolic compounds and antioxidant activity

The extraction of total phenolic compounds was performed by the procedure described by Parry¹² and they were quantified using the Folin-Ciocalteu reagent, according to a methodology described by Singleton and Rossi¹³. Total phenolic compounds were detected at λ = 765 nm (UV-VIS mini model 1240, Shimadzu, Chiyoda-ku, Tokyo, Japan). Gallic acid was used to plot the standard curve (R2 = 0.9999), and the results were expressed as mg EAG/kg.

The evaluation of antioxidant activity was carried out by two different spectrophotometer methodologies (Model UV-VIS mini 1240, Shimadzu, Chiyoda-ku, Tokyo, Japan). The DPPH analysis, which consists of evaluating the scavenging activity of the free radical 2,2-diphenyl-1-picrylhydrazyl, was carried out according to the method of Kalantzakis¹⁴, through which the sample absorbance was measured at λ = 517 nm and the result expressed as percentage.

The antioxidant activity analysis was also conducted through the FRAP method, which is based on the ability of phenols to reduce Fe+ TPTZ^-3 (tripyridil-s-triazine ferric iron) complex into Fe+ TPTZ^-2 (tripyridil iron-s-ferrous triazine) complex at pH 3.6. This methodology has been described by Szydlowska-Czerniak¹⁵ and the results were expressed as μM trolox/100 g.

Statistical analysis

The results obtained from the analytical determinations, in triplicate, were submitted to analysis of variance and the differences between means were tested at 5% probability by the Tukey test¹⁶, through the ESTAT program, version 2.0.

RESULTS AND DISCUSSION

Physicochemical properties

According to Codex Alimentarius¹⁷, refined and crude vegetable oils should have a maximum of 10 and 15 meq/kg of peroxides, respectively, in order to be considered good quality oils. Thus, it was possible to observe that, initially, all the oils presented peroxide value within the established limits (Table 1).

Table 1
Physicochemical properties of oils under storage.

Properties	Oils	Days of storage
		0	10	20
Peroxide (meq/kg)	L	1.80 ± 0.04cA	31.22 ± 3.80bA	47.71 ± 2.45aA
	A	3.30 ± 0.27bA	4.06 ± 0.02abC	6.73 ± 0.06aE
	C	0.66 ± 0.04aA	1.20 ± 0.04aC	1.75 ± 0.21aF
	LA	1.17 ± 0.06cA	4.87 ± 0.43bC	21.94 ± 1.91aD
	LC	0.70 ± 0.03cA	14.89 ± 0.06bB	40.50 ± 0.04aB
	LAC	1.11 ± 0.06bA	2.70 ± 0.05bC	28.59 ± 1.97aC
Conjugated dienes (%)	L	0.14 ± 0.01cD	0.40 ± 0.01bB	0.61 ± 0.002aA
	A	0.42 ± 0.01cA	0.54 ± 0.01aA	0.52 ± 0.002bB
	C	0.09 ± 0.001bE	0.11 ± 0.002aF	0.12 ± 0.01aE
	LA	0.25 ± 0.01cB	0.34 ± 0.01bC	0.46 ± 0.01aC
	LC	0.14 ± 0.01cD	0.17 ± 0.002bE	0.42 ± 0.004aD
	LAC	0.23 ± 0.01cC	0.22 ± 0.003bD	0.45 ± 0.003aC
ρ-anisidine	L	0.33 ± 0.03cD	11.81 ± 0.41bB	28.19 ± 0.16aA
	A	5.77 ± 0.18bA	6.33 ± 0.11bC	7.33 ± 0.06aD
	C	2.99 ± 0.11bC	3.20 ± 0.19bD	5.73 ± 0.05aE
	LA	4.40 ± 0.33cB	5.71 ± 0.03bC	6.84 ± 0.44aDE
	LC	0.73 ± 0.01cD	13.76 ± 0.03bA	24.06 ± 1.06aB
	LAC	1.42 ± 0.01cD	11.43 ± 0.84bB	21.88 ± 0.50aC
Totox	L	3.93	74.25	123.61
	A	12.37	14.45	20.79
	C	4.31	5.60	9.23
	LA	6.74	15.45	50.72
	LC	2.13	43.54	105.06
	LAC	3.64	16.83	79.06
Oxidative stability (h)	L	0.9 ± 0.03aD	0.71 ± 0.03bE	0.77 ± 0.03bC
	A	15.40 ± 0.07aB	12.94 ± 0.25bB	9.85 ± 0.17cB
	C	53.79 ± 0.02aA	45.49 ± 0.69bA	45.66 ± 0.98bA
	LA	5.80 ± 0.14aC	5.31 ± 0.05aC	2.06 ± 0.13bC
	LC	1.01 ± 0.01aD	2.28 ± 0.04bD	0.76 ± 0.01cC
	LAC	5.84 ± 0.16aC	6.16 ± 0.01aC	1.13 ± 0.03bC

• The mean ± standard deviation followed by lowercase letters in the lines do not differ by Tukey test (p > 0.05). Means ± standard deviation followed by upper case letters in the columns do not differ by Tukey test (p > 0.05).

However, as the storage time increased, linseed oil showed high oxidation, reaching 47.71 meq/kg of peroxides in 20 days of storage. This increase may have influenced the compound oils peroxide content, as cotton (6.73 meq/kg) and coconut (1.75 meq/kg) oils remained stable, contributing to a lower peroxide formation compared to linseed oil.

Conjugated dienes are primary oxidation products of polyunsaturated fatty acids formed by the displacement of double bonds¹⁸. The oils showed low values of conjugated dienes at the beginning of storage. However, there was significant increase during storage at 60ºC, except in coconut oil, which was stable after 10 days of storage. It was also possible to verify, when comparing the compound oils, that LC showed lower formation of conjugated dienes, possibly due to the influence of coconut oil.

According to Guillén and Cabo¹⁹ and Marina et al.²⁰, oils of good quality must have ρ-anisidine index below 10. Thus, it is possible to infer that, initially, all the oils presented good quality. However, the values of ρ-anisidine increased during storage. In 20 days, only cotton, coconut, and LA oils remained below 10, while linseed oil showed the lowest quality, 28.19 of ρ-anisidine. This fact is possibly due to the fact that linseed oil is crude and constituted by a large amount of α-linolenic acid.

It is also considered that well-preserved fat must have a Totox value below 10²¹. According to Table 1, initially, all oils presented good conservation status, except for cotton oil (12.37). However, after 10 days of storage, only coconut oil remained below this limit, indicating that it is a stable oil.

In the oxidative stability test, coconut (53.79 h) and cotton (15.40 h) oils showed the highest rates. According to Michotte et al.²², the high content of unsaturated fatty acids of linseed oil makes it extremely sensitive to oxidative reactions. This information is confirmed in this study, since linseed oil presented the lowest oxidative stability during storage, in relation to the other oils. When comparing the compound oils, it was found that LA and LAC remained stable up to 10 days of storage, probably due to the synergism between linseed and cotton oils.

Ten different types of fatty acids were identified (Table 2). Initially, coconut oil showed higher value of α-linolenic acid (47.9%) than linseed oil (47.1%). With heating, lauric, palmitic, and stearic fatty acids were found to remain stable for up to 20 days of storage. However, α-linolenic acid showed significant reduction in compound oils, especially in LC, in which it was reduced by 47.41%.

Table 2
Fatty acid composition of oils under storage.

Fatty acids (%)	Oils	Days of storage
		0	20
Caproic (6:0)	L	nd	nd
	A	nd	nd
	C	2.07 ± 0.02bA	2.24 ± 0.01aA
	LA	nd	Nd
	LC	0.58 ± 0.01bB	1.36 ± 0.01aB
	LAC	0.32 ± 0.01bC	0.55 ± 0.01aC
Caprylic (8:0)	L	nd	nd
	A	nd	nd
	C	2.44 ± 0.01bA	2.50 ± 0.01aA
	LA	nd	nd
	LC	0.70 ± 0.01bB	1.56 ± 0.02aB
	LAC	0.36 ± 0.01bC	0.61 ± 0.01aC
Lauric (12:0)	L	nd	nd
	A	0.27 ± 0.02aD	0.29 ± 0.02aD
	C	47.10 ± 0.01aA	47.11 ± 0.01aA
	LA	0.18 ± 0.0aE	0.19 ± 0.01aE
	LC	13.62 ± 0.02bB	29.87 ± 0.03aB
	LAC	7.14 ± 0.01bC	11.56 ± 0.02aC
Myrístic (14:0)	L	nd	nd
	A	0.83 ± 0.01aD	0.83 ± 0.02aD
	C	13.33 ± 0.01bA	14.92 ± 0.02aA
	LA	0.24 ± 0.01bE	0.54 ± 0.01aE
	LC	4.44 ± 0.01bB	9.62 ± 0.03aB
	LAC	2.59 ± 0.01bC	4.05 ± 0.01aC
Palmitic (16:0)	L	11.44 ± 0.01aF	11.64 ± 0.01aF
	A	40.31 ± 0.01aA	40.04 ± 0.01aA
	C	12.75 ± 0.01aD	12.54 ± 0.01aD
	LA	25.19 ± 0.02bB	29.15 ± 0.01aB
	LC	11.93 ± 0.03aE	12.31 ± 0.01aE
	LAC	20.51 ± 0.01aC	20.13 ± 0.01aC
Stearic (18:0)	L	5.34 ± 0.02aA	5.09 ± 0.01aB
	A	3.21 ± 0.01aE	3.22 ± 0.02aF
	C	3.76 ± 0.01aD	3.66 ± 0.02aE
	LA	3.22 ± 0.03aE	3.99 ± 0.02aD
	LC	4.92 ± 0.02aB	4.35 ± 0.01aC
	LAC	4.33 ± 0.04bC	5.36 ± 0.03aA
Fatty acids (%)	Oils	Days of storage
		0	20
Oleic (18:1n9c)	L	26.98 ± 0.01aA	27.37 ± 0.02aA
	A	18.72 ± 0.02aE	18.87 ± 0.01aE
	C	15.40 ± 0.01aF	15.11 ± 0.01aF
	LA	19.29 ± 0.04bD	22.06 ± 0.01aB
	LC	23.56 ± 0.01aB	19.02 ± 0.03bD
	LAC	23.07 ± 0.04aC	21.54 ± 0.02bC
Linoleic (18:2n6c)	L	13.31 ± 0.01aD	13.34 ± 0.01aD
	A	36.23 ± 0.01aA	36.23 ± 0.04aA
	C	1.29 ± 0.01aF	1.28 ± 0.02aF
	LA	23.50 ± 0.03bB	26.71 ± 0.01aB
	LC	9.81 ± 0.01aE	5.77 ± 0.02bE
	LAC	18.49 ± 0.01aC	16.33 ± 0.04bC
Arachidic (20:0)	L	nd	nd
	A	0.15 ± 0.01aC	0.15 ± 0.01aB
	C	1.89 ± 0.02aA	0.67 ± 0.04bA
	LA	nd	nd
	LC	0.57 ± 0.02aB	0.13 ± 0.03bB
	LAC	0.14 ± 0.04aC	nd
α-Linolenic (18:3n3)	L	42.93 ± 0.01aA	42.56 ± 0.01aA
	A	0.30 ± 0.01aE	0.37 ± 0.01aE
	C	nd	nd
	LA	28.39 ± 0.02aC	17.34 ± 0.05bC
	LC	30.48 ± 0.06aB	16.03 ± 0.11bD
	LAC	23.06 ± 0.03aD	19.89 ± 0.09bB
Ʃ Saturated	L	16.78 ± 0.03aF	16.73 ± 0.02aF
	A	44.62 ± 0.01aB	44.38 ± 0.01aC
	C	81.45 ± 0.01aA	82.97 ± 0.01aA
	LA	28.83 ± 0.03aE	33.87 ± 0.02aE
	LC	36.19 ± 0.05bC	59.07 ± 0.03aB
	LAC	35.25 ± 0.04bD	42.26 ± 0.03aD
∑ Monounsaturated	L	26.98 ± 0.07aA	27.37 ± 0.02aA
	A	18.72 ± 0.02aE	18.87 ± 0.01aE
	C	15.40 ± 0.01aF	15.11 ± 0.01aF
	LA	19.29 ± 0.03bD	22.06 ± 0.01aB
	LC	23.56 ± 0.01aB	19.02 ± 0.03bD
	LAC	23.07 ± 0.04aC	21.54 ± 0.02bC
Ʃ Polyunsaturated	L	56.24 ± 0.03aA	55.90 ± 0.04aA
	A	36.68 ± 0.01aE	36.75 ± 0.01aC
	C	3.18 ± 0.01aF	1.95 ± 0.02bF
	LA	51.89 ± 0.01aB	44.05 ± 0.04bB
	LC	40.86 ± 0.04aD	21.93 ± 0.06bE
	LAC	41.69 ± 0.01aC	36.22 ± 0.06bD

• The mean ± standard deviation followed by lowercase letters in the lines do not differ by Tukey test (p > 0.05). Means ± standard deviation followed by upper case letters in the columns do not differ by Tukey test (p > 0.05). nd: not detected.

The compound oils showed to be constituted mainly of unsaturated fatty acids, representing 64 to 71% of the totality (Table 2). These oils can help lower cholesterol and triglyceride levels, regulate blood pressure, and reduce chronic inflammation and the development of cancer, heart diseases, and stroke^23,24. During storage, saturated fatty acids did not change significantly, except in LC and LAC, probably due to the influence of linseed and coconut oils. On the other hand, the compound oils showed reduction in relation to unsaturated fatty acids, especially in LC (46.32%).

As for the phytosterols profile, the oils showed four different isomers: campesterol, stigmasterol, β-sitosterol, and stigmastanol (Table 3). The presence of β-sitosterol was higher than the other isomers in all oils, and stigmastanol was not detected in cotton and coconut oils.

Initially, linseed oil had higher amounts of campesterol (6.51 mg/kg) and stigmastanol (56.09 mg/kg), while cotton had higher amount of β-sitosterol (90.27 mg/kg), and coconut oil had higher stigmasterol content (8.50 mg/kg).

According to Ferrari et al.²⁵, phytosterols are susceptible to oxidation by reaction with oxygen, light, metal ions, and high temperature, depending on their degree of unsaturation. Thus, it was observed that all phytosterol isomers decreased during storage. However, linseed oil presented greater degradation of total phytosterols than cotton and coconut oils, which remained stable after 10 days of storage. When comparing the compound oils, it was verified that LC retained greater amount of total phytosterols, 95.1%, in 20 days.

Table 3
Phytosterol profile of oils under storage.

Phytosterol (mg/kg)	Oils	Days of storage
		0	10	20
Campesterol	L	6.51 ± 0.04aA	5.38 ± 0.04bA	4.86 ± 0.14cA
	A	4.81 ± 0.06aD	3.53 ± 0.04bE	3.31 ± 0.27bB
	C	5.71 ± 0.03aB	4.94 ± 0.05bB	4.89 ± 0.04bA
	LA	4.13 ± 0.04aF	3.93 ± 0.04aD	3.59 ± 0.12bB
	LC	5.54 ± 0.02aC	5.46 ± 0.09abA	5.23 ± 0.04bA
	LAC	4.61 ± 0.01aE	4.25 ± 0.06bC	3.54 ± 0.06cB
β-sitosterol	L	58.13 ± 0.03aC	55.44 ± 0.05bB	43.25 ± 0.07cE
	A	90.27 ± 0.05aA	72.11 ± 0.02bA	71.72 ± 0.59bA
	C	27.07 ± 0.04aF	19.75 ± 0.11bF	19.82 ± 0.04bF
	LA	58.42 ± 0.04aB	54.07 ± 0.04bC	53.57 ± 0.04cB
	LC	49.37 ± 0.04aE	49.33 ± 0.33aD	47.37 ± 0.04bC
	LAC	50.49 ± 0.04aD	46.44 ± 0.06bE	44.64 ± 0.06cD
Stigmasterol	L	4.28 ± 0.04bC	4.10 ± 0.03cC	4.87 ± 0.04aB
	A	6.57 ± 0.04aB	5.27 ± 0.05bB	5.15 ± 0.22bB
	C	8.50 ± 0.03aA	6.75 ± 0.04bA	6.74 ± 0.03bA
	LA	3.68 ± 0.04aD	3.65 ± 0.01aD	3.12 ± 0.02bD
	LC	4.31 ± 0.04aC	4.22 ± 0.03aC	3.83 ± 0.06bC
	LAC	3.50 ± 0.06aE	2.80 ± 0.15bE	2.15 ± 0.07cE
Stigmastanol	L	56.09 ± 0.02aA	48.08 ± 0.03bA	24.39 ± 0.16cD
	A	nd	nd	nd
	C	nd	nd	nd
	LA	37.04 ± 0.03aC	34.19 ± 0.02bC	31.32 ± 0.12cB
	LC	47.97 ± 0.04aB	46.13 ± 0.16bB	45.44 ± 0.09cA
	LAC	36.89 ± 0.04aD	30.74 ± 0.23bD	27.45 ± 0.64cC
Total	L	124.99 ± 0.01aA	112.99 ± 0.14bA	77.37 ± 0.13cD
	A	101.65 ± 0.05aD	80.91 ± 0.03bE	80.17 ± 1.07bC
	C	41.28 ± 0.03aF	31.44 ± 0.11bF	31.44 ± 0.03bE
	LA	103.26 ± 0.01aC	95.83 ± 0.01bC	91.66 ± 0.06cB
	LC	107.17 ± 0.01aB	105.14 ± 0.04bB	101.87 ± 0.04cA
	LAC	95.49 ± 0.04aE	84.22 ± 0.26bD	77.78 ± 0.83cD

• The mean ± standard deviation followed by lowercase letters in the lines do not differ by Tukey test (p > 0.05). Means ± standard deviation followed by upper case letters in the columns do not differ by Tukey test (p > 0.05). nd: not detected.
• Limits of detection: campesterol ≤ 52 mg/kg; stigmasterol ≤ 56 mg/kg e stigmastanol ≤ 42,5 mg/kg.

The oils presented α-, γ-, and δ-tocopherol isomers (Table 4). Initially, regarding α-tocopherol, cotton oil (86.05 mg/kg) stood out, whereas in LA and linseed oils, γ- and δ-tocopherol were higher, 79.60 and 17.65 mg/kg, respectively.

Table 4
Quantification of tocopherols and vitamin E of oils under storage.

Tocopherols (mg/kg)	Oils	Days of storage
		0	10	20
α-tocol	L	29.53 ± 0.11aC	27.50 ± 0.14bC	8.20 ± 0.14cD
	A	86.05 ± 0.36aA	85.35 ± 0.35aA	83.90 ± 0.28bA
	C	nd	nd	nd
	LA	55.00 ± 0.14aB	48.25 ± 0.07bB	47.35 ± 0.07cB
	LC	24.70 ± 0.57D	nd	nd
	LAC	21.80 ± 0.14aE	18.45 ± 0.07bD	17.45 ± 0.07cC
γ-tocol	L	76.05 ± 0.49aB	71.20 ± 0.14bB	39.65 ± 0.49cD
	A	59.85 ± 0.07aE	59.50 ± 0.28aC	53.25 ± 0.21bB
	C	nd	nd	nd
	LA	79.60 ± 0.14aA	78.90 ± 0.14aA	75.35 ± 0.77bA
	LC	63.15 ± 0.35aD	9.25 ± 0.21bD	nd
	LAC	65.15 ± 0.21aC	59.85 ± 0.21bC	42.45 ± 0.21cC
δ-tocol	L	17.65 ± 0.07aA	16.55 ± 0.35bA	nd
	A	12.55 ± 0.07aD	12.15 ± 0.21aB	12.45 ± 0.07aA
	C	nd	nd	nd
	LA	11.25 ± 0.07aE	10.40 ± 0.14bC	9.75 ± 0.21cB
	LC	16.60 ± 0.28B	nd	nd
	LAC	14.50 ± 0.14aC	9.45 ± 0.21bC	9.05 ± 0.07bC
Total	L	123.22 ± 0.67aC	115.25 ± 0.35bC	47.85 ± 0.35cD
	A	158.44 ± 0.50aA	157.00 ± 0.42aA	149.60 ± 0.01bA
	C	nd	nd	nd
	LA	145.85 ± 0.21aB	137.55 ± 0.07bB	132.45 ± 0.49cB
	LC	104.45 ± 0.64aD	9.25 ± 0.21bE	nd
	LAC	101.45 ± 0.07aE	87.75 ± 0.35bD	68.95 ± 0.07cC
Vitamin E*	L	39.98 ± 0.28aC	37.36 ± 0.13bC	13.61 ± 0.07cD
	A	94.16 ± 0.44aA	93.47 ± 0.39aA	91.16 ± 0.26bA
	C	nd	nd	nd
	LA	65.96 ± 0.16aB	59.11 ± 0.09bB	57.57 ± 0.03cB
	LC	33.46 ± 0.61aD	1.26 ± 0.03bE	nd
	LAC	30.82 ± 0.11aE	26.70 ± 0.04bD	23.32 ± 0.04cC

• The mean ± standard deviation followed by lowercase letters in the lines do not differ by Tukey test (p > 0.05). Means ± standard deviation followed by upper case letters in the columns do not differ by Tukey test (p > 0.05). nd: not detected. Limits of detection: δ-tocol < 2,30 mg/kg.*Expressed as α-tocol.

It can be observed that γ-tocopherol isomer was the one in highest amount in all oils. Tocopherols and vitamin E were not detected in coconut oil. Concerning vitamin E, the highest amount was found in cotton (94.16 mg/kg) and LA (65.96 mg/kg) oils, mainly due to the amount of α-tocopherol present in cotton oil.

It was observed that the isomers of tocopherols and vitamin E decreased during storage. According to Lampi et al.²⁶, oxidizing agents, especially in the presence of heat, light, metals, and alkali, easily oxidize tocopherols. In relation to total tocopherols, cotton oil remained stable, with retention of 94.42% at the end of the process. Among compound oils, LA retained higher amount of total tocopherols (90.81%) and vitamin E (87.28%) at the end of storage.

Phenolic compounds and antioxidant activity

In relation to phenolic compounds (Table 5), at the beginning of storage, coconut oil stood out with 220.40 mg/kg.

Table 5
Phenolic compounds and antioxidant activity of oils under storage.

	Oils	Days of storage
		0	10	20
Phenolic compounds (mg/kg)	L	214.14±15.41bA	235.77 ± 16.32bD	218,.7 ± 10.01aCD
	A	142.84 ± 4.44cB	371.07 ± 19.23aA	213.51 ± 19.02bE
	C	220.40 ± 2.00cA	378.85 ± 4.02bA	515.29 ± 4.33aB
	LA	213.95 ± 20.00bA	278.62 ± 1.54aC	238.84±28.07abDE
	LC	150.40 ± 1.76cB	371.29 ± 5.67bA	667.29 ± 9.05aA
	LAC	211.51 ± 1.68bA	318.85 ± 15.67aB	330.84 ± 24.24aC
DPPH (%)	L	57.20 ± 1.19aB	55.79 ± 1.69aC	47.07 ± 0.73bC
	A	92.76 ± 1.13aA	88.05 ± 2.26aA	88.64 ± 2.19aA
	C	26.01 ± 0.67aD	26.66 ± 0.52aD	25.0 ± 0.18aDE
	LA	59.18 ± 7.12cB	80.91 ± 1.98aB	71.75 ± 2.12bB
	LC	42.42 ± 7.20aC	27.99 ± 1.66bD	29.46 ± 0.67bD
	LAC	38.71 ± 1.77aC	19.51 ± 0.96bE	19.51 ± 0.41bE
FRAP (µM/100 g)	L	97.23 ± 6.69bA	107.95 ± 0.39aA	103.05 ± 0.13abA
	A	102.59 ± 2.44aA	74.09 ± 0.45bC	71.27 ± 3.15bB
	C	89.73 ± 2.51aB	66.68 ± 3.21bC	57.0 ± 0.32cC
	LA	75.55 ± 0.32bC	90.14 ± 1.67aB	76.55 ± 1.61bB
	LC	99.09 ± 1.22bA	109.41 ± 0.51aA	109.86 ± 0.64aA
	LAC	74.32 ± 3.34cC	84.64 ± 0.19bB	109.0 ± 1.74aA

• The mean ± standard deviation followed by lowercase letters in the lines do not differ by Tukey test (p > 0.05). Means ± standard deviation followed by upper case letters in the columns do not differ by Tukey test (p > 0.05).

Throughout storage, there was an increase in the levels of phenolic compounds in all oils. Such increase may have occurred due to the reaction of Folin-Ciocalteu reagent with phenol groups resulting from the degradation of tocopherols, since the presence of these easily oxidizable compounds results in the formation of blue complexes, causing overestimation of the total phenolic compounds²⁷.

In the test for the reduction of DPPH radical, cotton oil (92.76%), followed by LA (59.18%) and linseed (57.20%) oils, stood out showing higher antioxidant activity at the beginning of storage. This may be due to the greater presence of natural antioxidants in these oils. Cotton and coconut oils showed stable antioxidant activity up to 20 days of storage. Yet, it was possible to detect a decrease in antioxidant substances in the other oils after 10 days of storage.

Regarding the FRAP method, cotton (102.59 μM/100 g), LC (99.09 μM/100 g), and linseed (97.23 μM/100 g) oils showed higher antioxidant power. However, oscillations occurred during storage, possibly due to the presence of pro-oxidant compounds from crude linseed oil.

CONCLUSIONS

The oils underwent degradation during storage at 60°C/20 days, as peroxide, conjugated dienes, ρ-anisidine, and Totox values increased. However, it was found that the formation of the degradation compounds was lower in the compound oils, especially in LA. In the compound oils, unsaturated fatty acids predominated, and α-linolenic acid stood out. The oils showed reduction of phytosterols and tocopherols during storage, with higher retention in the compound oils, especially LC, with 95.1% of phytosterols, and LA, with 90.81% of tocopherols, at the end of storage. The oils showed significant antioxidant activity, probably due to the presence of phenolic compounds, phytosterols, and tocopherols. It is possible to conclude that cotton and coconut oils can be used in the formulation of compound oils, adding oxidative stability to linseed oil and, consequently, helping in the retention of compounds such as phytosterols and tocopherols.

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Publication Dates

• Publication in this collection
  2018

History

• Received
  03 Feb 2018
• Accepted
  27 Sept 2018