The influence of a microwave (MV) pre-treatment (3, 6, 9 min, 800W) on the physicochemical properties of high-oleic rapeseed oil prepared from intact (HORO) and de-hulled seeds (DHORO) was investigated in this study. A control DHORO contained higher levels of total tocopherols and carotenoids, while higher concentrations of total phenolic compounds and chlorophylls were detected in the HORO. The MV pre-treatment caused a decrease in the unsaturated fatty acids content that was more evident for the DHOROs. The microwaving time significantly affected phytochemical contents and the color of both types of oils. A vast increase in canolol concentration was noticeable following 9 min of microwaving, which increased 506- and 155-fold in the HORO and DHORO, respectively. At the same time, the antioxidant capacity of oil produced from MV pre-treated seeds for 9 min was nearly 4 times higher than that of the control oil for both types of oils.
Rapeseed/canola now occupies the third position in rank order for the production of oils and fats after palm and soybean oil. EU-27, China and India dominate the production and consumption of this oil, whereas Canada is an important grower and exporter of rapeseeds (FAOSTAT,
The studies investigating the effect of low-erucic rapeseed (LEAR, canola) hulling are few and contradictory. Wroniak
The effect of soybean and sunflower seed microwave pre-treatment on the bioactive compound contents, oxidative stability and color change in the oil has been well documented (Anjum
This study was performed to provide information on the effect of hulling of high-oleic rapeseed on the physicochemical properties of the oil. De-hulling can improve the color, flavor and nutritional value of the oil, and therefore, the phytochemical contents and oil color change following rapeseed hulling were determined. Since high-oleic rapeseed oils are recommended for processes where high stability of the oil is required such as deep-fat frying, this work focused on the impact of the intact and de-hulled seeds’ thermal pre-treatment by microwave on the extent of the oil’s oxidation, measured by quantitative changes in fatty acid content, formation of primary and secondary oxidation products and the oxidative stability.
Therefore, the aims of this study were to (1) investigate the effect of hulling on the physicochemical composition of the resulting oil; (2) investigate the oxidative resistance to oxidation of the oil pressed from high-oleic rapeseeds.
Seeds of high-oleic rapeseed were provided by the Złoto Polskie CLP (Kalisz, Poland). Seeds were harvested at optimum maturity, and did not contain any impurities or broken seeds. The moisture content of the rapeseeds was 6.1%.
Analytical standards of tocopherols (> 95%), HPLC-grade n-hexane, methanol, acetonitrile (ACN), orthophosphoric acid, and 1,4-dioxane were provided by Calbiochem-Merck Biosciences (Darmstadt, Germany). KOH in methanol (1M), hexane, sodium methoxide (0.4N), 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) and (±)-6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox), were purchased from Sigma-Aldrich (St. Louis, MO, USA). All other solvents and chemicals used in this study were of analytical grade.
The moisture content of the seeds was determined based on a precision weighing balance, using an Electronic Moisture Analyzer (Kern & Sohn GmbH, Germany). The seeds (batches of 500g) were sprayed with a pre-calculated amount of water, mixed thoroughly, sealed in polyethylene bags and equilibrated at 4 ± 2 °C for 72 h. Based on the results presented in our previous work (Rękas
Mechanical hulling of the rapeseeds was performed using a shearing disc sheller equipped with cylindrical blades, developed by Anders (
For each microwave (MV) pre-treatment, 500 g of seeds were placed in a glass beaker (16-cm diameter) inside the microwave (Model: NN-J155W). The seeds were exposed to microwave irradiation for 3, 6, and 9 min (2450 MHz, 800 W). Intact and de-hulled high-oleic rapeseed samples without microwave radiation (0 min radiation time) were used a control sample. Each experiment was performed in triplicate for all variants of the microwave radiation. Following each heating run, seeds were allowed to cool to ambient temperature and thoroughly mixed to obtain a homogenous sampling.
Pressing was carried out by applying the screw press method (Farmet, Czech Republic), where the temperature was kept below 40 °C. Once produced, the oil was stored at 4 °C overnight in the dark until analyzed.
Tocochromanols (α-, β-, γ-, and δ-tocopherol and plastochromanol-8) and canolol were determined according to the method described by Siger
The total phenolic content was determined by the Folin-Ciocalteau colorimetric method (Koski
Fatty acid methyl esters (FAMEs) were prepared using sodium methoxide (CH3ONa, 0.4N) as a catalyst, following the method described previously (Rękas
The total chlorophyll content (mg/kg) was quantified by spectrophotometry according to the AOCS Method (
The non-enzymatic browning index was assayed spectrophotometrically for oil samples diluted in chloroform at 420 nm (Yoshida
The CIE Lab coordinates (L*, a*, b*) were directly read with a spectrophotometer (CM-3600d, Konica Minolta, Japan). Color difference (∆E) was calculated as ∆E = [(L0*-L*)2+(a0*-a*)2+(b0*-b*)2]1/2, where L0*, a0*, and b0* are the color parameters of the control HORO and DHORO.
The peroxide value (PV), p-anisidine value (p-AnV), and K232 and K268 extinction coefficients, were measured following ISO standard methods (3960:2005; 6885:2008; 3656:2011).
Oxidative stability, expressed as the oxidation induction period (hours), was measured with the Rancimat apparatus (model 743 Metrohm Co, Herisau, Switzerland) using an oil sample of 2.5 g warmed to 120 °C, and an air flow of 20 l/h. All determinations were carried out in triplicate.
The radical scavenging capacity (RSC) of the oil sample was analyzed using the DPPH radical-scavenging following the method described by Tuberoso
A statistical analysis was carried out with Statistica v. 12 software (StatSoft, Inc., Tulsa, OK, USA). Statistical differences among the oil samples were estimated by applying one-way ANOVA and using the Tukey test at a significance level of p < 0.05.
The contents of the individual tocopherol homologues in the HOROs and DHOROs prepared at the different MV conditions, as well as in the cold-pressed oil, are given in
Tocochromanols, canolol and total phenolic compound concentrations (mg 100g−1) of high-oleic rapeseed oils produced from microwave pre-treated intact and de-hulled seeds
Oil source | HORO | DHORO | ||||||
---|---|---|---|---|---|---|---|---|
Microwaving time (min) | 0 | 3 | 6 | 9 | 0 | 3 | 6 | 9 |
α-Tocopherol | 25.58 ± 0.07 |
25.40 ± 0.02 |
25.59 ± 0.03 |
24.70 ± 0.14 |
25.33 ± 0.02 |
25.81 ± 0.07 |
25.57 ± 0.11 |
25.11 ± 0.00 |
β-Tocopherol | 0.12 ± 0.01 |
0.09 ± 0.01 |
0.09 ± 0.01 |
0.08 ± 0.01 |
0.11 ± 0.01 |
0.11 ± 0.01 |
0.10 ± 0.01 |
0.06 ± 0.01 |
γ-Tocopherol | 36.44 ± 0.03 |
36.81 ± 5.17 |
36.32 ± 0.20 |
44.60 ± 0.24 |
36.60 ± 0.12 |
36.76 ± 0.07 |
37.31 ± 0.05 |
45.37 ± 0.12 |
δ-Tocopherol | 0.58 ± 0.01 |
0.60 ± 0.03 |
0.69 ± 0.02 |
0.65 ± 0.01 |
0.72 ± 0.01 |
0.71 ± 0.01 |
0.74 ± 0.01 |
0.66 ± 0.02 |
Total tocopherols | 62.70 ± 0.06 |
61.89 ± 0.23 |
62.68 ± 0.23 |
70.03 ± 0.40 |
62.74 ± 0.10 |
63.38 ± 0.14 |
63.71 ± 0.07 |
71.19 ± 0.10 |
Plastochromanol-8 | 2.49 ± 0.05 |
3.49 ± 0.05 |
5.46 ± 0.11 |
5.49 ± 0.03 |
2.78 ± 0.06 |
4.29 ± 0.04 |
5.68 ± 0.20 |
5.76 ± 0.08 |
Canolol | 0.32 ± 0.09 |
0.74 ± 0.09 |
9.59 ± 0.09 |
162.00 ± 0.74 |
0.41 ± 0.04 |
0.85 ± 0.17 |
3.64 ± 0.16 |
63.52 ± 4.27 |
Total phenolic compounds | 0.57 ± 2.31 |
0.98 ± 3.02 |
11.45 ± 1.12 |
165.45 ± 5.05 |
0.76 ± 0.34 |
1.15 ± 4.31 |
5.65 ± 4.04 |
68.67 ± 3.34 |
Denotes statistically significant differences (p < 0.05) within a group of oils pressed from intact seeds.
Denotes statistically significant differences (p < 0.05) within a group of oils pressed from de-hulled seeds.
Since tocopherols are thermal-sensitive, it was expected that the oil from MV seeds would have lower tocopherol concentrations. However, as the seeds’ MV pre-treatment time increased, the total tocopherol content of the oil increased. A maximum total tocopherol concentration in the oil was achieved following 9 min of seed microwaving, where 70.03 and 71.19 mg·100g−1 of total tocopherols were determined in the HORO and DHORO, respectively. However, α- and β-T were found to decrease with longer seed exposure to MV. At the same time, an increase in γ- and δ-T was noticed (
Although the levels of total phenolics and canolol were higher in the control DHORO (0.41 and 0.76 mg 100g−1, respectively), with a longer seed exposure to MV, a greater increase in these compound concentrations was found in the HOROs (
Color development of high-oleic rapeseed oils produced from microwave pre-treated intact and de-hulled seeds
Oil source | HORO | DHORO | ||||||
---|---|---|---|---|---|---|---|---|
Microwaving time (min) | 0 | 3 | 6 | 9 | 0 | 3 | 6 | 9 |
1.08 ± 0.09 |
3.03 ± 0.09 |
4.13 ± 0.11 |
5.17 ± 0.13 |
0.61 ± 0.04 |
2.07 ± 0.12 |
2.75 ± 0.22 |
4.32 ± 0.17 |
|
6.31 ± 0.31 |
7.98 ± 0.02 |
8.81 ± 0.12 |
8.63 ± 0.05 |
7.82 ± 0.34 |
7.99 ± 0.12 |
10.31 ± 0.04 |
10.02 ± 0.34 |
|
0.090 ± 0.03 |
0.144 ± 0.05 |
0.166 ± 0.00 |
0.196 ± 0.01 |
0.100 ± 0.00 |
0.145 ± 0.01 |
0.167 ± 0.09 |
0.176 ± 0.05 |
|
96.99 ± 0.02 |
96.06 ± 0.01 |
95.55 ± 0.01 |
95.34 ± 0.00 |
97.21 ± 0.01 |
97.03 ± 0.02 |
96.15 ± 0.00 |
95.59 ± 0.01 |
|
-3.31 ± 0.01 |
-3.66 ± 0.01 |
-4.15 ± 0.00 |
-4.84 ± 0.01 |
-3.06 ± 0.01 |
-3.16 ± 0.01 |
-3.57 ± 0.01 |
-3.81 ± 0.01 |
|
27.82 ± 0.07 |
30.05 ± 0.01 |
49.86 ± 0.04 |
45.28 ± 0.03 |
29.88 ± 0.13 |
33.43 ± 0.03 |
47.87 ± 0.04 |
52.63 ± 0.11 |
|
- | 2.44 ± 0.03 |
22.10 ± 0.03 |
17.60 ± 0.02 |
- | 3.56 ± 0.02 |
18.03 ± 0.03 |
22.82 ± 0.06 |
L* lightness of the sample (0 = black, 100 = white); a* indicates redness by positive or greenness by negative; b* indicates yellowness by positive or blueness by negative; ∆E color difference
Denotes statistically significant differences (p < 0.05) within a group of oils pressed from intact seeds.
Denotes statistically significant differences (p < 0.05) within a group of oils pressed from de-hulled seeds.
Chlorophyll and carotenoids, together with membrane proteins and membrane lipids, form a membrane-bound compartment called thylakoid. Crude rapeseed oil may contain as much as 95mg kg−1 total carotenoids. Unlike carotenoids, chlorophyll and its derivative (pheophytin) are not wanted in oils because they produce an undesirable green hue in the oil, in addition to the pro-oxidative effect of chlorophylls in oil exposed to light. The level of chlorophylls in the crude rapeseed oil may vary from 5 to 25 ppm (Przybylski,
A color change in the oil was the most noticeable effect of rapeseed de-hulling. The DHORO had a bright yellowish color, suggesting that the coloring compounds, especially those responsible for the undesirable green hue, were significantly removed from the seeds by de-hulling (Abou-Gharbia
High oleic rapeseed oil combines high oxidation stability with the lowest saturated fatty acid contents among other commercial edible oils (Merrill
The de-hulling of rapeseed prior to microwaving affected fatty acids’ susceptibility to thermal degradation (
The effect of microwave pre-treatment on the changes in fatty acid contents (mg 100g−1) of high-oleic rapeseed oils.
As shown in
Oxidative stability parameters and antioxidant capacity (mmol TEAC l−1) of rapeseed oils produced from microwave pre-treated intact and de-hulled seeds.
Oxidative stability parameters |
Antioxidant capacity (mmol TEAC l−1) |
||||||||
---|---|---|---|---|---|---|---|---|---|
Oil source | Microwaving time (min) | PV (meq O2·kg−1) | IP (h) | HF | LF | TF | |||
HORO | 0 | 0.93 ± 0.09 |
0.17 ± 0.03 |
1.31 ± 0.046 |
0.067 ± 0.001 |
6.72 ± 0.01 |
0.62 ± 0.05 |
0.86 ± 0.04 |
1.54 ± 0.01 |
3 | 1.25 ± 0.16 |
0.13 ± 0.03 |
1.35 ± 0.052 |
0.074 ± 0.009 |
7.99 ± 0.08 |
0.98 ± 0.11 |
1.13 ± 0.04 |
2.21 ± 0.08 |
|
6 | 1.31 ± 0.03 |
0.26 ± 0.05 |
1.41 ± 0.023 |
0.128 ± 0.013 |
8.74 ± 0.23 |
2.33 ± 0.04 |
1.32 ± 0.03 |
3.86 ± 0.04 |
|
9 | 1.69 ± 0.18 |
1.26 ± 0.09 |
2.03 ± 0.014 |
0.565 ± 0.004 |
11.94 ± 0.31 |
5.17 ± 0.06 |
1.49 ± 0.09 |
6.89 ± 0.06 |
|
DHORO | 0 | 1.05 ± 0.12 |
0.06 ± 0.03 |
1.28 ± 0.052 |
0.066 ± 0.003 |
5.69 ± 0.04 |
0.54 ± 0.01 |
0.94 ± 0.04 |
1.62 ± 0.04 |
3 | 1.42 ± 0.09 |
0.25 ± 0.06 |
1.33 ± 0.025 |
0.072 ± 0.007 |
6.59 ± 0.23 |
0.93 ± 0.03 |
1.18 ± 0.07 |
2.36 ± 0.13 |
|
6 | 1.51 ± 0.04 |
0.61 ± 0.07 |
1.42 ± 0.023 |
0.132 ± 0.002 |
8.49 ± 0.08 |
2.15 ± 0.09 |
1.28 ± 0.01 |
3.52 ± 0.04 |
|
9 | 3.12 ± 0.20 |
2.27 ± 0.01 |
2.58 ± 0.011 |
0.975 ± 0.011 |
10.58 ± 0.04 |
4.14 ± 0.12 |
1.72 ± 0.02 |
5.94 ± 0.02 |
PV peroxide value;
Antioxidant activity of: hydrophilic fraction (HF); lipophilic fraction (LF) and whole oil (TF)
Denotes statistically significant differences (p < 0.05) within a group of oils pressed from intact seeds.
Denotes statistically significant differences (p < 0.05) within a group of oils pressed from de-hulled seeds.
The antioxidant capacity of the methanol soluble phase (HF) was higher for the control HORO, while a higher TEAC value of the insoluble in methanol fraction (LF) was calculated for the control DHORO (
In general, the rapeseed oil lipophilic fraction contains mainly tocopherols, carotenoids and phospholipids, whereas phenolic compounds are the major constituents of the hydrophilic fraction. The relative antioxidant activity
The antioxidant activity of phenolic acids and their esters depends on the number of hydroxyl groups in the molecule. Monophenols are less efficient antioxidants than polyphenols, the presence of a secondary hydroxyl group in the
The heat pre-treatment of oilseeds leads to the formation of new antioxidants, including canolol and Maillard reaction products. Canolol (2,6-dimethoxy-4-vinylphenol) classified as phenolic compound has gained interest in recent years due to antioxidant, anticancer, anti-inflammatory, and antibacterial activities. There are a number of scientific evidences that canolol is a potent antioxidant and anti-mutagenic compound. Galano
Iimproved oxidative stability and antioxidant capacity of the mustard and rapeseed oils were also found to correlate well with increased contents of phosphorus and phospholipids (Shrestha
De-hulling high-oleic rapeseed prior to pressing significantly affected extractability of the bioactive compounds, no such effect was found when the oxidative state of the oil was analyzed. The applied microwave pre-treatment altered the contents of tocopherols, plastochromanol-8, total carotenoids, whereas a remarkable increase in the canolol concentration was noted with longer seed exposition to microwaves. After 9 min of seed MV pre-treatment, a marked increase in the oxidative stability was noted, which was approx. 2-fold higher in relation to the control oils prepared from intact and de-hulled seeds. At the same time, the antioxidant capacity of the oils produced from MV pre-treated seeds for 9 min was nearly 4-fold higher than that of the control HORO and DHORO. With the increase in MV pre-treatment time the formation of hydroperoxides and their degradation products was noticeable. Additionally, prolonged seed heating resulted in unsaturated fatty acid degradation, which was higher in DHOROs than in the HOROs. Although high-oleic rapeseed de-hulling in conjunction with microwaving enabled the production of oil which was high in antioxidant compounds, undesirable changes such as lipid oxidation, darkened color and altered flavor must be monitored to ensure high quality of the oil.
The authors would like to thank Dr. Andrzej Anders from University of Warmia and Mazury for the performance of rapeseed mechanical hulling. The authors also thank the Złoto Polskie CLP for providing the high-oleic rapeseed samples.
The authors have declared no conflict of interest.