Abstract

Purpose: In this study, sunflower seeds (Helianthus annuus) were treated with an industrial microwave oven under 700 W for 8, 12, 16 and 20 min and oil was extracted using mechanical press technique. A suitable roasting treatment (20 min) is advantageous to oil extraction yield and tocopherol contents. The extracted oil results showed no variations in the contents of fiber, ash, and protein that were attributable to the roasting. However, the color, FFA, p-anisidine, saponification and density values of oils were increased significantly as the roasting time increased. The iodine values of the oils were noticeably decreased. The oxidative stability revealed that, as the roasting time increased, the oxidative stability of sunflower oil decreased. Tocopherol contents were identified, namely, α- and β- tocopherols, whereas no δ-tocopherol was detected. The main tocopherol found in sunflower oil was α-tocopherol. The content of α-tocopherol in sunflower oil at 15 min of roasting gradually increased from 895.1 to 1108.83 mg/kg as roasting time increased. The fatty acid compositions of sunflower oil did not change with the roasting time. The major fatty acid was linoleic acid.

1. Introduction

One of the most famous techniques of food preparation today is Microwave heating because of its suitability, rapidity and low cost ¹. Recently, the design of the Microwave is improving, and then the cost of electricity should stimulate new development trends and models of microwave heating and food properties as the reason for demonstrating work in research. Microwave heating system has been utilized in the food industry for normal heating, boiling, cooking, drying, pasteurization, sterilization and thawing various foodstuffs ^{2, 3}. George and others showed the application of Microwave heating in food processing that identified benefits and restrictions for an extensive variety of food manufacturing processes ⁴.

The mechanism of food substance warming in a microwave oven is resulted by interaction of electromagnetic field and the chemical constituents of food. The electromagnetic interactions instantaneously create heat regeneration due to molecular friction and excitation ^{5, 6}. The effect of microwaves on food ingredients have been conducted on numerous studies ⁷, and the influence of microwave and conventional heating on the nutrients, flavors and colors in foods have also been studied ⁸.

The effect of microwave heating on various animal and vegetable lipid constituents was well studied and confirmed ^{9, 10}. The level of value of quality deterioration such as oxidative degradation was influenced by the measure of polyunsaturated fatty acids amount in the oil ¹¹. The change in the chemical composition and levels of minor components affected the functional and nutritional properties of oils ¹². Several studies reported that; the maintenance of supplements and nutrients such as vitamins in food samples during microwave heating is enhanced when the roasting time is reduced.

On the other side, further reports demonstrated that the nutrients retention during microwave treatment was not much better than that of exposure to conventional heating ¹³.

Amount of tocopherol homologs (α-, β-, γ-, and δ-) in food acts as free radical scavengers during oil oxidation. The unsaturated fatty acids (FA) contents determines their amount in vegetable oils ¹⁴. Up to 1 g/kg⁻¹ of extra of tocopherols can be found in unsaturated oils, although most saturated oils contain nearly none. The determination of tocopherol homologs in sunflower oil is important because of its antioxidant effects and positive healthful physiological effects in human metabolism. Microwave roasting has been displayed to effect the thermal oxidation of tocopherols ^{12, 15}.

Sunflower oil is characterized to contain high amount of tocopherols (up to 935 ppm) higher than those of other oils such as soybean and peanut. It is considered as an oil with high stability due to its great content of natural antioxidants ^{16, 17}, although sunflower oil is sensitive and it can be easily oxidized during frying and roasting.

The objectives of this research were to determine the effects of microwave radiation on the chemical composition of sunflower seeds oil and to extend our understanding in regards to the changes in the distribution of fatty acids (FA) and tocopherols of oil, and to evaluate the relationship between oxidative oil index and minor components contents in extracted sunflower oil by various microwave roasting times

2. Materials and Methods

In this study Sunflower seeds samples were obtained from Shandong Lu hua Group LTD (Shandong, China). 300-500 g of the seed was regularly obtained. The crude oil of sunflower was extracted by mechanical method and kept away from the light, high temperature and oxygen in order to avoid auto-oxidation. Duplicates of seeds samples were collected.

Standards of tocopherols [DL-α-tocopherol, (+)-δ-tocopherol, and (+)-γ-tocopherol] and FAME were purchased from Roche Vitamins Inc –Parsippany, New Jersey, 07054-USA. All other chemicals and solvents used were of analytical grade.

Each sample (500 g) was placed in a turn vessel and then was roasted in an industrial microwave oven (Nanjing Jiequan Microwave Development Co, Ltd) at a frequency of 700 MHz (oven capable of generating 130°C) for 8, 12, 16 and 20 min. After roasting, sunflower seeds were permitted to cool to ambient temperature and blended before grinding and oil extraction.

Moisture content of raw and roasted samples were determined after each exposure time and considered as obvious moisture, and then were calculated by dividing the weight of moisture loss by the weight of the sample before roasting.

The microwaved batch (500g) of sunflower seeds were quickly subjected to the mechanical press (Korea Hydraulic Oil Press) and pressed at 600 kg/cm for 20 min to obtain the sunflower oil. The unroasted sunflower oil was prepared by the same process as described above. The extracted sunflower oil was filtered with filter bag under vacuum to eliminate impurities.

Protein content was determined according to the AOAC VA, Method 28.110 ²² using a micro-Kjeldhal apparatus. Each meal sample (2 g) was digested for about an hour with 2 g of digestion mixture (Cu, Fe, and K sulfates in the ratio of 9:1:90 by wt.) and 10 mL of concentrated H₂SO_4. When the digestion was completed, the solution became clear and was then made up to 100.0 mL in a volumetric flask with distilled water.

For the nitrogen determination, 5 mL of 2% boric acid solution was first taken in a beaker with a few drops of methyl red as indicator. Then 10 mL of the digested mixture, 10 mL of 40% NaOH solution, and 10 mL of distilled water were transferred to the distillation chamber. Ammonia was liberated, and it combined with NaOH to form NH₄OH, which was then received into the boric acid solution to form ammonium borate (from pink color to yellow). Distillate (ammonium borate) was then titrated with 0.1 N H₂SO₄. The volume of acid that had been added at the point when the color of the distillate changed from yellow to pink was recorded. Protein was calculated according to the following formula: %protein = %N ×6.25.

Fiber contents were determined according to the standard International Standard Organization procedure, Standard No. 5983 ²³. Two grams of finely ground defatted meal were weighed and then boiled with 250 mL of 0.1275 N H₂SO₄, followed by the separation and washing of insoluble residues. The residues were then boiled with 250 mL of 0.313 92 N NaOH followed by the separation, washing, and drying of residues. The dried residues were weighed and ashed in a muffle furnace (Shanghai Instruments Co., Ltd) at 600°C, and the loss of mass was determined gravimetrically.

Ash contents were determined by the standard ISO method, Standard No. 749 ²⁴. Two grams of meal were carbonized by heating on a gas flame and then ashed in an electric muffle furnace at 600°C until a constant mass was achieved.

Determination of density, refractive index, FFA, PV, iodine value, saponification value, and p-anisidine value of the extracted oil was carried out according to the standard IUPAC, Method 2.301 ²¹. Color was determined in duplicate, using Lovibond PFX880 Tintometer according to the official AOCS method. The optical path length of the glass cell was ‘1’.

A Rancimat model 743 (Metrohm–Swiss) was used to determine the oxidative oil index. The tests were done with 3 g oil samples at temperatures of 120°C and an airflow rate of 20 L/h ¹⁹.

A chromatographic system consisting of a Waters 1525 binary pump (Waters, Milford, USA), 40µL injection loop and photodiode array detector (Waters, USA) was used to determine tocopherols content (VE). Lichrospher Si-60 column (25092.0, 5µm, Hanbon Science and Technology, China) was used for separation. The mobile phase used was a mixture of n-hexane–isopropanol (98.5:1.5 v/v) at a flow rate of 0.15 mL/min.

The signal was measured at wavelength 295 nm ²⁰. Mixed tocopherols (α-, β-, γ-, and δ-) were used as standard to measure the retention time of each of these compounds. The standard was prepared using n-hexane at a concentration of 0.01 g mL−¹ .One gram of each sample was weighed into a 10-mL n-hexane actinic glass volumetric flask. The samples were brought to volume with n-hexane before injection of 20µL onto HPLC column. By the end of each analysis, isopropanol was pumped through the HPLC column for 30 min and n-hexane also for 30 min and this removed the more polar oil components on the column during the detection. Afterwards this washing step, the mobile phase was pumped through the column to get equilibration before injection of samples.

FAME were prepared by IUPAC standard, Method 2.301 ²¹ and analyzed on a Shimadzu gas chromatograph model 2010PLUS equipped with a TR-FAME 260M154P (Thermo Scientific) methyl lignocerate-coated (film thickness 0.25 µm) polar capillary column (60 m ×0.25 mm) and an FID. The carrier gas was nitrogen, and the total gas flow rate was 25 mL/min. Other conditions were as follows: initial oven temperature, 60°C; ramp rate, 5°C/min; final temperature, 220°C; injector temperature 250°C; detector temperature, 250°C. FAME were identified by comparing their relative and absolute retention times with those of authentic standards. Quantification was done by a Chromatography Station for Windows (CSW32) data-handling program (Data APEX Ltd., Prague, The Czech Republic). The FA composition was reported as a relative percentage of the total peak area. The internal standard was nonadecanoic acid.

Each reported value is the mean of determinations for triplicate samples prepared from each roasting condition, and the data were analyzed by ANOVA and Duncan’s multiple range test (Duncan’s test). Statistical significance was accepted at a level of P < 0.05 using SPSS 16.0 for Windows (SPSS Inc., Chicago, USA).

3. Results and Discussion

The proximate analysis results of the unroasted (control sample) and roasted sunflower seeds were shown in Table 1, the petroleum ether-extracted oil contents of unroasted seeds was 37.93%. In roasting seeds at interval times 8, 12, 16 and 20 min, the oil content increased significantly by time (P < 0.05).

The moisture content of the unroasted sunflower seeds oil was 5.69 %. Contents of protein, fiber, and ash were 24.20, 00, and 2.43%, respectively.

Microwave roasting of seeds did not influence the fiber, ash, and protein contents significantly (P > 0.05). The loss in the weight of seeds after roasting process at interval times 8, 12, 16 and 20 min amounted to 1.71, 0.68, 0.69, and 0.41, %, respectively. With increasing roasting time, the loss in weight of the seeds was clearly higher. ^{5, 26} reported that an increasing in the roasting time of sunflower seeds resulted in significant increasing in weight loss, which is reliable with our results. This loss in weight might reveal total volatile compounds, however it was considered to be mostly due to the loss in moisture content. In the present investigation, the loss in weight of sunflower seeds oil was found to be larger if compared with those results reported by Yoshida ⁵, which might be attributable to deviations in the genetic manipulation and the original moisture contents of the types of sunflower seeds oil inspected. Some authors reported that after roasting for frequent times, the weight variations of peanut and sesame seeds might have occurred due to the existence of moisture and volatile constituents ^{15, 26}. In contrast, previous investigation studied showed no significant differences in the weight loss between different cultivars of soybeans at different times of roasting ¹⁵. Oomah & Mazza, reported that microwave oven drying might be used as a rapid method for moisture determination in seeds oil such as flax, canola, and mustard. Statistical analysis presented weight loss increasing significantly according to increasing in roasting time ¹⁵.

Physical and chemical parameters of the sunflower seed oils before and after microwave treatment were summarized in Table 2, the refractive index of the control oil from raw sunflower seeds was 1.4675. As predictable, no significant differences in refractive index between raw and roasted sunflower seeds at different roasting times (8−20 min) were recorded in this study.

The density of the control oils was 0.9081 mg·mL⁻¹. The densities of the oils gradually increased as roasting time increased. After 20 min of roasting, the densities increased beyond the original value. This increase in values might be due to the occurrence of polymerization, which makes the oil denser.

The color progress of sunflower oil which was extracted at different roasting times, changed gradually from light yellow to yellow and then to a brown color (16 and 20 min of roasting). Therefore, with an increase in the roasting time, browning substances were significantly increased. Browning substances were very polar because of active radicals. The longer the microwave treatment time, the larger the strengthening of the color. The formation of browning substances in numerous thermally processed foods resulted from Maillard-non-enzymatic reactions type, caramelization and phospholipids degradation increased with the increasing roasting time ³⁰. Previous studies have reported that the positive change in the roasting time and temperature of seeds such as rice germ and sesame seeds resulted in a significant increase in the oil’s color ^{26, 31, 32}. Megahed reported that oil extracted from peanuts revealed gradual darkening and higher Lovibond color changed with increasing of heating time ³³. Hafez proved that TAG were slightly hydrolyzed by microwaves to produce FFA; and an increase in microwave roasting time was accompanied by an increase in the browning substances and phospholipids degradation, which might be attributed to the increase of polar lipids ³⁴. Phospholipids were demonstrated to cause browning of the oil during roasting treatment. Subsequently, increase in browning substances might be attributable to the increase in contents of other lipids, such as glyceroglycolipids in the oil. A previous study reported that color intensity increases with the formation of browning substances, as a result of phospholipid degradation during microwave heating, which was in range with our results ³⁶.

Saponification value of the control sunflower oil sample was 173.02 mg·g⁻¹. After 20 min of microwave heating of sunflower seeds, the saponification values of the extracted oils were lower than the original values. The more the roasting time, the lesser the saponification value. Saponification value significantly decreased (P < 0.05), after 8, 12, 16 and 20 min of microwaved sunflower oil.

The free fatty acids (FFA) tests evidently revealed that, as the roasting time increased, the FFA contents of sunflower oil increased significantly (P < 0.05), these findings are in agreement with Yoshida, who reported that FFA in sunflower seed oil increased with increasing of roasting time ³⁷. The increase in FFA of the oil might be attributed to hydrolysis of TAG by microwaves to produce FFA and DAG, as reported for olive oil, peanuts and sesame seeds ^{6, 12, 27}. Fukuda reported that heated sesame oil contained high FFA than other refined vegetable oils ³⁹.

Iodine value of control oil was 122 g of I/100 g of oil. The iodine values of the extracted oils were decreased after roasting. Anjum reported that the decrease in iodine value of the oil might be attributed to the reduction in the number of unsaturation sites due to oxidation, polymerization, or breakage of the long-chain FA ⁴⁰. Nevertheless, Jung reported that the iodine values of red pepper seeds oils did not change by roasting time ⁴¹. Some authors found that for sunflower seeds and peanut oils, roasting caused a significant decrease (P < 0.05) in molecular species containing more than four double bonds ^{25, 42}.

Iodine value is frequently utilized as measurement of stability of oil after roasting. Most of unsaturated oils are unstable to thermal oxidation. More exact, the amounts of PUFA such as linoleic and linolenic acids act as indication of stability, which have more than one double bond in the molecule structure. Linoleic and linolenic oxidize faster than oleic acid, which has one double bond. Therefore, if there are two oils with the same iodine value, the oil with the higher linoleic acid content will oxidize more rapidly than the oil with the higher oleic acid content. The roasting influenced the iodine values of sunflower oils significantly (P < 0.05).

Figure 1 elucidates the oil oxidation stability index of the oils obtained from control and roasted sunflower seeds samples. OSI tests evidently showed that as the roasting time increased, the oxidative stability of sunflower oil was improved. The OSI of control sunflower oil was higher than the microwaved oil, because it contains high amounts of VE, anti-oxidative phenolic compounds and might be attributable to its fatty acids composition being more resistant to oxidation. The OSI was significantly decreased after roasting.

Figure 2 shows the peroxide and p-anisidine values, PV considered as indicator for the extent of formation of initial oxidation factors in oils while the p-anisidine value reflects the degree of secondary oxidation compounds development ²⁸. There was noticeable increase in PV and p-anisidine values after 12 min of roasting, and more clearly differences (P < 0.05) were detected after 20 min of roasting. Yoshida demonstrated that a slight enhancement in PV and p-anisidine value in peanut seed oil after 30 min of roasting, and a progressive increase with longer roasting time in sesame seed oil ^{12, 26, 27}. Commonly, PV does not elucidate the entire state of oxidation of oil because hydro-peroxide was unstable during heating; this resulted in rapid transformation to secondary products. On the other hand, the release of secondary oxidation compounds during microwave roasting was obviously low. The most sensitive parameters that are responsible for changing the chemical properties of refined fats that are heated in a microwave oven is p-anisidine value ⁴⁰. Lee proposed that an increase in the roasting time, PV of safflower oil was developed. Some of the undesirable and unsafe compounds such as oxidation products and pigments can be produced during microwave roasting of peanuts ³⁰.

The contents of the individual tocopherols in oils are arranged at different roasting times that were included in Figure 3, the major tocopherol that was found in sunflower oil was α-tocopherol, but no presence of δ tocopherols was detected. The content of α-tocopherol in sunflower oil gradually (P < 0.05) increased as roasting time amplified. For example, the contents of α-tocopherol in sunflower oils which was roasted at 8, 12, 16 and 20 min were 863.04 ±3.72, 1047.4±15.84, and 1108.83±1.14 and 1286.83±5.71 mg/kg respectively. Comparable trends were detected in β-, and γ-tocopherols. Yoshida reported that the content of tocopherol in sesame oil that was prepared by microwave oven heating decayed during the time ¹². Yen reported that the level of tocopherol in sesame oils prepared by electric oven and heating increased by roasting at temperatures up to 200°C ³¹. Kim also reported that the content of α-tocopherol in rice germ oil increased as roasting temperature and time increased. These results propose that the tissues damage occurred due to the heating, and this caused rapid release of tocopherols ³².

The oil stability, physical properties, and nutritional quality can be determined through FA composition. There were almost no differences in FA structure of microwaved sunflower oils (Figure 4, Figure 5, Figure 6 and Figure 7). Sunflower oil (control) contained was 4.76% palmitic, 6.51% stearic, 28.36% oleic and 57.91% linoleic. Sunflower oil from roasted seeds for 20 min contained 4.65% palmitic, 6.55% stearic, 28.45% oleic and 57.12% linoleic. Previous studies have revealed that there was no differences in FA compositions of rice germ and sesame seed oils which were prepared at different roasting temperatures with time interval ^{31, 32, 45}. Our results of the FA analysis exhibited no formation of any trans FA during microwave roasting.

4. Conclusion

The chemical composition and oxidative stability of sunflower oil obtained from the roasted seeds at different roasting times (8, 12, 16 and 20 min) were assessed and compared with that of unroasted sunflower oil. Color, FFA, p-anisidine and tocopherols contents of oils were varied through the roasting time. Nevertheless, fatty acids composition of sunflower oils did not change with microwave heating. Commercial sunflower with 37.93% oil content is consider one of the important types of sunflower grown in China. The oil stability index showed that, as the roasting time increased, the oil stability index of sunflower oil increased.

Acknowledgements

This work was supported by the Natural Science Foundation of China (31671786), the Research Fund of National 13th Five-Year Plan of China (2016YFD0401404), Northern Jiangsu province science and technology projects (BN2016137), and the Fundamental Research Funds for the Central Universities (JUSRP51501).

References