بررسی کیفیت انواع روغن‌های زیتون موجود در بازار براساس ویژگی‏های تعیین شده در استاندارد ملی ایران

Introduction: Today olive oil is widely produced and consumed in different countries due to its good taste and valuable nutritional properties (Moldao-Martins et al. 2004). Olive oil fatty acid profiles consist of saturated fatty acids (about 11%), monounsaturated fatty acids (in particular oleic acid) (about 80%), and polyunsaturated fatty acids (approximately 9%). In addition, olive oil contains antioxidant and anti-inflammatory compounds such as squalene, phytosterol, tocopherols, polyphenols, aromatic compounds and coloring pigments (Rafehi et al. 2012; Hernaez et al. 2015). These compounds have important effects on human health, nutritional value, sensory properties and oil stability (Kalua et al. 2007). Based on definition provided by Iranian national standard (1446) olive oil is obtained solely from the fruit of the olive tree to the exclusion of oils obtained using solvents or re-esterification processes and of any mixture with other kind of oil. It is marketed in accordance with the following designations and definitions: virgin olive oils, refined olive oil, olive-pomace oil and olive oil. Virgin olive oils are more expensive than other kind of olive oils because of their good taste and good nutritional values; therefore sometimes virgin olive oil sold is adulterated, cut with poor quality olive oil or cheaper oils such as sunflower and soybean oil (Moreda et al. 2003). Recently, there is a suspicion among consumers and regulatory agencies that some manufacturers (to increase their profits) have labeled lower quality olive oils as a virgin or extra virgin olive oil. A comprehensive study of food fraud over a 30-year period showed that the highest amount of food fraud was related to the olive oil. The study used scientific papers published between 1980 and 2010 (Moore et al. 2012). Therefore the aim of the current study was to investigate the properties of different types of olive oils (including virgin, refined and olive oil), distributed in the Iranian market.
Material and methods: Total of 18 samples of olive oil including, virgin olive oil (5 samples), ordinary virgin (3 samples), refined olive oil (5 samples) and olive oil (5 samples) were collected from the market (Table 1). The samples were transferred to the laboratory and kept in the refrigerator (4 °C) until they were analyzed. physicochemical and qualitative experiments (including acidity, peroxide, moisture content, insoluble impurities, extinction coefficient (K270, K232, k)); and oil purity (including identification and determination of sis and trans fatty acid, sterols, erythrodiol and uvaol content, 2-glyceryl monopalmitate, unsaponifiable matter, and ECN 42) were determined. Each sample was tested three times and the results were reported as "mean±standard deviation". Physicochemical properties including acidity, peroxide, moisture and insoluble impurities were measured according to the national standards No 4178, 4179, 4291, and 4095 respectively. The extinction coefficient (K232, K270, K) was determined using a spectrophotometer (EU 2200, ONLAB). Fatty acid profile was determined according to standard No 13126 (2016). Oil samples were sterified in methanol solution (2 M) for 30 min at 50 °C and methyl ester of fatty acids were determined by gas chromatography (GC-Yung Lin 6500) equipped with flame ionization detector (FID). Nitrogen gas (at the flow rate of 4 ml/min) was used as the carrier gas. The ECN 42 was calculated based on Iranian National Standard No17379. The 2-monoglycerol palmitate was measured using a gas chromatography equipped with a flame ionization detector (FID). The chromatographic column used was DB-5 (15m*0.25mm*0.53µm). Sterol and triterpene dialcohol composition including, desmethylsterol composition (% total sterols), Total sterol content, and Erythrodiol and uvaol content were determined using GC based on method provided by Iranian national standard (ISIRI 16324). All obtained results for olive oils were subjected to statistical analysis using statistical software (Minitab, 17:0, PA., State College, USA). The significant differences between mean values were determined by analysis of variance (ANOVA) with Post Hoc Tukey’s test. A probability value of 0.05 was used to determine the statistical significance. All data are reported as means ± standard deviations.
Results and discussion: Results showed that 8 out of 18 samples (44%), did not comply with standard 1446 (at least in one of the specifications). In 4 samples (about 22%) acidity was higher than the maximum permitted level in national standard No 1446 (Table 2). Acidity is one of the important criteria in the classification of olive oil types. This characteristic indicates the quality of the fruit used for oil production and its production conditions. One reason for the increase in acidity is the activation of enzymes by damage to olive fruit (Zegane et al. 2015). All samples were in compliance with the national standard 1446 in terms of peroxide values, moisture content, volatiles compound, and extinction coefficients (Table 2). The results of insoluble impurities in light petroleum showed that in two samples of olive oil and one sample of refined oil this value was higher than the maximum permitted level set by Iranian national standard (Table 2). Unsaponifiable matter in all samples varied between 0.71 and 1.82 g/kg and was in agreeing with related standard (Table 5). The fatty acid profiles of the investigated olive oil samples (C16 to C20) are shown in Table 3. In all samples, oleic and palmitic acids were the most abundant fatty acids followed by linoleic and stearic acid, respectively. The highest mean oleic acid (72.04±2.2) was related to refined olive oil. The results of this study were in agreement with those of other researchers (Matthaus et al., 2011). The trans fatty acids were not detected in any of the samples. The results obtained for sterol and triterpene dialcohol composition including, desmethylsterol composition (% total sterols), Total sterol content (mg/kg), erythrodiol and uvaol content (% total sterols) are presented in Tables 4 and 5. Desmethylsterol is composed of cholesterol, brassicasterol, campesterol, stigmasterol, delta-7-stigmastenol, and apparent beta-sitosterol. In two oil samples (43 and 14), the amount of Brassica sterol was higher than the standard limit (maximum 0.1%), showing that the other vegetable oils may have been added to olive oil. Maximum difference between the actual and theoretical ECN 42 triacylglycerol content are presented inTable 5. Results showed in 3 out of 18 samples (about 17%) ΔECN 42 was higher than standard limit.In all 18 samples, the amount of 2-monoglycerol palmitate ranged from 0.1% to 0.8% and was in accordance with national standard 1446 (Table 5).
Conclusion: Examination of 18 olive oil samples showed that olive oils distributed in the market, has an acceptable level of quality. The highest amount of non-conformity was related to the amount of sterols followed by acidity. Trans fatty acids were not detected in any of the samples.However, the characteristics studied in this study were not able to detect the virgin olive oil from those with lower quality (refined and olive oil). Therefore other experiments such as wax content and aliphatic alcohols are necessary to determine the small amount of olive pomace oil and stigmastadiene is needed to determine whether the olive oil is mixed with other vegetable oils. This study showed that it is difficult to distinguish between different types of olive oils and further research is necessary.