2.1 Materials

The research test samples were sesame oilseed (Sesamum indicum L.) and BARI Sarisha-14 mustard oilseed (Brassica rapa), both locally grown and purchased from Katakhali Bazar, Rajshahi, Bangladesh. Most chemicals used were of analytical grade, with some being GC grade. A FAME mix standard of 37 compounds was obtained from Supelco (Sigma-Aldrich, USA).

2.2 Extraction of oils and preparation of blends

Locally purchased oil seeds were manually cleaned, sundried and ground. The oil extraction process was performed using a Soxhlet extraction unit (Model D3165, KOTTERMANN, Germany) and yield percentages was determined as following (Agroindustriais 2013): $$\mathrm{O}\mathrm{i}\mathrm{l}\mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}=\frac{\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{o}\mathrm{i}\mathrm{l}}{\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{l}}\times 100$$

Sesame oil (SO) and mustard oil (MO) were weighed separately and mixed at four different ratios: 50:50, 60:40, 70:30 and 80:20 (SO:MO); following the method (Yeasmin et al. 2021) (Scheme 1).

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Scheme 1

Experimental setup and schematic diagram for the preparation of blended oil.

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2.3 Characterization

Physico-chemical characterization was carried out following the AOAC method (Agroindustriais 2013) with slight modification in some cases, to monitor oil quality and edibility. The instruments used to measure flashpoint, fatty acids and tocopherols, trace metals and p-anisidine value were flash point apparatus (NCL 440, NORMALAB, Belgium), GCMS (GCMS-QP-2020, Shimadju, Japan), ICPMS (NexION 2000, PerkinElmer, USA) and oil fat analyzer (CDR Food Lab, Italy) respectively. Aflatoxin of oil was detected applying effective Enzyme Linked Immunosorbent Assay (ELISA) test following by the method (Qi et al. 2019).

2.4 Statistical analysis

Statistical analyses were performed applying SPSS software version 22 to understand significant difference to all parameters. Variables were compared by one way analysis of variance (ANOVA) with post-hoc Duncan Multiple Rank Test (DMRT).

3.1 Fatty acid percentage and fatty acids ratio analyses

Table 1 presents the fatty acid percentages and ratios for each individual oil and their four blends. Sesame oil comprises six fatty acids, while mustard oil contains nine. Major common fatty acids in both oils include palmitic (16:0), linoleic (18:2), oleic (18:1), and stearic (18:0), with minor common ones being 13-octadecenoic (18:1) and heneicosanoic acids (21:0). Notably, erucic (22:1), gondoic (20:1), and linolenic (18:3) acids are exclusive to mustard oil. Linolenic acid, an essential ω-3 polyunsaturated fatty acid crucial for brain health and heart function, is absent in sesame oil. Gondoic and erucic acids, monounsaturated omega-9 fatty acids, offer diverse health benefits such as skin radiance, neuroprotection, and anti-inflammatory properties (Farag and Gad 2022), enhancing the appeal of sesame oil when blended with mustard oil.

Sesame oil exhibits higher levels of saturated palmitic (22 %) and stearic (19.3 %) acids compared to mustard oil, which contains lower amounts of saturated palmitic (8.32 %) and stearic (3.6 %) acids. Conversely, mustard oil shows higher concentrations of unsaturated oleic acid (23.52 %), linolenic acid (5.31 %), and erucic acid (38.52 %). In the blends, palmitic, linoleic, and oleic acids significantly (p < 0.05) increased from 17.50 % to 19.74 %, 18.58 % to 31.05 %, and 21.86 % to 33.25 %, respectively. The blends also showed variations in other fatty acids such as 13-octadecenoic, stearic, linolenic, gondoic, and heneicosanoic acids. Erucic acid content in the blends ranged from 19.44 % to 4.22 %, with the SO80:MO20 blend containing the lowest level, well below the permissible range (5–5.1 %) (Downey 1983; Khansili and Rattu 2017), making it superior for edible purposes.

The percentages of SFA, MUFA and PUFA also depicted graphically in Fig. 1. Sesame oil exhibits a higher proportion of SFA (43.774 %), a lower proportion of PUFA (25.127 %), and a moderate proportion of MUFA (31.098 %). Conversely, mustard oil contains a very low percentage of SFA (13.199 %) and PUFA (22.076 %), but a notably high percentage of MUFA (64.725 %). Blending two oils at different ratios results in a significant decrease (p < 0.05) in SFA content in sesame oil and MUFA content in mustard oil, while PUFA content significantly increases (p < 0.05) from the SO50:MO50 blend to the SO80:MO20 blend. At the SO80:MO20 blend, the percentages of SFA and PUFA are almost equal (29.58 % and 31.781 %, respectively), with MUFA slightly higher (38.639 %). Fatty acid composition plays a crucial role in oil stability, with oils containing higher unsaturation and lower saturation being more prone to oxidation and rapid spoilage (de Camargo et al. 2016).

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Fig. 1

Comparative analysis of fatty acid percentages in various oil samples.

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The fatty acid ratios (SFA:MUFA:PUFA) are presented in Table 2 for sesame oil, mustard oil, SO50:MO50, SO60:MO40, SO70:MO30 and SO80:MO20 blended oils. Among these, blend SO80:MO20 exhibits a balanced fatty acid ratio (1:1.3:1) closer to the recommended guidelines for health agencies and is thus more suitable for edible purposes. Based on the results, it can be concluded that among the four blends of sesame oil and mustard oil, the SO80:MO20 blend achieved an optimal fatty acid ratio. Additionally, its erucic acid content of 4.22 % was below the permissible threshold (<5%). Numerous studies have highlighted the potential toxic effects of high doses of erucic acid on the heart, emphasizing the importance of limiting its concentration to 5–5.1 % (Downey 1983; Khansili and Rattu 2017). Blending two or more different oils can yield advantageous effects due to their distinct fatty acid compositions and functions. Similar observations have been made by various researchers, underscoring that no single indigenous oil is nutritionally optimal for edible purposes (Chugh and Dhawan 2014).

3.2 Physico-chemical properties

For each oil and their SO80:MO20 blend, various physical and chemical parameters were measured and tabulated in Table 2. Understanding physical and chemical properties enables us to evaluate the oil’s suitability for human consumption or industrial use. These metrics serve as important indicators of edible oil quality, as highlighted by previous studies (Azimah et al. 2017; Hashempour-Baltork et al. 2016).

3.3 Concentration of toxic trace metals

Monitoring toxic trace metals in edible oils is crucial for food safety and shelf-life. Metals like As, Cd, Cr, Pb, and Hg accelerate oil oxidation and pose health risks if consumed in excess. Recent research worldwide has focused on determining heavy metal levels in edible oils (Alrajhi and Idriss 2020).

The concentrations of toxic metals (Hg, As, Pb, Ni, Cr, Cd, Be, Se) in the SO80:MO20 blend and individual oils were measured and compared to WHO/FAO permissible levels (Table 3). Results showed that all metals in SO, MO, and BO were within the recommended limits established by WHO/FAO (Berg and Licht 2002). There were no significant differences among the oils, and all values were well below the maximum tolerable range for indigenous oils. Initial metal content affects oil stability during storage (List et al., 2005). The prepared blended oil had very low concentrations of toxic trace metals, making it suitable for consumption.

3.4 Aflatoxin determination

Aflatoxin level was determined in blended oil (SO80:MO20) and compared them with other edible oils (Table 4). Two soybean oil brands (Rupchanda and Teer) and three mustard oil brands (Hasan Wali, Radhuni, and Suresh) were tested. Aflatoxin levels varied significantly (p < 0.05). Teer Soybean Oil had the highest aflatoxin content at 2.959 ppb, followed by Radhuni Mustard Oil at 2.060 ppb. Rupchanda Soybean Oil, Hasan Wali Mustard Oil, and Suresh Mustard Oil had lower levels at 0.986, 1.153, and 0.125 ppb respectively. Blended oil had the lowest aflatoxin level at 0.015 ppb, well below the EU limit of 4 ppb for indigenous oils (Egmond et al., 2007). The result for blended oil was significantly lowered as compared to European’s limit. Yang et al., (2011) investigated a range of aflatoxin contamination (0.15 to 2.72 ppb) in edible oil grown in China.

3.5 Total antioxidant as DPPH free radicals scavenging capacity percentage

DPPH scavenging percentages were determined at different concentrations, as shown in Fig. 2A. Results indicated that scavenging percentages increased with concentration up to 1.0 mg/mL, then plateaued. IC₅₀ values, representing the concentration needed to scavenge 50 % of DPPH radicals, were calculated and presented in Fig. 2B. Lower IC₅₀ values indicate higher antioxidant activity (Khatoon et al. 2013).

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Fig. 2A

DPPH radical scavenging percentage across a varieties of samples.

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Fig. 2B

IC₅₀ value of different samples.

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At 1.5 mg/mL, blended oil showed the highest DPPH scavenging capacity at 95 % with an IC₅₀ of 0.5750 mg/mL, outperforming sesame and mustard oils. Mustard oil had a scavenging capacity of 94 % and an IC₅₀ of 0.5896 mg/mL, while sesame oil showed 93 % scavenging with an IC₅₀ of 0.6125 mg/mL. BHT and BHA demonstrated higher antioxidant capacities (97.6 % and 96.5 %, respectively) with IC₅₀ values of 0.4148 and 0.4637 mg/mL. The blended oil’s radical scavenging capacity was comparable to BHT and BHA at 1.5 mg/mL. A diet rich in antioxidants protects cells from free radicals, potentially preventing cancer, heart disease, and Alzheimer’s (Umamaheswari and Chatterjee 2008). Therefore, blended oil, with its additional antioxidants, is superior to single oils.

3.6 Shelf-life study

Table 5 shows the results, indicating a slight increase in moisture content for all samples, with a significant rise (p < 0.05) after 180 days, but all remained below the Codex Alimentarius limit of 0.2 % (Codex 2011).

The specific gravity of the blended oil (BO) decreased more slowly compared to soybean oil (SO) and mustard oil (MO), with non-significant decreases at 25 °C. The refractive index of BO increased slowly over 6 months, with no significant differences among the oils over time.

Free fatty acid (FFA) values ranged from 0.26 to 0.60 for SO and 0.15 to 0.46 for MO, showing a significant increase after 120 days. BO showed a very slow FFA increase (0.14 to 0.19) over 180 days, all within the safety limit of 0.6 % (Codex 2011).

Peroxide values increased over time, reaching 8.05 meqO₂/kg for SO, 7.99 meqO₂/kg for MO, and 5.22 meqO₂/kg for BO at 180 days. BO had a slower rise in peroxide value, with significant differences (p < 0.05) in all oils over time, consistent with previous studies on olive oil (Méndez and Falqué 2007). Peroxide levels above 10 meqO₂/kg indicate oxidation and off-flavors (Chugh and Dhawan 2014).

Over 180 days, SO had the highest p-anisidine value (1.90 to 12.5), followed by MO (1.24 to 10.1) and BO (1.0 to 9.5), with significant differences (p < 0.05). Peroxides break down into harmful chemicals, increasing the p-anisidine value and leading to oxidative rancidity.

The TOTOX value of the stored SO80:MO20 blended oil ranged from 3.52 to 19.94 meqO₂/kg oil, staying within the Codex standard limit of 26 meqO₂/kg oil (Codex 2011). In contrast, the TOTOX values for soybean oil (5.82 to 28.60 meqO₂/kg oil) and mustard oil (3.34 to 26.08 meqO₂/kg oil) exceeded the permissible range after 180 days, indicating rapid oxidation.

Iodine values (IV) decreased significantly (p < 0.05) from 100.67 to 86.91 gI₂/100 g for SO and from 119.30 to 115.4 for MO after 120 days, reflecting the decomposition of double bonds in the oil. BO showed a non-significant decrease in IV from 110 to 104 over 180 days. The saponification value decreased slightly for all oils over 180 days, with significant differences (p < 0.05) observed.

After 180 days of storage, the moisture content, refractive index, FFA, peroxide value, p-anisidine value, saponification value, iodine value, and TOTOX value of BO showed a slower increase compared to SO and MO. Despite some significant and non-significant changes, BO maintained consistent quality without developing an unpleasant smell over six months. The natural antioxidants in sesame and mustard oils likely contributed to this stability by preventing rapid oxidation.

Overall, the study found that the designed oil blend had superior shelf life and stability compared to individual oils. The desired fatty acid ratio in the blend may enhance its nutritional value and durability, consistent with findings from other research on indigenous oil blends (Murthi et al. 1987).